Author name code: nakariakov
ADS astronomy entries on 2022-09-14
author:"Nakariakov, Valery" 
------------------------------------------------------------------------  

Title: Two-Spacecraft Detection of Short-period Decayless Kink
    Oscillations of Solar Coronal Loops
Authors: Zhong, Sihui; Nakariakov, Valery M.; Kolotkov, Dmitrii Y.;
   Verbeeck, Cis; Berghmans, David
Bibcode: 2022arXiv220901917Z
Altcode:
  Decayless kink oscillations of an ensemble of loops are captured
  simultaneously by the High Resolution Imager (HRI) of the Extreme
  Ultraviolet Imager (EUI) and the Atmospheric Imaging Assembly (AIA)
  from 22:58 UT on 5 November to 00:27 UT on 6 November 2021. Oscillations
  are analysed by processing image sequences taken by the two instruments
  with a motion magnification technique. The analysed loops are around 51
  Mm in length, and oscillate with short periods of 1-3 min (1.6 min in
  average) and displacement amplitudes of 27-83 km. The signals recorded
  by AIA are delayed by 66 s as compared to HRI, which coincides with
  the light travel time difference from the Sun to each instrument. After
  correction of this time difference, the cross-correlation coefficient
  between the signals from the two data varies from 0.82 to 0.97,
  indicating that they are well consistent. This work confirms that HRI
  sees the same oscillations as AIA, which is the necessary first step
  before proceeding to the detection of shorter time scales by EUI. In
  addition, our results indicate the robustness of the de-jittering
  procedure in the study of kink oscillations with HRI.

Title: Probing the Density Fine Structuring of the Solar Corona with
    Comet Lovejoy
Authors: Nisticò, Giuseppe; Zimbardo, Gaetano; Perri, Silvia;
   Nakariakov, Valery M.; Duckenfield, Timothy J.; Druckmueller, Miloslav
Bibcode: 2022arXiv220904051N
Altcode:
  The passage of sungrazing comets in the solar corona can be a powerful
  tool to probe the local plasma properties. Here, we carry out a
  study of the striae pattern appearing in the tail of sungrazing Comet
  Lovejoy, as observed by the Atmospheric Imaging Assembly (AIA) aboard
  the Solar Dynamics Observatory (SDO) during the inbound and outbound
  phases of the comet orbit. We consider the images in EUV in the 171
  Å bandpass, where emission from oxygen ions O$^{4+}$ and O$^{5+}$
  is found. The striae are described as due to a beam of ions injected
  along the local magnetic field, with the initial beam velocity decaying
  because of collisions. Also, ion collisional diffusion contributes to
  ion propagation. Both the collision time for velocity decay and the
  diffusion coefficient for spatial spreading depend on the ambient plasma
  density. A probabilistic description of the ion beam density along the
  magnetic field is developed, where the beam position is given by the
  velocity decay and the spreading of diffusing ions is described by
  a Gaussian probability distribution. Profiles of emission intensity
  along the magnetic field are computed and compared with the profiles
  along the striae observed by AIA, showing a good agreement for most
  considered striae. The inferred coronal densities are then compared
  with a hydrostatic model of the solar corona. The results confirm that
  the coronal density is strongly spatially structured.

Title: Seismology of the solar corona and extensions to stellar
    coronae
Authors: Nakariakov, Valery
Bibcode: 2022cosp...44.2498N
Altcode:
  Wave motions detected in the upper part of the solar atmosphere, the
  corona, become a commonly used tool for probing physical conditions
  in the waveguiding plasma structures. Properties of the waves are
  sensitive to macroscopic parameters of the plasma, as well as transport
  coefficients and geometry and directivity of the magnetic field. Waves
  observed in the corona are of the magnetohydrodynamic (MHD) type, and
  are well described by MHD wave theory. The combination of high-precision
  observations with an array of spaceborne and ground-based instruments,
  and state-of-the-art theoretical modelling gives us estimations of
  physical parameters which are not open to direct observations, such
  as the absolute value of the magnetic field and its twist and free
  magnetic energy, electric currents, fine perpendicular structuring,
  the coronal heating function, and channels of the slow solar wind. A
  specific interest is paid to quasi-periodic (QPP) processes in
  flaring energy releases, as, on one hand it is natural to search for
  oscillatory processes in the vicinity of impulsive energy releases and,
  on the other, the enhanced radiation power can significantly improve
  the signal-to-noise ratio. We discuss possible links of QPP in solar
  and stellar flares with MHD wave processes, and implications of these
  links to plasma diagnostics.

Title: Probing the solar corona with sungrazing comets: magnetic
    field and coronal density estimates from the EUV striae in the tail
    of Comet Lovejoy
Authors: Nisticò, Giuseppe; Nakariakov, Valery; Zimbardo, Gaetano;
   Perri, Silvia; Duckenfield, Timothy; Druckmüller, Miloslav
Bibcode: 2022cosp...44.2501N
Altcode:
  The transit of Comet Lovejoy C/2011 W3 across the solar atmosphere in
  December 2011 offers us the opportunity to probe the physical conditions
  of the solar corona at distances that are still inaccessible to current
  spacecraft. The tail of Comet Lovejoy, as observed in EUV with the
  Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory
  (SDO), appeared in the form of striae caused by the emission of cometary
  ions injected along the local coronal magnetic field lines intersected
  by the comet. We studied the spatial and time evolution of such striae
  as observed in the channel at 171 \AA~ of SDO/AIA to acquire information
  on the local magnetic field and plasma density. The striae are modelled
  as a beam of oxygen ions, with the injection beam velocity decaying
  because of collisions. Also, oxygen ions spread in space because of
  collisional diffusion. Both the collision time for velocity decay and
  the diffusion coefficient depend on the ambient plasma density. We
  developed a probabilistic description of the ion beam density along
  the magnetic field. In such a model, the beam position is determined by
  the velocity decay and the spreading of diffusing ions is described by
  a Gaussian probability distribution. Profiles of the ion density along
  the magnetic field are computed and conveniently compared with the 171
  \AA~ intensity profiles along the striae to constrain the values of
  the local plasma density. Furthermore, transverse displacements of the
  striae observed in the data are interpreted in terms of propagating
  kink waves, whose properties allow us to estimate the magnitude of
  the local magnetic field.

Title: A new look at the frequency-dependent damping of slow-mode
    waves in the solar corona
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.
Bibcode: 2022MNRAS.514L..51K
Altcode: 2022MNRAS.tmpL..51K; 2022arXiv220505346K
  Being directly observed in the Doppler shift and imaging data and
  indirectly as quasi-periodic pulsations in solar and stellar flares,
  slow magnetoacoustic waves offer an important seismological tool
  for probing many vital parameters of the coronal plasma. A recently
  understood active nature of the solar corona for magnetoacoustic
  waves, manifested through the phenomenon of wave-induced thermal
  misbalance, led to the identification of new natural mechanisms for the
  interpretation of observed properties of waves. A frequency-dependent
  damping of slow waves in various coronal plasma structures
  remains an open question, as traditional wave damping theories
  fail to match observations. We demonstrate that accounting for the
  back-reaction caused by thermal misbalance on the wave dynamics
  leads to a modification of the relationship between the damping
  time and oscillation period of standing slow waves, prescribed by
  the linear theory. The modified relationship is not of a power-law
  form and has the equilibrium plasma conditions and properties of the
  coronal heating/cooling processes as free parameters. It is shown
  to readily explain the observed scaling of the damping time with
  period of standing slow waves in hot coronal loops. Functional forms
  of the unknown coronal heating process, consistent with the observed
  frequency-dependent damping, are seismologically revealed.

Title: High frequency waves in chromospheric spicules
Authors: Bate, William; Nakariakov, Valery; Jafarzadeh, Shahin; Jess,
   David; Stangalini, Marco; Grant, Samuel; Keys, Peter; Christian,
   Damian; Keenan, Francis
Bibcode: 2022cosp...44.2548B
Altcode:
  Using high cadence observations from the Hydrogen-alpha Rapid
  Dynamics camera imaging system on the Dunn Solar Telescope, we
  present an investigation of the statistical properties of transverse
  oscillations in spicules captured above the solar limb. At five equally
  separated atmospheric heights, spanning approximately $4900-7500$~km,
  we have detected a total of $15{\,}959$ individual wave events,
  with a mean displacement amplitude of $151\pm 124$~km, a mean
  period of $54\pm 45$~s, and a mean projected velocity amplitude
  of $21\pm 13$~km{\,}s$^{-1}$. We find that both the displacement
  and velocity amplitudes increase with height above the solar limb,
  ranging from $132\pm 111$~km and $17.7\pm 10.6$~km{\,}s$^{-1}$ at
  $\approx4900$~km, and $168\pm 125$~km and $26.3\pm 14.1$~km{\,}s$^{-1}$
  at $\approx7500$~km, respectively. Following the examination of
  neighboring oscillations in time and space, we find 45% of the waves
  to be upwardly propagating, 49% to be downwardly propagating, and 6%
  to be standing, with mean absolute phase velocities for the propagating
  waves on the order of $75-150$~km{\,}s$^{-1}$. While the energy flux
  of the waves propagating downwards does not appear to depend on height,
  we find the energy flux of the upwardly propagating waves decreases with
  atmospheric height at a rate of $-13{\,}200\pm6500$~W{\,}m$^{-2}$/Mm. As
  a result, this decrease in energy flux as the waves propagate upwards
  may provide significant thermal input into the local plasma.

Title: Long-term evolution of decayless kink oscillations of solar
    coronal loops
Authors: Zhong, Sihui; Nakariakov, Valery M.; Kolotkov, Dmitrii Y.;
   Anfinogentov, Sergey A.
Bibcode: 2022MNRAS.513.1834Z
Altcode: 2022MNRAS.tmp.1009Z
  Long-term evolution of instantaneous parameters of decayless kink
  oscillations of six solar coronal loops observed for longer than
  2 h each is studied. The oscillations are analysed by processing
  sequences of 171 Å images obtained with the Solar Dynamics Observatory
  (SDO)/Atmospheric Imaging Assembly (AIA) in the time interval from 2020
  December till 2021 June, with the motion magnification technique. It
  is established that decayless kink oscillations could exist for
  more than 30 or 40 oscillation cycles. Neither the loop brightness
  nor instantaneous parameters of the oscillations show a monotonic
  increase or decrease during the oscillation. The observed instantaneous
  oscillation periods and amplitudes are found to vary randomly in time,
  with distributions around the mean values that resemble Gaussian
  profiles. Mean values of the oscillation periods and amplitudes are
  consistent with previous observations of this phenomenon. A power-law
  dependence of the oscillation period on the displacement amplitude
  is found, with the power-law index of 0.41 and with the 95 per cent
  confidence interval of [0.39, 0.71]. In general, we established the
  lack of correlation between instantaneous oscillation parameters and
  loop brightness. One exception is an event with relatively strong
  anticorrelation of the amplitude and the loop's brightness, with
  the cross-correlation coefficient of about -0.81, but this effect
  requires a further study. Fourier power spectra of the envelopes of
  the time-evolving instantaneous amplitudes and periods are white noise,
  indicating that consecutive values of the instantaneous parameters are
  independent of each other. The results obtained provide an empirical
  ground for validating and comparing existing and future theoretical
  models of decayless kink oscillations of coronal loops.

Title: Kink Oscillation of a Flux Rope During a Failed Solar Eruption
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Karpen, Judith T.;
   Richard DeVore, C.; Cho, Kyung-Suk
Bibcode: 2022ApJ...932L...9K
Altcode: 2022arXiv220503480K
  We report a decaying kink oscillation of a flux rope during a confined
  eruptive flare, observed off the solar limb by the Solar Dynamics
  Observatory's Atmospheric Imaging Assembly (AIA), which lacked a
  detectable white-light coronal mass ejection. The erupting flux rope
  underwent kinking, rotation, and apparent leg-leg interaction during the
  event. The oscillations were observed simultaneously in multiple AIA
  channels at 304, 171, and 193 Å, indicating that multithermal plasma
  was entrained in the rope. After reaching the overlying loops in the
  active region, the flux rope exhibited large-amplitude, decaying kink
  oscillations with an apparent initial amplitude of 30 Mm, a period of
  about 16 minutes, and a decay time of about 17 minutes. We interpret
  these oscillations as a fundamental standing kink mode of the flux
  rope. The oscillation polarization has a clear vertical component,
  while the departure of the detected waveform from a sinusoidal signal
  suggests that the oscillation could be circularly or elliptically
  polarized. The estimated kink speed is 1080 km s<SUP>-1</SUP>,
  corresponding to an Alfvén speed of about 760 km s<SUP>-1</SUP>. This
  speed, together with the estimated electron density in the rope from our
  differential emission measure analysis, n <SUB> e </SUB> ≍ (1.5-2.0)
  × 10<SUP>9</SUP> cm<SUP>-3</SUP>, yields a magnetic-field strength of
  about 15 G. To the best of our knowledge, decaying kink oscillations of
  a flux rope with nonhorizontal polarization during a confined eruptive
  flare have not been reported before. These oscillations provide unique
  opportunities for indirect measurements of the magnetic-field strength
  in low-coronal flux ropes during failed eruptions.

Title: Slow Magnetoacoustic Oscillations in Stellar Coronal Loops
Authors: Lim, Daye; Nakariakov, Valery M.; Moon, Yong-Jae
Bibcode: 2022ApJ...931...63L
Altcode: 2022arXiv220511750L
  Slow magnetoacoustic oscillations in stellar coronal loops with
  gravitational stratification are analyzed with a numerical solution
  of the boundary value problem for eigenvalues and eigenfunctions. In
  this study, we only focus on the resonant periods. The effects of the
  gravitational stratification, star mass, loop temperature, and loop
  length on the properties of slow magnetoacoustic oscillations are
  investigated. It is shown that the discrepancy between stratified
  and nonstratified loops is higher in density perturbations than
  in velocity perturbations. When the star has a larger mass, higher
  coronal temperature, and longer loop, the density perturbations in
  the stratified loop are significantly different from the harmonic
  functions. The periods in the stratified loop are slightly longer than
  in the nonstratified loop. The periods calculated in our model (14-644
  minutes) are consistent with the periods of stellar quasi-periodic
  pulsations observed in both soft X-rays (2-70 minutes) and white lights
  (8-390 minutes).

Title: High-frequency Waves in Chromospheric Spicules
Authors: Bate, W.; Jess, D. B.; Nakariakov, V. M.; Grant, S. D. T.;
   Jafarzadeh, S.; Stangalini, M.; Keys, P. H.; Christian, D. J.; Keenan,
   F. P.
Bibcode: 2022ApJ...930..129B
Altcode: 2022arXiv220304997B
  Using high-cadence observations from the Hydrogen-alpha Rapid
  Dynamics camera imaging system on the Dunn Solar Telescope, we
  present an investigation of the statistical properties of transverse
  oscillations in spicules captured above the solar limb. At five equally
  separated atmospheric heights, spanning approximately 4900-7500 km,
  we have detected a total of 15,959 individual wave events, with a
  mean displacement amplitude of 151 ± 124 km, a mean period of 54
  ± 45 s, and a mean projected velocity amplitude of 21 ± 13 km
  s<SUP>-1</SUP>. We find that both the displacement and velocity
  amplitudes increase with height above the solar limb, ranging from
  132 ± 111 km and 17.7 ± 10.6 km s<SUP>-1</SUP> at ≍4900 km,
  and 168 ± 125 km and 26.3 ± 14.1 km s<SUP>-1</SUP> at ≍7500 km,
  respectively. Following the examination of neighboring oscillations in
  time and space, we find 45% of the waves to be upwardly propagating,
  49% to be downwardly propagating, and 6% to be standing, with mean
  absolute phase velocities for the propagating waves on the order of
  75-150 km s<SUP>-1</SUP>. While the energy flux of the waves propagating
  downwards does not appear to depend on height, we find the energy flux
  of the upwardly propagating waves decreases with atmospheric height
  at a rate of -13,200 ± 6500 W m<SUP>-2</SUP>/Mm. As a result, this
  decrease in energy flux as the waves propagate upwards may provide
  significant thermal input into the local plasma.

Title: Editorial to the Topical Collection: Oscillatory Processes
    in Solar and Stellar Coronae
Authors: Nakariakov, Valery M.; Banerjee, Dipankar; Li, Bo; Wang,
   Tongjiang; Zimovets, Ivan V.; Falanga, Maurizio
Bibcode: 2022SSRv..218...13N
Altcode:
  No abstract at ADS

Title: The dynamics and observability of circularly polarized
    kink waves
Authors: Magyar, N.; Duckenfield, T.; Van Doorsselaere, T.; Nakariakov,
   V. M.
Bibcode: 2022A&A...659A..73M
Altcode: 2021arXiv211214951M
  Context. Kink waves are routinely observed in coronal loops. Resonant
  absorption is a well-accepted mechanism that extracts energy
  from kink waves. Nonlinear kink waves are know to be affected by
  the Kelvin-Helmholtz instability. However, all previous numerical
  studies consider linearly polarized kink waves. <BR /> Aims: We study
  the properties of circularly polarized kink waves on straight plasma
  cylinders, for both standing and propagating waves, and we compare them
  to the properties of linearly polarized kink waves. <BR /> Methods:
  We used the code MPI-AMRVAC to solve the full 3D magnetohydrodynamic
  equations for a straight magnetic cylinder, excited by both standing and
  propagating circularly polarized kink (m = 1) modes. <BR /> Results: The
  damping due to resonant absorption is independent of the polarization
  state. The morphology or appearance of the induced resonant flow is
  different for the two polarizations; however, there are essentially
  no differences in the forward-modeled Doppler signals. For nonlinear
  oscillations, the growth rate of small scales is determined by the total
  energy of the oscillation rather than the perturbation amplitude. We
  discuss possible implications and seismological relevance. <P
  />Movies associated to Figs. 1, 2, and 5 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141945/olm">https://www.aanda.org</A>

Title: Hot Jets in the Solar Corona: Creating a Catalogue of Events
    Based on Multi-Instrumental Observations
Authors: Kaltman, T. I.; Stupishin, A. G.; Anfinogentov, S. A.;
   Nakariakov, V. M.; Loukitcheva, M. A.; Shendrik, A. V.
Bibcode: 2021Ge&Ae..61.1083K
Altcode:
  We present a catalogue of solar coronal plasma jets with a temperature
  above 0.5 MK, which includes primary information about the events,
  parameters of the diagnosed jets, as well as related eruptive
  phenomena. The catalogue (https://solar.sao.ru/coronal-jets-catalog/)
  contains data obtained using the spaceborne EUV high-precision
  telescope SDO/AIA and ground-based radio telescopes and spectrometers,
  including RATAN-600, SRH and NoRH. For a number of events data on
  the reconstructed magnetic field is also presented. The purpose
  of the catalogue is to provide summary information on coronal jets
  for further statistical analysis, determination of characteristic
  parameters of jets, and for in-depth study of the individual events
  by all interested researchers.

Title: Multiwavelength Quasi-periodic Pulsations in a Stellar
    Superflare
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Holt, Robin;
   Kuznetsov, Alexey A.
Bibcode: 2021ApJ...923L..33K
Altcode: 2021arXiv211207734K
  We present the first multiwavelength simultaneous detection of
  quasi-periodic pulsations (QPPs) in a superflare (more than a thousand
  times stronger than known solar flares) on a cool star, in soft X-rays
  (SXRs, with XMM-Newton) and white light (WL, with Kepler). It allowed
  for the first ever analysis of oscillatory processes in a stellar flare
  simultaneously in thermal and nonthermal emissions, conventionally
  considered to come from the corona and chromosphere of the star,
  respectively. The observed QPPs have periods 1.5 ± 0.15 hr (SXR)
  and 3 ± 0.6 hr (WL), and correlate well with each other. The unique
  relationship between the observed parameters of QPPs in SXR and WL
  allowed us to link them with oscillations of the electric current in the
  flare loop, which directly affect the dynamics of nonthermal electrons
  and indirectly (via ohmic heating) the thermal plasma. These findings
  could be considered in favor of the equivalent LCR contour model of a
  flare loop, at least in the extreme conditions of a stellar superflare.

Title: The solar corona as an active medium for magnetoacoustic waves
Authors: Kolotkov, D. Y.; Zavershinskii, D. I.; Nakariakov, V. M.
Bibcode: 2021PPCF...63l4008K
Altcode: 2021arXiv211102370K
  The presence and interplay of continuous cooling and heating processes
  maintaining the corona of the Sun at the observed one million K
  temperature were recently understood to have crucial effects on the
  dynamics and stability of magnetoacoustic waves. These essentially
  compressive waves perturb the coronal thermal equilibrium, leading to
  the phenomenon of a wave-induced thermal misbalance. Representing an
  additional natural mechanism for the exchange of energy between the
  plasma and the wave, thermal misbalance makes the corona an active
  medium for magnetoacoustic waves, so that the wave can not only lose
  but also gain energy from the coronal heating source (similarly to
  burning gases, lasers and masers). We review recent achievements in
  this newly emerging research field, focussing on the effects that
  slow-mode magnetoacoustic waves experience as a back-reaction of
  this perturbed coronal thermal equilibrium. The new effects include
  enhanced frequency-dependent damping or amplification of slow waves,
  and effective, not associated with the coronal plasma non-uniformity,
  dispersion. We also discuss the possibility to probe the unknown coronal
  heating function by observations of slow waves and linear theory of
  thermal instabilities. The manifold of the new properties that slow
  waves acquire from a thermodynamically active nature of the solar corona
  indicate a clear need for accounting for the effects of combined coronal
  heating/cooling processes not only for traditional problems of the
  formation and evolution of prominences and coronal rain, but also for
  an adequate modelling and interpretation of magnetohydrodynamic waves.

Title: The high-energy Sun - probing the origins of particle
    acceleration on our nearest star
Authors: Matthews, S. A.; Reid, H. A. S.; Baker, D.; Bloomfield, D. S.;
   Browning, P. K.; Calcines, A.; Del Zanna, G.; Erdelyi, R.; Fletcher,
   L.; Hannah, I. G.; Jeffrey, N.; Klein, L.; Krucker, S.; Kontar, E.;
   Long, D. M.; MacKinnon, A.; Mann, G.; Mathioudakis, M.; Milligan,
   R.; Nakariakov, V. M.; Pesce-Rollins, M.; Shih, A. Y.; Smith, D.;
   Veronig, A.; Vilmer, N.
Bibcode: 2021ExA...tmp..135M
Altcode:
  As a frequent and energetic particle accelerator, our Sun provides
  us with an excellent astrophysical laboratory for understanding
  the fundamental process of particle acceleration. The exploitation
  of radiative diagnostics from electrons has shown that acceleration
  operates on sub-second time scales in a complex magnetic environment,
  where direct electric fields, wave turbulence, and shock waves all
  must contribute, although precise details are severely lacking. Ions
  were assumed to be accelerated in a similar manner to electrons, but
  γ-ray imaging confirmed that emission sources are spatially separated
  from X-ray sources, suggesting distinctly different acceleration
  mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic
  understanding of accelerated particle spectra and the total energy
  budgets are therefore poorly constrained. Additionally, the recent
  detection of relativistic ion signatures lasting many hours, without
  an electron counterpart, is an enigma. We propose a single platform
  to directly measure the physical conditions present in the energy
  release sites and the environment in which the particles propagate and
  deposit their energy. To address this fundamental issue, we set out
  a suite of dedicated instruments that will probe both electrons and
  ions simultaneously to observe; high (seconds) temporal resolution
  photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV -
  30 MeV), polarization measurements (5-1000 keV) and high spatial and
  temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray)
  regimes. These instruments will observe the broad range of radiative
  signatures produced in the solar atmosphere by accelerated particles.

Title: Motion Magnification in Solar Imaging Data Sequences in the
    Sub-pixel Regime
Authors: Zhong, Sihui; Duckenfield, Timothy J.; Nakariakov, Valery M.;
   Anfinogentov, Sergey A.
Bibcode: 2021SoPh..296..135Z
Altcode:
  The capability of the motion-magnification technique for the detection
  of transverse oscillations, such as kink oscillations of solar coronal
  loops observed with an imaging telescope, in the sub-pixel regime is
  investigated. The technique is applied to artificial-image sequences
  imitating harmonic transverse displacements of the loop, observed
  in the optically thin regime. Motion magnification is found to work
  well on the analysis of sub-pixel, ≥0.01 pixel oscillations, and it
  is characterised by linear scaling between the magnified amplitude
  and input amplitude. Oscillations of loops with transverse density
  profiles of different steepness are considered. After magnification,
  the original transverse profiles are preserved sufficiently well. The
  motion-magnification performance is found to be robust in noisy data,
  for coloured noise with spectral indices ranging from 0 to 3, and
  additional Poisson noise with a signal-to-background-noise ratio down to
  unity. Our findings confirm the reliability of the motion-magnification
  technique for applications in magnetohydrodynamic seismology of the
  solar corona.

Title: Kink Oscillations of Coronal Loops
Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Antolin, P.; Jain, R.;
   Kolotkov, D. Y.; Kupriyanova, E. G.; Li, D.; Magyar, N.; Nisticò, G.;
   Pascoe, D. J.; Srivastava, A. K.; Terradas, J.; Vasheghani Farahani,
   S.; Verth, G.; Yuan, D.; Zimovets, I. V.
Bibcode: 2021SSRv..217...73N
Altcode: 2021arXiv210911220N
  Kink oscillations of coronal loops, i.e., standing kink waves, is
  one of the most studied dynamic phenomena in the solar corona. The
  oscillations are excited by impulsive energy releases, such as low
  coronal eruptions. Typical periods of the oscillations are from a
  few to several minutes, and are found to increase linearly with the
  increase in the major radius of the oscillating loops. It clearly
  demonstrates that kink oscillations are natural modes of the loops,
  and can be described as standing fast magnetoacoustic waves with the
  wavelength determined by the length of the loop. Kink oscillations are
  observed in two different regimes. In the rapidly decaying regime,
  the apparent displacement amplitude reaches several minor radii of
  the loop. The damping time which is about several oscillation periods
  decreases with the increase in the oscillation amplitude, suggesting a
  nonlinear nature of the damping. In the decayless regime, the amplitudes
  are smaller than a minor radius, and the driver is still debated. The
  review summarises major findings obtained during the last decade,
  and covers both observational and theoretical results. Observational
  results include creation and analysis of comprehensive catalogues of
  the oscillation events, and detection of kink oscillations with imaging
  and spectral instruments in the EUV and microwave bands. Theoretical
  results include various approaches to modelling in terms of the
  magnetohydrodynamic wave theory. Properties of kink oscillations are
  found to depend on parameters of the oscillating loop, such as the
  magnetic twist, stratification, steady flows, temperature variations
  and so on, which make kink oscillations a natural probe of these
  parameters by the method of magnetohydrodynamic seismology.

Title: Fast magnetoacoustic wave trains: from tadpoles to boomerangs
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Moss, Guy;
   Shellard, Paul
Bibcode: 2021MNRAS.505.3505K
Altcode: 2021arXiv210513696K
  Rapidly propagating fast magnetoacoustic wave trains guided by
  field-aligned plasma non-uniformities are confidently observed in the
  Sun's corona. Observations at large heights suggest that fast wave
  trains can travel long distances from the excitation locations. We
  study characteristic time signatures of fully developed, dispersive fast
  magnetoacoustic wave trains in field-aligned zero-β plasma slabs in the
  linear regime. Fast wave trains are excited by a spatially localized
  impulsive driver and propagate along the waveguide as prescribed by
  the waveguide-caused dispersion. In slabs with steeper transverse
  density profiles, developed wave trains are shown to consist of three
  distinct phases: a long-period quasi-periodic phase with the oscillation
  period shortening with time, a multiperiodic (peloton) phase in which
  distinctly different periods co-exist, and a short-lived periodic Airy
  phase. The appearance of these phases is attributed to a non-monotonic
  dependence of the fast wave group speed on the parallel wavenumber
  due to the waveguide dispersion, and is shown to be different for
  axisymmetric (sausage) and non-axisymmetric (kink) modes. In wavelet
  analysis, this corresponds to the transition from the previously known
  tadpole shape to a new boomerang shape of the wave train spectrum, with
  two well-pronounced arms at shorter and longer periods. We describe
  a specific previously published radio observation of a coronal fast
  wave train, highly suggestive of a change of the wavelet spectrum from
  a tadpole to a boomerang, broadly consistent with our modelling. The
  applicability of these boomerang-shaped fast wave trains for probing the
  transverse structuring of the waveguiding coronal plasma is discussed.

Title: Diagnostics of plasma jets in the solar corona
Authors: Anfinogentov, Sergey; Kaltman, Tatyana; Stupishin, Alexey;
   Nakariakov, Valery; Lukitcheva, Maria
Bibcode: 2021STP.....7b...3A
Altcode:
  The paper discusses the diagnostics of plasma jets in the solar corona
  with the use of data from modern space- and ground-based telescopes
  observing the Sun in the extreme ultraviolet (EUV) and micro- wave
  bands. We examine observational parameters of EUV and radio emission
  in events associated with plasma jets, depending on the mechanism
  of formation, initiation conditions, and evolution of the jets. The
  opportunities provided by the study of plasma jets, which relies on
  simultaneous observations in different bands, are highlighted. For
  a number of jets, we have measured their primary parameters; and in
  this paper we present preliminary results of statistical processing
  of the data obtained. Microwave observations of several specific
  events, made by ground-based instruments RATAN-600, SRH, and Nobeyama
  Radioheliograph, are considered in detail. The diagnostic capabilities
  of these instruments for studying coronal jets are shown. To analyze
  the three-dimensional structure of the coronal magnetic field, we
  have used SDO/HMI data, which allowed for the reconstruction of the
  field in the lower corona. The information gained is compared with the
  results of diagnostics of the magnetic field at the base of the corona
  according to RATAN-600 data. The purpose of the methods developed is
  to determine the physical mechanisms responsible for the generation,
  collimation, and dynamics of plasma jets in the solar atmosphere.

Title: Spectroscopic Detection of Alfvénic Waves in the Chromosphere
    of Sunspot Regions
Authors: Chae, Jongchul; Cho, Kyuhyoun; Nakariakov, Valery M.; Cho,
   Kyung-Suk; Kwon, Ryun-Young
Bibcode: 2021ApJ...914L..16C
Altcode:
  Transverse magnetohydrodynamic waves often called Alfvénic (or
  kink) waves have been often theoretically put forward to solve the
  outstanding problems of the solar corona like coronal heating, solar
  wind acceleration, and chemical abundance enhancement. Here we report
  the first spectroscopic detection of Alfvénic waves around a sunspot at
  chromospheric heights. By analyzing the spectra of the Hα line and Ca
  II 854.2 nm line, we determined line-of-sight velocity and temperature
  as functions of position and time. As a result, we identified transverse
  magnetohydrodynamic waves pervading the superpenumbral fibrils. These
  waves are characterized by the periods of 2.5 to 4.5 minutes, and
  the propagation direction parallel to the fibrils, the supersonic
  propagation speeds of 45 to 145 km s<SUP>-1</SUP>, and the close
  association with umbral oscillations and running penumbral waves in
  sunspots. Our results support the notion that the chromosphere around
  sunspots abounds with Alfvénic waves excited by the mode conversion
  of the upward-propagating slow magnetoacoustic waves.

Title: Could Switchbacks Originate in the Lower Solar
    Atmosphere? II. Propagation of Switchbacks in the Solar Corona
Authors: Magyar, Norbert; Utz, Dominik; Erdélyi, Robertus; Nakariakov,
   Valery M.
Bibcode: 2021ApJ...914....8M
Altcode: 2021arXiv210410126M
  The magnetic switchbacks observed recently by the Parker Solar Probe
  have raised the question about their nature and origin. One of the
  competing theories of their origin is the interchange reconnection
  in the solar corona. In this scenario, switchbacks are generated at
  the reconnection site between open and closed magnetic fields, and
  are either advected by an upflow or propagate as waves into the solar
  wind. In this paper we test the wave hypothesis, numerically modeling
  the propagation of a switchback, modeled as an embedded Alfvén wave
  packet of constant magnetic field magnitude, through the gravitationally
  stratified solar corona with different degrees of background magnetic
  field expansion. While switchbacks propagating in a uniform medium with
  no gravity are relatively stable, as reported previously, we find that
  gravitational stratification together with the expansion of the magnetic
  field act in multiple ways to deform the switchbacks. These include WKB
  effects, which depend on the degree of magnetic field expansion, and
  also finite-amplitude effects, such as the symmetry breaking between
  nonlinear advection and the Lorentz force. In a straight or radially
  expanding magnetic field the propagating switchbacks unfold into waves
  that cause minimal magnetic field deflections, while a super-radially
  expanding magnetic field aids in maintaining strong deflections. Other
  important effects are the mass uplift the propagating switchbacks
  induce and the reconnection and drainage of plasmoids contained within
  the switchbacks. In the Appendix, we examine a series of setups with
  different switchback configurations and parameters, which broaden the
  scope of our study.

Title: Probing the solar corona magnetic field with sungrazing comets
Authors: Nisticò, Giuseppe; Nakariakov, Valery M.; Duckenfield,
   Timothy; Druckmüller, Miloslav; Zimbardo, Gaetano
Bibcode: 2021EGUGA..2314653N
Altcode:
  Space telescopes of the SoHO, STEREO and SDO missions have occasionally
  acquired observations of comets, providing an interesting opportunity
  to investigate the structure and dynamics of the heliospheric
  plasma. Cometary plasma tails exhibit a wave-like motion, which is
  believed to be a response to the physical conditions of the local
  interplanetary medium. Furthermore, sungrazing comets diving in the
  solar atmosphere provide us with an unprecedented way to diagnose the
  coronal plasma at distances which are unaccessible from the current
  spacecraft. Here, we present observations of Comet Lovejoy C/2011 W3
  from SDO/AIA, which was seen to cross the EUV solar corona in December
  2011. The cometary ions produced by the sublimation of the comet
  nucleus were channelled along the magnetic field lines forming some
  filamented structures. Such structures appear to show small amplitude
  kink oscillations, which are used to determine the magnitude of the
  coronal magnetic field by coronal seismology.

Title: Kinematic Oscillations of Post-CME Blobs Detected by K-Cor
    on 2017 September 10
Authors: Lee, Jae-Ok; Cho, Kyung-Suk; Nakariakov, Valery M.; Lee,
   Harim; Kim, Rok-Soon; Jang, Soojeong; Yang, Heesu; Kim, Sujin; Kim,
   Yeon-Han
Bibcode: 2021JKAS...54...61L
Altcode:
  We investigate 20 post-coronal mass ejection (CME) blobs formed in
  the post-CME current sheet (CS) that were observed by K-Cor on 2017
  September 10. By visual inspection of the trajectories and projected
  speed variations of each blob, we find that all blobs except one
  show irregular "zigzag" trajectories resembling transverse oscillatory
  motions along the CS, and have at least one oscillatory pattern in their
  instantaneous radial speeds. Their oscillation periods are ranging from
  30 to 91 s and their speed amplitudes from 128 to 902 km/s. Among 19
  blobs, 10 blobs have experienced at least two cycles of radial speed
  oscillations with different speed amplitudes and periods, while 9 blobs
  undergo one oscillation cycle. To examine whether or not the apparent
  speed oscillations can be explained by vortex shedding, we estimate
  the quantitative parameter of vortex shedding, the Strouhal number,
  by using the observed lateral widths, linear speeds, and oscillation
  periods of the blobs. We then compare our estimates with theoretical
  and experimental results from MHD simulations and fluid dynamics
  experiments. We find that the observed Strouhal numbers range from 0.2
  to 2.1, consistent with those (0.15-3.0) from fluid dynamics experiments
  of bluff spheres, while they are higher than those (0.15-0.25) from MHD
  simulations of cylindrical shapes. We thus find that blobs formed in
  a post-CME CS undergo kinematic oscillations caused by fluid dynamic
  vortex shedding. The vortex shedding is driven by the interaction
  of the outward-moving blob having a bluff spherical shape with the
  background plasma in the post-CME CS.

Title: Could Switchbacks Originate in the Lower Solar
    Atmosphere? I. Formation Mechanisms of Switchbacks
Authors: Magyar, Norbert; Utz, Dominik; Erdélyi, Robertus; Nakariakov,
   Valery M.
Bibcode: 2021ApJ...911...75M
Altcode: 2021arXiv210303726M
  The recent rediscovery of magnetic field switchbacks or deflections
  embedded in the solar wind flow by the Parker Solar Probe mission lead
  to a huge interest in the modeling of the formation mechanisms and
  origin of these switchbacks. Several scenarios for their generation were
  put forth, ranging from lower solar atmospheric origins by reconnection,
  to being a manifestation of turbulence in the solar wind, and so
  on. Here we study some potential formation mechanisms of magnetic
  switchbacks in the lower solar atmosphere, using three-dimensional
  magnetohydrodynamic (MHD) numerical simulations. The model is that of an
  intense flux tube in an open magnetic field region, aiming to represent
  a magnetic bright point opening up to an open coronal magnetic field
  structure, e.g., a coronal hole. The model is driven with different
  plasma flows in the photosphere, such as a fast up-shooting jet,
  as well as shearing flows generated by vortex motions or torsional
  oscillations. In all scenarios considered, we witness the formation
  of magnetic switchbacks in regions corresponding to chromospheric
  heights. Therefore, photospheric plasma flows around the foot-points
  of intense flux tubes appear to be suitable drivers for the formation
  of magnetic switchbacks in the lower solar atmosphere. Nevertheless,
  these switchbacks do not appear to be able to enter the coronal heights
  of the simulation in the present model. In conclusion, based on the
  presented simulations, switchbacks measured in the solar wind are
  unlikely to originate from photospheric or chromospheric dynamics.

Title: On the Nature of Propagating Intensity Disturbances in Polar
    Plumes during the 2017 Total Solar Eclipse
Authors: Cho, Kyung-Suk; Cho, Il-Hyun; Madjarska, Maria S.; Nakariakov,
   Valery M.; Yang, Heesu; Choi, Seonghwan; Lim, Eun-Kyung; Lee,
   Kyung-Sun; Seough, Jung-Jun; Lee, Jaeok; Kim, Yeon-Han
Bibcode: 2021ApJ...909..202C
Altcode: 2021arXiv210202085C
  The propagating intensity disturbances (PIDs) in plumes are still
  poorly understood, and their identity (magnetoacoustic waves or flows)
  remains an open question. We investigate PIDs in five plumes located
  in the northern polar coronal hole observed during the 2017 total solar
  eclipse. Three plumes are associated with coronal bright points, jets,
  and macrospicules at their base (active plumes), and the other two
  plumes are not (quiet plumes). The electron temperature at the base of
  the plumes is obtained from the filter ratio of images taken with the
  X-ray Telescope on board Hinode and the passband ratio around 400 nm
  from an eclipse instrument, the Diagnostic Coronagraph Experiment. The
  phase speed (v<SUB>r</SUB>), frequency (ω), and wavenumber (k) of the
  PIDs in the plumes are obtained by applying a Fourier transformation to
  the spacetime (r - t plane) plots in images taken with the Atmospheric
  Imaging Assembly (AIA) in three different wavelength channels (171,
  193, and 211 Å). We found that the PIDs in the higher-temperature AIA
  channels, 193 and 211 Å, are faster than that of the cooler AIA 171
  Å channel. This tendency is more significant for the active plumes
  than the quiet ones. The observed speed ratio (∼1.3) between the
  AIA 171 and 193 Å channels is similar to the theoretical value (1.25)
  of a slow magnetoacoustic wave. Our results support the idea that PIDs
  in plumes represent a superposition of slow magnetoacoustic waves and
  plasma outflows that consist of dense cool flows and hot coronal jets.

Title: The effect of the magnetic field on the damping of slow waves
    in the solar corona
Authors: Duckenfield, T. J.; Kolotkov, D. Y.; Nakariakov, V. M.
Bibcode: 2021A&A...646A.155D
Altcode: 2020arXiv201110437D
  Context. Slow magnetoacoustic waves are routinely observed in
  astrophysical plasma systems such as the solar corona, and they are
  usually seen to damp rapidly. As a slow wave propagates through a
  plasma, it modifies the equilibrium quantities of density, temperature,
  and the magnetic field. In the corona and other plasma systems, the
  thermal equilibrium is comprised of a balance between continuous
  heating and cooling processes, the magnitudes of which vary with
  density, temperature and the magnetic field. Thus the wave may induce
  a misbalance between these competing processes. Its back reaction on
  the wave has been shown to lead to dispersion, and amplification or
  damping, of the wave. <BR /> Aims: This effect of heating and cooling
  misbalance has previously been studied in the infinite magnetic field
  approximation in a plasma whose thermal equilibrium is comprised of
  optically thin radiative losses and field-aligned thermal conduction,
  balanced by an (unspecified) heating process. In this work we extend
  this analysis by considering a non-zero β plasma. The importance
  of the effect of the magnetic field in the rapid damping of slow
  waves in the solar corona is evaluated and compared to the effects of
  thermal conduction. <BR /> Methods: A linear perturbation under the
  thin flux tube approximation is considered, and a dispersion relation
  describing the slow magnetoacoustic modes is found. The dispersion
  relation's limits of strong non-adiabaticity and weak non-adiabaticity
  are studied. The characteristic timescales were calculated for plasma
  systems with a range of typical coronal densities, temperatures, and
  magnetic field strengths. <BR /> Results: The number of timescales
  characterising the effect of the misbalance is found to remain at
  two, as with the infinite magnetic field case. In the non-zero β
  case, these two timescales correspond to the partial derivatives of
  the combined heating and cooling function with respect to constant
  gas pressure and with respect to constant magnetic pressure. The
  predicted damping times of slow waves from thermal misbalance in the
  solar corona are found to be of the order of 10-100 min, coinciding
  with the wave periods and damping times observed. Moreover, the slow
  wave damping by thermal misbalance is found to be comparable to the
  damping by field-aligned thermal conduction. The change in damping with
  plasma-β is complex and depends on the coronal heating function's
  dependence on the magnetic field in particular. Nonetheless, we show
  that in the infinite field limit, the wave dynamics is insensitive to
  the dependence of the heating function on the magnetic field, and this
  approximation is found to be valid in the corona so long as the magnetic
  field strength is greater than approximately 10 G for quiescent loops
  and plumes, and 100 G for hot and dense loops. <BR /> Conclusions:
  A thermal misbalance may damp slow magnetoacoustic waves rapidly in
  much of the corona, and its inclusion in our understanding of slow
  mode damping may resolve discrepancies between the observations and
  theory relying on compressive viscosity and thermal conduction alone.

Title: Three-dimensional Simulations of the Inhomogeneous Low
    Solar Wind
Authors: Magyar, N.; Nakariakov, V. M.
Bibcode: 2021ApJ...907...55M
Altcode: 2020arXiv201200811M
  In the near future, the Parker Solar Probe will put theories about
  the dynamics and nature of the transition between the solar corona and
  the solar wind to stringent tests. The most popular mechanism aimed to
  explain the dynamics of the nascent solar wind, including its heating
  and acceleration, is magnetohydrodynamic (MHD) turbulence. Most of the
  previous models focused on nonlinear cascade induced by interactions
  of outgoing Alfvén waves and their reflections, ignoring effects that
  might be related to perpendicular structuring of the solar coronal
  plasma, despite overwhelming evidence for it. In this paper, for the
  first time, we analyze through 3D MHD numerical simulations the dynamics
  of the perpendicularly structured solar corona and solar wind, from the
  low corona to 15 R<SUB>⊙</SUB>. We find that background structuring
  has a strong effect on the evolution of MHD turbulence, on much faster
  timescales than in the perpendicularly homogeneous case. On timescales
  shorter than nonlinear times, linear effects related to phase mixing
  result in a 1/f perpendicular energy spectrum. As the turbulent cascade
  develops, we observe a perpendicular (parallel) energy spectrum with a
  power-law index of -3/2 or -5/3 (-2), a steeper perpendicular magnetic
  field than velocity spectrum, and a strong build-up of negative
  residual energy. We conclude that the turbulence is most probably
  generated by the self-cascade of the driven transverse kink waves,
  referred to previously as "uniturbulence," which might represent the
  dominant nonlinear energy cascade channel in the pristine solar wind.

Title: Solar Bayesian Analysis Toolkit—A New Markov Chain Monte
    Carlo IDL Code for Bayesian Parameter Inference
Authors: Anfinogentov, Sergey A.; Nakariakov, Valery M.; Pascoe,
   David J.; Goddard, Christopher R.
Bibcode: 2021ApJS..252...11A
Altcode: 2020arXiv200505365A
  We present the Solar Bayesian Analysis Toolkit (SoBAT), which is a
  new easy to use tool for Bayesian analysis of observational data,
  including parameter inference and model comparison. SoBAT is aimed
  (but not limited) to be used for the analysis of solar observational
  data. We describe a new IDL code designed to facilitate the comparison
  of a user-supplied model with data. Bayesian inference allows prior
  information to be taken into account. The use of Markov Chain Monte
  Carlo sampling allows efficient exploration of large parameter spaces
  and provides reliable estimation of model parameters and their
  uncertainties. The Bayesian evidence for different models can be
  used for quantitative comparison. The code is tested to demonstrate
  its ability to accurately recover a variety of parameter probability
  distributions. Its application to practical problems is demonstrated
  using studies of the structure and oscillation of coronal loops.

Title: Effect of Higher Radial Harmonics in Sausage Oscillations in
    Coronal Loops
Authors: Lim, D.; Nakariakov, V. M.; Moon, Y. J.; Yu, D. J.; Cho, I. H.
Bibcode: 2020AGUFMSH0290006L
Altcode:
  Impulsively excited sausage oscillations of a plasma cylinder with a
  smooth radial profile of Alfvén speed are analyzed with a numerical
  solution of the initial-value problem for a partial differential
  equation of the Klein-Gordon type, describing linear magnetoacoustic
  oscillations with a fixed axial wavelength and an azimuthal mode
  number. The range of analyzed ratios of Alfvén speeds outside and
  inside the cylinder is from 2 to 10. Both trapped and leaky regimes
  of the oscillations are considered. It is shown that even in the
  long-wavelength limit, i.e., for axial wavenumbers much smaller than
  the cutoff values, damping times of higher radial sausage harmonics
  could be significantly greater than the oscillation periods, i.e.,
  several oscillation cycles could be present in the signal. The quality
  factors decrease with decreasing ratios of Alfvén speeds outside and
  inside the cylinder. Oscillation periods of the second and third radial
  harmonics remain practically independent of the axial wavelength even
  when the wavelength is shorter than the radius of the cylinder. The
  ratios of oscillation periods of fundamental and higher radial and axial
  harmonics are found to be significantly different, up to a factor of
  two in the long-wavelength limit. It is concluded that higher radial
  harmonics could be responsible for the departure of observed sausage
  oscillation signals from a harmonic shape, especially during the first
  several cycles of the oscillation. Even in the absence of spatially
  resolved data, higher axial and radial harmonics can be distinguished
  from each other by the period ratios. <P />This work was supported by
  Institute for Information &amp; communications Technology Promotion
  (IITP) grant funded by the Korea government (2018-0-01422, Study on
  analysis and prediction technique of solar flares).

Title: Seismological constraints on the solar coronal heating function
Authors: Kolotkov, D. Y.; Duckenfield, T. J.; Nakariakov, V. M.
Bibcode: 2020A&A...644A..33K
Altcode: 2020arXiv201003364K
  <BR /> Aims: The hot solar corona exists because of the balance
  between radiative and conductive cooling and some counteracting
  heating mechanism that remains one of the major puzzles in solar
  physics. <BR /> Methods: The coronal thermal equilibrium is perturbed
  by magnetoacoustic waves, which are abundantly present in the corona,
  causing a misbalance between the heating and cooling rates. As a
  consequence of this misbalance, the wave experiences a back-reaction,
  either losing or gaining energy from the energy supply that heats the
  plasma, at timescales comparable to the wave period. <BR /> Results:
  In particular, the plasma can be subject to wave-induced instability
  or over-stability, depending on the specific choice of the coronal
  heating function. In the unstable case, the coronal thermal equilibrium
  would be violently destroyed, which does not allow for the existence
  of long-lived plasma structures typical for the corona. Based on this,
  we constrained the coronal heating function using observations of slow
  magnetoacoustic waves in various coronal plasma structures.

Title: Accelerating and Supersonic Density Disturbances in Solar
    Polar Plumes
Authors: Cho, I. H.; Moon, Y. J.; Nakariakov, V. M.; Lee, J. Y.; Yu,
   D. J.; Cho, K. S. F.; Yurchyshyn, V.; Lee, H.
Bibcode: 2020AGUFMSH0290003C
Altcode:
  Propagating intensity disturbances in solar coronal holes are often
  considered as wave propagations or mass flows. By applying the
  differential emission measure technique for the extreme ultraviolet
  images taken by the Atmospheric Imaging Assembly on board the Solar
  Dynamics Observatory, we analyze the propagation speed of density
  disturbances of plume structure in an off-limb coronal hole for a
  given temperature. We construct the map of cross-correlation between
  density profile for a given height and the profile at the height
  of 50 Mm. The evolution of density disturbances is well fitted by
  the second-order polynomial. The acceleration is calculated to be
  36 m s<SUP>-2</SUP>. The initial speed is 134 km s<SUP>-1</SUP>
  which is comparable with the sound speed given by the DEM-weighted
  temperature. Hence, density disturbances are accelerating and supersonic
  at around the base of the solar corona. The excess speed relative
  to the sound speed is ∼ 30 km s<SUP>-1</SUP> at the height of 1.23
  solar radii, which is consistent with the Doppler speeds and Doppler
  dimming speeds observed by different instruments. The extrapolated
  sonic distance of the excess speed is ∼ 2.16 solar radii which is
  consistent with those of solar winds. The lower limit of the mass
  flux corresponds to 7% of the global solar wind. Hence, we interpret
  that the observed density disturbances are slow magnetoacoustic waves
  propagating in subsonic and accelerating solar winds.

Title: The Solar Orbiter magnetometer
Authors: Horbury, T. S.; O'Brien, H.; Carrasco Blazquez, I.; Bendyk,
   M.; Brown, P.; Hudson, R.; Evans, V.; Oddy, T. M.; Carr, C. M.; Beek,
   T. J.; Cupido, E.; Bhattacharya, S.; Dominguez, J. -A.; Matthews, L.;
   Myklebust, V. R.; Whiteside, B.; Bale, S. D.; Baumjohann, W.; Burgess,
   D.; Carbone, V.; Cargill, P.; Eastwood, J.; Erdös, G.; Fletcher,
   L.; Forsyth, R.; Giacalone, J.; Glassmeier, K. -H.; Goldstein, M. L.;
   Hoeksema, T.; Lockwood, M.; Magnes, W.; Maksimovic, M.; Marsch, E.;
   Matthaeus, W. H.; Murphy, N.; Nakariakov, V. M.; Owen, C. J.; Owens,
   M.; Rodriguez-Pacheco, J.; Richter, I.; Riley, P.; Russell, C. T.;
   Schwartz, S.; Vainio, R.; Velli, M.; Vennerstrom, S.; Walsh, R.;
   Wimmer-Schweingruber, R. F.; Zank, G.; Müller, D.; Zouganelis, I.;
   Walsh, A. P.
Bibcode: 2020A&A...642A...9H
Altcode:
  The magnetometer instrument on the Solar Orbiter mission is designed
  to measure the magnetic field local to the spacecraft continuously
  for the entire mission duration. The need to characterise not only
  the background magnetic field but also its variations on scales from
  far above to well below the proton gyroscale result in challenging
  requirements on stability, precision, and noise, as well as magnetic
  and operational limitations on both the spacecraft and other
  instruments. The challenging vibration and thermal environment has
  led to significant development of the mechanical sensor design. The
  overall instrument design, performance, data products, and operational
  strategy are described.

Title: Accelerating and Supersonic Density Fluctuations in Coronal
Hole Plumes: Signature of Nascent Solar Winds
Authors: Cho, Il-Hyun; Nakariakov, Valery M.; Moon, Yong-Jae; Lee,
   Jin-Yi; Yu, Dae Jung; Cho, Kyung-Suk; Yurchyshyn, Vasyl; Lee, Harim
Bibcode: 2020ApJ...900L..19C
Altcode: 2020arXiv200807848C
  Slow magnetoacoustic waves in a static background provide a
  seismological tool to probe the solar atmosphere in the analytic
  frame. By analyzing the spatiotemporal variation of the electron
  number density of plume structure in coronal holes above the limb for
  a given temperature, we find that the density perturbations accelerate
  with supersonic speeds in the distance range from 1.02 to 1.23 solar
  radii. We interpret them as slow magnetoacoustic waves propagating at
  about the sound speed with accelerating subsonic flows. The average
  sonic height of the subsonic flows is calculated to be 1.27 solar
  radii. The mass flux of the subsonic flows is estimated to be 44.1%
  relative to the global solar wind. Hence, the subsonic flow is likely
  to be the nascent solar wind. In other words, the evolution of the
  nascent solar wind in plumes at the low corona is quantified for the
  first time from imaging observations. Based on the interpretation,
  propagating density perturbations present in plumes could be used as
  a seismological probe of the gradually accelerating solar wind.

Title: Magnetohydrodynamic Waves in the Solar Corona
Authors: Nakariakov, Valery M.; Kolotkov, Dmitrii Y.
Bibcode: 2020ARA&A..58..441N
Altcode:
  The corona of the Sun is a unique environment in which
  magnetohydrodynamic (MHD) waves, one of the fundamental processes of
  plasma astrophysics, are open to a direct study. There is striking
  progress in both observational and theoretical research of MHD wave
  processes in the corona, with the main recent achievements summarized
  as follows: Both periods and wavelengths of the principal MHD modes
  of coronal plasma structures, such as kink, slow and sausage modes,
  are confidently resolved. Scalings of various parameters of detected
  waves and waveguiding plasma structures allow for the validation of
  theoretical models. In particular, kink oscillation period scales
  linearly with the length of the oscillating coronal loop, clearly
  indicating that they are eigenmodes of the loop. Damping of decaying
  kink and standing slow oscillations depends on the oscillation
  amplitudes, demonstrating the importance of nonlinear damping. The
  dominant excitation mechanism for decaying kink oscillations is
  associated with magnetized plasma eruptions. Propagating slow waves are
  caused by the leakage of chromospheric oscillations. Fast wave trains
  could be formed by waveguide dispersion. The knowledge gained in the
  study of coronal MHD waves provides ground for seismological probing
  of coronal plasma parameters, such as the Alfvén speed, the magnetic
  field and its topology, stratification, temperature, fine structuring,
  polytropic index, and transport coefficients.

Title: Excitation of Negative Energy Surface Magnetohydrodynamic
    Waves in an Incompressible Cylindrical Plasma
Authors: Yu, D. J.; Nakariakov, V. M.
Bibcode: 2020ApJ...896...21Y
Altcode: 2020arXiv200413552Y
  Negative energy wave (NEW) phenomena may appear in shear flows in the
  presence of a wave decay mechanism and external energy supply. We
  study the appearance of negative energy surface waves in a plasma
  cylinder in the incompressible limit. The cylinder is surrounded by an
  axial magnetic field and by a plasma of different density. Considering
  flow inside and viscosity outside the flux tube, we derive dispersion
  relations and obtain analytical solutions for the phase speed and growth
  rate (increment) of the waves. It is found that the critical speed
  shear for the occurrence of the dissipative instability associated
  with NEWs and the threshold of Kelvin-Helmholtz instability (KHI)
  depend on the axial wavelength. The critical shear for the appearance
  of sausage NEW is lowest for the longest axial wavelengths, while
  for kink waves the minimum value of the critical shear is reached for
  the axial wavelength comparable to the diameter of the cylinder. The
  range between the critical speed of the dissipative instability and
  the KHI threshold is shown to depend on the difference of the Alfvén
  speeds inside and outside of the cylinder. For all axial wavenumbers,
  NEW appears for the shear flow speeds lower than the KHI threshold. It
  is easier to excite NEW in an underdense cylinder than in an overdense
  one. The negative energy surface waves can be effectively generated
  for an azimuthal number m = 0 with a large axial wavenumber and for
  higher modes (m &gt; 0) with a small axial wavenumber.

Title: Standing Kink Waves in Sigmoid Solar Coronal Loops:
    Implications for Coronal Seismology
Authors: Magyar, N.; Nakariakov, V. M.
Bibcode: 2020ApJ...894L..23M
Altcode: 2020arXiv200414083M
  Using full three-dimensional magnetohydrodynamic numerical simulations,
  we study the effects of magnetic field sigmoidity or helicity on
  the properties of the fundamental kink oscillation of solar coronal
  loops. Our model consists of a single denser coronal loop, embedded
  in a plasma with dipolar force-free magnetic field with a constant
  α-parameter. For the loop with no sigmoidity, we find that the
  numerically determined oscillation period of the fundamental kink
  mode matches the theoretical period calculated using WKB theory. In
  contrast, with increasing sigmoidity of the loop, the actual period is
  increasingly smaller than the one estimated by WKB theory. Translated
  through coronal seismology, increasing sigmoidity results in magnetic
  field estimates that are increasingly shifting toward higher
  values, and even surpassing the average value for the highest α
  value considered. Nevertheless, the estimated range of the coronal
  magnetic field value lies within the minimal/maximal limits, proving
  the robustness coronal seismology. We propose that the discrepancy in
  the estimations of the absolute value of the force-free magnetic field
  could be exploited seismologically to determine the free energy of
  coronal loops, if averages of the internal magnetic field and density
  can be reliably estimated by other methods.

Title: Quasi-Periodic Pulsations in Solar and Stellar Flares. Review
Authors: Kupriyanova, Elena; Kolotkov, Dmitrii; Nakariakov, Valery;
   Kaufman, Anastasiia
Bibcode: 2020STP.....6a...3K
Altcode: 2020SZF.....6a...3K
  This paper provides an overview of the state-of-the-art studies of
  oscillatory processes in solar and stellar flares, based on modern
  observational data from ground-based and space-borne instruments
  with high temporal, spatial, and spectral resolution in different
  bands of the electromagnetic spectrum. We examine the mecha-nisms
  that generate flare emission and its quasi-periodic modulation. We
  discuss similarities and differences between solar and stellar flares,
  and address associated problems of superflares on the Sun and space
  weather. Quasi-periodic pulsations (QPPs) of flare emission are shown to
  be an effective tool for diagnosing both the flare processes themselves
  and the parameters of flaring plasmas and accelerated particles. We
  consider types of QPPs, their statistical properties, and methods
  of analysis, taking into account the non-stationarity of the QPP'
  parameters. We review the proposed mechanisms of QPPs and summarize
  open questions.

Title: Higher Radial Harmonics of Sausage Oscillations in Coronal
    Loops
Authors: Lim, Daye; Nakariakov, Valery M.; Yu, Dae Jung; Cho, Il-Hyun;
   Moon, Yong-Jae
Bibcode: 2020ApJ...893...62L
Altcode:
  Impulsively excited sausage oscillations of a plasma cylinder with a
  smooth radial profile of Alfvén speed are analyzed with a numerical
  solution of the initial-value problem for a partial differential
  equation of the Klein-Gordon type, describing linear magnetoacoustic
  oscillations with a fixed axial wavelength and an azimuthal mode
  number. The range of analyzed ratios of Alfvén speeds outside and
  inside the cylinder is from 2 to 10. Both trapped and leaky regimes
  of the oscillations are considered. It is shown that even in the
  long-wavelength limit, I.e., for axial wavenumbers much smaller than
  the cutoff values, damping times of higher radial sausage harmonics
  could be significantly greater than the oscillation periods, I.e.,
  several oscillation cycles could be present in the signal. The quality
  factors decrease with decfreasing ratios of Alfvén speeds outside and
  inside the cylinder. Oscillation periods of the second and third radial
  harmonics remain practically independent of the axial wavelength even
  when the wavelength is shorter than the radius of the cylinder. The
  ratios of oscillation periods of fundamental and higher radial and axial
  harmonics are found to be significantly different, up to a factor of
  two in the long-wavelength limit. It is concluded that higher radial
  harmonics could be responsible for the departure of observed sausage
  oscillation signals from a harmonic shape, especially during the first
  several cycles of the oscillation. Even in the absence of spatially
  resolved data, higher axial and radial harmonics can be distinguished
  from each other by the period ratios.

Title: Magnetic Connectivity between the Light Bridge and Penumbra
    in a Sunspot
Authors: Feng, Song; Miao, Yuhu; Yuan, Ding; Qu, Zhongquan; Nakariakov,
   Valery M.
Bibcode: 2020ApJ...893L...2F
Altcode: 2020arXiv200303976F
  A light bridge is a prominent structure commonly observed within a
  sunspot. Its presence usually triggers a wealth of dynamics in a sunspot
  and has a lasting impact on sunspot evolution. However, the fundamental
  structure of light bridges is still not well understood. In this study,
  we used the high-resolution spectropolarimetry data obtained by the
  Solar Optical Telescope on board the Hinode satellite to analyze the
  magnetic and thermal structure of a light bridge at AR 12838. We
  also combined the high-cadence $1700\,\mathring{\rm A} $ channel
  data provided by the Atmospheric Imaging Assembly on board the Solar
  Dynamics Observatory to study the dynamics on this bridge. We found
  a pair of blue and red Doppler shift patches at two ends of this
  bridge; this pattern appears to be the convective motion directed
  by the horizontal component of the magnetic field aligned with the
  spine of the bridge. Paired upward and downward motions imply that
  the light bridge could have a two-legged or undulating magnetic
  field. Significant 4 minute oscillations in the emission intensity
  of the $1700\,\mathring{\rm A} $ bandpass were detected at two ends,
  which overlapped the paired blue- and redshift patches. The oscillatory
  signals at the light bridge and the penumbra were highly correlated with
  each other. Although they are separated in space at the photosphere,
  the periodicity seems to have a common origin from underneath the
  sunspot. Therefore, we infer that the light bridge and penumbra could
  share a common magnetic source and become fragmented at the photosphere
  by magnetoconvection.

Title: Editorial: Magnetohydrodynamic Waves in the Solar Atmosphere:
    Heating and Seismology
Authors: Van Doorsselaere, Tom; Nakariakov, Valery M.; Li, Bo;
   Antolin, Patrick
Bibcode: 2020FrASS...6...79V
Altcode:
  No abstract at ADS

Title: Excitation of decay-less transverse oscillations of coronal
    loops by random motions
Authors: Afanasyev, A. N.; Van Doorsselaere, T.; Nakariakov, V. M.
Bibcode: 2020A&A...633L...8A
Altcode: 2019arXiv191207980A
  Context. The relatively large-amplitude decaying regime of transverse
  oscillations of coronal loops has been known for two decades and
  has been interpreted in terms of magnetohydrodynamic kink modes of
  cylindrical plasma waveguides. In this regime oscillations decay
  in several cycles. Recent observational analysis has revealed
  so-called decay-less, small-amplitude oscillations, in which a
  multi-harmonic structure has been detected. Several models have been
  proposed to explain these oscillations. In particular, decay-less
  oscillations have been described in terms of standing kink waves
  driven with continuous mono-periodic motions of loop footpoints,
  in terms of a simple oscillator model of forced oscillations due
  to harmonic external force, and as a self-oscillatory process due
  to the interaction of a loop with quasi-steady flows. However,
  an alternative mechanism is needed to explain the simultaneous
  excitation of several longitudinal harmonics of the oscillation. <BR
  /> Aims: We study the mechanism of random excitation of decay-less
  transverse oscillations of coronal loops. <BR /> Methods: With a
  spatially one-dimensional and time-dependent analytical model taking
  into account effects of the wave damping and kink speed variation
  along the loop, we considered transverse loop oscillations driven by
  random motions of footpoints. The footpoint motions were modelled by
  broad-band coloured noise. <BR /> Results: We found the excitation of
  loop eigenmodes and analysed their frequency ratios as well as the
  spatial structure of the oscillations along the loop. The obtained
  results successfully reproduce the observed properties of decay-less
  oscillations. In particular, excitation of eigenmodes of a loop as
  a resonator can explain the observed quasi-monochromatic nature of
  decay-less oscillations and the generation of multiple harmonics
  detected recently. <BR /> Conclusions: We propose a mechanism that
  can interpret decay-less transverse oscillations of coronal loops in
  terms of kink waves randomly driven at the loop footpoints.

Title: Quasi-periodic Pulsations of Gamma-Ray Emissions from a Solar
    Flare on 2017 September 6
Authors: Li, D.; Kolotkov, D. Y.; Nakariakov, V. M.; Lu, L.; Ning,
   Z. J.
Bibcode: 2020ApJ...888...53L
Altcode: 2019arXiv191201145L
  We investigate quasi-periodic pulsations (QPPs) of high-energy
  nonthermal emissions from an X9.3 flare (SOL2017-Sep-06T11:53),
  the most powerful flare since the beginning of solar cycle 24. The
  QPPs are identified as a series of regular and repeating peaks in the
  light curves in the γ- and hard X-ray (HXR) channels recorded by the
  Konus-Wind, as well as the radio and microwave fluxes measured by the
  CALLISTO radio spectrograph during the impulsive phase. The periods
  are determined from the global wavelet and Fourier power spectra
  as 24-30 s in the HXR and microwave channels, which are associated
  with nonthermal electrons, and ∼20 s in the γ-ray band related to
  nonthermal ions. Both nonthermal electrons and ions may be accelerated
  by repetitive magnetic reconnection during the impulsive phase. However,
  we could not rule out other mechanisms, such as magnetohydrodynamic
  oscillation in a sausage mode. The QPP detected in this study is useful
  for understanding the particle acceleration and dynamic process in
  solar flares and also bridging the gap between stellar and solar flares
  because the energy realm of the X9.3 solar flare is almost comparable
  to a typical stellar flare.

Title: Observational signatures of the third harmonic in a decaying
    kink oscillation of a coronal loop
Authors: Duckenfield, T. J.; Goddard, C. R.; Pascoe, D. J.; Nakariakov,
   V. M.
Bibcode: 2019A&A...632A..64D
Altcode:
  <BR /> Aims: An observation of a coronal loop standing kink mode
  is analysed to search for higher harmonics, aiming to reveal the
  relation between different harmonics' quality factors. <BR /> Methods:
  Observations of a coronal loop were taken by the Atmospheric Imaging
  Assembly (AIA) of the Solar Dynamics Observatory (SDO). The loop's
  axis was tracked at many spatial positions along the loop to generate
  time series data. <BR /> Results: The distribution of spectral power
  of the oscillatory transverse displacements throughout the loop
  reveals the presence of two harmonics, a fundamental at a period of
  ∼8 min and its third harmonic at ∼2.6 min. The node of the third
  harmonic is seen at approximately a third of the way along the length
  of the loop, and cross correlations between the oscillatory motion
  on opposing sides of the node show a change in phase behaviour. The
  ratio of periods P<SUB>1</SUB>/3P<SUB>3</SUB> was found to be ∼0.87,
  indicating a non-uniform distribution of kink speed through the
  loop. The quality factor for the fundamental mode of oscillation
  was measured to be ∼3.4. The quality factor of the third harmonic
  was measured for each spatial location and, where data was reliable,
  yielded a value of ∼3.6. For all locations, the quality factors for
  the two harmonics were found to agree within error as expected from
  1d resonant absorption theory. This is the first time a measurement
  of the signal quality for a higher harmonic of a kink oscillation has
  been reported with spatially resolved data.

Title: A Blueprint of State-of-the-art Techniques for Detecting
    Quasi-periodic Pulsations in Solar and Stellar Flares
Authors: Broomhall, Anne-Marie; Davenport, James R. A.; Hayes, Laura
   A.; Inglis, Andrew R.; Kolotkov, Dmitrii Y.; McLaughlin, James A.;
   Mehta, Tishtrya; Nakariakov, Valery M.; Notsu, Yuta; Pascoe, David J.;
   Pugh, Chloe E.; Van Doorsselaere, Tom
Bibcode: 2019ApJS..244...44B
Altcode: 2019arXiv191008458B
  Quasi-periodic pulsations (QPPs) appear to be a common feature observed
  in the light curves of both solar and stellar flares. However, their
  quasi-periodic nature, along with the fact that they can be small
  in amplitude and short-lived, makes QPPs difficult to unequivocally
  detect. In this paper, we test the strengths and limitations of
  state-of-the-art methods for detecting QPPs using a series of
  hare-and-hounds exercises. The hare simulated a set of flares,
  both with and without QPPs of a variety of forms, while the hounds
  attempted to detect QPPs in blind tests. We use the results of these
  exercises to create a blueprint for anyone who wishes to detect QPPs
  in real solar and stellar data. We present eight clear recommendations
  to be kept in mind for future QPP detections, with the plethora of
  solar and stellar flare data from new and future satellites. These
  recommendations address the key pitfalls in QPP detection, including
  detrending, trimming data, accounting for colored noise, detecting
  stationary-period QPPs, detecting QPPs with nonstationary periods,
  and ensuring that detections are robust and false detections are
  minimized. We find that QPPs can be detected reliably and robustly
  by a variety of methods, which are clearly identified and described,
  if the appropriate care and due diligence are taken.

Title: A New Type of Jet in a Polar Limb of the Solar Coronal Hole
Authors: Cho, Il-Hyun; Moon, Yong-Jae; Cho, Kyung-Suk; Nakariakov,
   Valery M.; Lee, Jin-Yi; Kim, Yeon-Han
Bibcode: 2019ApJ...884L..38C
Altcode: 2019arXiv191009737C
  A new type of chromospheric jet in a polar limb of a coronal hole is
  discovered in the Ca II filtergram of the Solar Optical Telescope on
  board the Hinode. We identify 30 jets in a filtered Ca II movie with
  a duration of 53 minutes. The average speed at their maximum heights
  is found to be 132 ± 44 km s<SUP>-1</SUP> ranging from 57 to 264 km
  s<SUP>-1</SUP> along the propagation direction. The average lifetime
  is 20 ± 6 ranging from 11 to 36 s. The speed and lifetime of the
  jets are located at end-tails of those parameters determined for type
  II spicules, hence implying a new type of jets. To confirm whether
  these jets are different from conventional spicules, we construct
  a time-height image averaged over a horizontal region of 1″, and
  calculate lagged cross-correlations of intensity profiles at each height
  with the intensity at 2 Mm. From this, we obtain a cross-correlation
  map as a function of lag and height. We find that the correlation
  curve as a function of lag time is well fitted into three different
  Gaussian functions whose standard deviations of the lag time are 193,
  42, and 17 s. The corresponding propagation speeds are calculated to be
  9 km s<SUP>-1</SUP>, 67 km s<SUP>-1</SUP>, and 121 km s<SUP>-1</SUP>,
  respectively. The kinematic properties of the former two components
  seem to correspond to the 3-minute oscillations and type II spicules,
  while the latter component to the jets is addressed in this study.

Title: Magnetohydrodynamic Seismology of Quiet Solar Active Regions
Authors: Anfinogentov, Sergey A.; Nakariakov, V. M.
Bibcode: 2019ApJ...884L..40A
Altcode: 2019arXiv191003809A
  The ubiquity of recently discovered low-amplitude decayless kink
  oscillations of plasma loops allows for the seismological probing
  of the corona on a regular basis. In particular, in contrast to
  traditionally applied seismology that is based on the large-amplitude
  decaying kink oscillations excited by flares and eruptions, decayless
  oscillations can potentially provide the diagnostics necessary for
  their forecasting. We analyzed decayless kink oscillations in several
  distinct loops belonging to active region NOAA 12107 on 2010 July 10
  during its quiet time period, when it was observed on the west limb in
  extreme ultraviolet by the Atmospheric Imaging Assembly on board the
  Solar Dynamics Observatory. The oscillation periods were estimated
  with the use of the motion magnification technique. The lengths of
  the oscillating loops were determined within the assumption of its
  semicircular shape by measuring the position of their footpoints. The
  density contrast in the loops was estimated from the observed intensity
  contrast accounting for the unknown spatial scale of the background
  plasma. The combination of those measurements allows us to determine
  the distribution of kink and Alfvén speeds in the active region. Thus,
  we demonstrate the possibility of obtaining seismological information
  about coronal active regions during the quiet periods of time.

Title: DTCWT based motion magnification v0.5.0
Authors: Anfinogentov, Sergey; Nakariakov, Valery; Kosak, Katie
Bibcode: 2019zndo...3368774A
Altcode:
  This is the first release.

Title: Formation of quasi-periodic slow magnetoacoustic wave trains
    by the heating/cooling misbalance
Authors: Zavershinskii, D. I.; Kolotkov, D. Y.; Nakariakov, V. M.;
   Molevich, N. E.; Ryashchikov, D. S.
Bibcode: 2019PhPl...26h2113Z
Altcode: 2019arXiv190708168Z
  Slow magnetoacoustic waves are omnipresent in both natural and
  laboratory plasma systems. The wave-induced misbalance between plasma
  cooling and heating processes causes the amplification or attenuation,
  and also dispersion, of slow magnetoacoustic waves. The wave dispersion
  could be attributed to the presence of characteristic time scales in
  the system, connected with the plasma heating or cooling due to the
  competition of the heating and cooling processes in the vicinity of
  thermal equilibrium. We analyzed linear slow magnetoacoustic waves in
  a plasma in thermal equilibrium formed by a balance of optically thin
  radiative losses, field-align thermal conduction, and an unspecified
  heating process. The dispersion is manifested by the dependence of the
  effective adiabatic index of the wave on the wave frequency, making the
  phase and group speeds frequency-dependent. The mutual effect of the
  wave amplification and dispersion is shown to result in the occurrence
  of an oscillatory pattern in an initially broadband slow wave, with
  the characteristic period determined by the thermal misbalance time
  scales, i.e., by the derivatives of the combined radiation loss
  and heating function with respect to the density and temperature,
  evaluated at the equilibrium. This effect is illustrated by estimating
  the characteristic period of the oscillatory pattern, appearing because
  of thermal misbalance in the plasma of the solar corona. It is found
  that by an order of magnitude, the period is about the typical periods
  of slow magnetoacoustic oscillations detected in the corona.

Title: Damping of slow magnetoacoustic oscillations by the misbalance
    between heating and cooling processes in the solar corona
Authors: Kolotkov, D. Y.; Nakariakov, V. M.; Zavershinskii, D. I.
Bibcode: 2019A&A...628A.133K
Altcode: 2019arXiv190707051K
  Context. Rapidly decaying slow magnetoacoustic waves are regularly
  observed in the solar coronal structures, offering a promising tool
  for a seismological diagnostics of the coronal plasma, including its
  thermodynamical properties. <BR /> Aims: The effect of damping of
  standing slow magnetoacoustic oscillations in the solar coronal loops
  is investigated accounting for field-aligned thermal conductivity and a
  wave-induced misbalance between radiative cooling and some unspecified
  heating rates. <BR /> Methods: The non-adiabatic terms were allowed to
  be arbitrarily large, corresponding to the observed values. The thermal
  conductivity was taken in its classical form, and a power-law dependence
  of the heating function on the density and temperature was assumed. The
  analysis was conducted in the linear regime and in the infinite magnetic
  field approximation. <BR /> Results: The wave dynamics is found to
  be highly sensitive to the characteristic timescales of the thermal
  misbalance. Depending on certain values of the misbalance, timescales
  three regimes of the wave evolution were identified, namely the regime
  of a suppressed damping, enhanced damping in which the damping rate
  drops down to observational values, and acoustic over-stability. The
  specific regime is determined by the dependences of the radiative
  cooling and heating functions on thermodynamical parameters of the
  plasma in the vicinity of the perturbed thermal equilibrium. <BR />
  Conclusions: The comparison of the observed and theoretically derived
  decay times and oscillation periods allows us to constrain the coronal
  heating function. For typical coronal parameters, the observed
  properties of standing slow magnetoacoustic oscillations could be
  readily reproduced with a reasonable choice of the heating function.

Title: Evidence for Vortex Shedding in the Sun's Hot Corona
Authors: Samanta, Tanmoy; Tian, Hui; Nakariakov, Valery M.
Bibcode: 2019PhRvL.123c5102S
Altcode: 2019arXiv190708930S
  Vortex shedding is an oscillating flow that is commonly observed
  in fluids due to the presence of a blunt body in a flowing
  medium. Numerical simulations have shown that the phenomenon of
  vortex shedding could also develop in the magnetohydrodynamic (MHD)
  domain. The dimensionless Strouhal number, the ratio of the blunt body
  diameter to the product of the period of vortex shedding and the speed
  of a flowing medium, is a robust indicator for vortex shedding, and,
  generally of the order of 0.2 for a wide range of Reynolds number. Using
  an observation from the Atmospheric Imaging Assembly on board the
  Solar Dynamics Observatory, we report a wavelike or oscillating plasma
  flow propagating upward against the Sun's gravitational force. A newly
  formed shrinking loop in the postflare region possibly generates the
  oscillation of the upflow in the wake of the hot and dense loop through
  vortex shedding. The computed Strouhal number is consistent with the
  prediction from previous MHD simulations. Our observation suggests
  the possibility of vortex shedding in the solar corona.

Title: Oscillation of a Small Hα Surge in a Solar Polar Coronal Hole
Authors: Cho, Kyung-Suk; Cho, Il-Hyun; Nakariakov, V. M.; Yurchyshyn,
   Vasyl B.; Yang, Heesu; Kim, Yeon-Han; Kumar, Pankaj; Magara, Tetsuya
Bibcode: 2019ApJ...877L...1C
Altcode:
  Hα surges (i.e., cool/dense collimated plasma ejections) may act as
  a guide for a propagation of magnetohydrodynamic waves. We report
  a high-resolution observation of a surge observed with 1.6 m Goode
  Solar Telescope (GST) on 2009 August 26, from 18:20 UT to 18:45
  UT. Characteristics of plasma motions in the surge are determined
  with the normalizing radial gradient filter and the Fourier motion
  filter. The shape of the surge is found to change from a “C”
  shape to an inverse “C” shape after a formation of a cusp, a
  signature of reconnection. There are apparent upflows seen above
  the cusp top and downflows below it. The upflows show rising and
  rotational motions in the right-hand direction, with the rotational
  speed decreasing with height. Near the cusp top, we find a transverse
  oscillation of the surge, with the period of ∼2 minutes. There is no
  change of the oscillation phase below the cusp top, but above the top
  a phase change is identified, giving a vertical phase speed about 86
  km s<SUP>-1</SUP>. As the height increases, the initial amplitude of
  the oscillation increases, and the oscillation damping time decreases
  from 5.13 to 1.18 minutes. We conclude that the oscillation is a
  propagating kink wave that is possibly excited by the repetitive
  spontaneous magnetic reconnection.

Title: The Physical Nature of Spiral Wave Patterns in Sunspots
Authors: Kang, Juhyung; Chae, Jongchul; Nakariakov, Valery M.; Cho,
   Kyuhyoun; Kwak, Hannah; Lee, Kyeore
Bibcode: 2019ApJ...877L...9K
Altcode: 2019arXiv190508908K
  Recently, spiral wave patterns (SWPs) have been detected in 3 minute
  oscillations of sunspot umbrae, but the nature of this phenomenon has
  remained elusive. We present a theoretical model that interprets the
  observed SWPs as the superposition of two different azimuthal modes
  of slow magnetoacoustic waves driven below the surface in an untwisted
  and non-rotating magnetic cylinder. We apply this model to SWPs of the
  line-of-sight (LOS) velocity in a pore observed by the Fast Imaging
  Solar Spectrograph installed at the 1.6 m Goode Solar Telescope. One-
  and two-armed SWPs were identified in instantaneous amplitudes of
  LOS Doppler velocity maps of 3 minute oscillations. The associated
  oscillation periods are about 160 s, and the durations are about 5
  minutes. In our theoretical model, the observed spiral structures are
  explained by the superposition of non-zero azimuthal modes driven 1600
  km below the photosphere in the pore. The one-armed SWP is produced
  by the slow-body sausage (m = 0) and kink (m = 1) modes, and the
  two-armed SWP is formed by the slow-body sausage (m = 0) and fluting
  (m = 2) modes of the magnetic flux tube forming the pore.

Title: Scaling laws of quasi-periodic pulsations in solar flares
Authors: Pugh, C. E.; Broomhall, A. -M.; Nakariakov, V. M.
Bibcode: 2019A&A...624A..65P
Altcode: 2019arXiv190209627P
  Context. Quasi-periodic pulsations (QPPs) are a common feature of
  solar flares, but there has previously been a lack of observational
  evidence to support any of the theoretical models that might explain
  the origin of these QPPs. <BR /> Aims: We aimed to determine if there
  are any relationships between the QPP period and other properties
  of the flaring region, using a previously assembled sample of flares
  with QPPs. If any relationships exist, then these can be compared with
  scaling laws for the theoretical QPP mechanisms. <BR /> Methods: To
  obtain the flaring region properties, we made use of the Atmospheric
  Imaging Assembly (AIA) 1600 Å and Helioseismic and Magnetic Imager
  (HMI) data. The flare ribbons are visible in AIA 1600 Å images, and the
  positive and negative magnetic polarity ribbons can be distinguished
  and the magnetic properties determined in the HMI magnetograms. The
  ribbon properties calculated in this study were the ribbon separation
  distance, area, total unsigned magnetic flux, and average magnetic field
  strength. Only the flares that occurred within ±60° of the solar
  disc centre were included, which meant a sample of 20 flares with 22
  QPP signals. <BR /> Results: Positive correlations were found between
  the QPP period and the ribbon properties. The strongest correlations
  were with the separation distance and magnetic flux. Because these
  ribbon properties also correlate with the flare duration and because the
  relationship between the QPP period and flare duration may be influenced
  by observational bias, we also made use of simulated data to determine
  whether artificial correlations were introduced. These simulations
  show that although QPPs cannot be detected for certain combinations
  of QPP period and flare duration, this does not introduce an apparent
  correlation. <BR /> Conclusions: There is evidence of relationships
  between the QPP period and flare ribbon properties, and in the future,
  the derived scaling laws between these properties can be compared to
  equivalent scaling laws for theoretical QPP mechanisms.

Title: Catalog of Decaying Kink Oscillations of Coronal Loops in
    the 24th Solar Cycle
Authors: Nechaeva, Alena; Zimovets, Ivan V.; Nakariakov, V. M.;
   Goddard, C. R.
Bibcode: 2019ApJS..241...31N
Altcode:
  A catalog of kink oscillations of solar coronal loops, which spans
  during almost all of solar cycle 24, is presented. The catalog is
  based on the observations made in the extreme ultraviolet band at
  171 Å with Solar Dynamics Observatory/Atmospheric Imaging Assembly
  and includes parameters of 223 oscillating loops in 96 oscillation
  events. The catalog provides the information about the oscillation
  locations, time, and dates of the events, associated flare, initial
  displacement, oscillation period, exponential damping time, and
  apparent amplitude. The vast majority of the oscillation detections,
  84%, were made in the loops situated near or off the solar limb. The
  oscillation periods are found to range from 1 to 28 minutes, with 74%
  of the events that have the period in the range of 2-10 minutes. About
  90% of the oscillations have the apparent amplitude in the range
  of 1-10 Mm. The oscillating loop lengths are 70-600 Mm. The typical
  apparent amplitude is about 1% of the loop length. The oscillation
  period scales linearly with the loop length, and the damping time
  scales linearly with the period, which confirm previous findings. The
  oscillation quality factor scales with the amplitude to the power of
  minus 0.7. No statistically significant evidence of correlation was
  found between both the oscillation period and the mean sunspot number,
  and the loop length and mean sunspot number. The catalog provides the
  research community with the foundation for the further statistical
  study of kink oscillations and their use for coronal seismology.

Title: Fast magnetoacoustic wave trains with time-dependent drivers
Authors: Goddard, C. R.; Nakariakov, V. M.; Pascoe, D. J.
Bibcode: 2019A&A...624L...4G
Altcode:
  Context. Frequent observations of quasi-periodic rapidly-propagating
  wave trains in coronal structures have been made in the last decade. The
  dispersive evolution of fast magnetohydrodynamic waves propagating in
  coronal waveguides can provide a physical interpretation for many of
  these observations. <BR /> Aims: Previous studies have considered the
  generation of fast wave trains by impulsive drivers which deposit energy
  instantaneously. The signatures of dispersively formed wave trains
  must depend on the temporal nature of the driver. We investigate the
  effect of varying the temporal width of the driving perturbation. <BR
  /> Methods: 2D magnetohydrodynamic numerical simulations of
  impulsively generated wave trains in a guiding field-aligned density
  enhancement were performed with the novel addition of a time-dependant
  driver. <BR /> Results: The final spatial and spectral signatures of
  the guided wave trains are found to depend strongly on the temporal
  duration of the initial perturbation. In particular, the wavelength
  (or frequency) of highest spectral amplitude is found to increase
  (decrease) with increasing temporal duration, whereas the spectral
  width decreases. Additionally, the efficiency of generation of fast wave
  trains is found to decrease strongly with increasing temporal width of
  the driver, with a cut-off at twice the internal Alfvén crossing time.

Title: Properties of Slow Magnetoacoustic Oscillations of Solar
    Coronal Loops by Multi-instrumental Observations
Authors: Nakariakov, V. M.; Kosak, M. K.; Kolotkov, D. Y.;
   Anfinogentov, S. A.; Kumar, P.; Moon, Y. -J.
Bibcode: 2019ApJ...874L...1N
Altcode:
  Rapidly decaying oscillations of the thermal emission detected in the
  decay phase of solar and stellar flares are usually interpreted as
  standing or sloshing (reflecting) slow magnetoacoustic oscillations. We
  determine the scalings of the oscillation periods, damping times, and
  amplitudes with the temperature, considering both standing and sloshing
  oscillations detected with different instruments. In addition, the time
  evolution of different spatial harmonics of a sloshing oscillation is
  considered. Parameters of slow oscillations observed in the EUV, X-ray,
  and microwave bands, and published in the literature, are used. The
  damping time of slow oscillations is found to scale almost linearly
  with the oscillation period, as the period to 0.87 ± 0.1, giving the
  average Q-factor determined as the ratio of the damping time to the
  period, of about 1. The Q-factor is found to scale with the relative
  amplitude to the power of {0.33}<SUB>-0.11</SUB><SUP>+0.10</SUP> with
  95% confidence. The amplitudes of different spatial harmonics forming
  a sloshing pulse show similar time evolution, suggesting that the
  period-dependent dissipation is counteracted by another mechanism. The
  results obtained indicate that the damping of slow oscillations depends
  on the oscillation amplitude, and that the competition of nonlinear
  and dissipative effects could allow for the existence of wave pulses
  of a sustained shape.

Title: Seismological Determination of the Alfvén Speed and Plasma
    Beta in Solar Photospheric Bright Points
Authors: Cho, Il-Hyun; Moon, Yong-Jae; Nakariakov, Valery M.; Yu,
   Dae Jung; Lee, Jin-Yi; Bong, Su-Chan; Kim, Rok-Soon; Cho, Kyung-Suk;
   Kim, Yeon-Han; Lee, Jae-Ok
Bibcode: 2019ApJ...871L..14C
Altcode: 2019arXiv190104144C
  The Alfvén speed and plasma beta in photospheric bright points
  (BPs) observed by the Broadband Filter Imager (BFI) of the Solar
  Optical Telescope on board the Hinode satellite are estimated
  seismologically. The diagnostics is based on the theory of slow
  magnetoacoustic waves in a non-isothermally stratified photosphere
  with a uniform vertical magnetic field. We identify and track BPs in a
  G-band movie by using the 3D region growing method, and align them with
  blue continuum images to derive their brightness temperatures. From the
  Fourier power spectra of 118 continuum light curves made in the BPs,
  we find that light curves of 91 BPs have oscillations with properties
  that are significantly different from oscillation in quiet regions,
  with the periods ranging 2.2-16.2 minutes. We find that the model
  gives a moderate value of the plasma beta when γ lies at around
  5/3. The calculated Alfvén speed is 9.68 ± 2.02 km s<SUP>-1</SUP>,
  ranging in 6.3-17.4 km s<SUP>-1</SUP>. The plasma beta is estimated
  to be of 0.93 ± 0.36, ranging in 0.2-1.9.

Title: Three-dimensional Oscillations of 21 Halo Coronal Mass
    Ejections Using Multi-spacecraft Data
Authors: Lee, Harim; Moon, Y. -J.; Nakariakov, V. M.; Na, Hyeonock;
   Cho, Il-Hyun; Park, Eunsu
Bibcode: 2018ApJ...868...18L
Altcode:
  We investigate the 3D structure of kinematic oscillations of full halo
  coronal mass ejections (FHCMEs) using multi-spacecraft coronagraph
  data from two non-parallel lines of sight. For this, we consider 21
  FHCMEs which are simultaneously observed by the Solar and Heliospheric
  Observatory and the Solar TErrestrial RElations Observatory A or B,
  from 2010 June to 2012 August when the spacecraft were roughly in
  quadrature. Using sequences of running difference images, we estimate
  the instantaneous projected speeds of the FHCMEs at 24 different
  azimuthal angles in the planes of the sky of those coronagraphs. We
  find that all these FHCMEs have experienced kinematic oscillations
  characterized by quasi-periodic variations of the instantaneous
  projected radial velocity with periods ranging from 24 to 48 min. The
  oscillations detected in the analyzed events are found to show distinct
  azimuthal wave modes. Thirteen events (about 62%) are found to oscillate
  with the azimuthal wave number m = 1. The oscillating directions
  of the nodes of the m = 1 mode for these FHCMEs are consistent with
  those of their position angles (or the direction of eruption), with a
  mean difference of about 23°. The oscillation amplitude is found to
  correlate well with the projected radial speed of the CME. An estimation
  of Lorentz accelerations shows that they are dominant over other forces,
  implying that the magnetic force is responsible for the kinematic
  oscillations of CMEs. However, we cannot rule out other possibilities:
  a global layer of enhanced current around the CMEs or the nonlinear
  nature of its driver, for example the effect of vortex shedding.

Title: Sausage oscillations in a plasma cylinder with a surface
    current
Authors: Lim, Daye; Nakariakov, Valery M.; Moon, Yong-Jae
Bibcode: 2018JASTP.175...49L
Altcode:
  Linear sausage oscillations of a cylinder embedded in a plasma with
  an azimuthal magnetic field, created by a current on the surface
  of the cylinder, are studied. Such a plasma configuration could be
  applied to modelling flaring loops, and magnetic ropes in coronal mass
  ejections. The plasma is assumed to be cold everywhere. Dispersion
  relations demonstrate that the lowest radial harmonic of the sausage
  mode is in the trapped regime for all values of the parallel wave
  number. In the long-wavelength limit, phase and group speeds of this
  mode are equal to the Alfvén speed in the external medium. It makes
  the oscillation period to be determined by the ratio of the parallel
  wavelength, e.g. double the length of an oscillating loop, to the
  external Alfvén speed, allowing for its seismological estimations. The
  application of the results obtained to the interpretation of long-period
  (longer than a minute) oscillations of emission intensity detected in
  solar coronal structures, gives reasonable estimations of the external
  Alfvén speed. Cutoff values of the parallel wavenumber for higher
  radial harmonics are determined analytically. Implications of this
  finding to the observational signatures of fast magnetoacoustic wave
  trains guided by cylindrical plasma non-uniformities are discussed.

Title: Magnetic structure of solar flare regions producing hard
    X-ray pulsations
Authors: Zimovets, I. V.; Wang, R.; Liu, Y. D.; Wang, C.; Kuznetsov,
   S. A.; Sharykin, I. N.; Struminsky, A. B.; Nakariakov, V. M.
Bibcode: 2018JASTP.174...17Z
Altcode: 2017arXiv170801869Z
  We present analysis of the magnetic field in seven solar flare regions
  accompanied by the pulsations of hard X-ray (HXR) emission. These
  flares were studied by Kuznetsov et al. (2016) (Paper I), and chosen
  here because of the availability of the vector magnetograms for their
  parent active regions (ARs) obtained with the SDO/HMI data. In Paper I,
  based on the observations only, it was suggested that a magnetic flux
  rope (MFR) might play an important role in the process of generation
  of the HXR pulsations. The goal of the present paper is to test this
  hypothesis by using the extrapolation of magnetic field with the
  non-linear force-free field (NLFFF) method. Having done this, we found
  that before each flare indeed there was an MFR elongated along and
  above a magnetic polarity inversion line (MPIL) on the photosphere. In
  two flare regions the sources of the HXR pulsations were located at
  the footpoints of different magnetic field lines wrapping around the
  central axis, and constituting an MFR by themselves. In five other
  flares the parent field lines of the HXR pulsations were not a part
  of an MFR, but surrounded it in the form of an arcade of magnetic
  loops. These results show that, at least in the analyzed cases, the
  "single flare loop" models do not satisfy the observations and magnetic
  field modeling, while are consistent with the concept that the HXR
  pulsations are a consequence of successive episodes of energy release
  and electron acceleration in different magnetic flux tubes (loops)
  of a complex AR. An MFR could generate HXR pulsations by triggering
  episodes of magnetic reconnection in different loops in the course of
  its non-uniform evolution along an MPIL. However, since three events
  studied here were confined flares, actual eruptions may not be required
  to trigger sequential particle acceleration episodes in the magnetic
  systems containing an MFR.

Title: Oscillations of cometary tails: a vortex shedding phenomenon?
Authors: Nisticò, G.; Vladimirov, V.; Nakariakov, V. M.; Battams,
   K.; Bothmer, V.
Bibcode: 2018A&A...615A.143N
Altcode: 2018arXiv180400997N
  Context. During their journey to perihelion, comets may appear in the
  field of view of space-borne optical instruments, showing in some
  cases a nicely developed plasma tail extending from their coma and
  exhibiting an oscillatory behaviour. <BR /> Aims: The oscillations of
  cometary tails may be explained in terms of vortex shedding because of
  the interaction of the comet with the solar wind streams. Therefore,
  it is possible to exploit these oscillations in order to infer the
  value of the Strouhal number S t, which quantifies the vortex shedding
  phenomenon, and the physical properties of the local medium. <BR />
  Methods: We used the Heliospheric Imager (HI) data of the Solar
  TErrestrial Relations Observatory (STEREO) mission to study the
  oscillations of the tails of comets 2P/Encke and C/2012 S1 (ISON) during
  their perihelion in Nov 2013. We determined the corresponding Strouhal
  numbers from the estimates of the halo size, the relative speed of the
  solar wind flow, and the period of the oscillations. <BR /> Results:
  We found that the estimated Strouhal numbers are very small, and the
  typical value of S t 0.2 would be extrapolated for size of the halo
  larger than 10<SUP>6</SUP> km. <BR /> Conclusions: Although the vortex
  shedding phenomenon has not been unambiguously revealed, the findings
  suggest that some kind of magnetohydrodynamic (MHD) instability process
  is responsible for the observed behaviour of cometary tails, which
  can be exploited for probing the physical conditions of the near-Sun
  region. <P />The movies associated to Figs. 1 and 4 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201732474/olm">http://www.aanda.org</A>

Title: Two-Dimensional Solar Wind Speeds from 6 to 26 Solar Radii
    in Solar Cycle 24 by Using Fourier Filtering
Authors: Cho, Il-Hyun; Moon, Yong-Jae; Nakariakov, Valery M.; Bong,
   Su-Chan; Lee, Jin-Yi; Song, Donguk; Lee, Harim; Cho, Kyung-Suk
Bibcode: 2018PhRvL.121g5101C
Altcode: 2018arXiv180608540C
  Measurement of the solar wind speed near the Sun is important for
  understanding the acceleration mechanism of the solar wind. In this
  Letter, we determine 2D solar wind speeds from 6 to 26 solar radii
  by applying Fourier motion filters to SOHO/LASCO C3 movies observed
  from 1999 to 2010. Our method successfully reproduces the original flow
  speeds in the artificially generated data as well as streamer blobs. We
  measure 2D solar wind speeds from one-day to one-year timescales and
  their variation in solar cycle 24. We find that the solar wind speeds
  at timescales longer than a month in the solar maximum period are
  relatively uniform in the azimuthal direction, while they are clearly
  bimodal in the minimum period, as expected from the Ulysses observations
  and interplanetary radio scintillation reconstruction. The bimodal
  structure appears at around 2006, becomes most distinctive in 2009,
  and abruptly disappears in 2010. The radial evolution of the solar
  wind speeds resembles the Parker's solar wind solution.

Title: Probing the inner heliosphere with comets
Authors: Nisticò, Giuseppe; Vladimirov, Vangelis; Nakariakov, Valery
   M.; Battams, Karl; Bothmer, Volker
Bibcode: 2018shin.confE..41N
Altcode:
  Optical instruments aboard space missions have recently provided
  us with exciting observations of comets in the vicinity of their
  perihelion. At this stage, a tail of dust and ions from the comet
  nucleus is formed, which interacts with the local solar wind flow and
  exhibits an oscillatory dynamics. The observed phenomenon is attributed
  to the formation of a Kármán vortex street in the wake of the cometary
  coma, whose properties depend upon the characteristics of the comet
  itself and the local medium. We present observations of the comets
  Encke and ISON detected in 2013 with the Heliospheric Imager aboard
  the STEREO spacecraft, and discuss the possibility to exploit comets
  as natural probes of the inner heliosphere, by relating the physical
  behaviour of cometary tails with the local conditions of the solar wind.

Title: Origin of the Modulation of the Radio Emission from the Solar
    Corona by a Fast Magnetoacoustic Wave
Authors: Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Kontar, Eduard P.
Bibcode: 2018ApJ...861...33K
Altcode: 2018arXiv180508282K
  Observational detection of quasi-periodic drifting fine structures in a
  type III radio burst associated with a solar flare SOL2015-04-16T11:22,
  with the LOw Frequency ARray (LOFAR), is presented. Although similar
  modulations of the type III emission have been observed before and
  were associated with the plasma density fluctuations, the origin
  of those fluctuations was unknown. Analysis of the striae of the
  intensity variation in the dynamic spectrum allowed us to reveal two
  quasi-oscillatory components. The shorter component has an apparent
  wavelength of ∼2 Mm, phase speed of ∼657 km s<SUP>-1</SUP>, which
  gives an oscillation period of ∼3 s, and a relative amplitude of
  ∼0.35%. The longer component has a wavelength of ∼12 Mm and relative
  amplitude of ∼5.1%. The short frequency range of the detection does
  not allow us to estimate its phase speed. However, the properties of
  the shorter oscillatory component allowed us to interpret it as a fast
  magnetoacoustic wave guided by a plasma nonuniformity along the magnetic
  field outwards from the Sun. The assumption that the intensity of the
  radio emission is proportional to the amount of plasma in the emitting
  volume allowed us to show that the superposition of the plasma density
  modulation by a fast wave and a longer-wavelength oscillation of an
  unspecified nature could readily reproduce the fine structure of the
  observed dynamic spectrum. The observed parameters of the fast wave
  give an absolute value for the magnetic field in the emitting plasma
  of ∼1.1 G, which is consistent with the radial magnetic field model.

Title: Finite amplitude transverse oscillations of a magnetic rope
Authors: Kolotkov, Dmitrii Y.; Nisticò, Giuseppe; Rowlands, George;
   Nakariakov, Valery M.
Bibcode: 2018JASTP.172...40K
Altcode: 2018arXiv180305195K
  The effects of finite amplitudes on the transverse oscillations of a
  quiescent prominence represented by a magnetic rope are investigated
  in terms of the model proposed by Kolotkov et al. (2016). We consider
  a weakly nonlinear case governed by a quadratic nonlinearity, and
  also analyse the fully nonlinear equations of motion. We treat the
  prominence as a massive line current located above the photosphere and
  interacting with the magnetised dipped environment via the Lorentz
  force. In this concept the magnetic dip is produced by two external
  current sources located at the photosphere. Finite amplitude horizontal
  and vertical oscillations are found to be strongly coupled between
  each other. The coupling is more efficient for larger amplitudes and
  smaller attack angles between the direction of the driver and the
  horizontal axis. Spatial structure of oscillations is represented
  by Lissajous-like curves with the limit cycle of a hourglass shape,
  appearing in the resonant case, when the frequency of the vertical
  mode is twice the horizontal mode frequency. A metastable equilibrium
  of the prominence is revealed, which is stable for small amplitude
  displacements, and becomes horizontally unstable, when the amplitude
  exceeds a threshold value. The maximum oscillation amplitudes are
  also analytically derived and analysed. Typical oscillation periods
  are determined by the oscillation amplitude, prominence current,
  its mass and position above the photosphere, and the parameters of
  the magnetic dip. The main new effects of the finite amplitude are
  the coupling of the horizontally and vertically polarised transverse
  oscillations (i.e. the lack of a simple, elliptically polarised regime)
  and the presence of metastable equilibria of prominences.

Title: In Situ Generation of Transverse Magnetohydrodynamic Waves
    from Colliding Flows in the Solar Corona
Authors: Antolin, Patrick; Pagano, Paolo; De Moortel, Ineke;
   Nakariakov, Valery M.
Bibcode: 2018ApJ...861L..15A
Altcode: 2018arXiv180700395A
  Transverse magnetohydrodynamic (MHD) waves permeate the solar
  atmosphere and are a candidate for coronal heating. However, the
  origin of these waves is still unclear. In this Letter, we analyze
  coordinated observations from Hinode/Solar Optical Telescope (SOT) and
  Interface Region Imaging Spectrograph ( IRIS) of a prominence/coronal
  rain loop-like structure at the limb of the Sun. Cool and dense
  downflows and upflows are observed along the structure. A collision
  between a downward and an upward flow with an estimated energy flux
  of 10<SUP>7</SUP>-10<SUP>8</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>
  is observed to generate oscillatory transverse perturbations of the
  strands with an estimated ≈40 km s<SUP>-1</SUP> total amplitude, and
  a short-lived brightening event with the plasma temperature increasing
  to at least 10<SUP>5</SUP> K. We interpret this response as sausage
  and kink transverse MHD waves based on 2D MHD simulations of plasma
  flow collision. The lengths, density, and velocity differences between
  the colliding clumps and the strength of the magnetic field are major
  parameters defining the response to the collision. The presence of
  asymmetry between the clumps (angle of impact surface and/or offset
  of flowing axis) is crucial for generating a kink mode. Using the
  observed values, we successfully reproduce the observed transverse
  perturbations and brightening, and show adiabatic heating to coronal
  temperatures. The numerical modeling indicates that the plasma β
  in this loop-like structure is confined between 0.09 and 0.36. These
  results suggest that such collisions from counter-streaming flows can
  be a source of in situ transverse MHD waves, and that for cool and dense
  prominence conditions such waves could have significant amplitudes.

Title: Quasi-periodic Pulsations in a Solar Microflare
Authors: Nakariakov, V. M.; Anfinogentov, S.; Storozhenko, A. A.;
   Kurochkin, E. A.; Bogod, V. M.; Sharykin, I. N.; Kaltman, T. I.
Bibcode: 2018ApJ...859..154N
Altcode:
  Irregular time evolution of the radio emission generated in a B2-class
  microflare (SOL2017-01-25T10:15), occurring on 2017 January 25 in
  active region 12,628, is studied. The microflare was apparently
  initiated by an appearance of an s-shaped loop, observed in the EUV
  band. The radio emission is associated with the nonthermal electrons
  detected with Ramaty High Energy Solar Spectroscopic Imager, and
  originates simultaneously from two opposite footpoints of a magnetic
  fan structure beginning at a sunspot. According to the active region
  geometry, the footpoints are situated in the meridional direction,
  and hence are observed by RATAN-600 simultaneously. The radio emission
  intensity signal, as well as the left-hand and right-hand circular
  polarization signals in the low-frequency band (3-4 GHz) show good
  correlation with each other, with the average characteristic time
  of the variation 1.4 ± 0.3 s. The polarization signal shows a time
  variation with the characteristic time of about 0.7 ± 0.2 s. The
  irregular quasi-periodic pulsations of the radio emission are likely
  to be caused by the superposition of the signals generated at the local
  electron plasma frequencies by the interaction of nonthermal electrons
  with the plasma at the footpoints. In this scenario, the precipitation
  rate of the nonthermal electrons at the opposite footpoints could be
  modulated by the superposition of fundamental and second harmonic
  modes of sausage oscillations, resulting in the observed different
  characteristic times of the intensity and polarization signals. However,
  other mechanisms, e.g., the oscillatory regime of loop coalescence or
  magnetic null point oscillation could not be rigorously excluded.

Title: Spatiotemporal Analysis of Coronal Loops Using Seismology
    of Damped Kink Oscillations and Forward Modeling of EUV Intensity
    Profiles
Authors: Pascoe, D. J.; Anfinogentov, S. A.; Goddard, C. R.;
   Nakariakov, V. M.
Bibcode: 2018ApJ...860...31P
Altcode:
  The shape of the damping profile of kink oscillations in coronal loops
  has recently allowed the transverse density profile of the loop to be
  estimated. This requires accurate measurement of the damping profile
  that can distinguish the Gaussian and exponential damping regimes,
  otherwise there are more unknowns than observables. Forward modeling
  of the transverse intensity profile may also be used to estimate the
  width of the inhomogeneous layer of a loop, providing an independent
  estimate of one of these unknowns. We analyze an oscillating loop for
  which the seismological determination of the transverse structure is
  inconclusive except when supplemented by additional spatial information
  from the transverse intensity profile. Our temporal analysis describes
  the motion of a coronal loop as a kink oscillation damped by resonant
  absorption, and our spatial analysis is based on forward modeling the
  transverse EUV intensity profile of the loop under the isothermal and
  optically thin approximations. We use Bayesian analysis and Markov chain
  Monte Carlo sampling to apply our spatial and temporal models both
  individually and simultaneously to our data and compare the results
  with numerical simulations. Combining the two methods allows both
  the inhomogeneous layer width and density contrast to be calculated,
  which is not possible for the same data when each method is applied
  individually. We demonstrate that the assumption of an exponential
  damping profile leads to a significantly larger error in the inferred
  density contrast ratio compared with a Gaussian damping profile.

Title: Quasi-periodic Pulsations in the Most Powerful Solar Flare
    of Cycle 24
Authors: Kolotkov, Dmitrii Y.; Pugh, Chloe E.; Broomhall, Anne-Marie;
   Nakariakov, Valery M.
Bibcode: 2018ApJ...858L...3K
Altcode: 2018arXiv180404955K
  Quasi-periodic pulsations (QPPs) are common in solar flares and are
  now regularly observed in stellar flares. We present the detection
  of two different types of QPP signals in the thermal emission light
  curves of the X9.3-class solar flare SOL2017-09-06T12:02, which is the
  most powerful flare of Cycle 24. The period of the shorter-period QPP
  drifts from about 12 to 25 s during the flare. The observed properties
  of this QPP are consistent with a sausage oscillation of a plasma loop
  in the flaring active region. The period of the longer-period QPP is
  about 4 to 5 minutes. Its properties are compatible with standing slow
  magnetoacoustic oscillations, which are often detected in coronal
  loops. For both QPP signals, other mechanisms such as repetitive
  reconnection cannot be ruled out, however. The studied solar flare
  has an energy in the realm of observed stellar flares, and the fact
  that there is evidence of a short-period QPP signal typical of solar
  flares along with a long-period QPP signal more typical of stellar
  flares suggests that the different ranges of QPP periods typically
  observed in solar and stellar flares is likely due to observational
  constraints, and that similar physical processes may be occurring in
  solar and stellar flares.

Title: Nonlinear waves in the terrestrial quasi-parallel foreshock
Authors: Hnat, Bogdan; Kolotkov, Dmitri; O'Connell, Daniel; Nakariakov,
   Valery; Rowlands, George
Bibcode: 2018EGUGA..20.4956H
Altcode:
  Upstream regions of quasiparallel collisionless astrophysical shocks are
  among the most complex plasma systems. When the magnetic field direction
  is quasiparallel to the shock normal, a fraction of incoming ions are
  reflected upstream generating an extended and turbulent foreshock. We
  provide strongly conclusive evidence that the cubic nonlinearity plays
  an important part in the evolution of the large amplitude magnetic
  structures in the terrestrial foreshock. Large amplitude nonlinear
  wave trains at frequencies above the proton cyclotron frequency are
  identified after non harmonic slow variations are filtered out by
  applying the empirical mode decomposition. Numerical solutions of the
  derivative nonlinear Schrodinger equation, predicted analytically by
  the use of a pseudo-potential approach, are found to be consistent
  with the observed wave forms. The approximate phase speed of these
  nonlinear waves, indicated by the parameters of numerical solutions,
  is of the order of the local Alfven speed. We suggest that the feedback
  of the large amplitudec fluctuations on background plasma is reflected
  in the evolution of the pseudo-potential.

Title: Detection of the Second Harmonic of Decay-less Kink
    Oscillations in the Solar Corona
Authors: Duckenfield, T.; Anfinogentov, S. A.; Pascoe, D. J.;
   Nakariakov, V. M.
Bibcode: 2018ApJ...854L...5D
Altcode:
  EUV observations of a multi-thermal coronal loop, taken by the
  Atmospheric Imaging Assembly of the Solar Dynamics Observatory, which
  exhibits decay-less kink oscillations are presented. The data cube of
  the quiet-Sun coronal loop was passed through a motion magnification
  algorithm to accentuate transverse oscillations. Time-distance maps are
  made from multiple slits evenly spaced along the loop axis and oriented
  orthogonal to the loop axis. Displacements of the intensity peak are
  tracked to generate time series of the loop displacement. Fourier
  analysis on the time series shows the presence of two periods within
  the loop: {P}<SUB>1</SUB>={10.3}<SUB>-1.7</SUB><SUP>+1.5</SUP>
  minutes and {P}<SUB>2</SUB>={7.4}<SUB>-1.3</SUB><SUP>+1.1</SUP>
  minutes. The longer period component is greatest in amplitude at the
  apex and remains in phase throughout the loop length. The shorter
  period component is strongest further down from the apex on both legs
  and displays an anti-phase behavior between the two loop legs. We
  interpret these results as the coexistence of the fundamental and
  second harmonics of the standing kink mode within the loop in the
  decay-less oscillation regime. An illustration of seismological
  application using the ratio P <SUB>1</SUB>/2P <SUB>2</SUB> ∼ 0.7
  to estimate the density scale height is presented. The existence of
  multiple harmonics has implications for understanding the driving and
  damping mechanisms for decay-less oscillations and adds credence to
  their interpretation as standing kink mode oscillations.

Title: Modelling Quasi-Periodic Pulsations in Solar and Stellar Flares
Authors: McLaughlin, J. A.; Nakariakov, V. M.; Dominique, M.; Jelínek,
   P.; Takasao, S.
Bibcode: 2018SSRv..214...45M
Altcode: 2018arXiv180204180M
  Solar flare emission is detected in all EM bands and variations in flux
  density of solar energetic particles. Often the EM radiation generated
  in solar and stellar flares shows a pronounced oscillatory pattern, with
  characteristic periods ranging from a fraction of a second to several
  minutes. These oscillations are referred to as quasi-periodic pulsations
  (QPPs), to emphasise that they often contain apparent amplitude and
  period modulation. We review the current understanding of quasi-periodic
  pulsations in solar and stellar flares. In particular, we focus on
  the possible physical mechanisms, with an emphasis on the underlying
  physics that generates the resultant range of periodicities. These
  physical mechanisms include MHD oscillations, self-oscillatory
  mechanisms, oscillatory reconnection/reconnection reversal, wave-driven
  reconnection, two loop coalescence, MHD flow over-stability, the
  equivalent LCR-contour mechanism, and thermal-dynamical cycles. We
  also provide a histogram of all QPP events published in the literature
  at this time. The occurrence of QPPs puts additional constraints on
  the interpretation and understanding of the fundamental processes
  operating in flares, e.g. magnetic energy liberation and particle
  acceleration. Therefore, a full understanding of QPPs is essential in
  order to work towards an integrated model of solar and stellar flares.

Title: Corrugation Instability of a Coronal Arcade
Authors: Klimushkin, D. Y.; Nakariakov, V. M.; Mager, P. N.;
   Cheremnykh, O. K.
Bibcode: 2017SoPh..292..184K
Altcode:
  We analyse the behaviour of linear magnetohydrodynamic perturbations
  of a coronal arcade modelled by a half-cylinder with an azimuthal
  magnetic field and non-uniform radial profiles of the plasma
  pressure, temperature, and the field. Attention is paid to the
  perturbations with short longitudinal (in the direction along the
  arcade) wavelengths. The radial structure of the perturbations, either
  oscillatory or evanescent, is prescribed by the radial profiles of the
  equilibrium quantities. Conditions for the corrugation instability of
  the arcade are determined. It is established that the instability growth
  rate increases with decreases in the longitudinal wavelength and the
  radial wave number. In the unstable mode, the radial perturbations of
  the magnetic field are stronger than the longitudinal perturbations,
  creating an almost circularly corrugated rippling of the arcade in the
  longitudinal direction. For coronal conditions, the growth time of the
  instability is shorter than one minute, decreasing with an increase in
  the temperature. Implications of the developed theory for the dynamics
  of coronal active regions are discussed.

Title: Properties of quasi-periodic pulsations in solar flares from
    a single active region
Authors: Pugh, C. E.; Nakariakov, V. M.; Broomhall, A. -M.; Bogomolov,
   A. V.; Myagkova, I. N.
Bibcode: 2017A&A...608A.101P
Altcode: 2017arXiv170909472P
  Context. Quasi-periodic pulsations (QPPs) are a common feature of
  solar and stellar flares, and so the nature of these pulsations should
  be understood in order to fully understand flares. <BR /> Aims: We
  investigate the properties of a set of solar flares originating from
  a single active region (AR) that exhibit QPPs, and in particular
  look for any indication of QPP periods relating to AR properties,
  as might be expected if the characteristic timescale of the pulsations
  corresponds to a characteristic length scale of the structure from which
  the pulsations originate. The three AR properties used for this study
  are the photospheric area, bipole separation distance, and average
  magnetic field strength at the photosphere. The AR studied, known as
  NOAA 12172/12192/12209, was unusually long-lived and persisted for over
  three Carrington rotations between September and November 2014. During
  this time a total of 181 flares were observed by GOES. <BR /> Methods:
  Data from the GOES/XRS, SDO/EVE/ESP, Fermi/GBM, Vernov/DRGE and Nobeyama
  Radioheliograph observatories were used to determine if QPPs were
  present in the flares. For the soft X-ray GOES/XRS and EVE/ESP data,
  the time derivative of the signal was used so that any variability
  in the impulsive phase of the flare was emphasised. Periodogram power
  spectra of the time series data, without any form of detrending, were
  inspected and flares with a peak above the 95% confidence level in the
  power spectrum were labelled as having candidate QPPs. The confidence
  levels were determined taking full account of data uncertainties
  and the possible presence of red noise. Active region properties
  were determined using SDO/HMI line of sight magnetogram data. <BR
  /> Results: A total of 37 flares, i.e. 20% of the sample, show good
  evidence of having stationary or weakly non-stationary QPPs, and some
  of the pulsations can be seen in data from multiple instruments and
  in different wavebands. Because the detection method used was rather
  conservative, this may be a lower bound for the true number of flares
  with QPPs. The QPP periods were found to show a weak correlation
  with the flare amplitude and duration, but this is likely due to an
  observational bias. A stronger correlation was found between the QPP
  period and duration of the QPP signal, which can be partially but not
  entirely explained by observational constraints. No correlations were
  found with the AR area, bipole separation distance, or average magnetic
  field strength. <BR /> Conclusions: The fact that a substantial fraction
  of the flare sample showed evidence of QPPs, using a strict detection
  method with minimal processing of the data, demonstrates that these
  QPPs are a real phenomenon that cannot be explained by the presence of
  red noise or the superposition of multiple unrelated flares. The lack
  of correlation between the QPP periods and AR properties implies that
  the small-scale structure of the AR is important and/or that different
  QPP mechanisms act in different cases.

Title: Effect of Local Thermal Equilibrium Misbalance on
    Long-wavelength Slow Magnetoacoustic Waves
Authors: Nakariakov, V. M.; Afanasyev, A. N.; Kumar, S.; Moon, Y. -J.
Bibcode: 2017ApJ...849...62N
Altcode:
  Evolution of slow magnetoacoustic waves guided by a cylindrical
  magnetic flux tube that represents a coronal loop or plume, is modeled
  accounting for the effects of finite gas pressure, weak nonlinearity,
  dissipation by thermal conduction and viscosity, and the misbalance
  between the cooling by optically thin radiation and unspecified heating
  of the plasma. An evolutionary equation of the Burgers-Malthus type is
  derived. It is shown that the cooling/heating misbalance, determined by
  the derivatives of the combined radiative cooling and heating function,
  with respect to the density, temperature, and magnetic field at the
  thermal equilibrium affect the wave rather strongly. This effect may
  either cause additional damping, or counteract it, or lead to the
  gradual amplification of the wave. In the latter case, the coronal
  plasma acts as an active medium for the slow magnetoacoustic waves. The
  effect of the cooling/heating misbalance could be important for coronal
  slow waves, and could be responsible for certain discrepancies between
  theoretical results and observations, in particular, the increased or
  decreased damping lengths and times, detection of the waves at certain
  heights only, and excitation of compressive oscillations. The results
  obtained open up a possibility for the diagnostics of the coronal
  heating function by slow magnetoacoustic waves.

Title: Seismology of contracting and expanding coronal loops using
    damping of kink oscillations by mode coupling
Authors: Pascoe, D. J.; Russell, A. J. B.; Anfinogentov, S. A.;
   Simões, P. J. A.; Goddard, C. R.; Nakariakov, V. M.; Fletcher, L.
Bibcode: 2017A&A...607A...8P
Altcode:
  <BR /> Aims: We extend recently developed seismological methods to
  analyse oscillating loops which feature a large initial shift in the
  equilibrium position and investigate additional observational signatures
  related to the loop environment and oscillation driver. <BR /> Methods:
  We model the motion of coronal loops as a kink oscillation damped by
  mode coupling, accounting for any change in loop length and the possible
  presence of parallel harmonics in addition to the fundamental mode. We
  apply our model to a loop which rapidly contracts due to a post-flare
  implosion (SOL2012-03-09) and a loop with a large lateral displacement
  (SOL2012-10-20). <BR /> Results: The seismological method is used to
  calculate plasma parameters of the oscillating loops including the
  transverse density profile, magnetic field strength, and phase mixing
  timescale. For SOL2012-03-09 the period of oscillation has a linear
  correlation with the contracting motion and suggests the kink speed
  remains constant during the oscillation. The implosion excitation
  mechanism is found to be associated with an absence of additional
  parallel harmonics. <BR /> Conclusions: The improved Bayesian
  analysis of the coronal loop motion allows for accurate seismology
  of plasma parameters, and the evolution of the period of oscillation
  compared with the background trend can be used to distinguish between
  loop motions in the plane of the loop and those perpendicular to
  it. The seismologically inferred kink speed and density contrast
  imply sub-Alfvénic (M<SUB>A</SUB> = 0.16 ± 0.03) propagation of the
  magnetic reconfiguration associated with the implosion, as opposed to
  triggering by a wave propagating at the Alfvén speed.

Title: Dispersive Evolution of Nonlinear Fast Magnetoacoustic
    Wave Trains
Authors: Pascoe, D. J.; Goddard, C. R.; Nakariakov, V. M.
Bibcode: 2017ApJ...847L..21P
Altcode:
  Quasi-periodic rapidly propagating wave trains are frequently observed
  in extreme ultraviolet observations of the solar corona, or are inferred
  by the quasi-periodic modulation of radio emission. The dispersive
  nature of fast magnetohydrodynamic waves in coronal structures
  provides a robust mechanism to explain the detected quasi-periodic
  patterns. We perform 2D numerical simulations of impulsively generated
  wave trains in coronal plasma slabs and investigate how the behavior
  of the trapped and leaky components depend on the properties of the
  initial perturbation. For large amplitude compressive perturbations,
  the geometrical dispersion associated with the waveguide suppresses
  the nonlinear steepening for the trapped wave train. The wave train
  formed by the leaky components does not experience dispersion once it
  leaves the waveguide and so can steepen and form shocks. The mechanism
  we consider can lead to the formation of multiple shock fronts by
  a single, large amplitude, impulsive event and so can account for
  quasi-periodic features observed in radio spectra.

Title: Toward a Next Generation Solar Coronagraph: Development of
    a Compact Diagnostic Coronagraph on the ISS
Authors: Cho, K. -S.; Bong, S. -C.; Choi, S.; Yang, H.; Kim, J.;
   Baek, J. -H.; Park, J.; Lim, E. -K.; Kim, R. -S.; Kim, S.; Kim,
   Y. -H.; Park, Y. -D.; Clarke, S. W.; Davila, J. M.; Gopalswamy, N.;
   Nakariakov, V. M.; Li, B.; Pinto, R. F.
Bibcode: 2017JKAS...50..139C
Altcode:
  The Korea Astronomy and Space Science Institute plans to develop
  a coronagraph in collaboration with National Aeronautics and Space
  Administration (NASA) and to install it on the International Space
  Station (ISS). The coronagraph is an externally occulted one-stage
  coronagraph with a field of view from 3 to 15 solar radii. The
  observation wavelength is approximately 400 nm, where strong Fraunhofer
  absorption lines from the photosphere experience thermal broadening and
  Doppler shift through scattering by coronal electrons. Photometric
  filter observations around this band enable the estimation of
  2D electron temperature and electron velocity distribution in the
  corona. Together with a high time cadence (&lt;12 min) of corona images
  used to determine the geometric and kinematic parameters of coronal
  mass ejections, the coronagraph will yield the spatial distribution
  of electron density by measuring the polarized brightness. For the
  purpose of technical demonstration, we intend to observe the total
  solar eclipse in August 2017 with the filter system and to perform a
  stratospheric balloon experiment in 2019 with the engineering model
  of the coronagraph. The coronagraph is planned to be installed on the
  ISS in 2021 for addressing a number of questions (e.g., coronal heating
  and solar wind acceleration) that are both fundamental and practically
  important in the physics of the solar corona and of the heliosphere.

Title: A statistical study of the inferred transverse density profile
    of coronal loop threads observed with SDO/AIA
Authors: Goddard, C. R.; Pascoe, D. J.; Anfinogentov, S.; Nakariakov,
   V. M.
Bibcode: 2017A&A...605A..65G
Altcode:
  <BR /> Aims: We carry out a statistical study of the inferred coronal
  loop cross-sectional density profiles using extreme ultraviolet (EUV)
  imaging data from the Atmospheric Imaging Assembly (AIA) on board
  the Solar Dynamics Observatory (SDO). <BR /> Methods: We analysed 233
  coronal loops observed during 2015/2016. We consider three models for
  the density profile; the step function (model S), the linear transition
  region profile (model L), and a Gaussian profile (model G). Bayesian
  inference is used to compare the three corresponding forward modelled
  intensity profiles for each loop. These are constructed by integrating
  the square of the density from a cylindrical loop cross-section
  along the line of sight, assuming an isothermal cross-section, and
  applying the instrumental point spread function. <BR /> Results:
  Calculating the Bayes factors for comparisons between the models,
  it was found that in 47% of cases there is very strong evidence for
  model L over model S and in 45% of cases very strong evidence for
  model G over S. Using multiple permutations of the Bayes factor the
  favoured density profile for each loop was determined for multiple
  evidence thresholds. There were a similar number of cases where model
  L or G are favoured, showing evidence for inhomogeneous layers and
  constantly varying density cross-sections, subject to our assumptions
  and simplifications. <BR /> Conclusions: For sufficiently well
  resolved loop threads with no visible substructure it has been shown
  that using Bayesian inference and the observed intensity profile we
  can distinguish between the proposed density profiles at a given AIA
  wavelength and spatial resolution. We have found very strong evidence
  for inhomogeneous layers, with model L being the most general, and a
  tendency towards thicker or even continuous layers.

Title: Quasi-periodic Radio Bursts Associated with Fast-mode Waves
    near a Magnetic Null Point
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
Bibcode: 2017ApJ...844..149K
Altcode: 2017arXiv170609988K
  This paper presents an observation of quasi-periodic rapidly propagating
  waves observed in the Atmospheric Image Assembly (AIA) 171/193 Å
  channels during the impulsive phase of an M1.9 flare that occurred on
  2012 May 7. The instant period was found to decrease from 240 to 120
  s, and the speed of the wavefronts was in the range of ∼664-1416
  km s<SUP>-1</SUP>. Almost simultaneously, quasi-periodic bursts with
  similar instant periods, ∼70 and ∼140 s, occur in the microwave
  emission and in decimetric type IV and type III radio bursts, and in
  the soft X-ray emission. The magnetic field configuration of the flare
  site was consistent with a breakout topology, I.e., a quadrupolar
  field along with a magnetic null point. The quasi-periodic rapidly
  propagating wavefronts of the EUV emission are interpreted as a fast
  magnetoacoustic wave train. The observations suggest that the fast-mode
  waves are generated during the quasi-periodic magnetic reconnection
  in the cusp region above the flare arcade loops. For the first time,
  we provide evidence of a tadpole wavelet signature at about 70-140
  s in decimetric (245/610 MHz) radio bursts, along with the direct
  observation of a coronal fast-mode wave train in EUV. In addition, at
  AIA 131/193 Å we observed quasi-periodic EUV disturbances with periods
  of 95 and 240 s propagating downward at apparent speeds of 172-273 km
  s<SUP>-1</SUP>. The nature of these downward propagating disturbances
  is not revealed, but they could be connected to magnetoacoustic waves
  or periodically shrinking loops.

Title: Significance testing for quasi-periodic pulsations in solar
    and stellar flares
Authors: Pugh, C. E.; Broomhall, A. -M.; Nakariakov, V. M.
Bibcode: 2017A&A...602A..47P
Altcode: 2017arXiv170307294P
  The robust detection of quasi-periodic pulsations (QPPs) in solar and
  stellar flares has been the topic of recent debate. In light of this,
  we have adapted a method described by Vaughan (2005, A&amp;A, 431,
  391) to aid with the search for QPPs in flare time series data. The
  method identifies statistically significant periodic signals in
  power spectra, and properly accounts for red noise as well as the
  uncertainties associated with the data. We show how the method
  can be further developed to be used with rebinned power spectra,
  allowing us to detect QPPs whose signal is spread over more than one
  frequency bin. An advantage of these methods is that there is no need
  to detrend the data prior to creating the power spectrum. Examples are
  given where the methods have been applied to synthetic data, as well
  as real flare time series data with candidate QPPs from the Nobeyama
  Radioheliograph. These show that, despite the transient nature of QPPs,
  peaks corresponding to the QPPs can be seen at a significant level in
  the power spectrum without any form of detrending or other processing
  of the original time series data, providing the background trends are
  not too steep.

Title: Nonlinear Evolution of Short-wavelength Torsional Alfvén Waves
Authors: Shestov, S. V.; Nakariakov, V. M.; Ulyanov, A. S.; Reva,
   A. A.; Kuzin, S. V.
Bibcode: 2017ApJ...840...64S
Altcode: 2017arXiv170502790S
  We analyze nonlinear evolution of torsional Alfvén waves in a straight
  magnetic flux tube filled in with a low-β plasma, and surrounded with
  a plasma of lower density. Such magnetic tubes model, in particular,
  a segment of a coronal loop or a polar plume. The wavelength is taken
  comparable to the tube radius. We perform a numerical simulation of
  the wave propagation using ideal magnetohydrodynamics. We find that a
  torsional wave nonlinearly induces three kinds of compressive flows:
  the parallel flow at the Alfvén speed, which constitutes a bulk
  plasma motion along the magnetic field, the tube wave, and also
  transverse flows in the radial direction, associated with sausage
  fast magnetoacoustic modes. In addition, the nonlinear torsional
  wave steepens and its propagation speed increases. The latter effect
  leads to the progressive distortion of the torsional wave front, I.e.,
  nonlinear phase mixing. Because of the intrinsic non-uniformity of the
  torsional wave amplitude across the tube radius, the nonlinear effects
  are more pronounced in regions with higher wave amplitudes. They
  are always absent at the axes of the flux tube. In the case of a
  linear radial profile of the wave amplitude, the nonlinear effects
  are localized in an annulus region near the tube boundary. Thus,
  the parallel compressive flows driven by torsional Alfvén waves in
  the solar and stellar coronae, are essentially non-uniform in the
  perpendicular direction. The presence of additional sinks for the
  wave energy reduces the efficiency of the nonlinear parallel cascade
  in torsional Alfvén waves.

Title: Multi-instrument observations of a failed flare eruption
    associated with MHD waves in a loop bundle
Authors: Nisticò, G.; Polito, V.; Nakariakov, V. M.; Del Zanna, G.
Bibcode: 2017A&A...600A..37N
Altcode: 2016arXiv161202077N
  Context. We present observations of a B7.9-class flare that occurred on
  the 24th January, 2015, using the Atmospheric Imaging Assembly (AIA)
  of the Solar Dynamics Observatory (SDO), the EUV Imaging Spectrometer
  (EIS) and the X-Ray Telescope of Hinode. The flare triggers the eruption
  of a dense cool plasma blob as seen in AIA 171 Å, which is unable
  to completely break out and remains confined within a local bundle
  of active region loops. During this process, transverse oscillations
  of the threads are observed. The cool plasma is then observed to
  descend back to the chromosphere along each loop strand. At the same
  time, a larger diffuse co-spatial loop observed in the hot wavebands
  of SDO/AIA and Hinode/XRT is formed, exhibiting periodic intensity
  variations along its length. <BR /> Aims: The formation and evolution
  of magnetohydrodynamic (MHD) waves depend upon the values of the local
  plasma parameters (e.g. density, temperature and magnetic field),
  which can hence be inferred by coronal seismology. In this study we
  aim to assess how the observed MHD modes are affected by the variation
  of density and temperature. <BR /> Methods: We combined analysis
  of EUV/X-ray imaging and spectroscopy using SDO/AIA, Hinode/EIS
  and XRT. <BR /> Results: The transverse oscillations of the cool
  loop threads are interpreted in terms of vertically polarised kink
  oscillations. The fitting procedure applied to the loop displacement
  time series gives a period of 3.5 to 4 min, and an amplitude of 5
  Mm. The oscillations are strongly damped showing very low quality
  factor (1.5-2), which is defined as the ratio of the damping time and
  the oscillation period. The weak variation of the period of the kink
  wave, which is estimated from the fitting analysis, is in agreement
  with the density variations due to the presence of the plasma blob
  inferred from the intensity light curve at 171 Å. The coexisting
  intensity oscillations along the hot loop are interpreted as a slow
  MHD wave with a period of 10 min and phase speed of approximately 436
  km s<SUP>-1</SUP>. Comparison between the fast and slow modes allows
  for the determination of the Alfvén speed, and consequently magnetic
  field values. The plasma-β inferred from the analysis is estimated to
  be approximately 0.1-0.3. <BR /> Conclusions: We show that the evolution
  of the detected waves is determined by the temporal variations of the
  local plasma parameters, caused by the flare heating and the consequent
  cooling. We apply coronal seismology to both waves obtaining estimates
  of the background plasma parameters. <P />Movies are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201629324/olm">http://www.aanda.org</A>

Title: Coronal loop seismology using damping of standing kink
    oscillations by mode coupling. II. additional physical effects and
    Bayesian analysis
Authors: Pascoe, D. J.; Anfinogentov, S.; Nisticò, G.; Goddard,
   C. R.; Nakariakov, V. M.
Bibcode: 2017A&A...600A..78P
Altcode:
  Context. The strong damping of kink oscillations of coronal loops can
  be explained by mode coupling. The damping envelope depends on the
  transverse density profile of the loop. Observational measurements of
  the damping envelope have been used to determine the transverse loop
  structure which is important for understanding other physical processes
  such as heating. <BR /> Aims: The general damping envelope describing
  the mode coupling of kink waves consists of a Gaussian damping
  regime followed by an exponential damping regime. Recent observational
  detection of these damping regimes has been employed as a seismological
  tool. We extend the description of the damping behaviour to account
  for additional physical effects, namely a time-dependent period of
  oscillation, the presence of additional longitudinal harmonics, and
  the decayless regime of standing kink oscillations. <BR /> Methods:
  We examine four examples of standing kink oscillations observed by
  the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
  Observatory (SDO). We use forward modelling of the loop position
  and investigate the dependence on the model parameters using Bayesian
  inference and Markov chain Monte Carlo (MCMC) sampling. <BR /> Results:
  Our improvements to the physical model combined with the use of Bayesian
  inference and MCMC produce improved estimates of model parameters and
  their uncertainties. Calculation of the Bayes factor also allows us to
  compare the suitability of different physical models. We also use a new
  method based on spline interpolation of the zeroes of the oscillation to
  accurately describe the background trend of the oscillating loop. <BR
  /> Conclusions: This powerful and robust method allows for accurate
  seismology of coronal loops, in particular the transverse density
  profile, and potentially reveals additional physical effects.

Title: Coronal loop density profile estimated by forward modelling
    of EUV intensity
Authors: Pascoe, D. J.; Goddard, C. R.; Anfinogentov, S.; Nakariakov,
   V. M.
Bibcode: 2017A&A...600L...7P
Altcode:
  <BR /> Aims: The transverse density structuring of coronal loops was
  recently calculated for the first time using the general damping
  profile for kink oscillations. This seismological method assumes
  a density profile with a linear transition region. We consider to
  what extent this density profile accounts for the observed intensity
  profile of the loop, and how the transverse intensity profile may
  be used to complement the seismological technique. <BR /> Methods:
  We use isothermal and optically transparent approximations for which
  the intensity of extreme ultraviolet (EUV) emission is proportional to
  the square of the plasma density integrated along the line of sight. We
  consider four different models for the transverse density profile; the
  generalised Epstein profile, the step function, the linear transition
  region profile, and a Gaussian profile. The effect of the point spread
  function is included and Bayesian analysis is used for comparison of
  the models. <BR /> Results: The two profiles with finite transitions are
  found to be preferable to the step function profile, which supports the
  interpretation of kink mode damping as being due to mode coupling. The
  estimate of the transition layer width using forward modelling is
  consistent with the seismological estimate. <BR /> Conclusions: For
  wide loops, that is those observed with sufficiently high spatial
  resolution, this method can provide an independent estimate of density
  profile parameters for comparison with seismological estimates. In
  the ill-posed case of only one of the Gaussian or exponential damping
  regimes being observed, it may provide additional information to allow
  a seismological inversion to be performed. Alternatively, it may be
  used to obtain structuring information for loops that do not oscillate.

Title: Determination of the Alfvén Speed and Plasma-beta Using the
    Seismology of Sunspot Umbra
Authors: Cho, I. -H.; Cho, K. -S.; Bong, S. -C.; Moon, Y. -J.;
   Nakariakov, V. M.; Park, J.; Baek, J. -H.; Choi, S.; Kim, Y. -H.;
   Lee, J.
Bibcode: 2017ApJ...837L..11C
Altcode:
  For 478 centrally located sunspots observed in the optical continuum
  with Solar Dynamics Observatory/Helioseismic Magnetic Imager,
  we perform seismological diagnostics of the physical parameters of
  umbral photospheres. The new technique is based on the theory of slow
  magnetoacoustic waves in a non-isothermally stratified photosphere
  with a uniform vertical magnetic field. We construct a map of the
  weighted frequency of three-minute oscillations inside the umbra
  and use it for the estimation of the Alfvén speed, plasma-beta,
  and mass density within the umbra. We find the umbral mean Alfvén
  speed ranges between 10.5 and 7.5 km s<SUP>-1</SUP> and is negatively
  correlated with magnetic field strength. The umbral mean plasma-beta
  is found to range approximately between 0.65 and 1.15 and does not
  vary significantly from pores to mature sunspots. The mean density
  ranges between (1-6) × 10<SUP>-4</SUP> kg m<SUP>-3</SUP> and shows
  a strong positive correlation with magnetic field strength.

Title: Long-period quasi-periodic oscillations of a small-scale
    magnetic structure on the Sun
Authors: Kolotkov, D. Y.; Smirnova, V. V.; Strekalova, P. V.;
   Riehokainen, A.; Nakariakov, V. M.
Bibcode: 2017A&A...598L...2K
Altcode:
  <BR /> Aims: Long-period quasi-periodic variations of the average
  magnetic field in a small-scale magnetic structure on the Sun are
  analysed. The structure is situated at the photospheric level and is
  involved in a facula formation in the chromosphere. <BR /> Methods:
  The observational signal obtained from the SDO/HMI line-of-sight
  magnetograms of the target structure has a non-stationary behaviour,
  and is therefore processed with the Hilbert-Huang Transform spectral
  technique. <BR /> Results: The empirical decomposition of the original
  signal and subsequent testing of the statistical significance of
  its intrinsic modes reveal the presence of the white and pink noisy
  components for the periods shorter and longer than 10 min, respectively,
  and a significant oscillatory mode. The oscillation is found to have
  a non-stationary period growing from approximately 80 to 230 min and
  an increasing relative amplitude, while the mean magnetic field in the
  oscillating structure is seen to decrease. The observed behaviour could
  be interpreted either by the dynamical interaction of the structure
  with the boundaries of supergranula cells in the region of interest
  or in terms of the vortex shedding appearing during the magnetic
  flux emergence.

Title: Observation of a Short Period Quasi-periodic Pulsation in
    Solar X-Ray, Microwave, and EUV Emissions
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
Bibcode: 2017ApJ...836..121K
Altcode: 2017arXiv170102159K
  This paper presents the multiwavelength analysis of a 13 s
  quasi-periodic pulsation (QPP) observed in hard X-ray (12-300 keV)
  and microwave (4.9-34 GHz) emissions during a C-class flare that
  occurred on 2015 September 21. Atmospheric Image Assembly (AIA) 304
  and 171 Å images show an emerging loop/flux tube (L1) moving radially
  outward, which interacts with the preexisting structures within the
  active region (AR). The QPP was observed during the expansion of and
  rising motion of L1. The Nobeyama Radioheliograph microwave images in
  17/34 GHz channels reveal a single radio source that was co-spatial
  with a neighboring loop (L2). In addition, using AIA 304 Å images,
  we detected intensity oscillations in the legs of L2 with a period
  of about 26 s. A similar oscillation period was observed in the GOES
  soft X-ray flux derivative. This oscillation period seems to increase
  with time. We suggest that the observed QPP is most likely generated
  by the interaction between L2 and L3 observed in the AIA hot channels
  (131 and 94 Å). The merging speed of loops L2 and L3 was ∼35 km
  s<SUP>-1</SUP>. L1 was destroyed possibly by its interaction with
  preexisting structures in the AR, and produced a cool jet with the
  speed of ∼106-118 km s<SUP>-1</SUP> associated with a narrow CME
  (∼770 km s<SUP>-1</SUP>). Another mechanism of the QPP in terms of
  a sausage oscillation of the loop (L2) is also possible.

Title: Diagnostics of Coronal Heating in Active-region Loops
Authors: Fludra, A.; Hornsey, C.; Nakariakov, V. M.
Bibcode: 2017ApJ...834..100F
Altcode:
  Understanding coronal heating remains a central problem in solar
  physics. Many mechanisms have been proposed to explain how energy
  is transferred to and deposited in the corona. We summarize past
  observational studies that attempted to identify the heating mechanism
  and point out the difficulties in reproducing the observations of
  the solar corona from the heating models. The aim of this paper is
  to study whether the observed extreme ultraviolet (EUV) emission in
  individual coronal loops in solar active regions can provide constraints
  on the volumetric heating function, and to develop a diagnostic for
  the heating function for a subset of loops that are found close to
  static thermal equilibrium. We reconstruct the coronal magnetic field
  from Solar Dynamics Observatory/HMI data using a nonlinear force-free
  magnetic field model. We model selected loops using a one-dimensional
  stationary model, with a heating rate dependent locally on the magnetic
  field strength along the loop, and we calculate the emission from
  these loops in various EUV wavelengths for different heating rates. We
  present a method to measure a power index β defining the dependence
  of the volumetric heating rate E<SUB>H</SUB> on the magnetic field,
  {E}<SUB>H</SUB>\propto {B}<SUP>β </SUP>, and controlling also
  the shape of the heating function: concentrated near the loop top,
  uniform and concentrated near the footpoints. The diagnostic is based
  on the dependence of the electron density on the index β. This method
  is free from the assumptions of the loop filling factor but requires
  spectroscopic measurements of the density-sensitive lines. The range of
  applicability for loops of different length and heating distributions
  is discussed, and the steps to solving the coronal heating problem
  are outlined.

Title: Preface to Topical Issue: Waves in the Solar Corona: From
    Microphysics to Macrophysics
Authors: Nakariakov, V. M.; Pascoe, D. J.; Sych, R.; van
   Driel-Gesztelyi, L.
Bibcode: 2016SoPh..291.3139N
Altcode: 2016SoPh..tmp..187N
  No abstract at ADS

Title: Motion Magnification in Coronal Seismology
Authors: Anfinogentov, Sergey; Nakariakov, Valery M.
Bibcode: 2016SoPh..291.3251A
Altcode: 2016SoPh..tmp..180A; 2016arXiv161101790A
  We introduce a new method for the investigation of low-amplitude
  transverse oscillations of solar plasma non-uniformities, such
  as coronal loops, individual strands in coronal arcades, jets,
  prominence fibrils, polar plumes, and other contrast features that
  have been observed with imaging instruments. The method is based on
  the two-dimensional dual-tree complex wavelet transform (DTℂWT). It
  allows us to magnify transverse, in the plane-of-the-sky, quasi-periodic
  motions of contrast features in image sequences. The tests performed
  on the artificial data cubes that imitated exponentially decaying,
  multi-periodic and frequency-modulated kink oscillations of coronal
  loops showed the effectiveness, reliability, and robustness of this
  technique. The algorithm was found to give linear scaling of the
  magnified amplitudes with the original amplitudes, provided these are
  sufficiently small. In addition, the magnification is independent of
  the oscillation period in a broad range of the periods. The application
  of this technique to SDO/AIA EUV data cubes of a non-flaring active
  region allowed for the improved detection of low-amplitude decay-less
  oscillations in the majority of loops.

Title: Quasi-periodic Acceleration of Electrons in the Flare on 2012
    July 19
Authors: Huang, Jing; Kontar, Eduard P.; Nakariakov, Valery M.;
   Gao, Guannan
Bibcode: 2016ApJ...831..119H
Altcode:
  Quasi-periodic pulsations (QPPs) of nonthermal emission in an M7.7
  class flare on 2012 July 19 are investigated with spatially resolved
  observations at microwave and HXR bands and with spectral observations
  at decimetric, metric waves. Microwave emission at 17 GHz of two
  footpoints, HXR emission at 20-50 keV of the north footpoint and
  loop top, and type III bursts at 0.7-3 GHz show prominent in-phase
  oscillations at 270 s. The microwave emission of the loop leg has less
  pulsation but stronger emission. Through the estimation of plasma
  density around the loop top from EUV observations, we find that the
  local plasma frequency would be 1.5 GHz or even higher. Thus, type
  III bursts at 700 MHz originate above the loop top. Quasi-periodic
  acceleration or injection of energetic electrons is proposed to
  dominate these in-phase QPPs of nonthermal emission from footpoints,
  loop top, and above. In the overlying region, drifting pulsations
  (DPS) at 200-600 MHz oscillate at a distinct period (200 s). Its global
  structure drifts toward lower frequency, which is closely related to
  upward plasmoids observed simultaneously from EUV emission. Hence,
  nonthermal emission from overlying plasmoids and underlying flaring
  loops show different oscillating periods. Two individual systems of
  quasi-periodic acceleration of electrons are proposed to coincide in
  the bi-direction outflows from the reconnection region.

Title: Dependence of Occurrence Rates of Solar Flares and Coronal
    Mass Ejections on the Solar Cycle Phase and the Importance of
    Large-scale Connectivity
Authors: Lee, Kangjin; Moon, Y. -J.; Nakariakov, V. M.
Bibcode: 2016ApJ...831..131L
Altcode:
  We investigate the dependence of the occurrence rates of major solar
  flares (M- and X-class) and front-side halo coronal mass ejections
  (FHCMEs), observed from 1996 to 2013, on the solar cycle (SC) phase
  for six active McIntosh sunspot group classes: Fkc, Ekc, Dkc, Fki,
  Eki, and Dki. We classify SC phases as follows: (1) ascending phase
  of SC 23 (1996-1999), (2) maximum phase of SC 23 (2000-2002), (3)
  descending phase of SC 23 (2003-2008), and (4) ascending phase of SC
  24 (2009-2013). We find that the occurrence rates of major flares and
  FHCMEs during the descending phase are noticeably higher than those
  during the other phases for most sunspot group classes. For the most
  active sunspot group class, Fkc, the occurrence rate of FHCMEs during
  the descending phase of SC 23 is three times as high as that during
  the ascending phase of SC 23. The potential of each McIntosh sunspot
  group class to produce major flares or FHCMEs is found to depend on
  the SC phase. The occurrence rates (R) of major flares and FHCMEs
  are strongly anti-correlated with the annual average latitude of the
  sunspot groups (L): R∼ {L}<SUP>-2.07</SUP> for major flares and R∼
  {L}<SUP>-2.42</SUP> for FHCMEs. This finding indicates the possible
  role of large-scale coronal connectivity, e.g., trans-equatorial loops,
  in powerful energy releases. Interestingly, this relationship is very
  similar to that between the volumetric coronal heating rate and X-ray
  loop lengths, indicating common energy release mechanisms.

Title: Observation of quasi-periodic solar radio bursts associated
    with propagating fast-mode waves
Authors: Goddard, C. R.; Nisticò, G.; Nakariakov, V. M.; Zimovets,
   I. V.; White, S. M.
Bibcode: 2016A&A...594A..96G
Altcode: 2016arXiv160804232G
  <BR /> Aims: Radio emission observations from the Learmonth and Bruny
  Island radio spectrographs are analysed to determine the nature of
  a train of discrete, periodic radio "sparks" (finite-bandwidth,
  short-duration isolated radio features) which precede a type II
  burst. We analyse extreme ultraviolet (EUV) imaging from SDO/AIA
  at multiple wavelengths and identify a series of quasi-periodic
  rapidly-propagating enhancements, which we interpret as a fast
  wave train, and link these to the detected radio features. <BR />
  Methods: The speeds and positions of the periodic rapidly propagating
  fast waves and the coronal mass ejection (CME) were recorded using
  running-difference images and time-distance analysis. From the frequency
  of the radio sparks the local electron density at the emission location
  was estimated for each. Using an empirical model for the scaling of
  density in the corona, the calculated electron density was used to
  obtain the height above the surface at which the emission occurs, and
  the propagation velocity of the emission location. <BR /> Results: The
  period of the radio sparks, δt<SUB>r</SUB> = 1.78 ± 0.04 min, matches
  the period of the fast wave train observed at 171 Å, δt<SUB>EUV</SUB>
  = 1.7 ± 0.2 min. The inferred speed of the emission location of the
  radio sparks, 630 km s<SUP>-1</SUP>, is comparable to the measured
  speed of the CME leading edge, 500 km s<SUP>-1</SUP>, and the speeds
  derived from the drifting of the type II lanes. The calculated height
  of the radio emission (obtained from the density) matches the observed
  location of the CME leading edge. From the above evidence we propose
  that the radio sparks are caused by the quasi-periodic fast waves, and
  the emission is generated as they catch up and interact with the leading
  edge of the CME. <P />The movie associated to Fig. 2 is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628478/olm">http://www.aanda.org</A>

Title: Comparison of Damped Oscillations in Solar and Stellar
    X-Ray flares
Authors: Cho, I. -H.; Cho, K. -S.; Nakariakov, V. M.; Kim, S.;
   Kumar, P.
Bibcode: 2016ApJ...830..110C
Altcode:
  We explore the similarity and difference of the quasi-periodic
  pulsations (QPPs) observed in the decay phase of solar and stellar
  flares at X-rays. We identified 42 solar flares with pronounced QPPs,
  observed with RHESSI, and 36 stellar flares with QPPs, observed
  with XMM-Newton. The empirical mode decomposition (EMD) method and
  least-squares fit by a damped sine function were applied to obtain
  the periods (P) and damping times (τ) of the QPPs. We found that (1)
  the periods and damping times of the stellar QPPs are 16.21 ± 15.86
  minutes and 27.21 ± 28.73 minutes, while those of the solar QPPs are
  0.90 ± 0.56 and 1.53 ± 1.10 minutes, respectively; (2) the ratios of
  the damping times to the periods (τ /P) observed in the stellar QPPs
  (1.69 ± 0.56) are statistically identical to those of solar QPPs
  (1.74 ± 0.77) and (3) the scalings of the QPP damping time with the
  period are well described by the power law in both solar and stellar
  cases. The power indices of the solar and stellar QPPs are 0.96 ±
  0.10 and 0.98+/- 0.05, respectively. This scaling is consistent with
  the scalings found for standing slow magnetoacoustic and kink modes in
  solar coronal loops. Thus, we propose that the underlying mechanism
  responsible for the stellar QPPs is the natural magnetohydrodynamic
  oscillation in the flaring or adjacent coronal loops, as in the case
  of solar flares.

Title: Spatially resolved observation of the fundamental and second
    harmonic standing kink modes using SDO/AIA
Authors: Pascoe, D. J.; Goddard, C. R.; Nakariakov, V. M.
Bibcode: 2016A&A...593A..53P
Altcode:
  <BR /> Aims: We consider a coronal loop kink oscillation observed by
  the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory
  (SDO) which demonstrates two strong spectral components. The period of
  the lower frequency component being approximately twice that of the
  shorter frequency component suggests the presence of harmonics. <BR
  /> Methods: We examine the presence of two longitudinal harmonics
  by investigating the spatial dependence of the loop oscillation. The
  time-dependent displacement of the loop is measured at 15 locations
  along the loop axis. For each position the displacement is fitted as
  the sum of two damped sinusoids, having periods P<SUB>1</SUB> and
  P<SUB>2</SUB>, and a damping time τ. The shorter period component
  exhibits anti-phase oscillations in the loop legs. <BR /> Results:
  We interpret the observation in terms of the first (global or
  fundamental) and second longitudinal harmonics of the standing kink
  mode. The strong excitation of the second harmonic appears connected
  to the preceding coronal mass ejection (CME) which displaced one of
  the loop legs. The oscillation parameters found are P<SUB>1</SUB>
  = 5.00±0.62 min, P<SUB>2</SUB> = 2.20±0.23 min, P<SUB>1</SUB>/
  2P<SUB>2</SUB> = 1.15±0.22, and τ/P = 3.35 ± 1.45. <P />A
  movie associated to Fig. 5 is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628784/olm">http://www.aanda.org</A>

Title: Standing sausage modes in curved coronal slabs
Authors: Pascoe, D. J.; Nakariakov, V. M.
Bibcode: 2016A&A...593A..52P
Altcode:
  Context. Magnetohydrodynamic waveguides such as dense coronal loops
  can support standing modes. The ratios of the periods of oscillations
  for different longitudinal harmonics depend on the dispersive
  nature of the waveguide and so may be used as a seismological tool to
  determine coronal parameters. <BR /> Aims: We extend models of standing
  sausage modes in low β coronal loops to include the effects of loop
  curvature. The behaviour of standing sausage modes in this geometry is
  used to explain the properties of observed oscillations that cannot be
  accounted for using straight loop models. <BR /> Methods: We perform
  2D numerical simulations of an oscillating coronal loop, modelled
  as a dense slab embedded in a potential magnetic field. The loop is
  field-aligned and so experiences expansion with height in addition
  to being curved. Standing sausage modes are excited by compressive
  perturbations of the loop and their properties are studied. <BR />
  Results: The spatial profiles of standing sausage modes are found to be
  modified by the expanding loop geometry typical for flaring loops and
  modelled by a potential magnetic field in our simulations. Longitudinal
  harmonics of order n &gt; 1 have anti-nodes that are shifted towards
  the loop apex and the amplitude of anti-nodes near the loop apex is
  smaller than those near the loop footpoints. <BR /> Conclusions: We
  find that the observation of standing sausage modes by the Nobeyama
  Radioheliograph in a flaring coronal loop on 12 January 2000 is
  consistent with interpretation in terms of the global mode (n = 1)
  and third harmonic (n = 3). This interpretation accounts for the period
  ratio and spatial structure of the observed oscillations.

Title: Empirical mode decomposition analysis of random processes in
    the solar atmosphere
Authors: Kolotkov, D. Y.; Anfinogentov, S. A.; Nakariakov, V. M.
Bibcode: 2016A&A...592A.153K
Altcode:
  Context. Coloured noisy components with a power law spectral energy
  distribution are often shown to appear in solar signals of various
  types. Such a frequency-dependent noise may indicate the operation
  of various randomly distributed dynamical processes in the solar
  atmosphere. <BR /> Aims: We develop a recipe for the correct usage of
  the empirical mode decomposition (EMD) technique in the presence of
  coloured noise, allowing for clear distinguishing between quasi-periodic
  oscillatory phenomena in the solar atmosphere and superimposed random
  background processes. For illustration, we statistically investigate
  extreme ultraviolet (EUV) emission intensity variations observed
  with SDO/AIA in the coronal (171 Å), chromospheric (304 Å), and
  upper photospheric (1600 Å) layers of the solar atmosphere, from
  a quiet sun and a sunspot umbrae region. <BR /> Methods: EMD has
  been used for analysis because of its adaptive nature and essential
  applicability to the processing non-stationary and amplitude-modulated
  time series. For the comparison of the results obtained with EMD,
  we use the Fourier transform technique as an etalon. <BR /> Results:
  We empirically revealed statistical properties of synthetic coloured
  noises in EMD, and suggested a scheme that allows for the detection of
  noisy components among the intrinsic modes obtained with EMD in real
  signals. Application of the method to the solar EUV signals showed that
  they indeed behave randomly and could be represented as a combination
  of different coloured noises characterised by a specific value of
  the power law indices in their spectral energy distributions. On
  the other hand, 3-min oscillations in the analysed sunspot were
  detected to have energies significantly above the corresponding noise
  level. <BR /> Conclusions: The correct accounting for the background
  frequency-dependent random processes is essential when using EMD for
  analysis of oscillations in the solar atmosphere. For the quiet sun
  region the power law index was found to increase with height above
  the photosphere, indicating that the higher frequency processes are
  trapped deeper in the quiet sun atmosphere. In contrast, lower levels
  of the sunspot umbrae were found to be characterised by higher values
  of the power law index, meaning the domination of lower frequencies
  deep inside the sunspot atmosphere. Comparison of the EMD results with
  those obtained with the Fourier transform showed good consistency,
  justifying the applicability of EMD.

Title: Statistically Determined Dispersion Relations of Magnetic
    Field Fluctuations in the Terrestrial Foreshock
Authors: Hnat, B.; O'Connell, D.; Nakariakov, V. M.; Sundberg, T.
Bibcode: 2016ApJ...827...91H
Altcode: 2016arXiv160108054H
  We obtain dispersion relations of magnetic field fluctuations for two
  crossings of the terrestrial foreshock by Cluster spacecraft. These
  crossings cover plasma conditions that differ significantly in their
  plasma β and in the density of the reflected ion beam, but not in the
  properties of the encountered ion population, both showing shell-like
  distribution function. Dispersion relations are reconstructed using
  two-point instantaneous wave number estimations from pairs of Cluster
  spacecraft. The accessible range of wave vectors, limited by the
  available spacecraft separations, extends to ≈2 × 10<SUP>4</SUP>
  km. Results show multiple branches of dispersion relations, associated
  with different powers of magnetic field fluctuations. We find that
  sunward propagating fast magnetosonic waves and beam resonant modes
  are dominant for the high plasma β interval with a dense beam, while
  the dispersions of the interval with low beam density include Alfvén
  and fast magnetosonic modes propagating sunward and anti-sunward.

Title: Statistical properties of quasi-periodic pulsations in
    white-light flares observed with Kepler
Authors: Pugh, C. E.; Armstrong, D. J.; Nakariakov, V. M.; Broomhall,
   A. -M.
Bibcode: 2016MNRAS.459.3659P
Altcode: 2016arXiv160403018P; 2016MNRAS.tmp..660P
  We embark on a study of quasi-periodic pulsations (QPPs) in the decay
  phase of white-light stellar flares observed by Kepler. Out of the
  1439 flares on 216 different stars detected in the short-cadence data
  using an automated search, 56 flares are found to have pronounced
  QPP-like signatures in the light curve, of which 11 have stable
  decaying oscillations. No correlation is found between the QPP period
  and the stellar temperature, radius, rotation period and surface
  gravity, suggesting that the QPPs are independent of global stellar
  parameters. Hence they are likely to be the result of processes
  occurring in the local environment. There is also no significant
  correlation between the QPP period and flare energy, however there is
  evidence that the period scales with the QPP decay time for the Gaussian
  damping scenario, but not to a significant degree for the exponentially
  damped case. This same scaling has been observed for MHD oscillations
  on the Sun, suggesting that they could be the cause of the QPPs in
  those flares. Scaling laws of the flare energy are also investigated,
  supporting previous reports of a strong correlation between the flare
  energy and stellar temperature/radius. A negative correlation between
  the flare energy and stellar surface gravity is also found.

Title: Effect of a Sausage Oscillation on Radio Zebra-pattern
    Structures in a Solar Flare
Authors: Yu, Sijie; Nakariakov, V. M.; Yan, Yihua
Bibcode: 2016ApJ...826...78Y
Altcode: 2016arXiv160804289Y
  Sausage modes that are axisymmetric fast magnetoacoustic oscillations
  of solar coronal loops are characterized by variation of the plasma
  density and magnetic field, and hence cause time variations of
  the electron plasma frequency and cyclotron frequency. The latter
  parameters determine the condition for the double plasma resonance
  (DPR), which is responsible for the appearance of zebra-pattern (ZP)
  structures in time spectra of solar type IV radio bursts. We perform
  numerical simulations of standing and propagating sausage oscillations
  in a coronal loop modeled as a straight, field-aligned plasma slab, and
  determine the time variation of the DPR layer locations. Instant values
  of the plasma density and magnetic field at the DPR layers allowed us
  to construct skeletons of the time variation of ZP stripes in radio
  spectra. In the presence of a sausage oscillation, the ZP structures are
  shown to have characteristic wiggles with the time period prescribed by
  the sausage oscillation. Standing and propagating sausage oscillations
  are found to have different signatures in ZP patterns. We conclude
  that ZP wiggles can be used for the detection of short-period sausage
  oscillations and the exploitation of their seismological potential.

Title: Effect of a Radiation Cooling and Heating Function on Standing
    Longitudinal Oscillations in Coronal Loops
Authors: Kumar, S.; Nakariakov, V. M.; Moon, Y. -J.
Bibcode: 2016ApJ...824....8K
Altcode: 2016arXiv160308335K
  Standing long-period (with periods longer than several minutes)
  oscillations in large, hot (with a temperature higher than 3 MK)
  coronal loops have been observed as the quasi-periodic modulation of
  the EUV and microwave intensity emission and the Doppler shift of
  coronal emission lines, and they have been interpreted as standing
  slow magnetoacoustic (longitudinal) oscillations. Quasi-periodic
  pulsations of shorter periods, detected in thermal and non-thermal
  emissions in solar flares could be produced by a similar mechanism. We
  present theoretical modeling of the standing slow magnetoacoustic
  mode, showing that this mode of oscillation is highly sensitive
  to peculiarities of the radiative cooling and heating function. We
  generalized the theoretical model of standing slow magnetoacoustic
  oscillations in a hot plasma, including the effects of the radiative
  losses and accounting for plasma heating. The heating mechanism is
  not specified and taken empirically to compensate the cooling by
  radiation and thermal conduction. It is shown that the evolution of
  the oscillations is described by a generalized Burgers equation. The
  numerical solution of an initial value problem for the evolutionary
  equation demonstrates that different dependences of the radiative
  cooling and plasma heating on the temperature lead to different regimes
  of the oscillations, including growing, quasi-stationary, and rapidly
  decaying. Our findings provide a theoretical foundation for probing the
  coronal heating function and may explain the observations of decayless
  long-period, quasi-periodic pulsations in flares. The hydrodynamic
  approach employed in this study should be considered with caution in
  the modeling of non-thermal emission associated with flares, because
  it misses potentially important non-hydrodynamic effects.

Title: Undamped transverse oscillations of coronal loops as a
    self-oscillatory process
Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Nisticò, G.; Lee,
   D. -H.
Bibcode: 2016A&A...591L...5N
Altcode:
  Context. Standing transverse oscillations of coronal loops are
  observed to operate in two regimes: rapidly decaying, large amplitude
  oscillations and undamped small amplitude oscillations. In the
  latter regime the damping should be compensated by energy supply,
  which allows the loop to perform almost monochromatic oscillations
  with almost constant amplitude and phase. Different loops oscillate
  with different periods. The oscillation amplitude does not show
  dependence on the loop length or the oscillation period. <BR /> Aims:
  We aim to develop a low-dimensional model explaining the undamped kink
  oscillations as a self-oscillatory process caused by the effect of
  negative friction. The source of energy is an external quasi-steady
  flow, for example, supergranulation motions near the loop footpoints
  or external flows in the corona. <BR /> Methods: We demonstrate that
  the interaction of a quasi-steady flow with a loop can be described
  by a Rayleigh oscillator equation that is a non-linear ordinary
  differential equation, with the damping and resonant terms determined
  empirically. <BR /> Results: Small-amplitude self-oscillatory solutions
  to the Rayleigh oscillator equation are harmonic signals of constant
  amplitude, which is consistent with the observed properties of undamped
  kink oscillations. The period of self-oscillations is determined by
  the frequency of the kink mode. The damping by dissipation and mode
  conversion is compensated by the continuous energy deposition at the
  frequency of the natural oscillation. <BR /> Conclusions: We propose
  that undamped kink oscillations of coronal loops may be caused by
  the interaction of the loops with quasi-steady flows, and hence are
  self-oscillations, which is analogous to producing a tune by moving
  a bow across a violin string.

Title: Coronal loop seismology using damping of standing kink
    oscillations by mode coupling
Authors: Pascoe, D. J.; Goddard, C. R.; Nisticò, G.; Anfinogentov,
   S.; Nakariakov, V. M.
Bibcode: 2016A&A...589A.136P
Altcode:
  Context. Kink oscillations of solar coronal loops are frequently
  observed to be strongly damped. The damping can be explained by mode
  coupling on the condition that loops have a finite inhomogeneous layer
  between the higher density core and lower density background. The
  damping rate depends on the loop density contrast ratio and
  inhomogeneous layer width. <BR /> Aims: The theoretical description
  for mode coupling of kink waves has been extended to include the
  initial Gaussian damping regime in addition to the exponential
  asymptotic state. Observation of these damping regimes would provide
  information about the structuring of the coronal loop and so provide
  a seismological tool. <BR /> Methods: We consider three examples
  of standing kink oscillations observed by the Atmospheric Imaging
  Assembly (AIA) of the Solar Dynamics Observatory (SDO) for which
  the general damping profile (Gaussian and exponential regimes) can be
  fitted. Determining the Gaussian and exponential damping times allows us
  to perform seismological inversions for the loop density contrast ratio
  and the inhomogeneous layer width normalised to the loop radius. The
  layer width and loop minor radius are found separately by comparing
  the observed loop intensity profile with forward modelling based on
  our seismological results. <BR /> Results: The seismological method
  which allows the density contrast ratio and inhomogeneous layer width
  to be simultaneously determined from the kink mode damping profile has
  been applied to observational data for the first time. This allows the
  internal and external Alfvén speeds to be calculated, and estimates
  for the magnetic field strength can be dramatically improved using the
  given plasma density. <BR /> Conclusions: The kink mode damping rate
  can be used as a powerful diagnostic tool to determine the coronal loop
  density profile. This information can be used for further calculations
  such as the magnetic field strength or phase mixing rate.

Title: Transverse oscillations and stability of prominences in a
    magnetic field dip
Authors: Kolotkov, D. Y.; Nisticò, G.; Nakariakov, V. M.
Bibcode: 2016A&A...590A.120K
Altcode:
  <BR /> Aims: We developed an analytical model of the global transverse
  oscillations and mechanical stability of a quiescent prominence in
  the magnetised environment with a magnetic field dip that accounts
  for the mirror current effect. <BR /> Methods: The model is based on
  the interaction of line currents through the Lorentz force. Within
  this concept the prominence is treated as a straight current-carrying
  wire, and the magnetic dip is provided by two photospheric current
  sources. <BR /> Results: Properties of both vertical and horizontal
  oscillations are determined by the value of the prominence current,
  its density and height above the photosphere, and the parameters of
  the magnetic dip. The prominence can be stable in both horizontal
  and vertical directions simultaneously when the prominence current
  dominates in the system and its height is less than the half-distance
  between the photospheric sources.

Title: Dependence of kink oscillation damping on the amplitude
Authors: Goddard, C. R.; Nakariakov, V. M.
Bibcode: 2016A&A...590L...5G
Altcode:
  Context. Kink oscillations of coronal loops are one of the most
  intensively studied oscillatory phenomena in the solar corona. In the
  large-amplitude rapidly damped regime, these oscillations are observed
  to have a low quality factor with only a few cycles of oscillation
  detected before they are damped. The specific mechanism responsible
  for rapid damping is commonly accepted to be associated with the
  linear coupling between collective kink oscillations and localised
  torsional oscillations, the phenomenon of resonant absorption of the
  kink mode. The role of finite amplitude effects, however, is still not
  clear. <BR /> Aims: We investigated the empirical dependence of the
  kink oscillation damping time and its quality factor, which is defined
  as the ratio of damping time to oscillation period, on the oscillation
  amplitude. <BR /> Methods: We analysed decaying kink oscillation events
  detected previously with TRACE, SDO/AIA and and STEREO/EUVI in the
  extreme ultraviolet (EUV) 171 Å band. <BR /> Results: We found that
  the ratio of the kink oscillation damping time to the oscillation period
  systematically decreases with the oscillation amplitude. We approximated
  the quality factor dependence on the oscillation displacement amplitude
  via the power-law dependence with the exponent of -1/2, however we
  stress that this is a by-eye estimate, and a more rigorous estimation
  of the scaling law requires more accurate measurements and increased
  statistics. We conclude that damping of kink oscillations of coronal
  loops depends on the oscillation amplitude, indicating the possible
  role of non-linear mechanisms for damping.

Title: Observation of a Quasiperiodic Pulsation in Hard X-Ray, Radio,
    and Extreme-ultraviolet Wavelengths
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
Bibcode: 2016ApJ...822....7K
Altcode: 2016arXiv160303121K
  We present a multiwavelength analysis of a quasiperiodic pulsation (QPP)
  observed in the hard X-ray (HXR), radio, and extreme-ultraviolet (EUV)
  channels during an M1.9 flare that occurred on 2011 September 23-24. The
  nonthermal HXR emission in 25-50 keV observed by RHESSI shows five
  distinct impulsive peaks of decaying amplitude with a period of about 3
  minutes. A similar QPP was observed in the microwave emission recorded
  by the Nobeyama Radioheliograph and Polarimeter in the 2, 3.75, 9.4,
  and 17 GHz channels. Interestingly, the 3-minute QPP was also observed
  in the metric and decimetric radio frequencies (25-180, 245, 610 MHz) as
  repetitive type III bursts. Multiwavelength observations from the Solar
  Dynamics Observatory/Atmospheric Image Assembly, Hinode/SOT, and Solar
  TErrestrial RElations Observatory/SECCHI suggest a fan-spine topology
  at the eruption site, associated with the formation of a quasi-circular
  ribbon during the flare. A small filament was observed below the fan
  loops before the flare onset. The filament rose slowly and interacted
  with the ambient field. This behavior was followed by an untwisting
  motion of the filament. Two different structures of the filament
  showed an approximately 3-minute periodic alternate rotation in the
  clockwise and counterclockwise directions. The 3-minute QPP was found
  to highly correlate with 3-minute oscillations in a nearby sunspot. We
  suggest that the periodic reconnection (modulated either by a sunspot
  slow-mode wave or by an untwisting filament) at a magnetic null point
  most likely causes the repetitive particle acceleration, generating
  the QPP observed in HXR, microwave, and type III radio bursts.

Title: Solar Science with the Atacama Large Millimeter/Submillimeter
    Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
   Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
   B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
   P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
   A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
   Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
   D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
   der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
   Selhorst, C. L.; Barta, M.
Bibcode: 2016SSRv..200....1W
Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
  The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
  powerful tool for observing the Sun at high spatial, temporal, and
  spectral resolution. These capabilities can address a broad range
  of fundamental scientific questions in solar physics. The radiation
  observed by ALMA originates mostly from the chromosphere—a complex
  and dynamic region between the photosphere and corona, which plays a
  crucial role in the transport of energy and matter and, ultimately,
  the heating of the outer layers of the solar atmosphere. Based on
  first solar test observations, strategies for regular solar campaigns
  are currently being developed. State-of-the-art numerical simulations
  of the solar atmosphere and modeling of instrumental effects can help
  constrain and optimize future observing modes for ALMA. Here we present
  a short technical description of ALMA and an overview of past efforts
  and future possibilities for solar observations at submillimeter and
  millimeter wavelengths. In addition, selected numerical simulations
  and observations at other wavelengths demonstrate ALMA's scientific
  potential for studying the Sun for a large range of science cases.

Title: Magnetohydrodynamic Oscillations in the Solar Corona and
Earth's Magnetosphere: Towards Consolidated Understanding
Authors: Nakariakov, V. M.; Pilipenko, V.; Heilig, B.; Jelínek,
   P.; Karlický, M.; Klimushkin, D. Y.; Kolotkov, D. Y.; Lee, D. -H.;
   Nisticò, G.; Van Doorsselaere, T.; Verth, G.; Zimovets, I. V.
Bibcode: 2016SSRv..200...75N
Altcode: 2016SSRv..tmp....2N
  Magnetohydrodynamic (MHD) oscillatory processes in different
  plasma systems, such as the corona of the Sun and the Earth's
  magnetosphere, show interesting similarities and differences, which
  so far received little attention and remain under-exploited. The
  successful commissioning within the past ten years of THEMIS, Hinode,
  STEREO and SDO spacecraft, in combination with matured analysis of data
  from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI)
  makes it very timely to survey the breadth of observations giving
  evidence for MHD oscillatory processes in solar and space plasmas,
  and state-of-the-art theoretical modelling. The paper reviews several
  important topics, such as Alfvénic resonances and mode conversion;
  MHD waveguides, such as the magnetotail, coronal loops, coronal
  streamers; mechanisms for periodicities produced in energy releases
  during substorms and solar flares, possibility of Alfvénic resonators
  along open field lines; possible drivers of MHD waves; diagnostics of
  plasmas with MHD waves; interaction of MHD waves with partly-ionised
  boundaries (ionosphere and chromosphere). The review is mainly oriented
  to specialists in magnetospheric physics and solar physics, but not
  familiar with specifics of the adjacent research fields.

Title: Low-Frequency Waves in Space Plasmas
Authors: Keiling, Andreas; Lee, Dong-Hun; Nakariakov, Valery
Bibcode: 2016GMS...216.....K
Altcode:
  Low-frequency waves in space plasmas have been studied for several
  decades, and our knowledge gain has been incremental with several
  paradigm-changing leaps forward. In our solar system, such waves
  occur in the ionospheres and magnetospheres of planets, and around our
  Moon. They occur in the solar wind, and more recently, they have been
  confirmed in the Sun's atmosphere as well. The goal of wave research
  is to understand their generation, their propagation, and their
  interaction with the surrounding plasma. Low-frequency Waves in Space
  Plasmas presents a concise and authoritative up-to-date look on where
  wave research stands: What have we learned in the last decade? What
  are unanswered questions? <P />While in the past waves in different
  astrophysical plasmas have been largely treated in separate books,
  the unique feature of this monograph is that it covers waves in many
  plasma regions, including: <P />Waves in geospace, including ionosphere
  and magnetosphere <P />Waves in planetary magnetospheres <P />Waves
  at the Moon <P />Waves in the solar wind <P />Waves in the solar
  atmosphere <P />Because of the breadth of topics covered, this volume
  should appeal to a broad community of space scientists and students,
  and it should also be of interest to astronomers/astrophysicists who
  are studying space plasmas beyond our Solar System.

Title: A statistical study of decaying kink oscillations detected
    using SDO/AIA
Authors: Goddard, C. R.; Nisticò, G.; Nakariakov, V. M.; Zimovets,
   I. V.
Bibcode: 2016A&A...585A.137G
Altcode: 2015arXiv151103558G
  Context. Despite intensive studies of kink oscillations of coronal
  loops in the last decade, a large-scale statistically significant
  investigation of the oscillation parameters has not been made using
  data from the Solar Dynamics Observatory (SDO). <BR /> Aims: We carry
  out a statistical study of kink oscillations using extreme ultraviolet
  imaging data from a previously compiled catalogue. <BR /> Methods:
  We analysed 58 kink oscillation events observed by the Atmospheric
  Imaging Assembly (AIA) on board SDO during its first four years of
  operation (2010-2014). Parameters of the oscillations, including the
  initial apparent amplitude, period, length of the oscillating loop,
  and damping are studied for 120 individual loop oscillations. <BR />
  Results: Analysis of the initial loop displacement and oscillation
  amplitude leads to the conclusion that the initial loop displacement
  prescribes the initial amplitude of oscillation in general. The
  period is found to scale with the loop length, and a linear fit of
  the data cloud gives a kink speed of C<SUB>k</SUB> = (1330 ± 50) km
  s<SUP>-1</SUP>. The main body of the data corresponds to kink speeds in
  the range C<SUB>k</SUB> = (800-3300) km s<SUP>-1</SUP>. Measurements
  of 52 exponential damping times were made, and it was noted that
  at least 21 of the damping profiles may be better approximated by a
  combination of non-exponential and exponential profiles rather than a
  purely exponential damping envelope. There are nine additional cases
  where the profile appears to be purely non-exponential and no damping
  time was measured. A scaling of the exponential damping time with the
  period is found, following the previously established linear scaling
  between these two parameters.

Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
   Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
   Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
   M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
   Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
   V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
   P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
   A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
Bibcode: 2016arXiv160100587W
Altcode:
  This document was created by the Solar Simulations for the Atacama
  Large Millimeter Observatory Network (SSALMON) in preparation of
  the first regular observations of the Sun with the Atacama Large
  Millimeter/submillimeter Array (ALMA), which are anticipated to start
  in ALMA Cycle 4 in October 2016. The science cases presented here
  demonstrate that a large number of scientifically highly interesting
  observations could be made already with the still limited solar
  observing modes foreseen for Cycle 4 and that ALMA has the potential
  to make important contributions to answering long-standing scientific
  questions in solar physics. With the proposal deadline for ALMA Cycle
  4 in April 2016 and the Commissioning and Science Verification campaign
  in December 2015 in sight, several of the SSALMON Expert Teams composed
  strategic documents in which they outlined potential solar observations
  that could be feasible given the anticipated technical capabilities
  in Cycle 4. These documents have been combined and supplemented
  with an analysis, resulting in recommendations for solar observing
  with ALMA in Cycle 4. In addition, the detailed science cases also
  demonstrate the scientific priorities of the solar physics community
  and which capabilities are wanted for the next observing cycles. The
  work on this White Paper effort was coordinated in close cooperation
  with the two international solar ALMA development studies led by
  T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
  further updated until the beginning of Cycle 4 in October 2016. In
  particular, we plan to adjust the technical capabilities of the solar
  observing modes once finally decided and to further demonstrate the
  feasibility and scientific potential of the included science cases by
  means of numerical simulations of the solar atmosphere and corresponding
  simulated ALMA observations.

Title: Damping profile of standing kink oscillations observed
    by SDO/AIA
Authors: Pascoe, D. J.; Goddard, C. R.; Nisticò, G.; Anfinogentov,
   S.; Nakariakov, V. M.
Bibcode: 2016A&A...585L...6P
Altcode:
  <BR /> Aims: Strongly damped standing and propagating kink oscillations
  are observed in the solar corona. This can be understood in terms
  of mode coupling, which causes the wave energy to be converted from
  the bulk transverse oscillation to localised, unresolved azimuthal
  motions. The damping rate can provide information about the loop
  structure, and theory predicts two possible damping profiles. <BR />
  Methods: We used the recently compiled catalogue of decaying standing
  kink oscillations of coronal loops to search for examples with high
  spatial and temporal resolution and sufficient signal quality to allow
  the damping profile to be examined. The location of the loop axis
  was tracked, detrended, and fitted with sinusoidal oscillations with
  Gaussian and exponential damping profiles. <BR /> Results: Using the
  highest quality data currently available, we find that for the majority
  of our cases a Gaussian profile describes the damping behaviour at
  least as well as an exponential profile, which is consistent with the
  recently developed theory for the damping profile due to mode coupling.

Title: Solar cycle variations in the powers and damping rates of
    low-degree solar acoustic oscillations
Authors: Broomhall, A. -M.; Pugh, C. E.; Nakariakov, V. M.
Bibcode: 2015AdSpR..56.2706B
Altcode:
  Helioseismology uses the Sun's natural resonant oscillations to study
  the solar interior. The properties of the solar oscillations are
  sensitive to the Sun'2019;s magnetic activity cycle. Here we examine
  variations in the powers, damping rates, and energy supply rates of the
  most prominent acoustic oscillations in unresolved, Sun-as-a-star data,
  obtained by the Birmingham Solar Oscillations Network (BiSON) during
  solar cycles 22, 23, and the first half of 24. The variations in the
  helioseismic parameters are compared to the 10.7 cm flux, a well-known
  global proxy of solar activity. As expected the oscillations are most
  heavily damped and the mode powers are at a minimum at solar activity
  maximum. The 10.7 cm flux was linearly regressed using the fractional
  variations of damping rates and powers observed during cycle 23. In
  general, good agreement is found between the damping rates and the 10.7
  cm flux. However, the linearly regressed 10.7 cm flux and fractional
  variation in powers diverge in cycles 22 and 24, indicating that
  the relationship between the mode powers and the 10.7 cm flux is not
  consistent from one cycle to the next. The energy supply rate of the
  oscillations, which is usually approximately constant, also decreases
  at this time. We have determined that this discrepancy is not because
  of the first-order bias introduced by an increase in the level of
  background noise or gaps in the data. Although we cannot categorically
  rule out an instrumental origin, the divergence observed in cycle 24,
  when the data were of high quality and the data coverage was over 80%,
  raises the possibility that the effect may be solar in origin.

Title: SSALMON - The Solar Simulations for the Atacama Large
    Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
   H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
   B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
   Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
   Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
   A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
Bibcode: 2015AdSpR..56.2679W
Altcode: 2015arXiv150205601W
  The Solar Simulations for the Atacama Large Millimeter Observatory
  Network (SSALMON) was initiated in 2014 in connection with two ALMA
  development studies. The Atacama Large Millimeter/submillimeter Array
  (ALMA) is a powerful new tool, which can also observe the Sun at
  high spatial, temporal, and spectral resolution. The international
  SSALMONetwork aims at co-ordinating the further development of solar
  observing modes for ALMA and at promoting scientific opportunities
  for solar physics with particular focus on numerical simulations,
  which can provide important constraints for the observing modes and
  can aid the interpretation of future observations. The radiation
  detected by ALMA originates mostly in the solar chromosphere - a
  complex and dynamic layer between the photosphere and corona, which
  plays an important role in the transport of energy and matter and the
  heating of the outer layers of the solar atmosphere. Potential targets
  include active regions, prominences, quiet Sun regions, flares. Here,
  we give a brief overview over the network and potential science cases
  for future solar observations with ALMA.

Title: Preface: Advances in solar physics
Authors: Georgoulis, Manolis K.; Nakariakov, Valery M.
Bibcode: 2015AdSpR..56.2677G
Altcode:
  The idea for this special issue of Advances in Space Research (ASR)
  was formulated during the 14th European Solar Physics Meeting (ESPM-14)
  that took place in Dublin, Ireland in September 2014. Since ASR does not
  publish conference proceedings, it was decided to extend a general call
  to the international solar-physics community for manuscripts pertinent
  to the following thematic areas: <P />New and upcoming heliospheric
  observational and data assimilation facilities.

Title: Fast Magnetoacoustic Wave Trains of Sausage Symmetry in
    Cylindrical Waveguides of the Solar Corona
Authors: Shestov, S.; Nakariakov, V. M.; Kuzin, S.
Bibcode: 2015ApJ...814..135S
Altcode: 2015arXiv151007908S
  Fast magnetoacoustic waves guided along the magnetic field by plasma
  non-uniformities, in particular coronal loops, fibrils, and plumes,
  are known to be highly dispersive, which lead to the formation of
  quasi-periodic wave trains excited by a broadband impulsive driver,
  e.g., a solar flare. We investigated the effects of cylindrical
  geometry on the fast sausage wave train formation. We performed
  magnetohydrodynamic numerical simulations of fast magnetoacoustic
  perturbations of a sausage symmetry, propagating from a localized
  impulsive source along a field-aligned plasma cylinder with a smooth
  radial profile of the fast speed. The wave trains are found to have
  pronounced period modulation, with the longer instant period seen
  in the beginning of the wave train. The wave trains also have a
  pronounced amplitude modulation. Wavelet spectra of the wave trains
  have characteristic tadpole features, with the broadband large-amplitude
  heads preceding low-amplitude quasi-monochromatic tails. The mean period
  of the wave train is about the transverse fast magnetoacoustic transit
  time across the cylinder. The mean parallel wavelength is about the
  diameter of the wave-guiding plasma cylinder. Instant periods are longer
  than the sausage wave cutoff period. The wave train characteristics
  depend on the fast magnetoacoustic speed in both the internal and
  external media, the smoothness of the transverse profile of the
  equilibrium quantities, and also the spatial size of the initial
  perturbation. If the initial perturbation is localized at the axis
  of the cylinder, the wave trains contain higher radial harmonics that
  have shorter periods.

Title: North-south asymmetry in the magnetic deflection of polar
    coronal hole jets
Authors: Nisticò, G.; Zimbardo, G.; Patsourakos, S.; Bothmer, V.;
   Nakariakov, V. M.
Bibcode: 2015A&A...583A.127N
Altcode: 2015arXiv150801072N
  Context. Measurements of the sunspots area, of the magnetic field in
  the interplanetary medium, and of the heliospheric current sheet (HCS)
  position, reveal a possible north-south (N-S) asymmetry in the magnetic
  field of the Sun. This asymmetry could cause the bending of the HCS of
  the order of 5-10 deg in the southward direction, and it appears to
  be a recurrent characteristic of the Sun during the minima of solar
  activity. <BR /> Aims: We study the N-S asymmetry as inferred from
  measurements of the deflection of polar coronal hole jets when they
  propagate throughout the corona. <BR /> Methods: Since the corona is
  an environment where the magnetic pressure is greater than the kinetic
  pressure (β ≪ 1), we can assume that the magnetic field controls the
  dynamics of plasma. On average, jets follow magnetic field lines during
  their propagation, highlighting their local direction. We measured
  the position angles at 1 R<SUB>⊙</SUB> and at 2 R<SUB>⊙</SUB> of
  79 jets, based on the Solar TErrestrial RElations Observatory (STEREO)
  ultraviolet and white-light coronagraph observations during the solar
  minimum period March 2007-April 2008. The average jet deflection is
  studied both in the plane perpendicular to the line of sight and, for
  a reduced number of jets, in 3D space. The observed jet deflection is
  studied in terms of an axisymmetric magnetic field model comprising
  dipole (g<SUB>1</SUB>), quadrupole (g<SUB>2</SUB>), and esapole
  (g<SUB>3</SUB>) moments. <BR /> Results: We found that the propagation
  of the jets is not radial, which is in agreement with the deflection
  due to magnetic field lines. Moreover, the amount of the deflection is
  different between jets over the north and those from the south pole. A
  comparison of jet deflections and field line tracing shows that a ratio
  g<SUB>2</SUB>/g<SUB>1</SUB> ≃ -0.5 for the quadrupole and a ratio
  g<SUB>3</SUB>/g<SUB>1</SUB> ≃ 1.6-2.0 for the esapole can describe
  the field. The presence of a non-negligible quadrupole moment confirms
  the N-S asymmetry of the solar magnetic field for the considered
  period. <BR /> Conclusions: We find that the magnetic deflection of
  jets is larger in the north than in the south of the order of 25-40%,
  with an asymmetry that is consistent with a southward deflection of
  the heliospheric current sheet of the order of 10 deg, consistent with
  that inferred from other independent datasets and instruments.

Title: A Comparison Between Global Proxies of the Sun's Magnetic
Activity Cycle: Inferences from Helioseismology
Authors: Broomhall, A. -M.; Nakariakov, V. M.
Bibcode: 2015SoPh..290.3095B
Altcode: 2015SoPh..tmp...82B; 2015arXiv150702854B
  The last solar minimum was, by recent standards, unusually deep and
  long. We are now close to the maximum of the subsequent solar cycle,
  which is relatively weak. In this article we make comparisons between
  different global (unresolved) measures of the Sun's magnetic activity
  to investigate how they are responding to this weak-activity epoch. We
  focus on helioseismic data, which are sensitive to conditions,
  including the characteristics of the magnetic field, in the solar
  interior. Also considered are measures of the magnetic field in
  the photosphere (sunspot number and sunspot area), the chromosphere
  and corona (10.7 cm radio flux and 530.3 nm green coronal index),
  and two measures of the Sun's magnetic activity closer to Earth
  (the interplanetary magnetic field and the galactic cosmic-ray
  intensity). Scaled versions of the activity proxies diverge from the
  helioseismic data around 2000, indicating a change in relationship
  between the proxies. The degree of divergence varies from proxy to
  proxy, with sunspot area and 10.7 cm flux showing only small deviations,
  while sunspot number, coronal index, and the two interplanetary proxies
  show much larger departures. In Cycle 24 the deviations in the solar
  proxies and the helioseismic data decrease, raising the possibility
  that the deviations observed in Cycle 23 are just symptomatic of a
  22-year Hale cycle. However, the deviations in the helioseismic data
  and the interplanetary proxies increase in Cycle 24. Interestingly,
  the divergence in the solar proxies and the helioseismic data are
  not reflected in the shorter-term variations (often referred to as
  quasi-biennial oscillations) observed on top of the dominant 11-year
  solar cycle. However, despite being highly correlated in Cycle 22,
  the short-term variations in the interplanetary proxies show very
  little correlation with the helioseismic data during Cycles 23 and 24.

Title: A Multi-period Oscillation in a Stellar Superflare
Authors: Pugh, C. E.; Nakariakov, V. M.; Broomhall, A. -M.
Bibcode: 2015ApJ...813L...5P
Altcode: 2015arXiv151003613P
  Flares that are orders of magnitude larger than the most energetic
  solar flares are routinely observed on Sun-like stars, raising the
  question of whether the same physical processes are responsible for both
  solar and stellar flares. In this Letter, we present a white-light
  stellar superflare on the star KIC 9655129, observed by NASA’s
  Kepler mission, with a rare multi-period quasi-periodic pulsation
  (QPP) pattern. Two significant periodic processes were detected using
  the wavelet and autocorrelation techniques, with periods of 78 ± 12
  minutes and 32 ± 2 minutes. By comparing the phases and decay times
  of the two periodicities, the QPP signal was found to most likely
  be linear, suggesting that the two periodicities are independent,
  possibly corresponding either to different magnetohydrodynamic (MHD)
  modes of the flaring region or different spatial harmonics of the same
  mode. The presence of multiple periodicities is a good indication
  that the QPPs were caused by MHD oscillations and suggests that the
  physical processes in operation during stellar flares could be the
  same as those in solar flares.

Title: Decayless low-amplitude kink oscillations: a common phenomenon
    in the solar corona?
Authors: Anfinogentov, S. A.; Nakariakov, V. M.; Nisticò, G.
Bibcode: 2015A&A...583A.136A
Altcode: 2015arXiv150905519A
  Context. We investigate the decayless regime of coronal kink
  oscillations recently discovered in the Solar Dynamics Observatory
  (SDO)/AIA data. In contrast to decaying kink oscillations that are
  excited by impulsive dynamical processes, this type of transverse
  oscillations is not connected to any external impulsive impact, such as
  a flare or coronal mass ejection, and does not show any significant
  decay. Moreover the amplitude of these decayless oscillations is
  typically lower than that of decaying oscillations. <BR /> Aims:
  The aim of this research is to estimate the prevalence of this
  phenomenon and its characteristic signatures. <BR /> Methods: We
  analysed 21 active regions (NOAA 11637-11657) observed in January
  2013 in the 171 Å channel of SDO/AIA. For each active region we
  inspected six hours of observations, constructing time-distance
  plots for the slits positioned across pronounced bright loops. The
  oscillatory patterns in time-distance plots were visually identified
  and the oscillation periods and amplitudes were measured. We also
  estimated the length of each oscillating loop. <BR /> Results:
  Low-amplitude decayless kink oscillations are found to be present
  in the majority of the analysed active regions. The oscillation
  periods lie in the range from 1.5 to 10 min. In two active regions
  with insufficient observation conditions we did not identify any
  oscillation patterns. The oscillation periods are found to increase
  with the length of the oscillating loop. <BR /> Conclusions: The
  considered type of coronal oscillations is a common phenomenon in the
  corona. The established dependence of the oscillation period on the loop
  length is consistent with their interpretation in terms of standing
  kink waves. <P />Appendix A is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201526195/olm">http://www.aanda.org</A>

Title: Cut-off period for slow magnetoacoustic waves in coronal
    plasma structures
Authors: Afanasyev, A. N.; Nakariakov, V. M.
Bibcode: 2015A&A...582A..57A
Altcode:
  Context. There is abundant observational evidence of longitudinal
  compressive waves in plasma structures of the solar corona, which are
  confidently interpreted in terms of slow magnetoacoustic waves. The
  uses of coronal slow waves in plasma diagnostics, as well as analysis
  of their possible contribution to coronal heating and the solar
  wind acceleration, require detailed theoretical modelling. <BR />
  Aims: We investigate the effects of obliqueness, magnetic field, and
  non-uniformity of the medium on the evolution of long-wavelength slow
  magnetoacoustic waves guided by field-aligned plasma non-uniformities,
  also called tube waves. Special attention is paid to the cut-off effect
  due to the gravity stratification of the coronal plasma. <BR /> Methods:
  We study the behaviour of linear tube waves in a vertical untwisted
  straight field-aligned isothermal plasma cylinder. We apply the thin
  flux tube approximation, taking into account effects of stratification
  caused by gravity. The dispersion due to the finite radius of the flux
  tube is neglected. We analyse the behaviour of the cut-off period
  for an exponentially divergent magnetic flux tube filled in with a
  stratified plasma. The results obtained are compared with the known
  cases of the constant Alfven speed and the pure acoustic wave. <BR />
  Results: We derive the wave equation for tube waves and reduce it to
  the form of the Klein-Gordon equation with varying coefficients, which
  explicitly contains the cut-off frequency. The cut-off period is found
  to vary with height, decreasing significantly in the low-beta plasma and
  in the plasma with the beta of the order of unity. The depressions in
  the cut-off period profiles can affect the propagation of longitudinal
  waves along coronal plasma structures towards the higher corona and
  can form coronal resonators.

Title: Coexisting fast and slow propagating waves of the extreme-UV
    intensity in solar coronal plasma structures
Authors: Zhang, Yuzong; Zhang, Jun; Wang, Jingxiu; Nakariakov,
   Valery M.
Bibcode: 2015A&A...581A..78Z
Altcode:
  Context. From 06:15 UT to 08:15 UT on 2011 June 2, a toroidal
  filament located at the joint of two active regions, 11226 and 11227,
  appeared to perform two eruptions. During this phenomenon, fast and
  slow magnetoacoustic waves were detected to propagate simultaneously
  along a funnel coronal loop system of AR 11227. <BR /> Aims: We aim
  to understand the relationship between fast and slow magnetoacoustic
  waves during their propagations and measure their properties, such
  as the propagating speed, path, amplitude, and period observed in the
  extreme ultraviolet (EUV) wavebands. <BR /> Methods: We analyse time
  sequences of EUV images acquired by the Atmospheric Imaging Assembly
  onboard the Solar Dynamics Observatory. By creating time-distance maps
  along selected directions, we measure the speeds and localisation of EUV
  intensity waves in different EUV wavebands. We determine the periods
  of the waves with wavelet analysis. <BR /> Results: The fast and slow
  magnetoacoustic waves, apparently propagating along the same path,
  are found to have different properties. Their apparent propagation
  speeds, travel distances, and periods are about 900 km s<SUP>-1</SUP>
  and 100 km s<SUP>-1</SUP>, 145 Mm and 36 Mm, and 2 min and 3 min,
  respectively. <P />The movie is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201525621/olm">http://www.aanda.org</A>

Title: Hilbert-Huang transform analysis of periodicities in the last
    two solar activity cycles
Authors: Kolotkov, D. Y.; Broomhall, A. -M.; Nakariakov, V. M.
Bibcode: 2015MNRAS.451.4360K
Altcode:
  We investigated periodicities associated with the last two-and-a-half
  solar activity cycles with the novel Hilbert-Huang transform (HHT)
  method. Raw data signals of five different observational proxies:
  the 10.7 cm radio flux intensity, the helioseismic frequency shift,
  and the sunspot area signals recorded from the whole solar disc, and
  separately from the Northern and Southern hemispheres, were expanded
  into a set of intrinsic modes with the ensemble empirical mode
  decomposition technique. Instant and mean periods of each empirical
  mode were obtained with the use of the Hilbert transform applied
  independently to each separate mode. The periodicities were allocated to
  three distinct groups: short-term variations (with periods shorter than
  0.5 yr), quasi-biennial oscillations (with typical periods from 0.5 yr
  to 3.9 yr), and longer periodicities, e.g. such as the 11 yr cycle. All
  periodicities detected in the examined solar cycles 22-24 are consistent
  with the well-known results found in the earlier solar epochs. We have
  demonstrated that the HHT method is a good tool for characterizing
  periodicities in the helioseismic data, which are necessarily relatively
  limited in terms of their time resolution. Periodicities obtained
  using the helioseismic data are, nevertheless, consistent with those
  found in other proxies. Since helioseismic oscillations are sensitive
  to the solar interior, this indicates that the behaviour of surface
  and atmospheric magnetic activity reflects that of the Sun's internal
  magnetic field. All identified intrinsic modes are seen to have clear
  amplitude modulation highly correlated with the 11 yr cycle. This
  amplitude modulation is most pronounced in the short-period modes. The
  short-term periodicities were found to be multiples of the shortest
  period, of 25 d. This ordering of the short-term periodicities is
  consistent with the previous findings. Signatures of the north-south
  asymmetry were detected in the individual hemisphere sunspot area
  indices. Furthermore, evidence of the last `extended' solar minimum
  was detected too.

Title: Oscillations in stellar superflares
Authors: Balona, L. A.; Broomhall, A. -M.; Kosovichev, A.; Nakariakov,
   V. M.; Pugh, C. E.; Van Doorsselaere, T.
Bibcode: 2015MNRAS.450..956B
Altcode: 2015arXiv150401491B
  Two different mechanisms may act to induce quasi-periodic pulsations
  (QPP) in whole-disc observations of stellar flares. One mechanism may be
  magnetohydromagnetic forces and other processes acting on flare loops
  as seen in the Sun. The other mechanism may be forced local acoustic
  oscillations due to the high-energy particle impulse generated by the
  flare (known as `sunquakes' in the Sun). We analyse short-cadence Kepler
  data of 257 flares in 75 stars to search for QPP in the flare decay
  branch or post-flare oscillations which may be attributed to either
  of these two mechanisms. About 18 per cent of stellar flares show a
  distinct bump in the flare decay branch of unknown origin. The bump
  does not seem to be a highly damped global oscillation because the
  periods of the bumps derived from wavelet analysis do not correlate
  with any stellar parameter. We detected damped oscillations covering
  several cycles (QPP), in seven flares on five stars. The periods of
  these oscillations also do not correlate with any stellar parameter,
  suggesting that these may be a due to flare loop oscillations. We
  searched for forced global oscillations which might result after
  a strong flare. To this end, we investigated the behaviour of the
  amplitudes of solar-like oscillations in eight stars before and after
  a flare. However, no clear amplitude change could be detected. We
  also analysed the amplitudes of the self-excited pulsations in two δ
  Scuti stars and one γ Doradus star before and after a flare. Again,
  no clear amplitude changes were found. Our conclusions are that a new
  process needs to be found to explain the high incidence of bumps in
  stellar flare light curves, that flare loop oscillations may have been
  detected in a few stars and that no conclusive evidence exists as yet
  for flare induced global acoustic oscillations (starquakes).

Title: X-Ray and EUV Observations of Simultaneous Short and Long
    Period Oscillations in Hot Coronal Arcade Loops
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Cho, Kyung-Suk
Bibcode: 2015ApJ...804....4K
Altcode: 2015arXiv150207117K
  We report decaying quasi-periodic intensity oscillations in the X-ray
  (6-12 keV) and extreme-ultraviolet (EUV) channels (131, 94, 1600, 304
  Å) observed by the Fermi Gamma-ray Burst Monitor and Solar Dynamics
  Observatory/Atmospheric Imaging Assembly (AIA), respectively, during a
  C-class flare. The estimated periods of oscillation and decay time in
  the X-ray channel (6-12 keV) were about 202 and 154 s, respectively. A
  similar oscillation period was detected at the footpoint of the
  arcade loops in the AIA 1600 and 304 Å channels. Simultaneously,
  AIA hot channels (94 and 131 Å) reveal propagating EUV disturbances
  bouncing back and forth between the footpoints of the arcade loops. The
  period of the oscillation and decay time were about 409 and 1121 s,
  respectively. The characteristic phase speed of the wave is about 560
  km s<SUP>-1</SUP> for about 115 Mm of loop length, which is roughly
  consistent with the sound speed at the temperature about 10-16 MK
  (480-608 km s<SUP>-1</SUP>). These EUV oscillations are consistent with
  the Solar and Heliospheric Observatory/Solar Ultraviolet Measurement
  of Emitted Radiation Doppler-shift oscillations interpreted as the
  global standing slow magnetoacoustic wave excited by a flare. The
  flare occurred at one of the footpoints of the arcade loops, where the
  magnetic topology was a 3D fan-spine with a null-point. Repetitive
  reconnection at this footpoint could have caused the periodic
  acceleration of non-thermal electrons that propagated to the opposite
  footpoint along the arcade and that are precipitating there, causing
  the observed 202 s periodicity. Other possible interpretations, e.g.,
  the second harmonics of the slow mode, are also discussed.

Title: Excitation of kink oscillations of coronal loops: statistical
    study
Authors: Zimovets, I. V.; Nakariakov, V. M.
Bibcode: 2015A&A...577A...4Z
Altcode:
  Context. Solar flares are often accompanied by kink (transverse)
  oscillations of coronal loops. Despite intensive study of these
  oscillations in recent years, the mechanisms that excite them are still
  not known. <BR /> Aims: We aim to clarify the excitation mechanisms for
  these kink oscillations of coronal loops. <BR /> Methods: We analysed
  58 kink-oscillation events observed by the Atmospheric Imaging Assembly
  (AIA) onboard the Solar Dynamics Observatory (SDO) during its first
  four years (2010-2014) with the use of the JHelioviewer. Association
  of these oscillation events with flares, lower coronal (r ≲ 1.4
  R<SUB>⊙</SUB>) eruptions and plasma ejections, coronal mass ejections
  (CMEs), and coronal Type-II radio bursts is studied. <BR /> Results:
  We find that 44 of these 58 oscillation events (76%) were associated
  with CMEs observed in the white light emission. Moreover, 57 events
  (98%) were accompanied by lower coronal eruptions/ejections (LCEs)
  observed in the extreme-ultraviolet band in the parental active
  regions. In the remaining event an LCE was not clearly seen, but it was
  definitely associated with a CME too. The main observational finding
  is that the kink oscillations were excited by the deviation of loops
  from their equilibria by a nearby LCE in 55 events (95%). In three
  remaining events, it was difficult to reliably determine the cause of
  the oscillations because of limitations in the observational data. We
  also found that 53 events (91%) were associated with flares. In
  five remaining events, the parental active regions were behind the
  limb and we could not directly see flare sites. It indicates that
  there is a close relationship between these two kinds of solar
  activity. However, the estimated speeds of a hypothetical driver
  of kink oscillations by flares were found to be lower than 500 km
  s<SUP>-1</SUP> in 80% of the cases. Such low speeds do not favour
  the association of the oscillation excitation with a shock wave,
  as usually assumed. That only 23 (40%) of the oscillation events
  were found to be associated with coronal Type-II radio bursts also
  goes against the shock wave mechanism for the excitation of kink
  oscillations. <BR /> Conclusions: The statistical analysis shows
  that the most probable mechanism for exciting the kink oscillations
  of coronal loops is the deviation of loops from their equilibrium by
  nearby eruptions or plasma ejections rather than a blast shock wave
  ignited by a flare. <P />Table 1 is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201424960/olm">http://www.aanda.org</A>

Title: Diagnostics of Coronal Heating in Solar Active Regions
Authors: Fludra, Andrzej; Hornsey, Christopher; Nakariakov, Valery
Bibcode: 2015TESS....120305F
Altcode:
  We aim to develop a diagnostic method for the coronal heating mechanism
  in active region loops. Observational constraints on coronal heating
  models have been sought using measurements in the X-ray and EUV
  wavelengths. Statistical analysis, using EUV emission from many active
  regions, was done by Fludra and Ireland (2008) who studied power-law
  relationships between active region integrated magnetic flux and
  emission line intensities. A subsequent study by Fludra and Warren
  (2010) for the first time compared fully resolved images in an EUV
  spectral line of OV 63.0 nm with the photospheric magnetic field,
  leading to the identification of a dominant, ubiquitous variable
  component of the transition region EUV emission and a discovery
  of a steady basal heating, and deriving the dependence of the basal
  heating rate on the photospheric magnetic flux density. In this study,
  we compare models of single coronal loops with EUV observations. We
  assess to what degree observations of individual coronal loops made
  in the EUV range are capable of providing constraints on the heating
  mechanism. We model the coronal magnetic field in an active region using
  an NLFF extrapolation code applied to a photospheric vector magnetogram
  from SDO/HMI and select several loops that match an SDO/AIA 171 image of
  the same active region. We then model the plasma in these loops using
  a 1D hydrostatic code capable of applying an arbitrary heating rate as
  a function of magnetic field strength along the loop. From the plasma
  parameters derived from this model, we calculate the EUV emission along
  the loop in AIA 171 and 335 bands, and in pure spectral lines of Fe
  IX 17.1 nm and Fe XVI 33.5 nm. We use different spatial distributions
  of the heating function: concentrated near the loop top, uniform and
  concentrated near the footpoints, and investigate their effect on the
  modelled EUV intensities. We find a diagnostics based on the dependence
  of the total loop intensity on the shape of the heating function and
  discuss its range of applicability for loops of different length.

Title: Solar and Heliospheric Physics with the Square Kilometre Array
Authors: Nakariakov, V.; Bisi, M. M.; Browning, P. K.; Maia,
   D.; Kontar, E. P.; Oberoi, D.; Gallagher, P. T.; Cairns, I. H.;
   Ratcliffe, H.
Bibcode: 2015aska.confE.169N
Altcode: 2015PoS...215E.169N; 2015arXiv150700516N
  The fields of solar radiophysics and solar system radio physics,
  or radio heliophysics, will benefit immensely from an instrument
  with the capabilities projected for SKA. Potential applications
  include interplanetary scintillation (IPS), radio-burst tracking,
  and solar spectral radio imaging with a superior sensitivity. These
  will provide breakthrough new insights and results in topics of
  fundamental importance, such as the physics of impulsive energy
  releases, magnetohydrodynamic oscillations and turbulence, the
  dynamics of post-eruptive processes, energetic particle acceleration,
  the structure of the solar wind and the development and evolution of
  solar wind transients at distances up to and beyond the orbit of the
  Earth. The combination of the high spectral, time and spatial resolution
  and the unprecedented sensitivity of the SKA will radically advance
  our understanding of basic physical processes operating in solar and
  heliospheric plasmas and provide a solid foundation for the forecasting
  of space weather events.

Title: Radial and Azimuthal Oscillations in Halo Coronal Mass
    Ejections
Authors: Lee, Harim; Moon, Yong-Jae; Nakariakov, Valery
Bibcode: 2015TESS....131002L
Altcode:
  We present the first observational detection of radial and azimuthal
  oscillations in full halo coronal mass ejections (HCMEs). We analyse
  nine HCMEs well-observed by the Large Angle and Spectrometric
  Coronagraph (LASCO) from February to June, 2011. Using the LASCO C3
  running difference images, we estimated the instantaneous apparent
  speeds of the HCMEs in different radial directions from the solar disk
  centre. We find that the development of all these HCMEs is accompanied
  with quasi-periodic variations of the instantaneous radial velocity
  with the periods ranging from 24 to 48 minutes. The amplitudes of the
  instant speed variations reach about a half of the projected speeds. The
  amplitudes are found to anti-correlate with the periods and correlate
  with the HCME speed, indicating the nonlinear nature of the process. The
  oscillations have a clear azimuthal structure in the heliocentric polar
  coordinate system. The oscillations in seven events are found to be
  associated with distinct azimuthal wave modes with the azimuthal wave
  number m=1 for six events and m=2 for one event. The polarisation of
  the oscillations in these seven HCMEs is broadly consistent with those
  of their position angles with the mean difference of 42.5 degree. The
  oscillations may be connected with natural oscillations of the plasmoids
  around a dynamical equilibrium, or self-oscillatory processes, e.g. the
  periodic shedding of Alfvénic vortices. Our results indicate the need
  for advanced theory of oscillatory processes in CMEs.

Title: Radial and Azimuthal Oscillations of Halo Coronal Mass
    Ejections in the Sun
Authors: Lee, Harim; Moon, Y. -J.; Nakariakov, V. M.
Bibcode: 2015ApJ...803L...7L
Altcode:
  We present the first observational detection of radial and azimuthal
  oscillations in full halo coronal mass ejections (HCMEs). We analyze
  nine HCMEs well-observed by the Large Angle and Spectrometric
  Coronagraph (LASCO) from 2011 February to June. Using the LASCO C3
  running difference images, we estimated the instantaneous apparent
  speeds of the HCMEs in different radial directions from the solar disk
  center. We find that the development of all these HCMEs is accompanied
  by quasi-periodic variations of the instantaneous radial velocity
  with the periods ranging from 24 to 48 minutes. The amplitudes of
  the instant speed variations reach about a half of the projected
  speeds. The amplitudes are found to anti-correlate with the periods
  and correlate with the HCME speed, indicating the nonlinear nature
  of the process. The oscillations have a clear azimuthal structure in
  the heliocentric polar coordinate system. The oscillations in seven
  events are found to be associated with distinct azimuthal wave modes
  with the azimuthal wave number m = 1 for six events and m = 2 for one
  event. The polarization of the oscillations in these seven HCMEs is
  broadly consistent with those of their position angles with the mean
  difference of 43°. The oscillations may be connected with natural
  oscillations of the plasmoids around a dynamical equilibrium, or
  self-oscillatory processes, e.g., the periodic shedding of Alfvénic
  vortices. Our results indicate the need for an advanced theory of
  oscillatory processes in coronal mass ejections.

Title: Intensity and Doppler Velocity Oscillations in Pore Atmospheres
Authors: Cho, K. -S.; Bong, S. -C.; Nakariakov, V. M.; Lim, E. -K.;
   Park, Y. -D.; Chae, J. C.; Yang, H. -S.; Park, H. -M.; Yurchyshyn, V.
Bibcode: 2015ApJ...802...45C
Altcode:
  We have investigated chromospheric traveling features running across two
  merged pores from their centers at speeds of about 55 km s<SUP>-1</SUP>,
  in the active region AR 11828. The pores were observed on 2013 August
  24 by using high-time, spatial, and spectral resolution data from the
  Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope. We
  infer a line-of-sight (LOS) velocity by applying the lambdameter method
  to the Ca ii 8542 Å band and Hα band, and investigate intensity and
  LOS velocity changes at different wavelengths and different positions
  at the pores. We find that they have three-minute oscillations, and
  the intensity oscillation from the line center (0.0 \overset{\circ}A
  ) is preceded by that from the core (-0.3 \overset{\circ}A ) of the
  bands. There is no phase difference between the intensity and the
  LOS velocity oscillations at a given wavelength. The amplitude of LOS
  velocity from the near core spectra ({Δ }λ =0.10-0.21 \overset{\circ}A
  ) is greater than that from the far core spectra ({Δ }λ =0.24-0.36
  \overset{\circ}A ). These results support the interpretation of the
  observed wave as a slow magnetoacoustic wave propagating along the
  magnetic field lines in the pores. The apparent horizontal motion and
  a sudden decrease of its speed beyond the pores can be explained by
  the projection effect caused by inclination of the magnetic field with
  a canopy structure. We conclude that the observed wave properties of
  the pores are quite similar to those from the sunspot observations.

Title: Multi-mode quasi-periodic pulsations in a solar flare
Authors: Kolotkov, D. Y.; Nakariakov, V. M.; Kupriyanova, E. G.;
   Ratcliffe, H.; Shibasaki, K.
Bibcode: 2015A&A...574A..53K
Altcode:
  Context. Quasi-periodic pulsations (QPP) of the electromagnetic
  radiation emitted in solar and stellar flares are often detected
  in microwave, white light, X-ray, and gamma-ray bands. Mechanisms
  for QPP are intensively debated in the literature. Previous studies
  revealed that QPP may manifest non-linear, non-stationary and, perhaps,
  multi-modal processes operating in flares. <BR /> Aims: We study QPP
  of the microwave emission generated in an X3.2-class solar flare on
  14 May, 2013, observed with the Nobeyama Radioheliograph (NoRH),
  aiming to reveal signatures of the non-linear, non-stationary,
  and multi-modal processes in the signal. <BR /> Methods: The NoRH
  correlation signal obtained at the 17 GHz intensity has a clear QPP
  pattern. The signal was analysed with the Hilbert-Huang transform
  (HHT) that allows one to determine its instant amplitude and frequency,
  and their time variation. <BR /> Results: It was established that the
  QPP consists of at least three well-defined intrinsic modes, with the
  mean periods of 15, 45, and 100 s. All the modes have quasi-harmonic
  behaviour with different modulation patterns. The 100 s intrinsic
  mode is a decaying oscillation, with the decay time of 250 s. The 15
  s intrinsic mode shows a similar behaviour, with the decay time of
  90 s. The 45 s mode has a wave-train behaviour. <BR /> Conclusions:
  Dynamical properties of detected intrinsic modes indicate that the
  100 s and 15 s modes are likely to be associated with fundamental
  kink and sausage modes of the flaring loop, respectively. The 100 s
  oscillation could also be caused by the fundamental longitudinal mode,
  while this interpretation requires the plasma temperature of about 30
  million K and hence is not likely. The 45 s mode could be the second
  standing harmonics of the kink mode.

Title: Evolution of Fast Magnetoacoustic Pulses in Randomly Structured
    Coronal Plasmas
Authors: Yuan, D.; Pascoe, D. J.; Nakariakov, V. M.; Li, B.;
   Keppens, R.
Bibcode: 2015ApJ...799..221Y
Altcode: 2014arXiv1411.4152Y
  We investigate the evolution of fast magnetoacoustic pulses in randomly
  structured plasmas, in the context of large-scale propagating waves in
  the solar atmosphere. We perform one-dimensional numerical simulations
  of fast wave pulses propagating perpendicular to a constant magnetic
  field in a low-β plasma with a random density profile across the
  field. Both linear and nonlinear regimes are considered. We study
  how the evolution of the pulse amplitude and width depends on their
  initial values and the parameters of the random structuring. Acting
  as a dispersive medium, a randomly structured plasma causes amplitude
  attenuation and width broadening of the fast wave pulses. After the
  passage of the main pulse, secondary propagating and standing fast
  waves appear. Width evolution of both linear and nonlinear pulses can
  be well approximated by linear functions; however, narrow pulses may
  have zero or negative broadening. This arises because narrow pulses are
  prone to splitting, while broad pulses usually deviate less from their
  initial Gaussian shape and form ripple structures on top of the main
  pulse. Linear pulses decay at an almost constant rate, while nonlinear
  pulses decay exponentially. A pulse interacts most efficiently with a
  random medium with a correlation length of about half of the initial
  pulse width. This detailed model of fast wave pulses propagating in
  highly structured media substantiates the interpretation of EIT waves
  as fast magnetoacoustic waves. Evolution of a fast pulse provides us
  with a novel method to diagnose the sub-resolution filamentation of
  the solar atmosphere.

Title: A Combined Analysis of the Observational Aspects of the
    Quasi-biennial Oscillation in Solar Magnetic Activity
Authors: Bazilevskaya, G.; Broomhall, A. -M.; Elsworth, Y.; Nakariakov,
   V. M.
Bibcode: 2015sac..book..359B
Altcode:
  No abstract at ADS

Title: Nonlinear slow magnetoacoustic waves in coronal plasma
    structures
Authors: Afanasyev, A. N.; Nakariakov, V. M.
Bibcode: 2015A&A...573A..32A
Altcode:
  Context. There is abundant observational evidence of longitudinal
  waves in the plasma structures of the solar corona. These essentially
  compressive waves are confidently interpreted as slow magnetoacoustic
  waves. The use of the slow waves in plasma diagnostics and estimating
  their possible contribution to plasma heating and acceleration
  require detailed theoretical modelling. <BR /> Aims: We investigate
  the role of obliqueness and magnetic effects in the evolution of slow
  magnetoacoustic waves, also called tube waves, in field-aligned plasma
  structures. Special attention is paid to the wave damping caused
  by nonlinear steepening. <BR /> Methods: We considered an untwisted
  straight axisymmetric field-aligned plasma cylinder and analysed the
  behaviour of the slow magnetoacoustic waves that are guided by this
  plasma structure. We adopted a thin flux tube approximation. We took
  into account dissipation caused by viscosity, resistivity and thermal
  conduction, and nonlinearity. Effects of stratification and dispersion
  caused by the finite radius of the flux tube were neglected. <BR />
  Results: We derive the Burgers-type evolutionary equation for tube
  waves in a uniform plasma cylinder. Compared with a plane acoustic
  wave, the formation of shock fronts in tube waves is found to occur at
  a larger distance from the source. In addition, tube waves experience
  stronger damping. These effects are most pronounced in plasmas with
  the parameter β at about or greater than unity. In a low-β plasma,
  the evolution of tube waves can satisfactorily be described with the
  Burgers equation for plane acoustic waves. <BR /> Conclusions:

Title: Vertical Kink Oscillation of a Magnetic Flux Rope Structure
    in the Solar Corona
Authors: Kim, S.; Nakariakov, V. M.; Cho, K. -S.
Bibcode: 2014ApJ...797L..22K
Altcode:
  Vertical transverse oscillations of a coronal magnetic rope, observed
  simultaneously in the 171 Å and 304 Å bandpasses of the Atmospheric
  Imaging Assembly on board the Solar Dynamics Observatory (SDO), are
  detected. The oscillation period is about 700 s and the displacement
  amplitude is about 1 Mm. The oscillation amplitude remains constant
  during the observation. Simultaneous observation of the rope in the
  bandpasses corresponding to the coronal and chromospheric temperatures
  suggests that it has a multi-thermal structure. Oscillatory patterns
  in 171 Å and 304 Å are coherent, which indicates that the observed
  kink oscillation is collective, in which the rope moves as a single
  entity. We interpret the oscillation as a fundamental standing
  vertically polarized kink mode of the rope, while the interpretation in
  terms of a perpendicular fast wave could not be entirely ruled out. In
  addition, the arcade situated above the rope and seen in the 171 Å
  bandpass shows an oscillatory motion with the period of about 1000 s.

Title: A Combined Analysis of the Observational Aspects of the
    Quasi-biennial Oscillation in Solar Magnetic Activity
Authors: Bazilevskaya, G.; Broomhall, A. -M.; Elsworth, Y.; Nakariakov,
   V. M.
Bibcode: 2014SSRv..186..359B
Altcode: 2014SSRv..tmp...34B
  Solar quasi-biennial oscillations (QBOs) with the time scale of 0.6-4
  yrs appear to be a basic feature of the Sun's activity. Observational
  aspects of QBOs are reviewed on the basis of recent publications. Solar
  QBOs are shown to be ubiquitous and very variable. We demonstrate
  that many features of QBOs are common to different observations. These
  features include variable periodicity and intermittence with signs of
  stochastisity, a presence at all levels of the solar atmosphere and
  even in the convective zone, independent development in the northern
  and southern solar hemispheres, most pronounced amplitudes during
  the maximum phase of the 11-yr cycle and the transition of QBOs into
  interplanetary space. Temporal weakening of solar activity around the
  maximum of the 11-yr cycle (Gnevyshev Gap) can be considered an integral
  part of QBOs. The exact mechanism by which the solar QBO is produced is
  poorly understood. We describe some of the most plausible theoretical
  mechanisms and discuss observational features that support/contradict
  the theory. QBOs have an important meaning as a benchmark of solar
  activity, not only for investigation of the solar dynamo but also in
  terms of space weather.

Title: Dynamics of a multi-thermal loop in the solar corona
Authors: Nisticò, G.; Anfinogentov, S.; Nakariakov, V. M.
Bibcode: 2014A&A...570A..84N
Altcode:
  Context. We present an observation of a long-living multi-thermal
  coronal loop, visible in different extreme ultra-violet wavebands of
  SDO/AIA in a quiet-Sun region close to the western solar limb. <BR />
  Aims: Analysis of persistent kink displacements of the loop seen in
  different bandpasses that correspond to different temperatures of the
  plasma allows sub-resolution structuring of the loop to be revealed. <BR
  /> Methods: A vertically oriented slit is taken at the loop top, and
  time-distance maps are made from it. Loop displacements in time-distance
  maps are automatically tracked with the Gaussian fitting technique and
  fitted with a sinusoidal function that is "guessed". Wavelet transforms
  are further used in order to quantify the periodicity variation in
  time of the kink oscillations. <BR /> Results: The loop strands are
  found to oscillate with the periods ranging between 3 and 15 min. The
  oscillations are observed in intermittent regime with temporal changes
  in the period and amplitude. The oscillations are different at three
  analysed wavelengths. <BR /> Conclusions: This finding suggests that
  the loop-like threads seen at different wavelengths are not co-spatial,
  hence that the loop consists of several multi-thermal strands. The
  detected irregularity of the oscillations can be associated with
  a stochastic driver acting at the footpoints of the loop. <P />A
  movie associated to Fig. 1 is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201423970/olm">http://www.aanda.org</A>

Title: Oscillations in a Sunspot with Light Bridges
Authors: Yuan, Ding; Nakariakov, Valery M.; Huang, Zhenghua; Li, Bo;
   Su, Jiangtao; Yan, Yihua; Tan, Baolin
Bibcode: 2014ApJ...792...41Y
Altcode: 2014arXiv1407.1544Y
  The Solar Optical Telescope on board Hinode observed a sunspot (AR
  11836) with two light bridges (LBs) on 2013 August 31. We analyzed
  a two-hour Ca II H emission intensity data set and detected strong
  five-minute oscillation power on both LBs and in the inner penumbra. The
  time-distance plot reveals that the five-minute oscillation phase
  does not vary significantly along the thin bridge, indicating that
  the oscillations are likely to originate from underneath it. The slit
  taken along the central axis of the wide LB exhibits a standing wave
  feature. However, at the center of the wide bridge, the five-minute
  oscillation power is found to be stronger than at its sides. Moreover,
  the time-distance plot across the wide bridge exhibits a herringbone
  pattern that indicates a counter-stream of two running waves, which
  originated at the bridge's sides. Thus, the five-minute oscillations
  on the wide bridge also resemble the properties of running penumbral
  waves. The five-minute oscillations are suppressed in the umbra, while
  the three-minute oscillations occupy all three cores of the sunspot's
  umbra, separated by the LBs. The three-minute oscillations were found
  to be in phase at both sides of the LBs. This may indicate that either
  LBs do not affect umbral oscillations, or that umbral oscillations
  at different umbral cores share the same source. It also indicates
  that LBs are rather shallow objects situated in the upper part of the
  umbra. We found that umbral flashes (UFs) follow the life cycles of
  umbral oscillations with much larger amplitudes. They cannot propagate
  across LBs. UFs dominate the three-minute oscillation power within
  each core; however, they do not disrupt the phase of umbral oscillation.

Title: Observation of a high-quality quasi-periodic rapidly
    propagating wave train using SDO/AIA
Authors: Nisticò, G.; Pascoe, D. J.; Nakariakov, V. M.
Bibcode: 2014A&A...569A..12N
Altcode:
  Context. We present a new event of quasi-periodic wave trains observed
  in EUV wavebands that rapidly propagate away from an active region
  after a flare. <BR /> Aims: We measured the parameters of a wave
  train observed on 7 December 2013 after an M1.2 flare, such as
  the phase speeds, periods and wavelengths, in relationship to the
  local coronal environment and the energy sources. <BR /> Methods:
  We compared our observations with a numerical simulation of fast
  magnetoacoustic waves that undergo dispersive evolution and leakage
  in a coronal loop embedded in a potential magnetic field. <BR />
  Results: The wave train is observed to propagate as several arc-shaped
  intensity disturbances for almost half an hour, with a speed greater
  than 1000 km s<SUP>-1</SUP> and a period of about 1 min. The wave
  train followed two different patterns of propagation, in accordance
  with the magnetic structure of the active region. The oscillatory
  signal is found to be of high-quality, i.e. there is a large number
  (10 or more) of subsequent wave fronts observed. The observations
  are found to be consistent with the numerical simulation of a fast
  wave train generated by a localised impulsive energy release. <BR />
  Conclusions: Transverse structuring in the corona can efficiently
  create and guide high-quality quasi-periodic propagating fast wave
  trains. <P />The movies are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201423763/olm">http://www.aanda.org</A>

Title: Wave dynamics in a sunspot umbra
Authors: Sych, R.; Nakariakov, V. M.
Bibcode: 2014A&A...569A..72S
Altcode: 2014arXiv1409.4530S
  Context. Sunspot oscillations are one of the most frequently studied
  wave phenomena in the solar atmosphere. Understanding the basic physical
  processes responsible for sunspot oscillations requires detailed
  information about their fine structure. <BR /> Aims: We aim to reveal
  the relationship between the fine horizontal and vertical structure,
  time evolution, and the fine spectral structure of oscillations in a
  sunspot umbra. <BR /> Methods: The high spatial and time resolution data
  obtained with SDO/AIA for the sunspot in active region NOAA 11131 on 08
  December 2010 were analysed with the time-distance plot technique and
  the pixelised wavelet filtering method. Different levels of the sunspot
  atmosphere were studied from the temperature minimum to the corona. <BR
  /> Results: Oscillations in the 3 min band dominate in the umbra. The
  integrated spectrum of umbral oscillations contains distinct narrowband
  peaks at 1.9 min, 2.3 min, and 2.8 min. The power significantly varies
  in time, forming distinct 12-20 min oscillation trains. The oscillation
  power distribution over the sunspot in the horizontal plane reveals
  that the enhancements of the oscillation amplitude, or wave fronts,
  have a distinct structure consisting of an evolving two-armed spiral
  and a stationary circular patch at the spiral origin, situated near
  the umbra centre. This structure is seen from the temperature minimum
  at 1700 Å to the 1.6 MK corona at 193 Å. In time, the spiral rotates
  anti-clockwise. The wave front spirality is most pronounced during the
  maximum amplitude phases of the oscillations, and in the bandpasses
  where umbral oscillations have the highest power, 304 Å and 171 Å. In
  the low-amplitude phases the spiral breaks into arc-shaped patches. The
  2D cross-correlation function shows that the oscillations at higher
  atmospheric levels occur later than at lower layers. The phase speed
  is estimated to be about 100 km s<SUP>-1</SUP>. The fine spectral
  analysis shows that the central patch corresponds to the high-frequency
  oscillations, while the spiral arms highlight the lower-frequency
  oscillations in the 3 min band. <BR /> Conclusions: The vertical and
  horizontal radial structure of the oscillations is consistent with the
  model that interprets umbral oscillations as slow magnetoacoustic waves
  filtered by the atmospheric temperature non-uniformity in the presence
  of the magnetic field inclination from the vertical. The mechanism
  for the polar-angle structure of the oscillations, in particular the
  spirality of the wave fronts, needs to be revealed.

Title: Erratum: "Three-minute Oscillations above Sunspot Umbra
    Observed with the Solar Dynamics Observatory/Atmospheric
    Imaging Assembly and Nobeyama Radioheliograph" <A
    href="/abs/2012ApJ...746..119R">(2012, ApJ, 746, 119)</A>
Authors: Reznikova, V. E.; Shibasaki, K.; Sych, R. A.; Nakariakov,
   V. M.
Bibcode: 2014ApJ...792...81R
Altcode:
  No abstract at ADS

Title: Fast magnetoacoustic wave trains in coronal holes
Authors: Pascoe, D. J.; Nakariakov, V. M.; Kupriyanova, E. G.
Bibcode: 2014A&A...568A..20P
Altcode:
  Context. Rapidly propagating coronal EUV disturbances recently
  discovered in the solar corona are interpreted in terms of guided
  fast magnetoacoustic waves. Fast magnetoacoustic waves experience
  geometric dispersion in waveguides, which causes localised, impulsive
  perturbations to develop into quasi-periodic wave trains. <BR /> Aims:
  We consider the formation of fast wave trains in a super-radially
  expanding coronal hole modelled by a magnetic funnel with a
  field-aligned density profile that is rarefied in comparison to the
  surrounding plasma. This kind of structure is typical of coronal
  holes, and it forms a fast magnetoacoustic anti-waveguide as a local
  maximum in the Alfvén speed. <BR /> Methods: We performed 2D MHD
  numerical simulations for impulsively generated perturbations to
  the system. Both sausage and kink perturbations are considered and
  the role of the density contrast ratio investigated. <BR /> Results:
  The anti-waveguide funnel geometry refracts wave energy away from the
  structure. However, in this geometry the quasi-periodic fast wave
  trains are found to appear, too, and so can be associated with the
  observed rapidly propagating coronal EUV disturbances. The wave trains
  propagate along the external edge of the coronal hole. The fast wave
  trains generated in coronal holes exhibit less dispersive evolution than
  in the case of a dense waveguide. <BR /> Conclusions: We conclude that
  an impulsive energy release localised in a coronal plasma inhomogeneity
  develops into a fast wave train for both kink and sausage disturbances
  and for both waveguide and anti-waveguide transverse plasma profiles.

Title: Editorial: solar radiophysics — recent results on
    observations and theories
Authors: Nakariakov, Valery M.; Kashapova, Larisa K.; Yan, Yi-Hua
Bibcode: 2014RAA....14....1N
Altcode:
  Solar radiophysics is a rapidly developing branch of solar physics
  and plasma astrophysics. Solar radiophysics has the goal of analyzing
  observations of radio emissions from the Sun and understanding basic
  physical processes operating in quiet and active regions of the solar
  corona. In the near future, the commissioning of a new generation
  of solar radio observational facilities, which include the Chinese
  Spectral Radio Heliograph (CSRH) and the upgrade of the Siberian
  Solar Radio Telescope (SSRT), and the beginning of solar observations
  with the Atacama Large Millimeter/submillimeter Array (ALMA), is
  expected to bring us new breakthrough results of a transformative
  nature. The Marie-Curie International Research Staff Exchange (MC
  IRSES) “RadioSun” international network aims to create a solid
  foundation for the successful exploitation of upcoming solar radio
  observational facilities, as well as intensive use of the existing
  observational tools, advanced theoretical modeling of relevant physical
  processes and observables, and training a new generation of solar
  radio physicists. The RadioSun network links research teams from China,
  Czech Republic, Poland, Russia and the UK. This mini-volume presents
  research papers based on invited reviews and contributed talks at
  the 1st RadioSun workshop in China. These papers cover a broad range
  of research topics and include recent observational and theoretical
  advances in solar radiophysics, MHD seismology of the solar corona,
  physics of solar flares, generation of radio emission, numerical
  modeling of MHD and plasma physics processes, charged-particle
  acceleration and novel instrumentation.

Title: Sausage oscillations of coronal plasma slabs
Authors: Hornsey, C.; Nakariakov, V. M.; Fludra, A.
Bibcode: 2014A&A...567A..24H
Altcode:
  Context. Sausage oscillations are observed in plasma non-uniformities
  of the solar corona as axisymmetric perturbations of the
  non-uniformity. Often, these non-uniformities can be modelled as
  field-aligned slabs of the density enhancement. <BR /> Aims: We
  perform parametric studies of sausage oscillations of plasma slabs,
  aiming to determine the dependence of the oscillation period on
  its parameters, and the onset of leaky and trapped regimes of the
  oscillations. <BR /> Methods: Slabs with smooth transverse profiles
  of the density of a zero-beta plasma are perturbed by an impulsive
  localised perturbation of the sausage symmetry. In particular, the
  slab can contain an infinitely thin current sheet in its centre. The
  initial value problem is then solved numerically. The numerical
  results are subject to spectral analysis. The results are compared
  with analytical solutions for a slab with a step-function profile and
  also with sausage oscillations of a plasma cylinder. <BR /> Results: We
  established that sausage oscillations in slabs generally have the same
  properties as in plasma cylinders. In the trapped regime, the sausage
  oscillation period increases with the increase in the longitudinal
  wavelength. In the leaky regime, the dependence of the period on the
  wavelength experiences saturation, and the period becomes independent
  of the wavelength in the long-wavelength limit. In the leaky regime
  the period is always longer than in the trapped regime. The sausage
  oscillation period in a slab is always longer than in a cylinder with
  the same transverse profile. In slabs with steeper transverse profiles,
  sausage oscillations have longer periods. The leaky regime occurs at
  shorter wavelengths in slabs with smoother profiles.

Title: Temperature Anisotropy in the Presence of Ultra Low Frequency
    Waves in the Terrestrial Foreshock
Authors: Selzer, L. A.; Hnat, B.; Osman, K. T.; Nakariakov, V. M.;
   Eastwood, J. P.; Burgess, D.
Bibcode: 2014ApJ...788L...5S
Altcode:
  We report the first study of the correlation between elevated solar wind
  core plasma temperatures and temperature anisotropy in the terrestrial
  foreshock. Plasma temperature is enhanced near the fire hose marginal
  stability threshold in the presence of ultra low frequency (ULF)
  large amplitude magnetic perturbations, which are intrinsically
  right-hand circularly polarized. Direct comparison of contemporaneous
  anisotropic temperatures in the upstream solar wind and the foreshock
  suggests that the net heating of plasma is mediated via increase of the
  parallel temperature in the foreshock region where the ULF waves are
  present. We consider the possibility that a mechanism based on Landau
  damping, where solar wind plasma temperature parallel to the background
  magnetic field is increased by interaction with oblique compressible
  fast magneto-acoustic ULF waves, influences temperature anisotropy.

Title: Fast magnetoacoustic wave trains in magnetic funnels of the
    solar corona
Authors: Pascoe, David; Nakariakov, Valery; Kupriyanova, Elena
Bibcode: 2014cosp...40E2453P
Altcode:
  Fast magnetoacoustic waves are highly dispersive in waveguides and
  so can generate quasi-periodic wave trains if a localised, impulsive
  driver is applied, such as flaring energy release. Wave trains have
  been observed propagating in the solar corona and may be of use as
  a seismological tool since they depend upon the plasma structuring
  perpendicular to the direction of propagation. We extend existing models
  of magnetoacoustic waveguides to consider the effects of an expanding
  magnetic field with a field-aligned density structure, responding to the
  recent discovery of fast wave trains of EUV intensity. We investigate
  the dependence of the wave train properties on variables such as the
  density contrast ratio, density stratification, and the spectral profile
  of the driver. The funnel geometry leads to novel wave behaviour, such
  as the formation of additional wave trains that propagate outside but
  along the density structure due to leakage and refraction. The results
  of our numerical simulations demonstrate properties similar to those
  of wave trains observed in the solar corona.

Title: Decaying long-period oscillations in flaring coronal loops
Authors: Nakariakov, Valery
Bibcode: 2014cosp...40E2227N
Altcode:
  Quasi-periodic rapidly-decaying variations of the Doppler shift of the
  emission lines associated with the hot plasma were detected in solar
  flares about ten years ago with the SUMER spectrograph operating
  in EUV. Later, similar field-aligned flows of the hot plasma in
  flaring loops were found in the data of Yohkoh/BCS. The oscillations
  characterised by relatively long periods, in the range 10-20 min,
  and very short decay times, 15-30 min, are known as “SUMER”
  oscillations. We present observations of SUMER oscillations in the
  microwave band with the Nobeyama Radioheliograph and SDO/AIA. Analysis
  of the microwave data, obtained in the 17 GHz channel during an
  M1.6 flare revealed the presence of 12.6-min oscillations of the
  emitting plasma density. The oscillations were seen to decay with the
  characteristic time of about 15 min. Simultaneously, these oscillations
  were detected in the variation of the EUV emission intensity measured
  in the 335A channel of SDO/AIA. Our observational findings support the
  interpretation of SUMER oscillations in terms of impulsively excited
  standing acoustic oscillations in flaring loops or arcades, based upon
  numerical radiative MHD simulations. Moreover, very recently a similar
  dynamical pattern was detected in the light-curve of a megaflare on
  the dM4.5e star YZ CMi in the white light band. This result indicates
  striking similarities between dynamical processes in moderate solar
  flares and stellar megaflares.

Title: Spatial Structure of Multimode Oscillations in a Solar Flare
    on 14 May 2013 in EUV and Radio Bands
Authors: Kolotkov, Dmitry; Nakariakov, Valery; Nisticò, Giuseppe;
   Shibasaki, Kiyoto; Kupriyanova, Elena
Bibcode: 2014cosp...40E1558K
Altcode:
  Quasi-periodic pulsations and coronal loop oscillations in an X-class
  solar flare on 14 May 2013 are considered. Rapidly decaying kink
  oscillations of coronal loops with periods of several minutes in the
  flaring active region detected in the EUV band with SDO/AIA after
  the impulsive phase of the flare. Oscillations of neighbouring loops
  are excited simultaneously, but get rapidly out of phase. In the
  impulsive phase, observations in the radio band with the Nobeyama
  Radioheliograph and Radiopolarimeter show quasi-periodic pulsations
  that are most pronounced in the 17 GHz band. In the correlation plots
  and the integrated flux the pulsations have a symmetric triangular
  shape. The period of pulsations is about 1 min. Analysis of the spatial
  locations of the radio sources reveal that the triangularity is likely
  to be caused by superposition of several harmonic modes.

Title: Multi-height observations of magnetoacoustic cut-off frequency
    in a sunspot atmosphere
Authors: Yuan, D.; Sych, R.; Reznikova, V. E.; Nakariakov, V. M.
Bibcode: 2014A&A...561A..19Y
Altcode: 2012arXiv1211.5196Y
  Context. The cut-off frequency of magnetoacoustic gravity (MAG) waves
  could be decreased by the inclined magnetic field, and therefore,
  low-frequency waves could penetrate into the upper atmosphere. <BR
  /> Aims: We observe the distribution of the cut-off frequency of
  compressive waves at various heights and reconstruct the magnetic
  field inclination, according to the MAG wave theory in a stratified
  atmosphere permeated by a uniform magnetic field. <BR /> Methods:
  We analysed the emission intensity oscillations of sunspot AR11131
  (08 Dec. 2010) observed at the 1700 Å, 1600 Å, and 304 Å bandpasses
  of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
  Observatory (SDO), and computed the narrow-band power maps with the
  pixelised wavelet filtering method. The distribution of the cut-off
  frequency was defined as the median contour in the azimuthally-averaged
  oscillation power. The magnetic field inclination was estimated with the
  local cut-off frequency according to the MAG wave theory in the low-β
  limit and was compared to the potential field extrapolation. <BR />
  Results: Shorter period oscillations dominate in the sunspot umbra,
  while longer period oscillations form an annular shape approximately
  concentric with the sunspot. Oscillations with longer periods are
  distributed further away from the sunspot centre. The 5 min oscillations
  appear to originate at or lower than the photosphere. The magnetic
  field inclinations determined with the cut-off frequency theory are
  about 30-40% larger than the values obtained by the potential field
  extrapolation. <BR /> Conclusions: The oscillation power distribution
  in a sunspot atmosphere reflects its magnetic and thermal structure. The
  cut-off frequency could be used to probe the magnetic field inclination,
  however, other factors have to be included to fully understand this
  phenomenon. The existence of return magnetic flux at the outer penumbra
  was evidenced by the cut-off frequency distribution.

Title: Observations of decay-less low-amplitude kink oscillations
    of EUV coronal loops
Authors: Nisticò, Giuseppe; Nakariakov, Valery; Anfinogentov, Sergey
Bibcode: 2014cosp...40E2294N
Altcode:
  The high spatial and temporal resolution observations at Extreme
  Ultra-Violet (EUV) wavelengths from the Atmospheric Imaging Assembly
  (AIA) of the Solar Dynamics Observatory (SDO) reveal new features in
  kink oscillations of coronal loops. We show that, in addition to the
  well-known rapidly decaying oscillations, a new type of kink waves is
  present, characterized by low-amplitude and undamped oscillations,
  that we define as decay-less. Typical periods range from 2.5 to 12
  min in both regimes and are different for different loops, increasing
  with the loop length. Estimates of the loop lengths are supported
  by three dimensional reconstruction of the loop geometry. The
  amplitude for the decay-less regime is about 1 Mm, close to the
  spatial resolution of the AIA instruments. The oscillation phase,
  measured by the cross-correlation method, is found to be constant
  along each analysed loop, and the spatial structure of the phase of the
  oscillations corresponds to the fundamental standing kink mode. We show
  that the observed behaviours are consistent with the empirical model
  of a damped linear oscillator excited by a continuous low-amplitude
  harmonic driver, in addition to an eventual impulsive high-amplitude
  driver. The observed life-time of the oscillations is likely to be
  determined by the observational conditions rather than any physical
  damping. However, the balance between the driving and damping is a
  necessary ingredient of this model. The properties of this type of
  transverse oscillations make them interesting object of study in the
  framework of resonant absorption theory and coronal heating process.

Title: Three-dimensional geometry of coronal loops inferred by the
    Principal Component Analysis
Authors: Nisticò, Giuseppe; Nakariakov, Valery
Bibcode: 2014cosp...40E2293N
Altcode:
  We propose a new method for the determination of the three dimensional
  (3D) shape of coronal loops from stereoscopy. The common approach
  requires to find a 1D geometric curve, as circumference or ellipse,
  that best-fits the 3D tie-points which sample the loop shape
  in a given coordinate system. This can be easily achieved by the
  Principal Component (PC) analysis. It mainly consists in calculating
  the eigenvalues and eigenvectors of the covariance matrix of the 3D
  tie-points: the eigenvalues give a measure of the variability of the
  distribution of the tie-points, and the corresponding eigenvectors
  define a new cartesian reference frame directly related to the loop. The
  eigenvector associated with the smallest eigenvalues defines the normal
  to the loop plane, while the other two determine the directions of
  the loop axes: the major axis is related to the largest eigenvalue,
  and the minor axis with the second one. The magnitude of the axes is
  directly proportional to the square roots of these eigenvalues. The
  technique is fast and easily implemented in some examples, returning
  best-fitting estimations of the loop parameters and 3D reconstruction
  with a reasonable small number of tie-points. The method is suitable
  for serial reconstruction of coronal loops in active regions, providing
  a useful tool for comparison between observations and theoretical
  magnetic field extrapolations from potential or force-free fields.

Title: Collective responses of coronal loops and CMEs to the eruptions
    of a filament
Authors: Zhang, Yuzong; Nakariakov, Valery; Wang, Jingxu; Zhang, Jun
Bibcode: 2014cosp...40E3810Z
Altcode:
  With the extreme ultraviolet data acquired by the Atmospheric Imaging
  Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we
  study the dramatic and multiple activities of coronal loops of the
  active regions (ARs) 11226 and 11227 during the double eruption event
  of a toroidal filament located at the joint of the two ARs on 2011
  June 2. It is found that 11 of 21 identified coronal loop systems
  demonstrated collective responses in five different ways, i.e., 1)
  shrinkage; 2) oscillation; 3) expansion; 4) eruption; 5) sideward
  motion, observed in six coronal wavebands of AIA. Moreover, an EUV
  wave triggered by the first filament eruption was seen to reflect back
  and also contributed to the shrinkage of the coronal loop systems in
  the two ARs. The different responding ways are closely related to the
  relative positions and the directions of the filament eruptions and
  the driven EUV wave propagation. Also, for the first time, both fast
  and slow magnetoacoustic waves are observed to propagate simultaneously
  along the same loop systems, with the speeds of ∼1,000 and tens of km
  s-1 , respectively. The slow mode waves were found to be unaffected by
  the fast ones. Furthermore, we shift to a more global scale to discuss
  the waves and some of the responses of the loops in conjunction with
  the CMEs.

Title: Fast magnetoacoustic wave trains in magnetic funnels of the
    solar corona
Authors: Pascoe, D. J.; Nakariakov, V. M.; Kupriyanova, E. G.
Bibcode: 2013A&A...560A..97P
Altcode:
  Context. Fast magneto-acoustic waves are highly dispersive in
  waveguides, so they can generate quasi-periodic wave trains if a
  localised, impulsive driver is applied. Such wave trains have been
  observed in the solar corona and may be of use as a seismological
  tool since they depend upon the plasma structuring perpendicular to
  the direction of propagation. <BR /> Aims: We extend existing models
  of magnetoacoustic waveguides to consider the effects of an expanding
  magnetic field. The funnel geometry employed includes a field-aligned
  density structure. <BR /> Methods: We performed 2D numerical simulations
  of impulsively generated fast magneto-acoustic perturbations. The
  effects of the density contrast ratio, density stratification,
  and spectral profile of the driver upon the excited wave trains
  were investigated. <BR /> Results: The density structure acts as a
  dispersive waveguide for fast magneto-acoustic waves and generates
  a quasi-periodic wave train similar to previous models. The funnel
  geometry leads to generating additional wave trains that propagate
  outside the density structure. These newly discovered wave trains are
  formed by the leakage of transverse perturbations, but they propagate
  upwards owing to the refraction caused by the magnetic funnel. <BR
  /> Conclusions: The results of our funnel model may be applicable to
  wave trains observed propagating in the solar corona. They demonstrate
  similar properties to those found in our simulations.

Title: Long-Period Oscillations of Sunspots by NoRH and SSRT
    Observations
Authors: Bakunina, Irina A.; Abramov-maximov, Vladimir E.; Nakariakov,
   Valery M.; Lesovoy, Sergei V.; Soloviev, Alexander A.; Tikhomirov,
   Yurii V.; Melnikov, Victor F.; Shibasaki, Kiyoto; Nagovitsyn, Yurii
   A.; Averina, Elena L.
Bibcode: 2013PASJ...65S..13B
Altcode:
  Long-term oscillations of microwave emission generated in sunspot
  magnetospheres are detected with the Nobeyama Radioheliograph (NoRH)
  at a frequency of 17 GHz, and the Siberian Solar Radio Telescope
  (SSRT) at 5.7 GHz. Significant periodicities in the range of 22-170
  min are found in the variation of the emission intensity, polarisation
  and the degree of circular polarisation. Periods of the oscillations
  are not stable: they are different in different sunspots and in the
  same sunspot on different days. A cross-correlation analysis shows the
  presence of common significant periods in both NoRH and SSRT data. The
  cross-correlation coefficients are typically lower than 0.5, which
  can be attributed to the different heights of the emission formation,
  and different mechanisms for the emission generation (gyroresonance
  and thermal bremstrahlung at 17 GHz, and pure gyroresonance at 5.7
  GHz). The observational results are consistent with the global sunspot
  oscillation model.

Title: Decay-less kink oscillations in coronal loops
Authors: Anfinogentov, S.; Nisticò, G.; Nakariakov, V. M.
Bibcode: 2013A&A...560A.107A
Altcode:
  Context. Kink oscillations of coronal loops in an off-limb active
  region are detected with the Imaging Assembly Array (AIA) instruments
  of the Solar Dynamics Observatory (SDO) at 171 Å. <BR /> Aims: We aim
  to measure periods and amplitudes of kink oscillations of different
  loops and to determinate the evolution of the oscillation phase along
  the oscillating loop. <BR /> Methods: Oscillating coronal loops were
  visually identified in the field of view of SDO/AIA and STEREO/EUVI-A:
  the loop length was derived by three-dimensional analysis. Several
  slits were taken along the loops to assemble time-distance maps. We
  identified oscillatory patterns and retrieved periods and amplitudes
  of the oscillations. We applied the cross-correlation technique to
  estimate the phase shift between oscillations at different segments of
  oscillating loops. <BR /> Results: We found that all analysed loops
  show low-amplitude undamped transverse oscillations. Oscillation
  periods of loops in the same active region range from 2.5 to 11 min,
  and are different for different loops. The displacement amplitude is
  lower than 1 Mm. The oscillation phase is constant along each analysed
  loop. The spatial structure of the phase of the oscillations corresponds
  to the fundamental standing kink mode. We conclude that the observed
  behaviour is consistent with the empirical model in terms of a damped
  harmonic resonator affected by a non-resonant continuously operating
  external force. <P />A movie is available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

Title: Quasi-periodic Wiggles of Microwave Zebra Structures in a
    Solar Flare
Authors: Yu, Sijie; Nakariakov, V. M.; Selzer, L. A.; Tan, Baolin;
   Yan, Yihua
Bibcode: 2013ApJ...777..159Y
Altcode: 2013arXiv1309.5777Y
  Quasi-periodic wiggles of microwave zebra pattern (ZP) structures with
  periods ranging from about 0.5 s to 1.5 s are found in an X-class
  solar flare on 2006 December 13 at the 2.6-3.8 GHz with the Chinese
  Solar Broadband Radio Spectrometer (SBRS/Huairou). Periodogram and
  correlation analysis show that the wiggles have two to three significant
  periodicities and are almost in phase between stripes at different
  frequencies. The Alfvén speed estimated from the ZP structures is about
  700 km s<SUP>-1</SUP>. We find the spatial size of the wave-guiding
  plasma structure to be about 1 Mm with a detected period of about 1
  s. This suggests that the ZP wiggles can be associated with the fast
  magnetoacoustic oscillations in the flaring active region. The lack of
  a significant phase shift between wiggles of different stripes suggests
  that the ZP wiggles are caused by a standing sausage oscillation.

Title: 3D Reconstruction of Coronal Loops by the Principal Component
    Analysis
Authors: Nisticò, Giuseppe; Verwichte, Erwin; Nakariakov, Valery
Bibcode: 2013Entrp..15.4520N
Altcode:
  Knowing the three dimensional structure of plasma filaments in the
  uppermost part of the solar atmosphere, known as coronal loops, and
  especially their length, is an important parameter in the wave-based
  diagnostics of this part of the Sun. The combination of observations of
  the Sun from different points of observations in space, thanks to the
  most recent missions, including the Solar Dynamics Observatory (SDO)
  and the Solar TErrestrial RElations Observatory (STEREO), allows us
  to infer information about the geometrical shape of coronal loops in
  3D space. Here, we propose a new method to reconstruct the loop shape
  starting from stereoscopically determined 3D points, which sample the
  loop length, by principal component analysis. This method is shown
  to retrieve in an easy way the main parameters that define the loop,
  e.g., the minor and major axes, the loop plane, the azimuthal and
  inclination angles, for the special case of a coplanar loop.

Title: The Decaying Long-period Oscillation of a Stellar Megaflare
Authors: Anfinogentov, S.; Nakariakov, V. M.; Mathioudakis, M.;
   Van Doorsselaere, T.; Kowalski, A. F.
Bibcode: 2013ApJ...773..156A
Altcode:
  We analyze and interpret the oscillatory signal in the decay phase of
  the U-band light curve of a stellar megaflare observed on 2009 January
  16 on the dM4.5e star YZ CMi. The oscillation is well approximated
  by an exponentially decaying harmonic function. The period of the
  oscillation is found to be 32 minutes, the decay time about 46 minutes,
  and the relative amplitude 15%. As this observational signature is
  typical of the longitudinal oscillations observed in solar flares at
  extreme ultraviolet and radio wavelengths, associated with standing
  slow magnetoacoustic waves, we suggest that this megaflare may be of a
  similar nature. In this scenario, macroscopic variations of the plasma
  parameters in the oscillations modulate the ejection of non-thermal
  electrons. The phase speed of the longitudinal (slow magnetoacoustic)
  waves in the flaring loop or arcade, the tube speed, of about 230
  km s<SUP>-1</SUP> would require a loop length of about 200 Mm. Other
  mechanisms, such as standing kink oscillations, are also considered.

Title: Distinct propagating fast wave trains associated with flaring
    energy releases
Authors: Yuan, D.; Shen, Y.; Liu, Y.; Nakariakov, V. M.; Tan, B.;
   Huang, J.
Bibcode: 2013A&A...554A.144Y
Altcode:
  Context. Large-scale fast waves with perturbation of the EUV emission
  intensity are well resolved in both temporal and spatial scale by
  SDO/AIA. These waves are prone to propagate along the magnetic field
  line. <BR /> Aims: We aim to probe the link between propagating
  fast wave trains and flaring energy releases. By measuring the wave
  parameters, we reveal their nature and investigate the potential
  to diagnose the energy source and waveguide. <BR /> Methods: The
  spatial and temporal evolution of the wave amplitude and propagating
  speed are studied. The correlation of individual wave trains with
  flare-generated radio bursts is tested. <BR /> Results: The propagating
  wave pattern comprises distinct wave trains with varying periods
  and wavelengths. This characteristic signature is consistent with
  the patterns formed by waveguide dispersion, when different spectral
  components propagate at different phase and group speeds. The wave
  train releases are found to be highly correlated in start time with
  the radio bursts emitted by the non-thermal electrons that were
  accelerated in bursty energy releases. The wave amplitude is seen to
  reach the maximum midway during its course. This can be caused by a
  combined effect of the waveguide spread in the transverse direction
  and density stratification. The transverse amplitude distribution
  perpendicular to the wave vector is found to follow approximately a
  Gaussian profile. The spatial structure is consistent with the kink
  mode that is polarised along the line-of-sight. The propagating speed
  is subject to deceleration from ~735-845 km s<SUP>-1</SUP> to ~600
  km s<SUP>-1</SUP>. This could be caused by the decrease in the local
  Alfvén speed and/or the projection effect.

Title: Decaying and decayless transverse oscillations of a coronal
    loop
Authors: Nisticò, G.; Nakariakov, V. M.; Verwichte, E.
Bibcode: 2013A&A...552A..57N
Altcode:
  <BR /> Aims: We investigate kink oscillations of loops observed in an
  active region with the Atmospheric Imaging Assembly (AIA) instrument on
  board the Solar Dynamics Observatory (SDO) spacecraft before and after
  a flare. <BR /> Methods: The oscillations were depicted and analysed
  with time-distance maps, extracted from the cuts taken parallel or
  perpendicular to the loop axis. Moving loops were followed in time
  with steadily moving slits. The period of oscillations and its time
  variation were determined by best-fitting harmonic functions. <BR />
  Results: We show that before and well after the occurrence of the flare,
  the loops experience low-amplitude decayless oscillations. The flare
  and the coronal mass ejection associated to it trigger large-amplitude
  oscillations that decay exponentially in time. The periods of the kink
  oscillations in both regimes (about 240 s) are similar. An empirical
  model of the phenomenon in terms of a damped linear oscillator
  excited by a continuous low-amplitude harmonic driver and by an
  impulsive high-amplitude driver is found to be consistent with the
  observations. <P />Two movies are available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

Title: Sausage Oscillations of Coronal Plasma Structures
Authors: Nakariakov, V. M.; Hornsey, C.; Melnikov, V. F.
Bibcode: 2012ApJ...761..134N
Altcode:
  The dependence of the period of sausage oscillations of coronal loops
  on length together with the depth and steepness of the radial profile
  are determined. We performed a parametric study of linear axisymmetric
  fast magnetoacoustic (sausage) oscillations of coronal loops modeled
  as a field-aligned low-β plasma cylinder with a smooth inhomogeneity
  of the plasma density in the radial direction. The density decreases
  smoothly in the radial direction. Sausage oscillations are impulsively
  excited by a perturbation of the radial velocity, localized at the
  cylinder axis and with a harmonic dependence on the longitudinal
  coordinate. The initial perturbation results in either a leaky or a
  trapped sausage oscillation, depending upon whether the longitudinal
  wavenumber is smaller or greater than a cutoff value, respectively. The
  period of the sausage oscillations was found to always increase with
  increasing longitudinal wavelength, with the dependence saturating in
  the long-wavelength limit. Deeper and steeper radial profiles of the
  Alfvén speed correspond to more efficient trapping of sausage modes:
  the cutoff value of the wavelength increases with the steepness and
  the density (or Alfvén speed) contrast ratio. In the leaky regime,
  the period is always longer than the period of a trapped mode of a
  shorter wavelength in the same cylinder. For shallow density profiles
  and shorter wavelengths, the period increases with wavelength. In the
  long-wavelength limit, the period becomes independent of the wavelength
  and increases with the depth and steepness of the radial profile of
  the Alfvén speed.

Title: Preface
Authors: Nakariakov, V. M.; Georgoulis, M. K.; Poedts, S.; van
   Driel-Gesztelyi, L.; Mandrini, C. H.; Leibacher, J.
Bibcode: 2012SoPh..280..295N
Altcode: 2012SoPh..tmp..226N
  No abstract at ADS

Title: Slow Magnetoacoustic Oscillations in the Microwave Emission
    of Solar Flares
Authors: Kim, S.; Nakariakov, V. M.; Shibasaki, K.
Bibcode: 2012ApJ...756L..36K
Altcode: 2013arXiv1310.2796K
  Analysis of the microwave data, obtained in the 17 GHz channel of the
  Nobeyama Radioheliograph during the M1.6 flare on 2010 November 4,
  revealed the presence of 12.6 minute oscillations of the emitting
  plasma density. The oscillations decayed with the characteristic time
  of about 15 minutes. Similar oscillations with the period of about
  13.8 minutes and the decay time of 25 minutes are also detected in the
  variation of EUV emission intensity measured in the 335 Å channel
  of the Solar Dynamics Observatory/Atmospheric Imaging Assembly. The
  observed properties of the oscillations are consistent with the
  oscillations of hot loops observed by the Solar and Heliospheric
  Observatory/Solar Ultraviolet Measurement of Emitted Radiation (SUMER)
  in the EUV spectra in the form of periodic Doppler shift. Our analysis
  presents the first direct observations of the slow magnetoacoustic
  oscillations in the microwave emission of a solar flare, complementing
  accepted interpretations of SUMER hot loop oscillations as standing
  slow magnetoacoustic waves.

Title: Nonlinear evolution of torsional Alfvén waves
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; Verwichte, E.;
   Van Doorsselaere, T.
Bibcode: 2012A&A...544A.127V
Altcode:
  <BR /> Aims: We study the efficiency of the energy transfer to shorter
  scales in the field-aligned direction - the parallel nonlinear cascade
  - that accompanies the propagation of torsional Alfvén waves along
  open magnetic fields in the solar and stellar coronae, and compare it
  with the same effects for the shear Alfvén wave. The evolution of the
  torsional Alfvén wave is caused by the back reaction of nonlinearly
  induced compressive perturbations on the Alfvén wave. <BR /> Methods:
  The evolution of upwardly propagating torsional Alfvén waves is
  considered in terms of the second-order thin flux-tube approximation
  in a straight untwisted and non-rotating magnetic flux-tube. The
  Cohen-Kulsrud equation for weakly nonlinear torsional waves is
  derived. In the model, the effect of the cubic nonlinearity on the
  propagation of long-wavelength axisymmetric torsional waves is compared
  with the similar effect that accompanies the propagation of plane
  linearly-polarised (shear) Alfvén waves of small amplitude. <BR />
  Results: The solution to the Cohen-Kulsrud type equation for torsional
  waves shows that their evolution is independent of the plasma-β,
  which is in contrast to the shear Alfvén wave. In a finite-β plasma,
  the nonlinear evolution of torsional Alfvén waves is slower and the
  parallel nonlinear cascade is less efficient than those of shear Alfvén
  waves. These results have important implications for the analysis of
  possible heating of the plasma and its acceleration in the upper layers
  of solar and stellar coronae. In particular, one-dimensional models of
  coronal heating and wave acceleration, which use shear Alfvén waves
  instead of torsional Alfvén waves, over-estimate the efficiency of
  these processes.

Title: Magnetohydrodynamic vortices in the solar corona
Authors: Nakariakov, Valery
Bibcode: 2012cosp...39.1329N
Altcode: 2012cosp.meet.1329N
  Interaction of coronal mass ejections with the plasma of the solar
  corona is accompanied with the generation of sharp shear flows that
  cause the induction of Alfvenic vortices. Numerical simulations revealed
  that for a broad range of parameters, the vortices are essentially
  compressible, with the perturbations of the density and the absolute
  value of the magnetic field in the vortex arms reaching 50% of the
  background density. The typical size of the vortex is about the size
  of the obstacle. The frequency of the vortex shedding is controlled
  by a dimensionless parameter known as the Strouhal number. We found
  that in collisionless low-beta plasma this number is about 0.2. Recent
  imaging observations with SDO/AIA revealed the generation of Alfvenic
  vortices at the flanks of emerging plasmoids. The vortices introduce
  a frictionless aerodynamic drag force, applied to the interacting
  plasmas. Implication of these findings for the excitation of kink
  oscillations of coronal loops and CME kinematics is discussed.

Title: Intensity variations associated with fast sausage modes
Authors: Gruszecki, M.; Nakariakov, V. M.; Van Doorsselaere, T.
Bibcode: 2012A&A...543A..12G
Altcode:
  <BR /> Aims: We determine the dependence of the observed properties of
  fast magnetoacoustic axisymmetric oscillations (the sausage mode) of a
  thick and dense flaring coronal loop, modelled by a magnetic cylinder,
  on the parameters of the equilibrium plasma configuration. The plasma
  inside and outside the cylinder is of low-beta, and penetrated by a
  straight magnetic field. The plasma density has a smooth profile across
  the magnetic field. <BR /> Methods: We use three-dimensional ideal
  magnetohydrodynamic equations to model numerically the development
  of the perturbations of the cylindrical equilibrium, considering
  both leaky and trapped regimes. <BR /> Results: Short-period sausage
  oscillations, trapped by the cylinder, are qualitatively consistent
  with the analytical results obtained in the models of a plasma slab
  or a cylinder with a step-function transverse profile. The period of
  trapped sausage oscillations is determined by the ratio of the phase
  speed, with the value between the internal and external Alfvén speeds,
  to the wavelength. Longer-period sausage oscillations are leaky, and
  their decay times are longer for higher density contrasts between the
  internal and external media. Leaky sausage oscillations have longer
  periods than trapped sausage oscillations of the same cylinder. In the
  coronal conditions, sausage oscillations are essentially compressible
  and transverse, hence produce modulation of the thermal optically
  thin emission intensity and periodic Doppler broadening of emission
  lines. However, if the oscillating plasma non-uniformity is poorly
  spatially resolved, the variation in the emission intensity is weak and
  proportional to the actual amplitude of the oscillation squared. The
  latter variation property is connected with the transverse nature of
  the oscillation, causing the conservation of mass in the transverse
  cross-section of the oscillating plasma structure.

Title: Slow magnetoacoustic waves in the solar corona
Authors: Nakariakov, Valery
Bibcode: 2012cosp...39.1330N
Altcode: 2012cosp.meet.1330N
  Quasi-monochromatic propagating disturbances of EUV intensity emission
  are often detected in fan-like structures and in polar plumes of the
  solar corona. We demonstrate that the observed characteristics of these
  propagating disturbances, including the apparent blue-wind asymmetry of
  the emission line, are consistent with their interpretation in terms of
  slow magnetoacoustic waves. The propagating waves are well-described by
  a one-dimensional evolutionary equation that accounts for the effects
  of thermal conduction and stratification. Moreover, the waves are shown
  to be a genuine counterpart of 3-min oscillations in the chromospheric
  slow-magnetoacoustic resonator. It is also found that the leaky slow
  magnetoacoustic waves provide a link between oscillations in sunspots
  and quasi-periodic pulsations in a solar flare occurring nearby,
  suggesting that the waves can play a role of external triggering of
  the energy release.

Title: Measuring the apparent phase speed of propagating EUV
    disturbances
Authors: Yuan, D.; Nakariakov, V. M.
Bibcode: 2012A&A...543A...9Y
Altcode:
  Context. Propagating disturbances of the EUV emission intensity are
  commonly observed over a variety of coronal structures. Parameters of
  these disturbances, particularly the observed apparent (image-plane
  projected) propagation speed, are important tools for MHD coronal
  seismology. <BR /> Aims: We design and test tools to reliably measure
  the apparent phase speed of propagating disturbances in imaging data
  sets. <BR /> Methods: We designed cross-fitting technique (CFT), 2D
  coupled fitting (DCF) and best similarity match (BSM) to measure the
  apparent phase speed of propagating EUV disturbances in the running
  differences of time-distance plots (R) and background-removed and
  normalised time-distance plots (D). <BR /> Results: The methods were
  applied to the analysis of quasi-periodic EUV disturbances propagating
  at a coronal fan-structure of active region NOAA11330 on 27 Oct. 2011,
  observed with the Atmospheric Imaging Assembly (AIA) on SDO in the
  171 Å bandpass. The noise propagation in the AIA image processing was
  estimated, resulting in the preliminary estimation of the uncertainties
  in the AIA image flux. This information was used in measuring the
  apparent phase speed of the propagating disturbances with the CFT, DCF
  and BSM methods, which gave consistent results. The average projected
  speed is measured at 47.6 ± 0.6 km s<SUP>-1</SUP> and 49.0 ± 0.7 km
  s<SUP>-1</SUP> for R and D, with the corresponding periods at 179.7 ±
  0.2 s and 179.7 ± 0.3 s, respectively. We analysed the effects of the
  lag time and the detrending time in the running difference processing
  and the background-removed plot, on the measurement of the speed,
  and found that they are fairly weak. <BR /> Conclusions: The CFT,
  DCF and BSM methods are found to be reliable techniques for measuring
  the apparent (projected) phase speed. The samples of larger effective
  spatial length are more suitable for these methods. Time-distance
  plots with background removal and normalisation allow for more robust
  measurements, with little effect of the choice of the detrending
  time. Cross-fitting technique provides reliable measurements on
  good samples (e.g. samples with large effective detection length and
  recurring features). 2D coupled-fitting is found to be sensitive to the
  initial guess for parameters of the 2D fitting function. Thus DCF is
  only optimised in measuring one of the parameters (the phase speed in
  our application), while the period is poorly measured. Best similarity
  measure is robust for all types of samples and very tolerant to image
  pre-processing and regularisation (smoothing).

Title: Nonlinear long-wavelength torsional Alfven waves
Authors: Nakariakov, Valery
Bibcode: 2012cosp...39.1331N
Altcode: 2012cosp.meet.1331N
  Long-wavelength torsional (Alfven) waves in solar and stellar coronal
  structures experience geometrical amplification with height and hence
  are subject to nonlinear effects. We analyse this regime in frames
  the second order thin flux-tube approximation of Zhugzhda, which
  describes axisymmetric (sausage) magnetohydrodynamic perturbations
  of a straight untwisted and non-rotating magnetic flux-tube,
  representing e.g. a polar plume or a jet, or a coronal loop or a
  prominence filament. Attention is paid to the compressible motions
  nonlinearly induced by long-wavelength torsional waves of small, but
  finite amplitude. We obtained that propagating torsional waves induce
  compressible perturbations oscillating with double the frequency
  of the torsional waves. In contrast with plane shear Alfven waves,
  the amplitude of compressible perturbations is independent of the
  plasma-beta. Moreover, nonlinear evolution of torsional waves is not
  affected by the singularity appearing at the height when the local
  Alfven speed is equal to the sound speed. This result significantly
  reduces the efficiency of nonlinear cascade, and hence suggests that the
  present theories of the solar and stellar wind heating and acceleration
  by Alfven waves, based upon the plane wave theory, require modification.

Title: Magnetohydrodynamic waves and coronal seismology: an overview
    of recent results
Authors: De Moortel, I.; Nakariakov, V. M.
Bibcode: 2012RSPTA.370.3193D
Altcode: 2012arXiv1202.1944D
  Recent observations have revealed that MHD waves and oscillations are
  ubiquitous in the solar atmosphere, with a wide range of periods. We
  give a brief review of some aspects of MHD waves and coronal seismology
  which have recently been the focus of intense debate or are newly
  emerging. In particular, we focus on four topics: (i) the current
  controversy surrounding propagating intensity perturbations along
  coronal loops, (ii) the interpretation of propagating transverse
  loop oscillations, (iii) the ongoing search for coronal (torsional)
  Alfven waves and (iv) the rapidly developing topic of quasi-periodic
  pulsations (QPP) in solar flares.

Title: Solar Particle Acceleration Radiation and Kinetics (SPARK). A
    mission to understand the nature of particle acceleration
Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig;
   Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam;
   Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova,
   Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa
   K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov,
   Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton,
   David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet,
   Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann,
   Gottfried; Aurass, Henri; Neukirch, Thomas
Bibcode: 2012ExA....33..237M
Altcode: 2011ExA...tmp..124M
  Energetic particles are critical components of plasma populations
  found throughout the universe. In many cases particles are accelerated
  to relativistic energies and represent a substantial fraction of
  the total energy of the system, thus requiring extremely efficient
  acceleration processes. The production of accelerated particles
  also appears coupled to magnetic field evolution in astrophysical
  plasmas through the turbulent magnetic fields produced by diffusive
  shock acceleration. Particle acceleration is thus a key component
  in helping to understand the origin and evolution of magnetic
  structures in, e.g. galaxies. The proximity of the Sun and the range
  of high-resolution diagnostics available within the solar atmosphere
  offers unique opportunities to study the processes involved in particle
  acceleration through the use of a combination of remote sensing
  observations of the radiative signatures of accelerated particles, and
  of their plasma and magnetic environment. The SPARK concept targets the
  broad range of energy, spatial and temporal scales over which particle
  acceleration occurs in the solar atmosphere, in order to determine how
  and where energetic particles are accelerated. SPARK combines highly
  complementary imaging and spectroscopic observations of radiation from
  energetic electrons, protons and ions set in their plasma and magnetic
  context. The payload comprises focusing-optics X-ray imaging covering
  the range from 1 to 60 keV; indirect HXR imaging and spectroscopy
  from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution
  LaBr<SUB>3</SUB> scintillators, and photometry and source localisation
  at far-infrared wavelengths. The plasma environment of the regions
  of acceleration and interaction will be probed using soft X-ray
  imaging of the corona and vector magnetography of the photosphere
  and chromosphere. SPARK is designed for solar research. However,
  in addition it will be able to provide exciting new insights into the
  origin of particle acceleration in other regimes, including terrestrial
  gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible
  existence of axions.

Title: Coronal Seismology: Waves and Oscillations in Stellar Coronae
    Flare Plasma
Authors: Stepanov, Alexander V.; Zaitsev, Valery V.; Nakariakov,
   Valery M.
Bibcode: 2012cswo.book.....S
Altcode:
  This concise and systematic account of the current state of this new
  branch of astrophysics presents the theoretical foundations of plasma
  astrophysics, magneto-hydrodynamics and coronal magnetic structures,
  taking into account the full range of available observation techniques
  -- from radio to gamma. <P />The book discusses stellar loops during
  flare energy releases, MHD waves and oscillations, plasma instabilities
  and heating and charged particle acceleration. Current trends and
  developments in MHD seismology of solar and stellar coronal plasma
  systems are also covered, while recent progress is presented in the
  observational study of quasi-periodic pulsations in solar and stellar
  flares with radio, optical, X and gamma rays. In addition, the authors
  investigate the origin of coherent radio emission from stellar loops,
  paying special attention to their fine structure. <P />For advanced
  students and specialists in astronomy, as well as theoretical and
  plasma physics.

Title: Frequency drifts of 3-min oscillations in microwave and EUV
    emission above sunspots
Authors: Sych, R.; Zaqarashvili, T. V.; Nakariakov, V. M.;
   Anfinogentov, S. A.; Shibasaki, K.; Yan, Y.
Bibcode: 2012A&A...539A..23S
Altcode: 2011arXiv1110.2556S
  <BR /> Aims: We analysed 3-min oscillations of microwave and extreme
  ultraviolet (EUV) emission generated at different heights of a sunspot
  atmosphere, studied the amplitude and frequency modulation of the
  oscillations, and its relationship with the variation of the spatial
  structure of the oscillations. <BR /> Methods: High-resolution data
  obtained with the Nobeyama Radioheliograph, TRACE and SDO/AIA were
  analysed with pixelised wavelet filtering (PWF) and wavelet skeleton
  techniques. <BR /> Results: Three-minute oscillations in sunspots
  appear in the form of recurring trains of 8-20 min duration (13 min
  in average). The typical interval between the trains is 30-50 min. The
  oscillation trains are transient in frequency and power. The relative
  amplitude of 3-min oscillations was about 3-8% and sometimes reached
  17%. Recurring frequency drifts of 3-min oscillations were detected
  during the development of individual trains, with the period varying
  in the range 90-240 s. A wavelet analysis showed that there are three
  types of oscillation trains: with positive drifts (to high frequencies),
  negative drifts, and without a drift. Negative drifts, i.e., when
  the 3-min oscillation period gradually increases, were found to occur
  more often. The start and end of the drifts coincides with the start
  time and end of the train. Sometimes two drifts co-exist, i.e. during
  the end of the previous drift, a new drift appears near 160 s, when
  the frequency is in the low-frequency part of the 3-min spectrum,
  near 200 s. This behaviour is seen at all levels of the sunspot
  atmosphere. The speed of the drift is 4-5 mHz/h in the photosphere,
  5-8 mHz/h in the chromosphere, and 11-13 mHz/h in the corona. There
  were also low-frequency peaks in the spectrum, corresponding to the
  periods of 10-20 min, and 30-60 min. The comparative study of the
  spatial structure of 3-min oscillations in microwave and EUV shows
  the appearance of new sources of the sunspot oscillations during the
  development of the trains. <BR /> Conclusions: These structures can
  be interpreted as waveguides that channel upward propagating waves,
  which in turn are responsible for the 3-min oscillations. A possible
  explanation of the observed properties are two simultaneously operating
  factors: dispersive evolution of the upward propagating wave pulses
  and the non-uniformity of the oscillation power distribution over
  the sunspot umbra with different wave sources that correspond to
  different magnetic flux tubes with different physical conditions and
  line-of-sight angles.

Title: Three-minute Oscillations above Sunspot Umbra Observed with
    the Solar Dynamics Observatory/Atmospheric Imaging Assembly and
    Nobeyama Radioheliograph
Authors: Reznikova, V. E.; Shibasaki, K.; Sych, R. A.; Nakariakov,
   V. M.
Bibcode: 2012ApJ...746..119R
Altcode: 2011arXiv1109.5434R
  Three-minute oscillations over a sunspot's umbra in AR 11131 were
  observed simultaneously in UV/EUV emission by the Solar Dynamics
  Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and in radio
  emission by the Nobeyama Radioheliograph (NoRH). We use 24 hr series of
  SDO and 8 hr series of NoRH observations to study spectral, spatial,
  and temporal variations of pulsations in the 5-9 mHz frequency range
  at different layers of the solar atmosphere. High spatial and temporal
  resolution of SDO/AIA in combination with long-duration observations
  allowed us to trace the variations of the cutoff frequency and
  spectrum of oscillations across the umbra. We found that higher
  frequency oscillations are more pronounced closer to the umbra's
  center, while the lower frequencies concentrate on the peripheral
  parts. We interpreted this discovery as a manifestation of variation
  of the magnetic field inclination across the umbra at the level of
  temperature minimum. Possible implications of this interpretation for
  the diagnostics of sunspot atmospheres are discussed.

Title: Slow magnetacoustic waves in magnetic arcades
Authors: Gruszecki, M.; Nakariakov, V. M.
Bibcode: 2011A&A...536A..68G
Altcode:
  <BR /> Aims: We investigate two-dimensional effects on the evolution of
  impulsively-generated slow magnetoacoustic waves in magnetic arcades
  of the solar corona. <BR /> Methods: We used a two-dimensional box
  model of a coronal arcade, neglecting the effects of gravity and
  magnetic curvature. Ideal finite-β magnetohydrodynamic equations were
  employed. The plasma was taken to be uniform, penetrated by a straight
  and uniform magnetic field. We applied line-tying boundary conditions
  at the magnetic footpoints. <BR /> Results: Running and standing slow
  magnetoacoustic waves develop across the magnetic field due to the
  reflection from the footpoints in the arcade. The perpendicular group
  speed is lower than both sound and Alfvén speeds. The speed grows
  with the increase in plasma-β, which is consistent with analytical
  theory. Slow magnetoacoustic pulse perturbs the magnetic field and
  current density; this effect is stronger for higher amplitudes. Standing
  waves spread across the field, forming a characteristic phase-mixing
  pattern of anti-parallel flows. The two-dimensional effects are more
  pronounced for higher β. Our results confirm the plausibility of the
  interpretation of the observed evolution of two-ribbon flares in terms
  of slow magnetoacoustic waves in magnetic arcades.

Title: Entropy mode at a magnetic null point as a possible tool for
    indirect observation of nanoflares in the solar corona
Authors: Murawski, K.; Zaqarashvili, T. V.; Nakariakov, V. M.
Bibcode: 2011A&A...533A..18M
Altcode:
  <BR /> Aims: We aim to explore the dynamics of the entropy mode
  perturbation excited by an energy release in the vicinity of a
  magnetic null point that is embedded in a gravitationally stratified
  solar corona. <BR /> Methods: We solve two-dimensional, time-dependent
  magnetohydrodynamic equations numerically to find spatial and temporal
  signatures of the entropy mode that is triggered impulsively by a
  spatially localized pulse of the gas pressure. <BR /> Results: We
  find that the properties of the entropy mode are determined by the
  sign of the initial pressure pulse. The initial increase in the gas
  pressure creates, together with the magnetoacoustic-gravity waves,
  a stationary void of the rarefied plasma at the launching place,
  associated with the entropy mode. In contrast, an initial decrease
  in the gas pressure, which corresponds to a rapid (or catastrophic)
  cooling, forms a blob of the dense plasma at the launching place. <BR
  /> Conclusions: The cool, dense blobs at magnetic null points may
  be observed in transition region and chromospheric spectral lines
  at coronal heights off the solar limb and may be associated with the
  places of nanoflare occurrence. Therefore, extensions of entropy mode
  studies may produce a diagnostic tool for indirect observations of
  nanoflares. The dense cool blobs may be affected by the gravity or
  carried by downflows, hence may initiate a coronal rain.

Title: Leakage of long-period oscillations from the chromosphere to
    the corona
Authors: Yuan, D.; Nakariakov, V. M.; Chorley, N.; Foullon, C.
Bibcode: 2011A&A...533A.116Y
Altcode:
  Long-period oscillations in a coronal diffuse structure are detected
  with the Transition Region And Coronal Explorer (TRACE). The EUV
  images of the NOAA active region 8253 are available in 171 Å and
  195 Å bandpasses from 30 June to 4 July 1998. The average intensity
  variation is found to be connected with the CCD temperature, which
  varies with the orbital motion of the spacecraft. Hence, oscillations
  with the orbital period and its higher harmonics appear as artifacts
  in the light curves. After the exclusion of the orbital effects, we
  identified several long-period oscillations in the diffuse fan-like
  structure of the active region. Similar periodicities were detected
  in the radio emission from the chromospheric part of that active
  region, observed with the ground-based Nobeyama Radioheliograph (NoRH)
  in the 17 GHz channel. It was found that 0.221, 0.312 and 0.573 mHz
  oscillations were present in both EUV emission lines in the corona and
  the radio signal from the sunspot in the chromosphere, just beneath
  the active region. From the frequency values, the 1st and 3rd detected
  oscillations could be associated with the l = 2, n = -3 or l = 3, n =
  -5 and l = 1 gravity-driven solar interior modes, respectively. The
  appearance of these oscillations in the coronal part of the active
  region can be connected with the wave leakage or the evanescence of
  chromospheric oscillations.

Title: Magnetoacoustic shock formation near a magnetic null point
Authors: Gruszecki, M.; Vasheghani Farahani, S.; Nakariakov, V. M.;
   Arber, T. D.
Bibcode: 2011A&A...531A..63G
Altcode:
  <BR /> Aims: We investigate the interaction of nonlinear fast
  magnetoacoustic waves with a magnetic null point in connection with the
  triggering of solar flares. <BR /> Methods: We model the propagation of
  fast, initially axisymmetric waves towards a two-dimensional isothermal
  magnetic null point in terms of ideal magnetohydrodynamic equations. The
  numerical simulations are carried out with the Lagrangian remap code
  Lare2D. <BR /> Results: Dynamics of initially axisymmetric fast pulses
  of small amplitude is found to be consistent with a linear analytical
  solution proposed earlier. The increase in the amplitude leads to the
  nonlinear acceleration of the compression pulse and deceleration of
  the rarefaction pulse and hence the distortion of the wave front. The
  pulse experiences nonlinear steepening in the radial direction either
  on the leading or the back slopes for the compression and rarefaction
  pulses, respectively. This effect is most pronounced in the directions
  perpendicular to the field. Hence, the nonlinear evolution of the fast
  pulse depends on the polar angle. The nonlinear steepening generates
  the sharp spikes of the electric current density. As in the uniform
  medium, the position of the shock formation also depends on the initial
  width of the pulse. Only sufficiently smooth and low-amplitude initial
  pulses can reach the vicinity of the null point, create there current
  density spikes, and initiate magnetic reconnection by seeding anomalous
  electrical resistivity. Steeper and higher amplitude initial pulses
  overturn at larger distance from the null point, and cannot trigger
  reconnection.

Title: Damped large amplitude transverse oscillations in an EUV
    solar prominence, triggered by large-scale transient coronal waves
Authors: Hershaw, J.; Foullon, C.; Nakariakov, V. M.; Verwichte, E.
Bibcode: 2011A&A...531A..53H
Altcode:
  <BR /> Aims: We investigate two successive trains of large amplitude
  transverse oscillations in an arched EUV prominence, observed
  with SoHO/EIT on the north-east solar limb on 30 July 2005. The
  oscillatory trains are triggered by two large scale coronal waves,
  associated with an X-class and a C-class flare occurring in the same
  remote active region. <BR /> Methods: The oscillations are tracked
  within rectangular slits parallel to the solar limb at different
  heights, which are taken to move with the apparent height profile
  of the prominence to account for solar rotation. Time series for
  the two prominence arch legs are extracted using Gaussian fitting
  on the 195 Å absorption features, and fitted to a damped cosine
  curve to determine the oscillatory parameters. <BR /> Results:
  Differing energies of the two triggering flares and associated
  waves are found to agree with the velocity amplitudes, of 50.6 ±
  3.2 and 15.9 ± 8.0 km s<SUP>-1</SUP> at the apex, for the first
  and second oscillatory trains respectively, as estimated in the
  transverse direction. The period of oscillation is similar for both
  trains, with an average of 99 ± 11 min, indicating a characteristic
  frequency as predicted by magnetohydrodynamics. Increasing velocity
  amplitude with height during the first oscillatory train, and in-phase
  starting motions of the two legs regardless of height, for each train,
  demonstrate that the prominence exhibits a global kink mode to a
  first approximation. However, discrepancies between the oscillatory
  characteristics of the two legs and an apparent dependence of period
  upon height, suggest that the prominence actually oscillates as a
  collection of separate but interacting threads. Damping times of around
  two to three cycles are observed. Combining our results with those
  of previously analysed loop oscillations, we find an approximately
  linear dependence of damping time upon period for kink oscillations,
  supporting resonant absorption as the damping mechanism despite
  limitations in testing this theory.

Title: Instrumental oscillations in RHESSI count rates during
    solar flares
Authors: Inglis, A. R.; Zimovets, I. V.; Dennis, B. R.; Kontar, E. P.;
   Nakariakov, V. M.; Struminsky, A. B.; Tolbert, A. K.
Bibcode: 2011A&A...530A..47I
Altcode: 2011arXiv1102.5349I
  <BR /> Aims: We seek to illustrate the analysis problems posed
  by RHESSI spacecraft motion by studying persistent instrumental
  oscillations found in the lightcurves measured by RHESSI's X-ray
  detectors in the 6-12 keV and 12-25 keV energy range during the
  decay phase of the flares of 2004 November 4 and 6. <BR /> Methods:
  The various motions of the RHESSI spacecraft which may contribute
  to the manifestation of oscillations are studied. The response of
  each detector in turn is also investigated. <BR /> Results: We find
  that on 2004 November 6 the observed oscillations correspond to the
  nutation period of the RHESSI instrument. These oscillations are of
  greatest amplitude for detector 5, while in the lightcurves of many
  other detectors the oscillations are small or undetectable. We also
  find that the variation in detector pointing is much larger during this
  flare than the counterexample of 2004 November 4. <BR /> Conclusions:
  Sufficiently large nutation motions of the RHESSI spacecraft lead to
  clearly observable oscillations in count rates, posing a significant
  hazard for data analysis. This issue is particularly problematic for
  detector 5 due to its design characteristics. Dynamic correction of
  the RHESSI counts, accounting for the livetime, data gaps, and the
  transmission of the bi-grid collimator of each detector, is required
  to overcome this issue. These corrections should be applied to all
  future oscillation studies.

Title: Period persistence of long period oscillations in sunspots
Authors: Chorley, N.; Foullon, C.; Hnat, B.; Nakariakov, V. M.;
   Shibasaki, K.
Bibcode: 2011A&A...529A.123C
Altcode:
  Long period oscillations in the microwave radiation intensity
  generated over the sunspot of NOAA AR 10330 are studied with the
  Nobeyama Radioheliograph as the sunspot passes over the solar disk,
  over the course of 9 days (06-15 April 2003). Periodogram, Fourier
  and global wavelet analyses reveal the presence of a significant
  oscillatory component in the range P ≈ 50-120 min over the course of
  the observations. The spectral amplitudes of five significant Fourier
  components in the range P = 50-150 min are also seen to be stable over
  the observations, when the data are not affected by changes in magnetic
  configuration in the region. The ground-based nature of the instrument
  naturally introduces long data gaps in such long duration observations
  and the presence of the gaps does not allow any conclusion as to the
  stability of the phases of the oscillations. As a model to explain the
  persistence of the dominant long periods, a simple oscillator with a
  nonlinear driving term is proposed. The spectral difference between
  distinct peaks within, e.g. the 3 min spectral band, is expected to
  be able to resonate with the long period one hour oscillations.

Title: Slow Magnetoacoustic Waves in Two-ribbon Flares
Authors: Nakariakov, V. M.; Zimovets, I. V.
Bibcode: 2011ApJ...730L..27N
Altcode:
  We demonstrate that disturbances observed to propagate along the axis
  of the arcade in two-ribbon solar flares at the speed of a few tens
  of km s<SUP>-1</SUP>, well below the Alfvén and sound speeds, can
  be interpreted in terms of slow magnetoacoustic waves. The waves can
  propagate across the magnetic field, parallel to the magnetic neutral
  line, because of the wave-guiding effect due to the reflection from
  the footpoints. The perpendicular group speed of the perturbation
  is found to be a fraction of the sound speed, which is consistent
  with observations. The highest value of the group speed grows with the
  increase in the ratio of the sound and Alfvén speeds. For a broad range
  of parameters, the highest value of the group speed corresponds to the
  propagation angle of 25°-28° to the magnetic field. This effect can
  explain the temporal and spatial structure of quasi-periodic pulsations
  observed in two-ribbon flares.

Title: Magnetic Kelvin-Helmholtz Instability at the Sun
Authors: Foullon, Claire; Verwichte, Erwin; Nakariakov, Valery M.;
   Nykyri, Katariina; Farrugia, Charles J.
Bibcode: 2011ApJ...729L...8F
Altcode:
  Flows and instabilities play a major role in the dynamics of magnetized
  plasmas including the solar corona, magnetospheric and heliospheric
  boundaries, cometary tails, and astrophysical jets. The nonlinear
  effects, multi-scale and microphysical interactions inherent to the
  flow-driven instabilities, are believed to play a role, e.g., in plasma
  entry across a discontinuity, generation of turbulence, and enhanced
  drag. However, in order to clarify the efficiency of macroscopic
  instabilities in these processes, we lack proper knowledge of their
  overall morphological features. Here we show the first observations
  of the temporally and spatially resolved evolution of the magnetic
  Kelvin-Helmholtz instability in the solar corona. Unprecedented
  high-resolution imaging observations of vortices developing at the
  surface of a fast coronal mass ejecta are taken by the new Solar
  Dynamics Observatory, validating theories of the nonlinear dynamics
  involved. The new findings are a cornerstone for developing a unifying
  theory on flow-driven instabilities in rarefied magnetized plasmas,
  which is important for understanding the fundamental processes at work
  in key regions of the Sun-Earth system.

Title: Nonlinear long-wavelength torsional Alfvén waves
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; van Doorsselaere,
   T.; Verwichte, E.
Bibcode: 2011A&A...526A..80V
Altcode:
  <BR /> Aims: We investigate the nonlinear phenomena accompanying
  long-wavelength torsional waves in solar and stellar coronae. <BR
  /> Methods: The second order thin flux-tube approximation is used
  to determine perturbations of a straight untwisted and non-rotating
  magnetic flux-tube, nonlinearly induced by long-wavelength axisymmetric
  magnetohydrodynamic waves of small, but finite amplitude. <BR />
  Results: Propagating torsional waves induce compressible perturbations
  oscillating with double the frequency of the torsional waves. In
  contrast with plane shear Alfvén waves, the amplitude of compressible
  perturbations is independent of the plasma-β and is proportional
  to the torsional wave amplitude squared. Standing torsional waves
  induce compressible perturbations of two kinds, that grow with
  the characteristic time inversely proportional to the sound speed,
  and that oscillate at double the frequency of the inducing torsional
  wave. The growing density perturbation saturates at the level, inversely
  proportional to the sound speed.

Title: Chromospheric Resonances above Sunspot Umbrae
Authors: Botha, G. J. J.; Arber, T. D.; Nakariakov, V. M.; Zhugzhda,
   Y. D.
Bibcode: 2011ApJ...728...84B
Altcode:
  Three-minute oscillations are observed in the chromosphere above
  sunspot umbrae. One of the models used to explain these oscillations
  is that of a chromospheric acoustic resonator, where the cavity
  between the photosphere and transition region partially reflects
  slow magnetoacoustic waves to form resonances in the lower sunspot
  atmosphere. We present a phenomenological study that compares simulation
  results with observations. The ideal magnetohydrodynamic equations
  are used with a uniform vertical magnetic field and a temperature
  profile that models sunspot atmospheres above umbrae. The simulations
  are initialized with a single broadband pulse in the vertical
  velocity inside the convection zone underneath the photosphere. The
  frequencies in the spectrum of the broadband pulse that lie below
  the acoustic cutoff frequency are filtered out so that frequencies
  equal and above the acoustic cutoff frequency resonate inside
  the chromospheric cavity. The chromospheric cavity resonates with
  approximately three-minute oscillations and is a leaky resonator, so
  that these oscillations generate traveling waves that propagate upward
  into the corona. Thus, there is no requirement that a narrowband
  three-minute signal is present in the photosphere to explain the
  narrowband three-minute oscillations in the chromosphere and corona. The
  oscillations in the chromospheric cavity have larger relative amplitudes
  (normalized to the local sound speed) than those in the corona and
  reproduce the intensity fluctuations of observations. Different umbral
  temperature profiles lead to different peaks in the spectrum of the
  resonating chromospheric cavity, which can explain the frequency shift
  in sunspot oscillations over the solar cycle.

Title: The First Measurement of the Adiabatic Index in the Solar
    Corona Using Time-dependent Spectroscopy of Hinode/EIS Observations
Authors: Van Doorsselaere, Tom; Wardle, Nick; Del Zanna, Giulio;
   Jansari, Kishan; Verwichte, Erwin; Nakariakov, Valery M.
Bibcode: 2011ApJ...727L..32V
Altcode:
  We use observations of a slow magnetohydrodynamic wave in the corona
  to determine for the first time the value of the effective adiabatic
  index, using data from the Extreme-ultraviolet Imaging Spectrometer
  on board Hinode. We detect oscillations in the electron density,
  using the CHIANTI atomic database to perform spectroscopy. From
  the time-dependent wave signals from multiple spectral lines the
  relationship between relative density and temperature perturbations is
  determined, which allows in turn to measure the effective adiabatic
  index to be γ<SUB>eff</SUB> = 1.10 ± 0.02. This confirms that the
  thermal conduction along the magnetic field is very efficient in the
  solar corona. The thermal conduction coefficient is measured from
  the phase lag between the temperature and density, and is shown to be
  compatible with Spitzer conductivity.

Title: Height distribution of the power of 3-min oscillations over
    sunspots
Authors: Kobanov, N. I.; Kolobov, D. Y.; Chupin, S. A.; Nakariakov,
   V. M.
Bibcode: 2011A&A...525A..41K
Altcode: 2011arXiv1111.6676K
  Context. The height structure of 3-min oscillations over sunspots is
  studied in the context of the recently discovered effect of height
  inversion: over the umbra, the spatial location of the maximum of
  chromospheric 3-min oscillation power corresponds to the relative
  decrease in the power of photospheric oscillations. <BR /> Aims:
  We investigate whether the height inversion of the power of 3-min
  oscillations is a common feature of the spatial structure of the
  oscillations for the majority of sunspots. <BR /> Methods: Spectrogram
  sequences of Hα 6563 Å and Fe i 6569 Å over sunspots, acquired with
  very high cadency (about 2 s or better) are obtained. The distribution
  of the oscillation power of the line-of-sight velocity signal is
  studied by using methods of wavelet frequency filtration and Fourier
  analysis. <BR /> Results: The effect of the height inversion is found
  in 9 of 11 analyzed active regions. The interpretation of this effect
  is possibly connected to both the decrease in the level of photosphere
  in sunspot umbrae and the magnetic field topology.

Title: Transient induced MHD oscillations: a tool to probe the solar
    active regions
Authors: Srivastava, Abhishek K.; Nakariakov, V. M.; Dwivedi, B. N.;
   Kumar, Pankaj
Bibcode: 2011ASInC...2..271S
Altcode: 2011arXiv1110.0705S
  Solar transients and eruptive phenomena which are ubiquitous in
  the solar atmosphere, can shed new light to the understanding of
  the outstanding problems like coronal heating and the solar wind
  acceleration. Observations in the entire electromagnetic spectrum of
  such dynamical processes of large and small-scale transient/eruptive
  events, with highly dynamic magnetic field configuration, and energetic
  particles, provide crucial information about the plasma processes
  at mega-Kelvin temperature embedded in a complex magnetic field,
  and also energy build-up/energy-release processes, taking place in
  such events. One of the most important phenomenological aspects of
  solar eruptive phenomena is the induced magnetohydrodynamic (MHD)
  waves generated during these energetic processes, which carry a
  potential signature to probing the solar active regions. In this
  paper, we briefly review the recent trends of the transient (e.g.,
  flares) induced quasi-periodic oscillations in the solar atmosphere
  and discuss their implications in diagnosing the solar active regions,
  providing the clue to understanding local plasma dynamics and heating.

Title: Oscillatory processes in solar flares
Authors: Nakariakov, V. M.; Inglis, A. R.; Zimovets, I. V.; Foullon,
   C.; Verwichte, E.; Sych, R.; Myagkova, I. N.
Bibcode: 2010PPCF...52l4009N
Altcode: 2010arXiv1010.0063N
  Electromagnetic (radio, visible-light, UV, EUV, x-ray and gamma-ray)
  emission generated by solar and stellar flares often contains pronounced
  quasi-periodic pulsations (QPPs). Physical mechanisms responsible
  for the generation of long-period QPP (with periods longer than
  1 s) are likely to be associated with MHD processes. The observed
  modulation depths, periods and anharmonicity of QPP suggest that
  they can be linked with some kind of MHD auto-oscillations, e.g. an
  oscillatory regime of magnetic reconnection. Such regimes, of both
  spontaneous and induced nature, have been observed in resistive-MHD
  numerical simulations. The oscillations are essentially nonlinear and
  non-stationary. We demonstrate that a promising novel method for their
  analysis is the empirical mode decomposition technique.

Title: Types of Microwave Quasi-Periodic Pulsations in Single
    Flaring Loops
Authors: Kupriyanova, E. G.; Melnikov, V. F.; Nakariakov, V. M.;
   Shibasaki, K.
Bibcode: 2010SoPh..267..329K
Altcode: 2010SoPh..tmp..179K
  Quasi-periodic pulsations (QPP) of microwave emission generated
  in single flaring loops observed with the Nobeyama Radioheliograph
  (NoRH) and Nobeyama Radio Polarimeters (NoRP) are studied. Specific
  features of the time profiles, i.e. the visible presence or absence
  of QPPs, are not accounted for in the selection. The time evolution
  of the periods of the QPPs is examined using wavelet and correlation
  analyses. In ten out of twelve considered events, at least one or more
  significant spectral components with periods from 5 - 60 s have been
  found. The quality of the oscillations is rather low: Q=πN, where
  N is the number of cycles, mostly varies in the range 12 to 40, with
  an average of 25. We suggest that the detected QPPs can be classified
  into four types: i) those with stable mean periods (e.g. of 15 - 20
  s or 8 - 9 s, the prevailing type); ii) those with spectral drift to
  shorter periods (mostly in the rise phase of the microwave emission);
  iii) those with drift to longer periods (mostly in the decay phase);
  iv) those with multiple periods showing an X-shaped drift (e.g. in
  the range from 20 - 40 s in the rise phase).

Title: Periodic Spectral Line Asymmetries in Solar Coronal Structures
    from Slow Magnetoacoustic Waves
Authors: Verwichte, E.; Marsh, M.; Foullon, C.; Van Doorsselaere,
   T.; De Moortel, I.; Hood, A. W.; Nakariakov, V. M.
Bibcode: 2010ApJ...724L.194V
Altcode:
  Recent spectral observations of upward moving quasi-periodic intensity
  perturbations in solar coronal structures have shown evidence of
  periodic line asymmetries near their footpoints. These observations
  challenge the established interpretation of the intensity perturbations
  in terms of propagating slow magnetoacoustic waves. We show that slow
  waves inherently have a bias toward enhancement of emission in the
  blue wing of the emission line due to in-phase behavior of velocity
  and density perturbations. We demonstrate that slow waves cause line
  asymmetries when the emission line is averaged over an oscillation
  period or when a quasi-static plasma component in the line of sight
  is included. Therefore, we conclude that slow magnetoacoustic waves
  remain a valid explanation for the observed quasi-periodic intensity
  perturbations.

Title: Web-Based Data Processing System for Automated Detection of
    Oscillations with Applications to the Solar Atmosphere
Authors: Sych, R. A.; Nakariakov, V. M.; Anfinogentov, S. A.; Ofman, L.
Bibcode: 2010SoPh..266..349S
Altcode: 2010SoPh..tmp..154S; 2010arXiv1005.3591S
  A web-based, interactive system for the remote processing of imaging
  data sets (i.e., EUV, X-ray, and microwave) and the automated
  interactive detection of wave and oscillatory phenomena in the solar
  atmosphere is presented. The system targets localized, but spatially
  resolved, phenomena such as kink, sausage, and longitudinal propagating
  and standing waves. The system implements the methods of Periodmapping
  for pre-analysis, and Pixelized Wavelet Filtering for detailed analysis
  of the imaging data cubes. The system is implemented on the dedicated
  data-processing server http://pwf.iszf.irk.ru, which is situated at
  the Institute of Solar-Terrestrial Physics, Irkutsk, Russia. Input
  data in the .sav, .fits, or .txt formats can be submitted via the
  local and/or global network (the Internet). The output data can be in
  the png, jpeg, and binary formats, on the user's request. The output
  data are periodmaps; narrowband amplitude, power, phase and correlation
  maps of the wave's sources at significant harmonics and in the chosen
  spectral intervals, and mpeg movies of their evolution. The system
  was tested by the analysis of the EUV and microwave emission from the
  active region NOAA 10756 on 4 May 2005 observed with TRACE and the
  Nobeyama Radioheliograph. The similarity of the spatial localization
  of three-minute propagating waves, near the footpoint of locally open
  magnetic-field lines determined by the potential-field extrapolation,
  in both the transition region and the corona was established. In the
  transition region the growth of the three-minute amplitude was found
  to be accompanied by the decrease in the line-of-sight angle to the
  wave-propagation direction.

Title: From Large-scale Loops to the Sites of Dense Flaring Loops:
    Preferential Conditions for Long-period Pulsations in Solar Flares
Authors: Foullon, C.; Fletcher, L.; Hannah, I. G.; Verwichte, E.;
   Cecconi, B.; Nakariakov, V. M.; Phillips, K. J. H.; Tan, B. L.
Bibcode: 2010ApJ...719..151F
Altcode:
  Long-period quasi-periodic pulsations (QPPs) of solar flares are
  a class apart from shorter period events. By involving an external
  resonator, the mechanism they call upon differs from traditional QPP
  models, but has wider applications. We present a multi-wavelength
  analysis of spatially resolved QPPs, with periods around 10 minutes,
  observed in the X-ray spectrum primarily at energies between 3 and 25
  keV. Complementary observations obtained in Hα and radio emission in
  the kHz to GHz frequency range, together with an analysis of the X-ray
  plasma properties provide a comprehensive picture that is consistent
  with a dense flaring loop subject to periodic energization and
  thermalization. The QPPs obtained in Hα and type III radio bursts,
  with similar periods as the QPPs in soft X-rays, have the longest
  periods ever reported for those types of data sets. We also report 1-2
  GHz radio emission, concurrent with but unrestricted to the QPP time
  intervals, which is multi-structured at regularly separated narrowband
  frequencies and modulated with ~18 minute periods. This radio emission
  can be attributed to the presence of multiple "quiet" large-scale loops
  in the background corona. Large scale but shorter inner loops below
  may act as preferential resonators for the QPPs. The observations
  support interpretations consistent with both inner and outer loops
  subject to fast kink magnetohydrodynamic waves. Finally, X-ray imaging
  indicates the presence of double coronal sources in the flaring sites,
  which could be the particular signatures of the magnetically linked
  inner loops. We discuss the preferential conditions and the driving
  mechanisms causing the repeated flaring.

Title: Phenomenon of Alfvénic Vortex Shedding
Authors: Gruszecki, M.; Nakariakov, V. M.; van Doorsselaere, T.;
   Arber, T. D.
Bibcode: 2010PhRvL.105e5004G
Altcode:
  Generation of Alfvénic (magnetohydrodynamic) vortices by the
  interaction of compressible plasma flows with magnetic-field-aligned
  blunt obstacles is modeled in terms of magnetohydrodynamics. It is
  found that periodic shedding of vortices with opposite vorticity
  is a robust feature of the interaction in a broad range of plasma
  parameters: for plasma beta from 0.025 to 0.5, and for the flow speeds
  from 0.1 to 0.99 of the fast magnetoacoustic speed. The Strouhal
  number is the dimensionless ratio of the blunt body diameter to the
  product of the period of vortex shedding and the inflow speed. It is
  found to be consistently in the range 0.15-0.25 in the whole range
  of parameters. The induced Alfvénic vortices are compressible and
  contain spiral-armed perturbations of the magnetic field strength
  and plasma mass density up to 50%-60% of the background values. The
  generated electric current also has the spiral-armed structuring.

Title: Long-wavelength torsional modes of solar coronal plasma
    structures
Authors: Vasheghani Farahani, S.; Nakariakov, V. M.; van Doorsselaere,
   T.
Bibcode: 2010A&A...517A..29V
Altcode:
  <BR /> Aims: We consider the effects of the magnetic twist and
  plasma rotation on the propagation of torsional m = 0 perturbations
  of cylindrical plasma structures (straight magnetic flux tubes)
  in the case when the wavelength is much longer than the cylinder
  diameter. <BR /> Methods: The second order thin flux tube approximation
  is used to derive dispersion relations and phase relations in linear
  long-wavelength axisymmetric magnetohydrodynamic waves in uniformly
  twisted and rotating plasma structures. <BR /> Results: Asymptotic
  dispersion relations linking phase speeds with the plasma parameters
  are derived. When twist and rotation are both present, the phase speed
  of torsional waves depends upon the direction of the wave propagation,
  and also the waves are compressible. The phase relations show that in
  a torsional wave the density and azimuthal magnetic field perturbations
  are in phase with the axial magnetic field perturbations and anti-phase
  with tube cross-section perturbations. In a zero-β non-rotating plasma
  cylinder confined by the equilibrium twist, the density perturbation is
  found to be about 66 percent of the amplitude of the twist perturbation
  in torsional waves.

Title: Independent Signals from the Influence of Internal Magnetic
    Layers on the Frequencies of Solar p-modes
Authors: Foullon, C.; Nakariakov, V. M.
Bibcode: 2010ApJ...714L..68F
Altcode:
  The discovery that p-mode frequencies of low degree do not follow
  changes of solar surface activity during the recent solar minimum
  offers the possibility of a new diagnostic signature of the responsible
  pressure perturbation in the wave guiding medium, potentially rich
  of information regarding the structure of the Sun and the cause
  of the unusually long solar minimum. Magnetic fields, as well as
  temperature changes, introduce equilibrium pressure deviations that
  modify the resonant frequencies of p-mode oscillations. Assuming the
  perturbation to be caused by a horizontal layer of magnetic field
  located in a plane-stratified model of the Sun, we compile analytical
  frequency shifts and process them to allow direct comparison with
  observations. The effect of magnetism itself on the central p-mode
  frequencies can be neglected in comparison with the thermal effect of
  a perturbative layer buried in the solar interior. A parametric study
  shows that a layer as thin as 2100 km at subsurface depths is able to
  reproduce reported mean anomalous frequency shifts (not correlated with
  the surface activity), while a layer of size around 4200 km increasing
  by a small amount at depths near 0.08 R <SUB>sun</SUB> can explain
  individual low-degree shifts. It is also possible to obtain the mean
  shifts via the upward motion through depths near 0.03 R <SUB>sun</SUB>
  of a rising perturbative layer of thickness around 7000 km. Hence,
  the anomalous frequency shifts are best explained by thermal effects
  in the upper regions of the convection zone. The effects of latitudinal
  distribution are not treated here.

Title: Quasi-Periodic Pulsations in the Gamma-Ray Emission of a
    Solar Flare
Authors: Nakariakov, V. M.; Foullon, C.; Myagkova, I. N.; Inglis, A. R.
Bibcode: 2010EGUGA..12.8615N
Altcode:
  Quasi-periodic pulsations (QPPs) of gamma-ray emission with a period
  of about 40 s are found in a single loop X-class solar flare on 2005
  January 1 at photon energies up to 2-6 MeV with the SOlar Neutrons
  and Gamma-rays (SONG) experiment aboard the CORONAS-F mission. The
  oscillations are also found to be present in the microwave
  emission detected with the Nobeyama Radioheliograph, and in the
  hard X-ray and low energy gamma-ray channels of RHESSI. Periodogram
  and correlation analysis shows that the 40 s QPPs of microwave,
  hard X-ray, and gamma-ray emission are almost synchronous in
  all observation bands. Analysis of the spatial structure of hard
  X-ray and low energy (80-225 keV) gamma-ray QPP with RHESSI reveals
  synchronous while asymmetric QPP at both footpoints of the flaring
  loop. The difference between the averaged hard X-ray fluxes coming
  from the two footpoint sources is found to oscillate with a period
  of about 13 s for five cycles in the highest emission stage of the
  flare. The proposed mechanism generating the 40 s QPP is a triggering
  of magnetic reconnection by a kink oscillation in a nearby loop. The
  13 s periodicity could be produced by the second harmonics of the
  sausage mode of the flaring loop.

Title: Long period oscillations in sunspots
Authors: Chorley, N.; Hnat, B.; Nakariakov, V. M.; Inglis, A. R.;
   Bakunina, I. A.
Bibcode: 2010A&A...513A..27C
Altcode:
  Long period oscillations of the gyroresonant emission from sunspot
  atmospheres are studied. Time series data generated from the sequences
  of images obtained by the Nobeyama Radioheliograph operating at a
  frequency of 17 GHz for three sunspots have been analysed and are
  found to contain significant periods in the range of several tens
  of minutes. Wavelet analysis shows that these periods are persistent
  throughout the observation periods. The presence of the oscillations
  is confirmed by several methods (periodogram, wavelets, Fisher
  randomisation and empirical mode decomposition). Spatial analysis
  using the techniques of period, power, correlation and time lag
  mapping reveals regions of enhanced oscillatory power in the umbral
  regions. Also seen are two regions of coherent oscillation of about 25
  pixels in size, that oscillate in anti-phase with each other. Possible
  interpretation of the observed periodicities is discussed, in terms
  of the shallow sunspot model and the leakage of the solar g-modes.

Title: Spatially-resolved X-ray observations of the global sausage
    mode oscillations in a coronal loop
Authors: Zimovets, Ivan; Nakariakov, Valery
Bibcode: 2010cosp...38.1813Z
Altcode: 2010cosp.meet.1813Z
  We present the first identification of the global (or fundamental) fast
  magnetoacoustic sausage mode of a coronal loop in the X-ray band, based
  upon the detailed analysis of multi-wavelength spatially-resolving
  observations of the M9.3 solar flare on 6 November 2004 by
  RHESSI. High quality harmonic oscillations with the time period of
  about 78 s and Q=240 have been detected in the thermal X-ray (with
  energies lower than 25 keV) emission flux in the decay phase of the
  flare. Similar quasi-periodic pulsations were also observed in the
  decimetric-centimetric radio emission flux (as pulsations of type
  IV radio burst). In the non-thermal hard X-ray emission (higher than
  25 keV) the oscillations were less pronounced. It is found that the
  area of the quasi-stationary and cooling soft X-ray source (lower
  than 15 keV), which was situated near the top of the flaring loop,
  varied in anti-phase with the oscillating flux of its radiation. The
  oscillation period remained constant during all the oscillations. The
  observed properties of the oscillations coincide with the theoretically
  predicted properties of standing sausage modes. Cooling of heated plasma
  during the oscillations is explained mainly by the conductive losses.

Title: Quasi-Periodic Pulsations in the Gamma-Ray Emission of a
    Solar Flare
Authors: Nakariakov, V. M.; Foullon, C.; Myagkova, I. N.; Inglis, A. R.
Bibcode: 2010ApJ...708L..47N
Altcode:
  Quasi-periodic pulsations (QPPs) of gamma-ray emission with a period
  of about 40 s are found in a single loop X-class solar flare on 2005
  January 1 at photon energies up to 2-6 MeV with the SOlar Neutrons
  and Gamma-rays (SONG) experiment aboard the CORONAS-F mission. The
  oscillations are also found to be present in the microwave
  emission detected with the Nobeyama Radioheliograph, and in the
  hard X-ray and low energy gamma-ray channels of RHESSI. Periodogram
  and correlation analysis shows that the 40 s QPPs of microwave,
  hard X-ray, and gamma-ray emission are almost synchronous in
  all observation bands. Analysis of the spatial structure of hard
  X-ray and low energy (80-225 keV) gamma-ray QPP with RHESSI reveals
  synchronous while asymmetric QPP at both footpoints of the flaring
  loop. The difference between the averaged hard X-ray fluxes coming
  from the two footpoint sources is found to oscillate with a period
  of about 13 s for five cycles in the highest emission stage of the
  flare. The proposed mechanism generating the 40 s QPP is a triggering
  of magnetic reconnection by a kink oscillation in a nearby loop. The
  13 s periodicity could be produced by the second harmonics of the
  sausage mode of the flaring loop.

Title: Foreword
Authors: Nakariakov, V. M.; Erdélyi, R.
Bibcode: 2009SSRv..149....1N
Altcode:
  No abstract at ADS

Title: Quasi-Periodic Pulsations in Solar Flares
Authors: Nakariakov, V. M.; Melnikov, V. F.
Bibcode: 2009SSRv..149..119N
Altcode:
  Quasi-periodic pulsations (QPP) are a common feature of flaring
  energy releases in the solar atmosphere, observed in all bands, from
  radio to hard X-ray. In this review we concentrate on QPP with the
  periods longer than one second. Physical mechanisms responsible for
  the generation of long QPP split into two groups: “load/unload”
  mechanisms and MHD oscillations. Load/unload mechanisms are repetitive
  regimes of flaring energy releases by magnetic reconnection or by
  other means. MHD oscillations can affect all elements of the flaring
  emission generation: triggering of reconnection and modulation of its
  rate, acceleration and dynamics of non-thermal electrons, and physical
  conditions in the emitting plasmas. In the case of MHD oscillations,
  the periodicity of QPP is determined either by the presence of some
  resonances, e.g. standing modes of plasma structures, or by wave
  dispersion. Periods and other parameters of QPP are linked with
  properties of flaring plasmas and their morphology. Observational
  investigation of the QPP generation mechanisms based upon the use of
  spatial information, broadband spectral coverage and multi-periodicity
  is discussed.

Title: Relationship between wave processes in sunspots and
    quasi-periodic pulsations in active region flares
Authors: Sych, R.; Nakariakov, V. M.; Karlicky, M.; Anfinogentov, S.
Bibcode: 2009A&A...505..791S
Altcode: 2010arXiv1005.3594S
  A phenomenological relationship between oscillations in a sunspot
  and quasi-periodic pulsations (QPP) in flaring energy releases at an
  active region (AR) above the sunspot is established. The analysis of
  the microwave emission recorded by the Nobeyama Radioheliograph at 17
  GHz shows a gradual increase in the power of the 3-min oscillation
  train in the sunspot associated with AR 10756 before flares in this
  AR. The flaring light curves are found to be bursty with a period of 3
  min. Our analysis of the spatial distribution of the 3-min oscillation
  power implies that the oscillations follow from sunspots along coronal
  loops towards the flaring site. It is proposed that QPP in the flaring
  energy releases can be triggered by 3-min slow magnetoacoustic waves
  leaking from sunspots.

Title: Ultra-long-period Oscillations in EUV Filaments Near to
Eruption: Two-wavelength Correlation and Seismology
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
Bibcode: 2009ApJ...700.1658F
Altcode:
  We investigate whether or not ultra-long-period oscillations in EUV
  filaments can be related to their eruption. We report new observations
  of long-period (~10-30 hr) oscillatory motions in an apparently
  quiescent filament, as it crosses the solar disk in a 12 minute cadence
  SOHO/Extreme-Ultraviolet Imaging Telescope (EIT) 195 Å uninterrupted
  data set. This data set is chosen to explore characteristics of the
  filament oscillations depending on its eruptive behavior, which is
  observed while the filament is still on the disk. The periods are
  found to increase in a near-stable regime prior to eruption. For the
  two sequences reported so far, we compare and link the EUV filament
  oscillations with pulsations in full-disk solar EUV irradiance from
  SOHO/CELIAS/SEM 304 Å flux measurements. In intervals with stationary
  periods, we find that the 304 Å pulsations and the 195 Å filament
  oscillations have similar periodicities, but are phase-shifted by
  about a quarter of period. The two-wavelength correlation serves to
  show that, when the filament is the dominant dynamical feature but
  can no longer be tracked on the disk, the full-disk irradiance may
  provide a mean to identify the period increase prior to the filament
  eruption. We use the periods thus obtained to estimate the height
  increase of filaments' suspending coronal magnetic field lines,
  based on a magnetohydrodynamic (MHD) wave interpretation of the
  oscillations. The results are consistent with changes in prominence
  heights detected off-limb and thus support the seismological tool
  employed. Other interpretations connected with thermal overstability or
  MHD piston effect are possible. These theoretical predictions however
  do not explain the quarter-period shift between the two EUV-wavelength
  signals. In any case, the detected variations may provide a powerful
  diagnostic tool for the forecasting of prominence eruptions.

Title: Characteristics of magnetoacoustic sausage modes
Authors: Inglis, A. R.; van Doorsselaere, T.; Brady, C. S.; Nakariakov,
   V. M.
Bibcode: 2009A&A...503..569I
Altcode: 2013arXiv1303.6301I
  Aims: We perform an advanced study of the fast magnetoacoustic
  sausage oscillations of coronal loops in the context of MHD coronal
  seismology to establish the dependence of the sausage mode period
  and cut-off wavenumber on the plasma-β of the loop-filling plasma. A
  parametric study of the ratios for different harmonics of the mode is
  also carried out. <BR />Methods: Full magnetohydrodynamic numerical
  simulations were performed using Lare2d, simulating hot, dense loops in
  a magnetic slab environment. The symmetric Epstein profile and a simple
  step-function profile were both used to model the density structure of
  the simulated loops. Analytical expressions for the cut-off wavenumber
  and the harmonic ratio between the second longitudinal harmonic and the
  fundamental were also examined. <BR />Results: It was established that
  the period of the global sausage mode is only very weakly dependent on
  the value of the plasma-β inside a coronal loop, which justifies the
  application of this model to hot flaring loops. The cut-off wavenumber
  k<SUB>c</SUB> for the global mode was found to be dependent on both
  internal and external values of the plasma-β, again only weakly. By
  far the most important factor in this case was the value of the density
  contrast ratio between the loop and the surroundings. Finally, the
  deviation of the harmonic ratio P_1/2P<SUB>2</SUB> from the ideal
  non-dispersive case was shown to be considerable at low k, again
  strongly dependent on plasma density. Quantifying the behaviour of the
  cut-off wavenumber and the harmonic ratio has significant applications
  to the field of coronal seismology.

Title: The possible role of vortex shedding in the excitation of
    kink-mode oscillations in the solar corona
Authors: Nakariakov, V. M.; Aschwanden, M. J.; van Doorsselaere, T.
Bibcode: 2009A&A...502..661N
Altcode:
  We propose a model for the excitation of horizontally polarised
  transverse (kink) magnetoacoustic oscillations of solar coronal loops
  by upflows associated with coronal mass ejections. If the magnetic
  field in the plasma that is dragged in the vertical direction by the
  flow is parallel to the loop, the phenomenon of vortex shedding causes
  the appearance of a quasi-periodic horizontal force that is applied to
  alternating sides of the loop. The period of the force is determined
  by the flow speed and the loop's minor radius. The oscillations are
  excited the most effectively when the force is in resonance with the
  natural frequency of the kink oscillations. This model can explain
  the selectivity of the excitation of the oscillations and the initial
  growth of the oscillation amplitude.

Title: Seismology of a Large Solar Coronal Loop from EUVI/STEREO
    Observations of its Transverse Oscillation
Authors: Verwichte, E.; Aschwanden, M. J.; Van Doorsselaere, T.;
   Foullon, C.; Nakariakov, V. M.
Bibcode: 2009ApJ...698..397V
Altcode:
  The first analysis of a transverse loop oscillation observed by both
  Solar TErrestrial RElations Observatories (STEREO) spacecraft is
  presented, for an event on the 2007 June 27 as seen by the Extreme
  Ultraviolet Imager (EUVI). The three-dimensional loop geometry is
  determined using a three-dimensional reconstruction with a semicircular
  loop model, which allows for an accurate measurement of the loop
  length. The plane of wave polarization is found from comparison with
  a simulated loop model and shows that the oscillation is a fundamental
  horizontally polarized fast magnetoacoustic kink mode. The oscillation
  is characterized using an automated method and the results from
  both spacecraft are found to match closely. The oscillation period
  is 630 ± 30 s and the damping time is 1000 ± 300 s. Also, clear
  intensity variations associated with the transverse loop oscillations
  are reported for the first time. They are shown to be caused by the
  effect of line-of-sight integration. The Alfvén speed and coronal
  magnetic field derived using coronal seismology are discussed. This
  study shows that EUVI/STEREO observations achieve an adequate accuracy
  for studying long-period, large-amplitude transverse loop oscillations.

Title: Propagating transverse waves in soft X-ray coronal jets
Authors: Vasheghani Farahani, S.; Van Doorsselaere, T.; Verwichte,
   E.; Nakariakov, V. M.
Bibcode: 2009A&A...498L..29V
Altcode:
  Aims: The theoretical model for magnetohydrodynamic (MHD) modes guided
  by a field-aligned plasma cylinder with a steady flow is adapted
  to interpret transverse waves observed in solar coronal hot jets,
  discovered with Hinode/XRT in terms of fast magnetoacoustic kink
  modes. <BR />Methods: Dispersion relations for linear magnetoacoustic
  perturbations of a plasma jet of constant cross-section surrounded by
  static magnetised plasma are used to determine the phase and group
  speeds of guided transverse waves and their relationship with the
  physical parameters of the jet and the background plasma. The structure
  of the perturbations in the macroscopic parameters of the plasma inside
  and outside the jet, and the phase relations between them are also
  established. <BR />Results: We obtained a convenient expansion for
  the long wave-length limit of the phase and group speeds and have
  shown that transverse waves observed in soft-X-ray solar coronal
  jets are adequately described in terms of fast magnetoacoustic kink
  modes by a magnetic cylinder model, which includes the effect of a
  steady flow. In the observationally determined range of parameters,
  the waves are not found to be subject to either the Kelvin-Helmholtz
  instability or the negative energy wave instability, and hence they
  are likely to be excited at the source of the jet.

Title: Sausage oscillations in loops with a non-uniform cross-section
Authors: Pascoe, D. J.; Nakariakov, V. M.; Arber, T. D.; Murawski, K.
Bibcode: 2009A&A...494.1119P
Altcode:
  Context: Flaring coronal loops are often observed in microwave
  and sometimes in soft X-rays, as extended structures of variable
  cross-section or width. The quasi-periodic pulsations associated with
  flaring coronal loops have been interpreted in terms of standing,
  fast sausage magnetoacoustic modes and suggested for the diagnostics
  of the magnetic field outside the oscillating loop. <BR />Aims: We
  investigate the effects of a non-uniform cross-section on a coronal loop
  with respect to the standing sausage modes it supports. <BR />Methods:
  Numerical simulations of standing sausage modes are performed for a
  straight, zero-β coronal loop with a varying cross-section. The global
  sausage mode and higher harmonics are considered, and simulations were
  performed for a range of density contrast ratios and loop divergence
  parameter. We consider modifications of the period of oscillation
  and the spatial profile of the standing modes. <BR />Results: As the
  loop divergence parameter increases, the period of the standing modes
  decreases. The fractional change period is independent of the density
  contrast ratio. The spatial profiles of the standing modes are modified
  by the cross-section inhomogeneity, but this effect is too weak to be
  observable by modern instruments. For the global sausage mode, the
  cross-section radius divergence by a factor of 2 is found to cause
  the decrease in the period of about 5%.

Title: A multi-periodic oscillatory event in a solar flare
Authors: Inglis, A. R.; Nakariakov, V. M.
Bibcode: 2009A&A...493..259I
Altcode:
  Aims: Ratios of different significant periods found in the light curves
  of a solar flare exhibiting quasi-periodic pulsations (QPP) are used
  to distinguish between the possible physical mechanisms responsible
  for such periodic behaviour. <BR />Methods: Time series data of the
  flaring event of 2002 July 3, observed via the Nobeyama Radioheliograph,
  Nobeyama Radiopolarimeters, and the RHESSI satellite, are investigated
  with the use of the Lomb-Scargle periodogram technique. Images of
  the event are also recovered via the RHESSI, SOHO, and Nobeyama
  Radioheliograph instruments. <BR />Results: Statistical analysis
  of the Lomb-Scargle periodogram results indicates three distinct
  periods above the 99% confidence level in Nobeyama Radioheliograph
  and Radiopolarimeter data, at 28 s, 18 s, and 12 s, respectively. The
  two longest of these periods were also observed in the RHESSI data at
  the same confidence level. Wavelet analysis demonstrated that multiple
  periods occurred simultaneously without any significant frequency shift
  over time. Reconstructed images of the event reveal a very compact flare
  structure unsuitable for spatially resolved analysis. Consideration
  of the period ratios leads to the conclusion that the cause of this
  multi-periodic event is likely to be a kink mode periodically triggering
  magnetic reconnection.

Title: Seismological demonstration of perpendicular density
    structuring in the solar corona
Authors: Van Doorsselaere, T.; Brady, C. S.; Verwichte, E.; Nakariakov,
   V. M.
Bibcode: 2008A&A...491L...9V
Altcode:
  The peculiarities of the propagating transverse waves observed in the
  solar corona with the Coronal Multi-channel Polarimeter (CoMP) indicate
  the existence of fine field structuring in the coronal density. We
  present results of numerical simulations studying the evolution of a
  localised transverse magneto-hydrodynamic wave in a uniform magnetic
  field. We consider two initial low plasma-beta equilibria: one with
  a homogeneous density, and one with a field-aligned dense structure
  (such as a loop or a plume). The perpendicular localisation of the
  wave strongly determines the angular distribution of the energy
  propagation. If the perpendicular scale of the wave is significantly
  smaller than the parallel scale (e.g. wavelength), as established by
  CoMP, the wave develops as an oblique fast magneto-acoustic wave. In
  an unstructured medium, the energy of such a wave is transferred
  mainly across the magnetic field. However, it is possible to channel
  the energy of the transverse wave along the magnetic field in the
  presence of a field-aligned density enhancement. We conclude that the
  CoMP results provide an independent seismological proof that the corona
  is structured in density in the perpendicular direction.

Title: Magnetohydrodynamic seismology of solar and stellar coronae
Authors: Nakariakov, V. M.
Bibcode: 2008JPhCS.118a2038N
Altcode:
  The review presents the study of long period (longer than a few seconds)
  wave and oscillation phenomena in the corona of the Sun in radio,
  VL, EUV and X-ray bands, interpreted in terms of MHD theory. This
  study provides us with the observational foundation for the remote
  diagnostics of solar and stellar coronae with MHD waves. Kink and
  sausage magnetoacoustic modes and longitudinal (or acoustic) modes of
  coronal loops have already been confidently identified in the data. At
  present, the main research emphasis is put on the determination
  of parametric relations in the wave and oscillatory phenomena, the
  study of multi-modal events, and on the identification of MHD modes
  in quasi-periodic pulsations of solar and stellar flares.

Title: Forward Modeling of EIS Spectroscopic Data Emitted by a
    Vertically Oscillating Loop
Authors: Doorsselaere, T. V.; Nakariakov, V. M.
Bibcode: 2008ASPC..397...58D
Altcode:
  This paper focuses on forward modeling of spectroscopic data, as would
  be emitted by a coronal loop oscillating in the vertical fast kink
  mode. The Doppler shift, broadening and deformation of the observed
  spectral lines are investigated. It is concluded that the vertical
  kink mode causes strong line broadening, either inside the loop or
  at the edges of the loop, depending on the time integration and the
  oscillatory phase observed.

Title: Detection of Waves in the Solar Corona: Kink or Alfvén?
Authors: van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.;
   Young, P. R.
Bibcode: 2008ESPM...12.2.81V
Altcode:
  Last year, Tomczyk et al. (2007) have conclusively proven that low
  amplitude (1km/s) waves are ubiquitously present in the corona. A few
  months later, this was followed up with the discovery that chromospheric
  spicules carry tremendous wave power (De Pontieu et al., 2007). These
  new developments form a basis for coronal seismological magnetic field
  mapping of the corona and chromosphere. However, this must be based
  upon a confident identification of the observed wave mode. <P />Using
  basic MHD wave theory, we demonstrate that the only way to interpret
  the observed wave motions is in terms of fast magnetoacoustic kink
  waves. We prove that the interpretation in terms of Alfven waves,
  as was put forward in the original articles, does not explain the
  observed phenomenology, while the fast magnetoacoustic kink waves
  reproduce the required observational constraints. <P />To underline the
  potential of magnetic field mapping, we discuss a recent observation
  of coronal loop kink oscillations observed with Hinode/EIS. Because
  of its spectroscopic and imaging capabilities, we are able to measure
  the loop density, simultaneously with the loop length. This allows us
  to determine the magnetic field with unprecedented accuracy.

Title: Multi-wavelength spatially resolved analysis of quasi-periodic
    pulsations in a solar flare
Authors: Inglis, A. R.; Nakariakov, V. M.; Melnikov, V. F.
Bibcode: 2008A&A...487.1147I
Altcode:
  Aims: We aim to perform a spatially resolved analysis of a
  quasi-periodic pulsation event from 8th May 1998 using microwave data
  from the Nobeyama Radioheliograph and Radiopolarimeter, and X-ray
  data from the Yohkoh satellite. <BR />Methods: Time spectra of the
  signals integrated over the emission source are constructed with the
  use of the Lomb-Scargle periodogram method, revealing the presence of
  a pronounced 16 s periodicity. The Pixon image reconstruction algorithm
  and Hanaoka algorithm are used to reconstruct images from the hard X-ray
  data from Yohkoh/HXT and Nobeyama Radioheliograph respectively. The
  phase relationship of the microwave emission was analysed with the
  use of cross-correlation techniques. <BR />Results: The flaring loop
  was resolved in the microwave band. The hard X-ray sources are found
  to be located near the footpoint and at the loop apex determined by
  the soft X-ray image. The apex source is much fainter than footpoint
  one. In microwave, all parts of the loop are seen to oscillate with the
  same period and almost in phase. It was not possible to determine the
  spatial structure of the oscillation in the hard X-ray band. The period
  and the coherent spatial structure of the oscillation are indicative
  of the presence of either an MHD sausage mode or a periodic regime of
  magnetic reconnection.

Title: Coronal magnetic field measurement using loop oscillations
    observed by Hinode/EIS
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Young, P. R.;
   Verwichte, E.
Bibcode: 2008A&A...487L..17V
Altcode:
  We report the first spectroscopic detection of a kink MHD oscillation
  of a solar coronal structure by the Extreme-Ultraviolet Imaging
  Spectrometer (EIS) on the Japanese Hinode satellite. The detected
  oscillation has an amplitude of 1 km s<SUP>-1</SUP> in the Doppler
  shift of the FeXII 195 Å spectral line (1.3~MK), and a period of
  296~s. The unique combination of EIS's spectroscopic and imaging
  abilities enables us to measure simultaneously the mass density and
  length of the oscillating loop. This enables us to measure directly
  the magnitude of the local magnetic field, the fundamental coronal
  plasma parameter, as 39 ± 8~G, with unprecedented accuracy. This
  proof of concept makes EIS an exclusive instrument for the full scale
  implementation of the MHD coronal seismological technique.

Title: Coronal loop seismology using multiple transverse loop
    oscillation harmonics
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
Bibcode: 2008IAUS..247..140V
Altcode: 2007IAUS..247..140V
  TRACE observations (23/11/1998 06:35:57-06:48:43UT) in the 171 Å
  bandpass of an active region are studied. Coronal loop oscillations are
  observed after a violent disruption of the equilibrium. The oscillation
  properties are studied to give seismological estimates of physical
  quantities, such as the density scale height. A loop segment is traced
  during the oscillation, and the resulting time series is analysed
  for periodicities. In the loop segment displacement, two periods are
  found: 435.6±4.5 s and 242.7±6.4 s, consistent with the periods
  of the fundamental and 2nd harmonic fast kink oscillation. The small
  uncertainties allow us to estimate the density scale height in the loop
  to be 109 Mm, which is about double the estimated hydrostatical value
  of 50 Mm. The eigenfunction is used to do spatial coronal seismology,
  but that method does not give any conclusive results.

Title: The Pixelised Wavelet Filtering Method to Study Waves and
    Oscillations in Time Sequences of Solar Atmospheric Images
Authors: Sych, R. A.; Nakariakov, V. M.
Bibcode: 2008SoPh..248..395S
Altcode:
  Pixelised wavelet filtering (PWF) for the determination of the spatial,
  temporal, and phase structure of oscillation sources in temporal
  sequences of 2D images, based upon the continuous wavelet transform,
  has been designed and tested. The PWF method allows us to obtain
  information about the presence of propagating and nonpropagating waves
  in the data and localise them precisely in time and in space. The
  method is tested on the data sets obtained in microwaves with the
  Nobeyama Radioheliograph and in the EUV with TRACE. The method reveals
  fine spatial structuring of the sources of 3-, 5-, and 15-minute
  periodicities in the microwave and EUV emission generated in sunspot
  atmospheres. In addition, the PWF method provides us with unique
  information about the temporal variability of the power, amplitude,
  and phase narrowband maps of the observed oscillations and waves. The
  applicability of the method to the analysis of coronal wave phenomena
  is discussed.

Title: Detection of Waves in the Solar Corona: Kink or Alfvén?
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
Bibcode: 2008ApJ...676L..73V
Altcode:
  Recently, the omnipresence of waves has been discovered in the corona
  using the CoMP instrument. We demonstrate that the observational
  findings can be explained in terms of guided kink magnetoacoustic
  modes. The interpretation of the observations in terms of Alfvén waves
  is shown to be inconsistent with MHD wave theory. The implications of
  the interpretation in terms of kink waves are discussed.

Title: Interactive remote data processing using Pixelize Wavelet
    Filtration (PWF-method) and PeriodMap analysis
Authors: Sych, Robert; Nakariakov, Valery; Anfinogentov, Sergey
Bibcode: 2008cosp...37.3106S
Altcode: 2008cosp.meet.3106S
  Wavelet analysis is suitable for investigating waves and oscillating in
  solar atmosphere, which are limited in both time and frequency. We have
  developed an algorithms to detect this waves by use the Pixelize Wavelet
  Filtration (PWF-method). This method allows to obtain information
  about the presence of propagating and non-propagating waves in the
  data observation (cube images), and localize them precisely in time as
  well in space. We tested the algorithm and found that the results of
  coronal waves detection are consistent with those obtained by visual
  inspection. For fast exploration of the data cube, in addition,
  we applied early-developed Period- Map analysis. This method based
  on the Fast Fourier Transform and allows on initial stage quickly
  to look for "hot" regions with the peak harmonic oscillations and
  determine spatial distribution at the significant harmonics. We
  propose the detection procedure of coronal waves separate on two
  parts: at the first part, we apply the PeriodMap analysis (fast
  preparation) and than, at the second part, use information about
  spatial distribution of oscillation sources to apply the PWF-method
  (slow preparation). There are two possible algorithms working with
  the data: in automatic and hands-on operation mode. Firstly we use
  multiply PWF analysis as a preparation narrowband maps at frequency
  subbands multiply two and/or harmonic PWF analysis for separate
  harmonics in a spectrum. Secondly we manually select necessary spectral
  subband and temporal interval and than construct narrowband maps. For
  practical implementation of the proposed methods, we have developed
  the remote data processing system at Institute of Solar-Terrestrial
  Physics, Irkutsk. The system based on the data processing server -
  http://pwf.iszf.irk.ru. The main aim of this resource is calculation
  in remote access through the local and/or global network (Internet)
  narrowband maps of wave's sources both in whole spectral band and at
  significant harmonics. In addition, we can obtain temporal dynamics
  (mpeg- files) of the main oscillation characteristics: amplitude,
  power and phase as a spatial-temporal coordinates. For periodogram
  mapping of data cubes as a method for the pre-analysis, we developed
  preparation of the color maps where the pixel's colour corresponds
  to the frequency of the power spectrum maximum. The computer system
  based on applications ION-scripts, algorithmic languages IDL and PHP,
  and Apache WEB server. The IDL ION-scripts use for preparation and
  configuration of network requests at the central data server with
  subsequent connection to IDL run-unit software and graphic output on
  FTP-server and screen. Web page is constructed using PHP language.

Title: Sausage Oscillations in Multishell Coronal Structures
Authors: Pascoe, D. J.; Nakariakov, V. M.; Arber, T. D.
Bibcode: 2007SoPh..246..165P
Altcode:
  The effect of fine multilayered structuring on the resonant periods of
  global sausage (m=0), fast magnetoacoustic oscillations of coronal loops
  is studied for the straight magnetic slab model filled with a zero-β
  plasma. The slab is considered to have a one-dimensional inhomogeneity
  of the Alfvén speed in the direction perpendicular to the axis of
  the slab, which coincides with the equilibrium, straight, magnetic
  field. This magnetic configuration models an oscillating coronal
  loop that is formed by a bundle of field-aligned, smooth, concentric,
  circular shells of variable density and width. The number of the layers
  (or shells), along with their density, width, and radial steepness,
  are randomly distributed. It is shown that the resonant properties of
  long-wavelength sausage standing modes of the slab are not sensitive
  to the details of fine structuring. It is concluded that the coronal
  seismological technique for the determination of the magnetic field
  strength by the period of global sausage oscillations is not sensitive
  to the fine multilayered or multishell structuring.

Title: Preface: A Topical Issue in Honor of Professor Bernard Roberts
Authors: Ballester, J. L.; Erdélyi, R.; Hood, A. W.; Leibacher,
   J. W.; Nakariakov, V. M.
Bibcode: 2007SoPh..246....1B
Altcode:
  No abstract at ADS

Title: Coronal loop seismology using multiple transverse loop
    oscillation harmonics
Authors: Van Doorsselaere, T.; Nakariakov, V. M.; Verwichte, E.
Bibcode: 2007A&A...473..959V
Altcode:
  Context: TRACE observations (23/11/1998 06:35:57-06:48:43 UT) in the 171
  Å bandpass of an active region are studied. Coronal loop oscillations
  are observed after a violent disruption of the equilibrium. <BR />Aims:
  The oscillation properties are studied to give seismological estimates
  of physical quantities, such as the density scale height. <BR />Methods:
  A loop segment is traced during the oscillation, and the resulting
  time series is analysed for periodicities. <BR />Results: In the
  loop segment displacement, two periods are found: 435.6 ± 4.5~s and
  242.7 ± 6.4~s, consistent with the periods of the fundamental and
  2nd harmonic fast kink oscillation. The small uncertainties allow us
  to estimate the density scale height in the loop to be 109~Mm, which
  is about double the estimated hydrostatical value of 50~Mm. Because
  a loop segment is traced, the amplitude dependence along the loop is
  found for each of these oscillations. The obtained spatial information
  is used as a seismological tool to give details about the geometry of
  the observed loop.

Title: Solar-B as a Tool for Coronal Wave Studies
Authors: Nakariakov, V. M.
Bibcode: 2007ASPC..369..221N
Altcode:
  Direct observational evidence of wave and oscillatory activity of the
  solar corona is abundant in all observational bands and includes recent
  discoveries of propagating compressible waves in polar plumes and near
  loop footpoints, flare-generated transverse oscillations of loops,
  and longitudinal and sausage standing oscillations within loops. These
  phenomena are successfully interpreted in terms of magnetohydrodynamic
  (MHD) waves. This observational breakthrough gave rise to the rapid
  development of a new method for the remote diagnostics of the coronal
  plasma, MHD coronal seismology, allowing for estimation of the absolute
  value of the magnetic field in coronal loops, Alfvén speeds, transport
  coefficients, fine structuring, heating function and other important
  coronal parameters. EIS and XRT instruments are excellent tools for
  the further development of coronal wave studies and especially flare
  generated oscillations and waves. We present the recent findings,
  theoretical estimations of the observability of different MHD modes
  with Solar-B instruments, theory-based observational and data analysis
  strategies and some ideas about the utilisation of the results for
  coronal seismology. The expected results are of particular interest
  for the revealing of coronal heating mechanisms.

Title: Coronal dynamics
Authors: Nakariakov, V. M.
Bibcode: 2007AIPC..919..214N
Altcode:
  The lectures present the foundation of solar coronal physics with
  the main emphasis on the MHD theory and on wave and oscillatory
  phenomena. We discuss major challenges of the modern coronal physics;
  the main plasma structures observed in the corona and the conditions for
  their equilibrium; phenomenology of large scale long period oscillatory
  coronal phenomena and their theoretical modelling as MHD waves. The
  possibility of the remote diagnostics of coronal plasmas with the use
  of MHD oscillations is demonstrated.

Title: Quasi-periodic oscillations associated with solar flares
Authors: Nakariakov, V. M.
Bibcode: 2007AGUSMSH22A..06N
Altcode:
  Mechanisms for the generation of quasi-periodic pulsations (QPP) in
  microwave, X-ray and visible light emission generated by solar and
  stellar flares, based upon magnetohydrodynamic (MHD) oscillations, are
  discussed. QPP can be produced by internal MHD oscillations of flaring
  magnetic structures, in particular by kink, sausage and longitudinal
  modes. Different modes have different observational signatures in
  different emission bands, allowing for the correct identification
  of the mode. Another mechanism is based upon periodic triggering of
  flaring energy releases by oscillations in a non-flaring loop situated
  nearby the flaring region. The use of observable parameters of QPP for
  remote diagnostics of coronal plasmas, in particular of the magnetic
  field in the flaring regions is demonstrated.

Title: Coronal Periodmaps
Authors: Nakariakov, V. M.; King, D. B.
Bibcode: 2007SoPh..241..397N
Altcode:
  The search for signatures of wave and oscillatory processes in the solar
  corona in the data obtained with imaging instruments can be automated by
  using the periodmap method. The method reduces a three-dimensional data
  cube to a two-dimensional map of the analysed field of view. The map
  reveals the presence and distribution of the most pronounced frequencies
  in the power spectrum of the time signal recorded at spatial pixels. We
  demonstrate the applicability of this method as a pre-analysis tool
  with the use of TRACE EUV coronal data, which contain examples of
  transverse and longitudinal oscillations of coronal loops. The main
  advantage of using periodmaps over other possible (more sophisticated)
  pre-analysis tools, such as wavelet analysis, is their robustness and
  efficiency (both in speed and computational power). The method can be
  implemented in the Hinode/XRT and SDO/AIA data pre-analysis.

Title: Sausage oscillations of coronal loops
Authors: Pascoe, D. J.; Nakariakov, V. M.; Arber, T. D.
Bibcode: 2007A&A...461.1149P
Altcode:
  Aims:Analytical theory predicts the existence of trapped global (or
  fundamental) sausage fast magnetoacoustic modes in thick and dense
  coronal loops only, with the periods estimated as the ratio of double
  the loop length and the Alfvén speed outside the loop. We extend
  this study to the leaking regime, considering global sausage modes of
  long loops with small density contrasts. <BR />Methods: Anti-symmetric
  fast magnetoacoustic perturbations (sausage, or m=0 modes) of a low β
  plasma slab with the symmetric Epstein profile of plasma density are
  modelled numerically. <BR />Results: It was found that long loops with
  sufficiently small density contrast can support global sausage leaky
  modes of detectable quality. The periods of the leaky modes are found to
  be approximately determined by the loop length and the external Alfvén
  speed. If the loop length can be estimated from imaging observations,
  the observed period of this mode provides us with the information about
  the Alfvén speed outside the loop. For typical flaring coronal loops,
  the estimated periods of the global sausage modes are about 5-60 s.

Title: A Magnetometer For The Solar Orbiter Mission
Authors: Carr, C. M.; Horbury, T. S.; Balogh, A.; Baumjohann, W.;
   Bavassano, B.; Breen, A.; Burgess, D.; Cargill, P. J.; Brooker, N.;
   Erdõs, G.; Fletcher, L.; Forsyth, R. J.; Giacalone, J.; Glassmeier,
   K. -H.; Hoeksema, J. T.; Goldstein, M. L.; Lockwood, M.; Magnes, W.;
   Masimovic, M.; Marsch, G.; Matthaeus, W. H.; Murphy, N.; Nakariakov,
   V. M.; Pacheco, J. R.; Pincon, J. -L.; Riley, P.; Russell, C. T.;
   Schwartz, S. J.; Szabo, A.; Thompson, M.; Vainio, R.; Velli, M.;
   Vennerstrom, S.; Walsh, R.; Wimmer-Schweingruber, R.; Zank, G.
Bibcode: 2007ESASP.641E..41C
Altcode:
  The magnetometer is a key instrument to the Solar Orbiter mission. The
  magnetic field is a fundamental parameter in any plasma: a precise
  and accurate measurement of the field is essential for understanding
  almost all aspects of plasma dynamics such as shocks and stream-stream
  interactions. Many of Solar Orbiter's mission goals are focussed
  around the links between the Sun and space. A combination of in situ
  measurements by the magnetometer, remote measurements of solar magnetic
  fields and global modelling is required to determine this link and
  hence how the Sun affects interplanetary space. The magnetic field
  is typically one of the most precisely measured plasma parameters and
  is therefore the most commonly used measurement for studies of waves,
  turbulence and other small scale phenomena. It is also related to the
  coronal magnetic field which cannot be measured directly. Accurate
  knowledge of the magnetic field is essential for the calculation of
  fundamental plasma parameters such as the plasma beta, Alfvén speed
  and gyroperiod. We describe here the objectives and context of magnetic
  field measurements on Solar Orbiter and an instrument that fulfils those
  objectives as defined by the scientific requirements for the mission.

Title: Quasi-periodic Pulsations as a Diagnostic Tool for Coronal
    Plasma Parameters
Authors: Nakariakov, V. M.; Stepanov, A. V.
Bibcode: 2007LNP...725..221N
Altcode:
  Quasi-periodic pulsations of radio emission generated in solar and
  stellar flares are interpreted in terms of MHD oscillations of coronal
  loops and in the frame of equivalent electric circuit approach,
  and compared with coronal wave and oscillatory phenomena recently
  discovered in the EUV, X-ray and visible light bands. Various methods
  of remote diagnostics of the coronal plasma, based upon the use of
  observationally detectable properties of quasi-periodic pulsations -
  their period, amplitude and quality - are discussed. The applicability
  of these methods to the diagnostics of stellar coronae is demonstrated.

Title: MHD oscillations in solar and stellar coronae: Current results
    and perspectives
Authors: Nakariakov, Valery M.
Bibcode: 2007AdSpR..39.1804N
Altcode:
  Wave and oscillatory activity is observed with modern imaging and
  spectral instruments in the visible light, EUV, X-ray and radio
  bands in all parts of the solar corona. Magnetohydrodynamic (MHD)
  wave theory gives satisfactory interpretation of these phenomena
  in terms of MHD modes of coronal structures. The paper reviews the
  current trends in the observational study of coronal oscillations,
  recent development of theoretical modelling of MHD wave interaction with
  plasma structures, and implementation of the theoretical results for the
  mode identification. Also the use of MHD waves for remote diagnostics of
  coronal plasmas is discussed. In particular, the applicability of this
  method to the estimation of the coronal magnetic field is demonstrated.

Title: MHD-Oscillation Modes of a Flaring Loop Using Microwave
    Observations With High Spatial Resolution
Authors: Reznikova, Veronika E.; Melnikov, Victor F.; Nakariakov,
   Valery M.; Shibasaki, Kiyoto
Bibcode: 2006AIPC..848..133R
Altcode:
  Study of an oscillating loop observed with high spatial resolution
  (NoRH) shows the presence of two Fourier spectral peaks: P1 = 14-17
  s, which is more pronounced at the loop apex and P2 = 8-11 s, that is
  stronger at the loop legs. We found the phase shift, P2/4, between the
  pulsations at the northern leg and at the loop top for the P2 spectral
  component and a lack of the phase shift between different parts of the
  flare loop for the P1 component. The computed phase speeds and periods
  of MHD modes of a coronal loop confirm identification of the first
  periodicity P1 as the global (fundamental) sausage mode. The second
  periodicity can be associated with several modes: the second and the
  third longitudinal harmonics of the kink mode, the third harmonics of
  the sausage mode and the second harmonics of ballooning mode.

Title: Quasi-periodic modulation of solar and stellar flaring emission
    by magnetohydrodynamic oscillations in a nearby loop
Authors: Nakariakov, V. M.; Foullon, C.; Verwichte, E.; Young, N. P.
Bibcode: 2006A&A...452..343N
Altcode:
  We propose a new model for quasi-periodic modulation of solar
  and stellar flaring emission. Fast magnetoacoustic oscillations
  of a non-flaring loop can interact with a nearby flaring active
  region. This interaction occurs when part of the oscillation situated
  outside the loop reaches the regions of steep gradients in magnetic
  field within an active region and produces periodic variations of
  electric current density. The modulation depth of these variations is
  a few orders of magnitude greater than the amplitude of the driving
  oscillation. The variations of the current can induce current-driven
  plasma micro-instabilities and thus anomalous resistivity. This can
  periodically trigger magnetic reconnection, and hence acceleration
  of charged particles, producing quasi-periodic pulsations of X-ray,
  optical and radio emission at the arcade footpoints.

Title: Seismology of curved coronal loops with vertically polarised
    transverse oscillations
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
Bibcode: 2006A&A...452..615V
Altcode:
  Aims.Using a model of vertically polarised fast magnetoacoustic waves
  in curved coronal loops, the method of coronal seismology is applied
  to observations of transverse loop oscillations. <BR /> Methods: . A
  coronal loop is modeled as a curved magnetic slab in the zero plasma-β
  limit. For an arbitrary piece-wise continuous power law equilibrium
  density profile, the dispersion relation governing linear vertically
  polarised fast magnetoacoustic kink waves is derived. The ways in which
  this model can be used for coronal seismology are explored and applied
  to two observational examples. <BR /> Results: . The Alfvén speed and
  equilibrium density profile are determined from observations. It is
  shown that the mechanism of lateral leakage of fast magnetoacoustic
  kink oscillations described in this model is efficient. In fact, the
  damping is so efficient that in order to match predicted values with
  observational ones, either the loop needs to be highly contrasted or the
  transverse Alfvén speed profile needs to be close to linear. Possible
  improvements to make the modeling of lateral wave leakage in loops more
  realistic, allowing a lower damping efficiency, are discussed. <BR />

Title: Fast magnetoacoustic waves in curved coronal
    loops. II. Tunneling modes
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
Bibcode: 2006A&A...449..769V
Altcode:
  Aims. Fast magnetoacoustic waves in curved coronal loops are
  investigated and the role of lateral leakage in wave damping, which
  includes the mechanism of wave tunneling, is explored. Methods. A
  coronal loop is modeled as a curved, magnetic slab in the zero
  plasma-β limit. In this model and for an arbitrary piece-wise
  continuous power law equilibrium density profile, the wave equation
  governing linear vertically polarised fast magnetoacoustic waves is
  solved analytically. An associated dispersion relation is derived
  and the frequencies and eigenfunctions of the wave modes are
  characterised. Results. For some equilibria, the waves are shown
  to be all damped due to lateral leakage. It is demonstrated that
  waves either leak straight out into the external medium or have to
  overcome an evanescent barrier, which is linked to wave tunneling. The
  wave solutions consist of alternating vertically polarised kink
  and sausage branches. Fast kink oscillations may have a non-zero
  density perturbation when averaged across the loop. The calculated
  damping rate of fast magnetoacoustic kink oscillations is shown to be
  consistent with related numerical simulations and show that lateral
  leakage may explain the observed damping of (vertically polarised)
  fast magnetoacoustic kink oscillations.

Title: Modulation of gyrosynchrotron emission in solar and stellar
    flares by slow magnetoacoustic oscillations
Authors: Nakariakov, V. M.; Melnikov, V. F.
Bibcode: 2006A&A...446.1151N
Altcode:
  Gyrosynchrotron emission generated by non-thermal electrons in solar
  and stellar coronal flares can be efficiently modulated by slow
  magnetoacoustic oscillations in the flaring loops. The modulation
  mechanism is based upon perturbation of the efficiency in the Razin
  suppression of optically thin gyrosynchrotron emission. Modulation
  of the emission is in anti-phase with the density perturbation in the
  slow wave. The observed emission modulation depth can be up to an order
  of magnitude higher than the slow wave amplitude. This effect is more
  pronounced at lower frequencies. Observations with spatial resolution,
  together with analysis of the modulation frequency, are shown to be
  sufficient for providing the information needed to identify the mode.

Title: Magnetohydrodynamic waves in coronal polar plumes
Authors: Nakariakov, Valery M.
Bibcode: 2006RSPTA.364..473N
Altcode:
  No abstract at ADS

Title: Hydromagnetic surface waves on a tangential discontinuity
Authors: Joarder, P. S.; Nakariakov, V. M.
Bibcode: 2006GApFD.100...59J
Altcode:
  Propagation of hydromagnetic surface waves on an MHD tangential
  discontinuity, separating two uniform magnetic plasmas, across which
  the magnetic field changes both its direction and field-strength,
  is examined. The study is arbitrarily confined to discontinuities one
  side of which has a plasma-ß value larger than unity. A classification
  scheme to identify the ‘fast’ and the ‘slow’ surface waves is
  employed thus enabling us to find new features in the modal structure
  of these waves. In the case of sufficiently small values for the
  density ratio between the higher and the lower Alfvén speed regions
  on opposite sides of the interface, this study reveals the presence
  of a ‘forbidden region’ for propagation of fast surface waves with
  values of their propagation angle being smaller than a certain critical
  value. The study also reveals the existence of two distinct slow surface
  waves with angles of propagation larger than a certain critical value
  but for two different ranges of values for the angle between magnetic
  field vectors across the interface. While revealing such new structures
  of the surface modes for intermediate parameter values, the present
  investigation also confirms earlier results regarding the asymptotic
  behaviour of the modes in the incompressible limit.

Title: Fast magnetoacoustic waves in curved coronal loops
Authors: Verwichte, E.; Foullon, C.; Nakariakov, V. M.
Bibcode: 2006A&A...446.1139V
Altcode:
  A study of vertically polarised fast magnetoacoustic waves in a curved
  coronal loop is presented. The loop is modeled as a semi-circular
  magnetic slab in the zero plasma-β limit. The governing equations for
  linear waves are derived. We show that the wave mode behaviour depends
  on the slope of the equilibrium density profile, which is modeled as
  a piece-wise continuous power law curve of index α. For all profiles,
  except for α=-4, wave modes are not trapped in the loop and leak out
  into the external medium through wave tunneling. The particular case of
  α=-4, which corresponds to a linearly increasing Alfvén speed profile,
  is examined in more detail as this is the only model that can support
  trapped wave modes. We compare the results with a straight slab model
  and find similar behaviour. Coupling between sausage and kink wave
  modes has not been found in the model.

Title: A parametric study of the effect of pressure anisotropy on
    slow nonlinear acoustic waves in high-beta plasmas
Authors: Simon, B.; Nakariakov, V.; Rowlands, G.; Chapman, S.;
   Dendy, R.
Bibcode: 2006cosp...36.1041S
Altcode: 2006cosp.meet.1041S
  Solitary wave solutions arise naturally from one dimensional
  Hall-Magnetohydrodynamic MHD equations These solutions have been
  recently identified as models for slow-mode solitary structures observed
  in the magnetosheath and other magnetospheric boundary layers and as a
  model for magnetic holes MH which are localized depressions of magnetic
  field observed in the solar wind We extend the analysis of the model of
  Stasiewicz which is based on one dimesional Hall-MHD with an explicit
  pressure anisotropy Applying the methods of the Sagdeev potential and
  direct numerical modelling we performed parametric studies of highly
  oblique magnetoacoustic nonlinear waves in a high- beta ratio of kinetic
  to magnetic pressures plasmas We found that anisotropy is crucial for
  the existence of solitary structures in this model Furthermore if the
  perpendicular pressure is greater than the parallel pressure there
  can be both field enhancing bright and field depleting dark solitary
  structures whereas increased parallel pressure compared to perpendicular
  pressure only allows for dark solitary structures Combinations of the
  plasma parameters corresponding to the different regimes are determined

Title: Coronal waves and oscillations
Authors: Nakariakov, Valery M.
Bibcode: 2006IAUS..233..464N
Altcode:
  Wave and oscillatory activity of the solar corona is confidently
  observed with modern imaging and spectral instruments in the
  visible light, EUV, X-ray and radio bands, and interpreted in
  terms of magnetohydrodynamic (MHD) wave theory. The review reflects
  the current trends in the observational study of coronal waves and
  oscillations, theoretical modelling of interaction of MHD waves with
  plasma structures, and implementation of the theoretical results for the
  mode identification. Also the use of MHD waves for remote diagnostics
  of coronal plasma - MHD coronal seismology - is discussed.

Title: Current results and perspectives of coronal wave investigations
Authors: Nakariakov, V. M.
Bibcode: 2006cosp...36..794N
Altcode: 2006cosp.meet..794N
  Wave and oscillatory activity is observed with modern imaging and
  spectral instruments in the visible light EUV X-ray and radio bands
  in all parts of the solar corona in particular in active regions and
  polar plumes Magnetohydrodynamic MHD wave theory gives satisfactory
  interpretation of these phenomena in terms of MHD modes of coronal
  structures The talk reviews the current trends in the observational
  study of coronal waves and oscillations with spaceborne and ground-based
  facilities with the main emphasis on the standing kink and sausage modes
  and propagating fast wave recent development of theoretical modelling
  of interaction of MHD waves with plasma structures and implementation
  of the theoretical results for the mode identification The use of MHD
  waves for remote diagnostics of coronal plasma is discussed and the
  applicability of this method for the estimation of coronal magnetic
  field and fine structuring is demonstrated

Title: Diagnostics of Mhd-Oscillation Modes of a Flaring Loop Using
    Microwave Observations with High Spatial Resolution
Authors: Reznikova, V. E.; Nakariakov, V. M.; Melnikov, V. F.;
   Shibasaki, K.
Bibcode: 2005ESASP.600E.140R
Altcode: 2005ESPM...11..140R; 2005dysu.confE.140R
  No abstract at ADS

Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
Bibcode: 2005ESASP.600E.101V
Altcode: 2005dysu.confE.101V; 2005ESPM...11..101V
  No abstract at ADS

Title: X-Ray Quasi-Periodic Pulsations in Solar Flares as
    Magnetohydrodynamic Oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
Bibcode: 2005ESASP.600E..33F
Altcode: 2005ESPM...11...33F; 2005dysu.confE..33F
  No abstract at ADS

Title: Magnetoacoustic Solitons in High-β Plasmas
Authors: Simon, B.; Nakariakov, V. M.; Chapman, S. C.; Dendy, R. O.
Bibcode: 2005AGUFMSH53A1238S
Altcode:
  Solitary wave solutions arise naturally from one dimensional
  Hall-Magnetohydrodynamic equations. These solutions have been recently
  identified as models for slow-mode solitary structures observed in the
  magnetosheath and other magnetospheric boundary layers, and as a model
  for magnetic holes (MH), which are localized depressions of magnetic
  field observed in the solar wind. We perform a parametric study of
  highly oblique magnetoacoustic solitons in frames of Stasiewicz's
  analytical model [1], taking plasma pressure anisotropy into account. We
  study the stability of these structures using a 1.5D hybrid code. In
  particular, we focus on magnetic structures in a high-β (ratio of
  kinetic to magnetic pressures) plasmas. These plasma conditions are
  typical in the region around MHs, and the study of stability is vital in
  verifying the theory that MHs are manifestations of such solitons. Our
  simulations are aimed at studying linear magnetic holes, where there
  is almost no variation in direction of the ambient magnetic field
  direction. The majority of MHs discovered in space plasmas fall into
  this category. [1] Stasiewicz,K., Phys. Rev. Lett., 93, 125004, 2004

Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
Bibcode: 2005ESASP.596E..38V
Altcode: 2005ccmf.confE..38V
  No abstract at ADS

Title: Short Quasi-Periodic MHD Waves in Coronal Structures
Authors: Nakariakov, V. M.; Pascoe, D. J.; Arber, T. D.
Bibcode: 2005SSRv..121..115N
Altcode:
  The possibility of remote diagnostics of coronal structures with
  impulsively-generated short-period fast magnetoacoustic wave trains is
  demonstrated. An initially broad-band, aperiodic fast magnetoacoustic
  perturbation guided by a 1D plasma inhomogeneity develops into
  a quasi-periodic wave train with a well-pronounced frequency and
  amplitude modulation. The quasi-periodicity results from the geometrical
  dispersion of the modes, determined by the transverse profile of the
  loop, and hence contains information about the profile. Wavelet images
  of the wave train demonstrate that their typical spectral signature
  is of a “crazy tadpole’’ shape: a narrow spectrum tail precedes
  a broad-band head. The instantaneous period of the oscillations in
  the wave train decreases gradually with time, with a mean value of
  several seconds for typical coronal values. The period and the spectral
  amplitude evolution are determined by the steepness of the transverse
  density profile and the density contrast ratio in the loop, which offers
  a tool for estimation of the sub-resolution structuring of the corona.

Title: X-Ray Quasi-Periodic Pulsations in Solar Flares as
    Magnetohydrodynamic Oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
Bibcode: 2005ESASP.596E..46F
Altcode: 2005ccmf.confE..46F
  No abstract at ADS

Title: X-ray quasi-periodic pulsations in solar flares as
    magnetohydrodynamic oscillations
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.; Fletcher, L.
Bibcode: 2005A&A...440L..59F
Altcode:
  We report the first observation at high spatial resolution of
  long-period quasi-periodic pulsations (QPP) of X-ray radiation during
  solar flares, made possible with the Reuven Ramaty High Energy Solar
  Spectroscopic Imager (RHESSI), supported by complementary data at other
  wavelengths from space-based and ground-based telescopes. Evidence
  for the presence of a transequatorial loop possibly responsible for
  the detected periodicity connected with its kink mode is found. Our
  findings suggest that QPP can be interpreted as a periodic pumping
  of electrons in a compact flaring loop, modulated by oscillations
  in a magnetically linked and larger loop acting as a long-period
  magnetohydrodynamic resonator.

Title: High-frequency Alfvén waves in multi-ion coronal plasma:
    Observational implications
Authors: Ofman, L.; Davila, J. M.; Nakariakov, V. M.; ViñAs, A. -F.
Bibcode: 2005JGRA..110.9102O
Altcode: 2005JGRA..11009102O
  We investigate the effects of high-frequency (of order ion
  gyrofrequency) Alfvén and ion-cyclotron waves on ion emission lines
  by studying the dispersion of these waves in a multi-ion coronal
  plasma. For this purpose we solve the dispersion relation of the
  linearized multifluid and Vlasov equations in a magnetized multi-ion
  plasma with coronal abundances of heavy ions. We also calculate the
  dispersion relation using nonlinear one-dimensional hybrid kinetic
  simulations of the multi-ion plasma. When heavy ions are present the
  dispersion relation of parallel propagating Alfvén cyclotron waves
  exhibits the following branches (in the positive Ω - k quadrant):
  right-hand polarized nonresonant and left-hand polarized resonant
  branch for protons and each ion. We calculate the ratio of ion to
  proton velocities perpendicular to the direction of the magnetic field
  for each wave modes for typical coronal parameters and find strong
  enhancement of the heavy ion perpendicular fluid velocity compared
  with proton perpendicular fluid velocity. The linear multifluid cold
  plasma results agree with linear warm plasma Vlasov results and with
  the nonlinear hybrid simulation model results. In view of our findings
  we discuss how the observed nonthermal line broadening of minor ions
  in coronal holes may relate to the high-frequency wave motions.

Title: Spatially resolved microwave pulsations of a flare loop
Authors: Melnikov, V. F.; Reznikova, V. E.; Shibasaki, K.; Nakariakov,
   V. M.
Bibcode: 2005A&A...439..727M
Altcode:
  A microwave burst with quasi-periodic pulsations was studied with high
  spatial resolution using observations with the Nobeyama Radioheliograph
  (NoRH). We found that the time profiles of the microwave emission at
  17 and 34 GHz exhibit quasi-periodic (with two well defined periods
  P_1= 14-17 s and P_2= 8-11 s) variations of the intensity at different
  parts of an observed flaring loop. Detailed Fourier analysis shows the
  P<SUB>1</SUB> spectral component to be dominant at the top, while the
  P<SUB>2</SUB> one near the feet of the loop. The 14-17 s pulsations
  are synchronous at the top and in both legs of the loop. The 8-11 s
  pulsations at the legs are well correlated with each other but the
  correlation is not so obvious with the pulsations at the loop top. For
  this P<SUB>2</SUB> spectral component, a definite phase shift, P_2/4≈
  2.2~s, between pulsations in the northern leg and loop top parts of
  the loop have been found. The length of the flaring loop is estimated
  as L = 25 Mm (≈ 34 arcsec) and its average width at half intensity
  at 34 GHz as about 6 Mm (≈ 8 arcsec). Microwave diagnostics shows
  the loop to be filled with a dense plasma with the number density
  n<SUB>0</SUB> ≈ 10<SUP>11</SUP> cm<SUP>-3</SUP>, penetrated by the
  magnetic field changing from B<SUB>0</SUB> ≈ 100 G near the loop top
  up to B<SUB>0</SUB> ≈ 200 G near the north footpoint. A comparative
  analysis of different MHD modes of the loop demonstrates the possibility
  of the simultaneous existence of two modes of oscillations in the loop:
  the global sausage mode, with the period P_1= 14-17 s and the nodes at
  the footpoints, and a higher harmonics mode (possibly with the radial
  wave number l&gt;1), with P_2= 8-11 s.

Title: Acoustic Oscillations in Solar and Stellar Flaring Loops
Authors: Kelly, A.; Nakariakov, V. M.; Arber, T. D.
Bibcode: 2005PADEU..15..125K
Altcode:
  Observations of flaring loops in radio, visible and x-ray bands
  show quasi-periodic pulsations with periods from a few seconds
  to several minutes. Recent numerical studies have shown that
  some of these oscillations can be interpreted as standing slow
  magnetoacoustic waves. Energy deposition from the flare excites the
  second standing harmonic, with a period determined by the temperature
  and loop length. The excited longitudinal oscillations can be
  practically dissipationless and can, possibly, be considered MHD
  autowaves. Numerical simulations with a wide range of flare durations
  and choices of heat deposition location show that the second harmonic
  is a common feature of flaring loops.

Title: Coronal Waves and Oscillations
Authors: Nakariakov, Valery M.; Verwichte, Erwin
Bibcode: 2005LRSP....2....3N
Altcode:
  Wave and oscillatory activity of the solar corona is confidently
  observed with modern imaging and spectral instruments in the
  visible light, EUV, X-ray and radio bands, and interpreted in terms
  of magnetohydrodynamic (MHD) wave theory. The review reflects
  the current trends in the observational study of coronal waves
  and oscillations (standing kink, sausage and longitudinal modes,
  propagating slow waves and fast wave trains, the search for torsional
  waves), theoretical modelling of interaction of MHD waves with plasma
  structures, and implementation of the theoretical results for the mode
  identification. Also the use of MHD waves for remote diagnostics of
  coronal plasma — MHD coronal seismology — is discussed and the
  applicability of this method for the estimation of coronal magnetic
  field, transport coefficients, fine structuring and heating function
  is demonstrated.

Title: Transverse waves in a post-flare supra-arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
Bibcode: 2005A&A...430L..65V
Altcode:
  Observations of propagating transverse waves in an open magnetic field
  structure with the Transition Region And Coronal Explorer (TRACE)
  are presented. Waves associated with dark tadpole-like sunward
  moving structures in the post-flare supra-arcade of NOAA active
  region 9906 on the 21st of April 2002 are analysed. They are seen as
  quasi-periodic transverse displacements of the dark tadpole tails, with
  periods in the range of 90-220 s. Their phase speeds and displacement
  amplitudes decrease as they propagate sunwards. At heights of 90 and
  60 Mm above the post-flare loop footpoints the phase speeds are in
  the ranges 200-700 km s<SUP>-1</SUP> and 90-200 km s<SUP>-1</SUP>
  respectively. Furthermore, for consecutive tadpoles the phase speeds
  decrease and periods increase as a function of time. The waves are
  interpreted as propagating fast magnetoacoustic kink waves guided by
  a vertical, evolving, open structure.

Title: Detection of Ultra-Long Oscillations in an EUV Filament
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
Bibcode: 2004ESASP.575..394F
Altcode: 2004soho...15..394F
  No abstract at ADS

Title: Transverse Oscillations in a Coronal Loop Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Ofman, L.; Deluca, E. E.
Bibcode: 2004ESASP.575..460V
Altcode: 2004soho...15..460V
  No abstract at ADS

Title: Coronal Waves: Coronal Heating and Coronal Seismology
Authors: Nakariakov, V. M.
Bibcode: 2004ASPC..325..253N
Altcode:
  SOHO and TRACE missions have demonstrated that wave phenomena are
  abundant in the corona and provided us with an observational basis for
  MHD coronal seismology. This review covers the recent observational
  finding of coronal wave phenomena and their theoretical models. The
  relevance of these observational and theoretical efforts to the coronal
  heating is discussed. Possible observability of these phenomena with
  EIS and XRT instruments is discussed.

Title: Transverse Waves in a Post-Flare Supra-Arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Cooper, F. C.
Bibcode: 2004ESASP.575..126V
Altcode: 2004soho...15..126V
  No abstract at ADS

Title: The Nature and Excitation Mechanisms of Acoustic Oscillations
    in Solar and Stellar Coronal Loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden,
   M. J.
Bibcode: 2004ESASP.575..114T
Altcode: 2004soho...15..114T; 2004astro.ph..9556T
  In the recent work of Nakariakov et al. (2004), it has been shown
  that the time dependences of density and velocity in a flaring loop
  contain pronounced quasi-harmonic oscillations associated with the
  2nd harmonic of a standing slow magnetoacoustic wave. That model
  used a symmetric heating function (heat deposition was strictly at
  the apex). This left outstanding questions: A) is the generation of
  the 2nd harmonic a consequence of the fact that the heating function
  was symmetric? B) Would the generation of these oscillations occur if
  we break symmetry? C) What is the spectrum of these oscillations? Is
  it consistent with a 2nd spatial harmonic? The present work (and
  partly Tsiklauri et al. (2004b)) attempts to answer these important
  outstanding questions. Namely, we investigate the physical nature
  of these oscillations in greater detail: we study their spectrum
  (using periodogram technique) and how heat positioning affects the
  mode excitation. We found that excitation of such oscillations is
  practically independent of location of the heat deposition in the
  loop. Because of the change of the background temperature and density,
  the phase shift between the density and velocity perturbations is
  not exactly a quarter of the period, it varies along the loop and is
  time dependent, especially in the case of one footpoint (asymmetric)
  heating. We also were able to model successfully SUMER oscillations
  observed in hot coronal loops.

Title: Detection of ultra-long-period oscillations in an EUV filament
Authors: Foullon, C.; Verwichte, E.; Nakariakov, V. M.
Bibcode: 2004A&A...427L...5F
Altcode:
  We report the first detection of long-period (8-27 h) oscillatory
  intensity variations in a coronal filament. The filament is observed
  continuously as it crosses the solar disk in a 12-min-cadence SoHO/EIT
  195 Å, uninterrupted data set. Cyclic intensity variations are found to
  be correlated along the filament, while the most pronounced oscillations
  are detected at its southern end for nearly 6 days. The dominant period
  of these oscillations is 12.1 h and the amplitude of the intensity
  variations reaches approximately 10% of the background intensity. The
  ultra-long-period oscillations may be interpreted in terms of slow
  string MHD modes or may be connected with thermal over-stability
  associated with peculiarities of the cooling/heating function and with
  the effect of neutrals. These theoretical predictions however do not
  explain the spatial structure of the oscillations along the filament.

Title: Meeting report: Waves in solar and magnetospheric plasmas
Authors: Nakariakov, V. M.; Roberts, B.; Wright, A. N.
Bibcode: 2004A&G....45e..32N
Altcode:
  This RAS discussion meeting was arranged with the idea of bringing
  together solar physicists and magnetospheric physicists, so as
  to understand better how the two disciplines approach waves, and
  ultimately to learn from each other. V M Nakariakov, B Roberts and A
  N Wright report.

Title: CDS wide slit time-series of EUV coronal bright points
Authors: Ugarte-Urra, I.; Doyle, J. G.; Nakariakov, V. M.; Foley, C. R.
Bibcode: 2004A&A...425.1083U
Altcode:
  Wide slit (90<SUP>''</SUP>×240<SUP>''</SUP>) movies of four Extreme
  Ultraviolet coronal bright points (BPs) obtained with the Coronal
  Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric
  Observatory (SoHO) have been inspected. The wavelet analysis of the He
  I 584.34 Å, O V 629.73 Å and Mg VII/IX 368 Å time-series confirms
  the oscillating nature of the BPs, with periods ranging between
  600 and 1100 s. In one case we detect periods as short as 236 s. We
  suggest that these oscillations are the same as those seen in the
  chromospheric network and that a fraction of the network bright points
  are most likely the cool footpoints of the loops comprising coronal
  bright points. These oscillations are interpreted in terms of global
  acoustic modes of the closed magnetic structures associated with BPs.

Title: Characteristics of transverse oscillations in a coronal
    loop arcade
Authors: Verwichte, E.; Nakariakov, V. M.; Ofman, L.; Deluca, E. E.
Bibcode: 2004SoPh..223...77V
Altcode:
  TRACE observations from 15 April 2001 of transverse oscillations in
  coronal loops of a post-flare loop arcade are investigated. They
  are considered to be standing fast kink oscillations. Oscillation
  signatures such as displacement amplitude, period, phase and damping
  time are deduced from 9 loops as a function of distance along the loop
  length. Multiple oscillation modes are found with different amplitude
  profile along the loop length, suggesting the presence of a second
  harmonic. The damping times are consistent with the hypothesis of
  phase mixing and resonant absorption, although there is a clear bias
  towards longer damping times compared with previous studies. The
  coronal magnetic field strength and coronal shear viscosity in the
  loop arcade are derived.

Title: Coronal seismology: Seismology of the corona of the Sun
Authors: Nakariakov, V. M.; Verwichte, E.
Bibcode: 2004A&G....45d..26N
Altcode:
  Seismology now includes study of the Sun's corona, a promising research
  target both in its own right and for its role in the relationship
  between the Sun and the Earth and its links to the solar magnetic
  field. In addition, the corona, as a natural plasma, is itself an
  objective for fundamental physics. Observations using spacecraft such
  as SOHO and TRACE highlight the interrelationship between the magnetic
  field and corona.

Title: Astrophysical Implications of Magnetoacoustic Autowaves in
    Thermally Unstable Plasmas
Authors: Kelly, A.; Nakariakov, V. M.
Bibcode: 2004plph.confE...1K
Altcode:
  The formation and evolution of magnetoacoustic autowaves in thermally
  unstable plasmas is discussed, with particular reference to solar
  and stellar flares. The competition of thermal overstability, thermal
  conductivity and radiative losses leads to self-organisation of weakly
  nonlinear magnetoacoustic perturbations into dissipative structures with
  the parameters independent of the initial perturbation. This evolution
  is modelled in terms of the extended Burgers equation. The properties
  of the autowaves are expressed through the plasma parameters such
  as the magnetic field, plasma temperature and density. The autowaves
  are saw-tooth shaped waves, with an eventual amplitude that depends
  upon the balance between the amplification due to radiative losses and
  decay due to thermal conductivity. Increased dissipation due to thermal
  conductivity or stronger non-linearity leads to a lower amplitude, while
  stronger thermal instability leads to a higher final amplitude. X-ray,
  white light and radio band observations and numerical simulations of
  flaring loops show quasi-periodic pulsations during the flares. These
  pulsations are standing acoustic waves (primarily the second harmonic)
  and show little decay over a number of periods despite the large
  theoretically predicted damping due to thermal conductivity. It is
  suggested that these waves are magnetoacoustic autowaves.

Title: Flare-generated acoustic oscillations in solar and stellar
    coronal loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden,
   M. J.
Bibcode: 2004A&A...422..351T
Altcode: 2004astro.ph..2261T
  Long period longitudinal oscillations of a flaring coronal loop are
  studied numerically. In the recent work of Nakariakov et al. (2004)
  it has been shown that the time dependence of density and velocity in a
  flaring loop contain pronounced quasi-harmonic oscillations associated
  with the 2nd harmonic of a standing slow magnetoacoustic wave. In
  this work we investigate the physical nature of these oscillations
  in greater detail, namely, their spectrum (using the periodogram
  technique) and how heat positioning affects mode excitation. We found
  that excitation of such oscillations is practically independent of the
  location of the heat deposition in the loop. Because of the change of
  the background temperature and density, the phase shift between the
  density and velocity perturbations is not exactly a quarter of the
  period; it varies along the loop and is time dependent, especially in
  the case of one footpoint (asymmetric) heating.

Title: Radiative hydrodynamic modeling of the Bastille-Day flare
    (14 July, 2000). I. Numerical simulations
Authors: Tsiklauri, D.; Aschwanden, M. J.; Nakariakov, V. M.; Arber,
   T. D.
Bibcode: 2004A&A...419.1149T
Altcode: 2004astro.ph..2260T
  A 1D loop radiative hydrodynamic model that incorporates the effects
  of gravitational stratification, heat conduction, radiative losses,
  external heat input, presence of helium, and Braginskii viscosity is
  used to simulate elementary flare loops. The physical parameters for the
  input are taken from observations of the Bastille-Day flare of 2000 July
  14. The present analysis shows that: a) the obtained maximum values of
  the electron density can be considerably higher (4.2 × 10<SUP>11</SUP>
  cm<SUP>-3</SUP> or more) in the case of footpoint heating than in
  the case of apex heating (2.5 × 10<SUP>11</SUP> cm<SUP>-3</SUP>);
  b) the average cooling time after the flare peak takes less time in
  the case of footpoint heating than in the case of apex heating; c)
  the peak apex temperatures are significantly lower (by about 10 MK)
  for the case of footpoint heating than for apex heating (for the
  same average loop temperature of about 30 MK). This characteristic
  would allow to discriminate between different heating positioning; d)
  in both cases (of apex and footpoint heating), the maximum obtained
  apex temperature T<SUP>max</SUP> is practically independent of the
  heating duration σ<SUB>t</SUB>, but scales directly with the heating
  rate E<SUB>H0</SUB>; e) the maximum obtained densities at the loop
  apex, n<SUB>e</SUB><SUP>max</SUP>, increase with the heating rate
  E<SUB>H0</SUB> and heating duration σ<SUB>t</SUB> for both footpoint
  and apex heating. In Paper II we will use the outputs of these
  hydrodynamic simulations, which cover a wide range of the parameter
  space of heating rates and durations, as an input for forward-fitting
  of the multi-loop arcade of the Bastille-day flare.

Title: Time signatures of impulsively generated coronal fast wave
    trains
Authors: Nakariakov, V. M.; Arber, T. D.; Ault, C. E.; Katsiyannis,
   A. C.; Williams, D. R.; Keenan, F. P.
Bibcode: 2004MNRAS.349..705N
Altcode:
  Impulsively generated short-period fast magneto-acoustic wave
  trains, guided by solar and stellar coronal loops, are numerically
  modelled. In the developed stage of the evolution, the wave trains
  have a characteristic quasi-periodic signature. The quasi-periodicity
  results from the geometrical dispersion of the guided fast modes,
  determined by the transverse profile of the loop. A typical feature
  of the signature is a tadpole wavelet spectrum: a narrow-spectrum
  tail precedes a broad-band head. The instantaneous period of the
  oscillations in the wave train decreases gradually with time. The period
  and the spectral amplitude evolution are shown to be determined by the
  steepness of the transverse density profile and the density contrast
  ratio in the loop. The propagating wave trains recently discovered
  with the Solar Eclipse Coronal Imaging System (SECIS) instrument are
  noted to have similar wavelet spectral features, which strengthens
  the interpretation of SECIS results as guided fast wave trains.

Title: Theoretical Aspects of MHD Coronal Seismology
Authors: Nakariakov, V. M.
Bibcode: 2004ESASP.547..407N
Altcode: 2004soho...13..407N
  The revolutionary breakthrough in observational detection of propagating
  and standing quasi-periodic disturbances of coronal plasma parameters,
  made in the EUV band with SOHO and TRACE mission instruments, toin
  terms of magnetohydrodynamic (MHD) waves, has given rise to MHD coronal
  seismology. This review presents the theoretical background of these
  studies and reflects current trends in the development of the theory.

Title: Global Sausage Magnetoacoustic Modes of Coronal Loops
Authors: Nakariakov, V. M.; Melnikov, V. F.; Reznikova, V. E.
Bibcode: 2004IAUS..223..653N
Altcode: 2005IAUS..223..653N
  A sufficiently thick and dense flaring magnetic loop is shown to be
  able to support global sausage magnetoacoustic modes. The oscillation
  period of this mode, calculated in the straight cylinder approximation,
  is determined by the length of the loop, not by its diameter, as
  it has previously been assumed. For the existence of this mode, the
  ratio of the loop length to its diameter is to be less than about a
  half of the square root of the density contrast ratio. This mode has a
  maximum of the magnetic field perturbation at the loop apex and nodes
  at the footpoints. We demonstrate that the 14-17 sec quasi-periodical
  pulsations of microwave emission, oscillating in phase at a loop apex
  and at its legs, as observed with the Nobeyama Radioheliograph, are
  interpreted in terms of the global sausage mode.

Title: Coronal Seismology by MHD Autowaves
Authors: Kelly, A.; Nakariakov, V. M.
Bibcode: 2004ESASP.547..483K
Altcode: 2004soho...13..483K
  Slow magnetoacoustic waves are routinely observed in solar coronal
  structures. These waves can be strongly affected by non-adiabatic
  effects leading to self-organising compressible propagating disturbances
  -- magnetoacoustic autowaves. Autowaves are propagating disturbances
  which have parameters independent of the excitation that are determined
  by the parameters of the medium only and, consequently, are potentially
  an ideal tool for coronal seismology. The influence of non-adiabatic
  effects is studied and magnetoacoustic autowaves are modelled with
  the extended Burgers equation. A numerical code is developed to
  study the evolution of such waves and a parametric study is carried
  out. Observable parameters of coronal magnetoacoustic autowaves could
  provide a tool for the determination of heat deposition mechanisms in
  the corona.

Title: Acoustic Oscillations in Solar and Stellar Flaring Loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Kelly, A.; Aschwanden,
   M. J.; Arber, T. D.
Bibcode: 2004ESASP.547..473T
Altcode: 2004soho...13..473T
  No abstract at ADS

Title: Acoustic oscillations in solar and stellar flaring loops
Authors: Nakariakov, V. M.; Tsiklauri, D.; Kelly, A.; Arber, T. D.;
   Aschwanden, M. J.
Bibcode: 2004A&A...414L..25N
Altcode: 2004astro.ph..2223N
  Evolution of a coronal loop in response to an impulsive energy release
  is numerically modelled. It is shown that the loop density evolution
  curves exhibit quasi-periodic perturbations with the periods given
  approximately by the ratio of the loop length to the average sound
  speed, associated with the second standing harmonics of an acoustic
  wave. The density perturbations have a maximum near the loop apex. The
  corresponding field-aligned flows have a node near the apex. We
  suggest that the quasi-periodic pulsations with periods in the range
  10-300 s, frequently observed in flaring coronal loops in the radio,
  visible light and X-ray bands, may be produced by the second standing
  harmonic of the acoustic mode.

Title: Magnetohydrodynamic Sausage-Mode Oscillations in Coronal Loops
Authors: Aschwanden, Markus J.; Nakariakov, Valery M.; Melnikov,
   Victor F.
Bibcode: 2004ApJ...600..458A
Altcode: 2003astro.ph..9493A
  A recent study by Nakariakov and coworkers pointed out that the
  dispersion relation of MHD sausage-mode oscillations has been
  incorrectly applied to coronal loops, neglecting the highly dispersive
  nature of the phase speed and the long-wavelength cutoff of the
  wavenumber. In the light of these new insights, we revisit previous
  observations that have been interpreted in terms of MHD sausage-mode
  oscillations in coronal loops and come to the following conclusions:
  (1) fast sausage MHD-mode oscillations require such a high electron
  density imposed by the wavenumber cutoff that they can only occur in
  flare loops, and (2) in the previously reported radio observations
  (ν~100 MHz to 1 GHz) with periods of P~0.5-5 s, the fast sausage
  MHD-mode oscillation is likely to be confined to a small segment
  (corresponding to a high harmonic node) near the apex of the loop,
  rather than involving a global oscillation over the entire loop
  length. The recent microwave and soft X-ray observations of fast periods
  (P~6-17 s) by Asai and coworkers and Melnikov and coworkers, however,
  are consistent with fast sausage MHD oscillations at the fundamental
  harmonic.

Title: 3D Numerical Simulations of Impulsively Generated MHD Waves
    in Solar Coronal Loops
Authors: Selwa, M.; Murawski, K.; Kowal, G.; Nakariakov, V.;
   Aschwanden, M.; Oliver, R.; Ballester, J. L.
Bibcode: 2004ESASP.547..495S
Altcode: 2004soho...13..495S
  Impulsively generated magnetohydrodynamic waves in a typical EUV solar
  coronal loop are studied numerically with a use of the three-dimensional
  FLASH code. Our results reveal several 3D effects such as distinctive
  time signatures which are collected at a detection point inside the
  loop. A slow magnetosonic wave generates a significant variation in a
  mass density profile with a time-scale of the order of s. A fast kink
  wave affects a mass density too but its magnitude is much lower than
  in the case of a slow wave. Time-scales which are associated with the
  fast kink wave are generally lower than in the case of a slow wave;
  they are in the range of a dozen or so seconds. Temporal signatures
  of a fast sausage wave reveal s oscillations in the quasi-periodic
  phase. Impulses which are launched outside the loop excite few seconds
  oscillations in the mass density. Time-signatures depend on a position
  of the detection point; they are usually more complex further out from
  the exciter.

Title: Observations of sausage mode oscillations in a flaring loop
Authors: Melnikov, V. F.; Reznikova, V. E.; Shibasaki, K.; Nakariakov,
   V. M.
Bibcode: 2004IAUS..223..647M
Altcode: 2005IAUS..223..647M
  We provide an observational proof of the existence of the global sausage
  mode oscillations in a flaring loop studying a microwave burst with
  quasi-periodical pulsations observed with the Nobeyama Radioheliograph
  (NoRH).

Title: The possible back-rotation of sunspots
Authors: Williams, D. R.; van Driel-Gesztelyi, L.; Nakariakov, V. M.
Bibcode: 2004cosp...35.4391W
Altcode: 2004cosp.meet.4391W
  The curious case of sunspot rotation in two senses, presented by
  Kucera (1982), provided an interesting challenge to the idea of sunspot
  rotation being due simply to the differential rotation of the Sun above
  the tachocline. As the rotation rate and direction, in that case, are
  seen to vary as a damped sinusoid, this presents the possibility that
  such motion is the result of torsional waves trapped in the sunspot's
  sub-photospheric flux tube. We therefore analyse the rotation of a
  number of sunspots using SoHO/MDI and track their rotation, presenting
  evidence of 'back-rotation' -- rotational motion which reverses --
  in a several of these spots. Intuitively, this raises the possibility
  of extending the field of 'Coronal Seismology' back down to beneath
  the photosphere (where helioseismology already exists).

Title: Alfvén Waves in Multi-ion Coronal Plasma: Observational
    Implications
Authors: Ofman, L.; Nakariakov, V. M.; Davila, J. M.
Bibcode: 2003AGUFMSH11D1129O
Altcode:
  We investigate low-frequency Alfvén waves in a multi-ion coronal plasma
  by deriving the dispersion relation of the linearized three-fluid
  equations in magnetized plasma with coronal parameters. We study
  the effect of collisions on the velocity amplitude of minor ions due
  to the Alfvén waves compared to the proton wave amplitude. We find
  that in the collisionless plasma the minor ion velocity is reduced
  by the factor Z/A compared to protons, where Z is the ion charge,
  and A is mass in units of proton mass. When the collision frequency
  is much larger then the Alfvén wave frequency the velocity amplitude
  of the minor ions is equal to the proton velocity amplitude. We show
  the effects of minor ions and collisions on the dispersion relation
  of Alfvén waves in the three fluid plasma. In view of our results we
  show how the observed nonthermal line broadening of minor ions relates
  to the wave motions of protons as a function of heliocentric distance,
  and the plasma physical parameters.

Title: Global sausage modes of coronal loops
Authors: Nakariakov, V. M.; Melnikov, V. F.; Reznikova, V. E.
Bibcode: 2003A&A...412L...7N
Altcode:
  Sufficiently thick and dense coronal loops can support global sausage
  magnetoacoustic modes. We demonstrate that the oscillation period
  of this mode, calculated in the straight cylinder approximation, is
  determined by the length of the loop, not by its diameter, as it was
  previously assumed. The existence condition for this mode is the ratio
  of the loop length to its diameter to be less than about a half of
  the square root of the density contrast ratio. This mode has a maximum
  of the magnetic field perturbation at the loop apex and nodes at the
  footpoints. We demonstrate that the 14-17 s quasi-periodic pulsations,
  oscillating in phase at a loop apex and at its legs, observed with
  the Nobeyama Radioheliograph, are interpreted in terms of the global
  sausage mode.

Title: Statistical characterisation of full-disk EUV/XUV solar
    irradiance and correlation with solar activity
Authors: Greenhough, J.; Chapman, S. C.; Dendy, R. O.; Nakariakov,
   V. M.; Rowlands, G.
Bibcode: 2003A&A...409L..17G
Altcode: 2003astro.ph..2277G
  We investigate the distribution of fluctuations in solar irradiance when
  integrated over the full disk, obtained using extreme ultraviolet/soft
  X-ray observations from the SOHO CELIAS/SEM instrument. This time
  series sums over both the contributions of single distinguishable
  flares, and of many other processes. By detrending we select events
  with timescales of less than a few hours such as waves, slow flows,
  and CMEs. The statistics generated by this range of phenomena can be
  characterised by power-law-tailed distributions. We show that (i)
  during the high activity period 2000 Jan.-June the tail exponent
  a<SUB>T</SUB>=1.5+/-0.1; (ii) during the low activity period 1996
  Jan.-June a<SUB>T</SUB>=3.0+/-0.2; and (iii) in general a<SUB>T</SUB>
  decreases with increasing activity.

Title: Short period fast waves in solar coronal loops
Authors: Cooper, F. C.; Nakariakov, V. M.; Williams, D. R.
Bibcode: 2003A&A...409..325C
Altcode:
  Short period fast magnetoacoustic waves propagating along solar
  coronal loops, perturbing the loop boundary along the line of sight
  (LOS), may be observed by imaging telescopes. The relationship between
  the difference in emission intensity, the angle between the LOS and
  the direction of propagation and the wave amplitude and wavelength,
  is explored for kink and sausage fast waves. It is shown that the
  compressibility of the plasma in the loop significantly affects the
  observability of the waves. For both wave types there is an optimal
  observation angle which is determined by the ratio of the wave length
  and the loop radius. The change of the observational conditions
  because of the loop curvature predicts a significant, up to an order
  of magnitude, change in the observed wave amplitude. This prediction
  is confirmed by the analysis of the evolution of the fast wave train
  amplitude, observed with the SECIS instrument. The wave train amplitude
  experiences a sharp increase and then a decrease along the loop. The
  observational results are in a good agreement with the theory.

Title: A Three Dimensional Magnetohydrodynamic Pulse in a Transversely
    Inhomogeneous Medium
Authors: Tsiklauri, D.; Nakariakov, V. M.
Bibcode: 2003AIPC..679..570T
Altcode:
  Interaction of impulsively generated MHD waves with a one-dimensional
  plasma inhomogeneity, transverse to the magnetic field, is considered in
  the three-dimensional regime. Because of the transverse inhomogeneity,
  MHD fluctuations, even if they do not include initially any
  density perturbation, evolve toward states where the compressible
  components tend to become predominant. The propagating MHD pulse
  asymptotically reaches a quasi-steady state with the final levels of
  density perturbation weakly depending on the degree of non-planeness
  of the pulse in the homogeneous transverse direction and somewhat
  stronger depending on plasma β. Our study demonstrates the necessity
  of incorporation of compressible and 3D effects in theory of Alfvén
  wave phase mixing. However, as far as the dynamics of weakly non-plane
  Alfvén waves is concerned it can still be qualitatively understood
  in terms of the previous 2.5D models.

Title: Propagating EUV disturbances in the Solar corona:
    Two-wavelength observations
Authors: King, D. B.; Nakariakov, V. M.; Deluca, E. E.; Golub, L.;
   McClements, K. G.
Bibcode: 2003A&A...404L...1K
Altcode:
  Quasi-periodic EUV disturbances simultaneously observed in 171 Å and
  195 Å TRACE bandpasses propagating outwardly in a fan-like magnetic
  structure of a coronal active region are analysed. Time series of
  disturbances observed in the different bandpasses have a relatively
  high correlation coefficient (up to about 0.7). The correlation has
  a tendency to decrease with distance along the structure: this is
  consistent with an interpretation of the disturbances in terms of
  parallel-propagating slow magnetoacoustic waves. The wavelet analysis
  does not show a significant difference between waves observed in
  different bandpasses. Periodic patterns of two distinct periods: 2-3 min
  and 5-8 min are detected in both bandpasses, existing simultaneously
  and at the same distance along the loop, suggesting the nonlinear
  generation of the second harmonics.

Title: Coronal `Tadpoles' Observed by TRACE
Authors: Seaton, D. B.; Deluca, E. E.; Cooper, F. C.; Nakariakov, V. M.
Bibcode: 2003SPD....34.1618S
Altcode: 2003BAAS...35..836S
  The X1.5 flare event observed by TRACE in 195 Å on 21 April 2002,
  showed dark, downward-propagating features over the flare arcade which
  we call `coronal tadpoles.' We analyzed these features and determined
  their distinguishing characteristics. Estimating plasma densities
  inside the features, we determined that they are of very low density
  and possibly &gt;10 million K. We propose that the coronal tadpoles
  my be interpreted in terms of kink modes of the flaring current sheet
  filled by hot, rarefied plasma.

Title: Nonlinear Multi-Scale Processes in the Heliosphere in the IHY
Authors: Chapman, S. C.; Nakariakov, V. M.; Arber, T. D.
Bibcode: 2003EAEJA....10486C
Altcode:
  University of Warwick's Space and Astrophysics Group have undertaken
  an intensive research programme in magnetospheric, heliospheric and
  solar physics, including analytical theory, large scale numerical
  simulation, advanced visualization techniques and theory led data
  analysis. The unifying theme is that these are nonlinear multiscale
  processes, which control the energy release and conversion processes,
  turbulence and self-organisation. The coordinated studies of solar,
  heliospheric and magnetospheric processes proposed in frames of the
  IHY provide a unique opportunity to unify knowledge that is presently
  fragmented. For example, remote sensing of physical processes in the
  solar corona with the use of ground-based and space-borne facilities
  (potentially, including the multiple vantage point observations with
  STEREO), remote (e.g. IPS) and in-situ (e.g. WIND, CLUSTER) studies of
  the development of these processes in the solar wind and the coupling to
  the dynamic Earth magnetosphere (as indicated by scaling in geomagnetic
  indices and POLAR UVI images of dynamic auroras) and ionosphere, will
  allow us to understand the chain of solar-terrestrial connections and,
  in particular, the nonlinear processes governing these connections.

Title: Phase mixing of a three dimensional magnetohydrodynamic pulse
Authors: Tsiklauri, D.; Nakariakov, V. M.; Rowlands, G.
Bibcode: 2003A&A...400.1051T
Altcode: 2002astro.ph.10117T
  Phase mixing of a three dimensional magnetohydrodynamic (MHD) pulse is
  studied in the compressive, three-dimensional (without an ignorable
  coordinate) regime. It is shown that the efficiency of decay of an
  Alfvénic part of a compressible MHD pulse is related linearly to
  the degree of localization of the pulse in the homogeneous transverse
  direction. In the developed stage of phase mixing (for large times),
  coupling to its compressive part does not alter the power-law decay
  of an Alfvénic part of a compressible MHD pulse. The same applies
  to the dependence upon the resistivity of the Alfvénic part of the
  pulse. All this implies that the dynamics of Alfvén waves can still be
  qualitatively understood in terms of the previous 2.5D models. Thus,
  the phase mixing remains a relevant paradigm for the coronal heating
  applications in the realistic 3D geometry and compressive plasma.

Title: Quasi-periodic behaviour of a flare ribbon system
Authors: Ludmány, A.; Major, B.; Nakariakov, V. M.
Bibcode: 2003PADEU..13..139L
Altcode:
  A series of quasi-periodic pulsations have been detected in the
  flare of 19th July 1999. The total area occupied by the flare ribbons
  exhibited temporary increases in the declining phase of the flare at
  certain moments, the period of the pulses was about 20-30 minutes. The
  phenomenon has been recognized in the center, as well as at 0.5 and
  1.0 Å wings on both sides of the H-alpha line.

Title: Coronal oscillations
Authors: Nakariakov, V. M.
Bibcode: 2003dysu.book..314N
Altcode:
  The detection of coronal waves provides us with a new tool for the
  determination of the unknown parameters of the corona - MHD seismology
  of the corona. The method is similar to helioseismology. But MHD coronal
  seismology is much richer as it is based upon three different wave modes
  - Alfvén, slow and fast magnetoacoustic modes. These MHD modes have
  quite different dispersive, polarization and propagation properties,
  which make this approach even more powerful. The delicate interplay of
  MHD wave theory and the observations of coronal waves and oscillations
  are presented, illustrating it with several examples.

Title: Line-of-sight effects on observability of kink and sausage
    modes in coronal structures with imaging telescopes
Authors: Cooper, F. C.; Nakariakov, V. M.; Tsiklauri, D.
Bibcode: 2003A&A...397..765C
Altcode: 2002astro.ph..7167C
  Kink modes of solar coronal structures, perturbing the loop in the
  direction along the line-of-sight (LOS), can be observed as emission
  intensity disturbances propagating along the loop provided the angle
  between the LOS and the structure is not ninety degrees. The effect
  is based upon the change of the column depth of the loop (along the
  LOS) by the wave. The observed amplitude of the emission intensity
  variations can be larger than the actual amplitude of the wave by a
  factor of two and there is an optimal angle maximizing the observed
  amplitude. For other angles this effect can also attenuate the observed
  wave amplitude. The observed amplitude depends upon the ratio of the
  wave length of kink perturbations to the width of the structure and on
  the angle between the LOS and the axis of the structure. Sausage modes
  are always affected negatively from the observational point of view,
  as the observed amplitude is always less than the actual one. This
  effect should be taken into account in the interpretation of wave
  phenomena observed in the corona with space-borne and ground-based
  imaging telescopes.

Title: Book Review: Current theoretical models and high resolution
    solar observations / Astronomical Society of the Pacific, 2003
Authors: Nakariakov, Valery
Bibcode: 2003Obs...123..391N
Altcode:
  No abstract at ADS

Title: Phase mixing of a three dimensional magnetohydrodynamic pulse
Authors: Tsiklauri, D.; Nakariakov, V. M.
Bibcode: 2002ESASP.506...67T
Altcode: 2002ESPM...10...67T; 2002svco.conf...67T
  Phase mixing of a three dimensional magnetohydrodynamic (MHD) pulse
  is studied in the compressive three-dimensional (without an ignorable
  coordinate) regime. It is shown that the coupling to the compressive
  modes dramatically enhances the efficiency of decay of an Alfvénic part
  of a compressible MHD pulse. However, the dynamics of weakly non-plane
  Alfvén waves can still be qualitatively understood in terms of the
  previous 2.5D models.

Title: Line-of-sight effects on observability of kink modes in
    coronal structures with imaging telescopes
Authors: Cooper, F. C.; Nakariakov, V. M.; Tsiklauri, D.
Bibcode: 2002ESASP.506..577C
Altcode: 2002svco.conf..577C; 2002ESPM...10..577C
  Kink modes of solar coronal structures, perturbing the loop in the
  direction along the line-of-sight (LOS), can be observed as emission
  intensity disturbances propagating along the loop provided the angle
  between the LOS and the structure is not ninety degrees. The phenomenon
  is based upon the change of the column depth of the loop (along the LOS)
  by the wave. The observed amplitude of the emission intensity variations
  can be larger than the actual amplitude of the wave by a factor of two
  and there is an optimal angle maximizing the observed amplitude. The
  observed amplitude depends upon the ratio of the wavelength of kink
  perturbations to the width of the structure and on the angle between
  the LOS and the axis of the structure. This phenomenon should be taken
  into account in the interpretation of wave phenomena observed in the
  corona with space-borne and ground-based imaging telescopes.

Title: Slow magnetoacoustic waves in coronal loops
Authors: Nakariakov, V. M.; King, D. B.; Tsiklauri, D.
Bibcode: 2002ESASP.506..705N
Altcode: 2002ESPM...10..705N; 2002svco.conf..705N
  Propagating disturbances of EUV emission, recently discovered in coronal
  loops with SOHO/EIT and TRACE, are interpreted as slow magnetoacoustic
  waves. This interpretation is based upon several observational findings:
  (a) the disturbances seem to propagate along the magnetic field
  (b) at a constant speed which is below the estimated sound speed,
  (c) the perturbations are compressible, (d) the periods are shorter
  than the acoustic cut-off period in the corona. A theoretical model
  describing the wave evolution and incorporating effects of dissipation,
  stratification and weak nonlinearity is created. In particular, the
  model explains why only the upwardly propagating waves are observed. The
  interpretation suggested is in a good agreement with obervational
  findings. It is shown that wide spectrum slow magnetoacoustic waves,
  consistent with currently available observations in the low frequency
  part of the spectrum, can provide the rate of heat deposition sufficient
  to heat the loop. In this scenario, the heat would be deposited near
  the loop footpoints which also agrees with the current observational
  data. The analysis of simultaneous observations of propagating EUV
  disturbances in 171 Å and 195 Å bandpasses of TRACE telescope shows
  high correlation which systematically decreases with the distance
  along the loop.

Title: Kink oscillations of coronal loops as a tool for the
    determination of coronal magnetic field
Authors: Nakariakov, Valery; Ofman, Leon
Bibcode: 2002ESASP.506..461N
Altcode: 2002svco.conf..461N; 2002ESPM...10..461N
  We develop a new method for the determination of the absolute value
  of the magnetic field strength in coronal closed magnetic structures,
  based on the analysis of flare-generated oscillations of coronal
  loops. Interpretation of the quasi-harmonic oscillations observed in
  terms of global standing kink waves allows to connect the period of
  the oscillations and the loops length with the magnetic field strength
  in the loops. Improved diagnostic of the loop length, the oscillation
  period, and the plasma density in the loop will significantly improve
  the method's precision.

Title: A strongly nonlinear Alfvénic pulse in a transversely
    inhomogeneous medium
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.
Bibcode: 2002A&A...395..285T
Altcode: 2002astro.ph.10625T
  We investigate the interaction of a plane, linearly polarized, Alfvénic
  pulse with a one-dimensional, perpendicular to the magnetic field,
  plasma density inhomogeneity in the strongly nonlinear regime. Our
  numerical study of the full MHD equations shows that: (i) Plasma
  density inhomogeneity substantially enhances (by about a factor of
  2) the generation of longitudinal compressive waves. (ii) Attained
  maximal values of the generated transverse compressive perturbations
  are insensitive to the strength of the plasma density inhomogeneity,
  plasma beta and the initial amplitude of the Alfvén wave. Typically,
  they reach about 40% of the initial Alfvén wave amplitude. (iii)
  Attained maximal values of the generated relative density perturbations
  are within 20-40% for 0.5 &lt;= beta &lt;= 2.0. They depend upon
  plasma beta strongly; and scale almost linearly with the initial
  Alfvén wave amplitude.

Title: A three dimensional magnetohydrodynamic pulse in a transversely
    inhomogeneous medium
Authors: Tsiklauri, D.; Nakariakov, V. M.
Bibcode: 2002A&A...393..321T
Altcode: 2002astro.ph..2489T
  Interaction of impulsively generated MHD waves with a one-dimensional
  plasma inhomogeneity, transverse to the magnetic field, is considered in
  the three-dimensional regime. Because of the transverse inhomogeneity,
  MHD fluctuations, even if they do not include initially any density
  perturbation, evolve toward states where the compressible components
  tend to become predominant. The propagating MHD pulse asymptotically
  reaches a quasi-steady state with the final levels of density
  perturbation weakly depending on the degree of non-planeness of the
  pulse in the homogeneous transverse direction and somewhat stronger
  depending on plasma beta . Our study demonstrates the necessity of
  incorporation of compressible and 3D effects in theory of Alfvén wave
  phase mixing. However, as far as the dynamics of weakly non-plane
  Alfvén waves is concerned it can still be qualitatively understood
  in terms of the previous 2.5D models.

Title: Nonlinear MHD waves in the solar wind plasma structures
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.
Bibcode: 2002ESASP.477..381T
Altcode: 2002scsw.conf..381T; 2001astro.ph.10494T
  We investigate the interaction of magnetohydrodnamic waves with
  plasma density inhomogeneities. Our numerical study of the full MHD
  equations shows that: (A) Plasma density inhomogeneities are a source
  of non-linear generation of transverse compressive waves by a plane
  Alfvén wave, and substantially enhance (by about a factor of 2) the
  generation of the longitudinal compressive waves. (B) Attained maximal
  values of the generated transverse compressive waves are insensitive
  to the strength of the plasma density inhomogeneity and the initial
  amplitude of the Alfvén wave. (C) Efficiency of the generation depends
  weakly upon the plasma β parameter. The maximum generated amplitude
  of transverse compressive wave, up to 32% of the initial Alfvén
  wave amplitude, is reached for about β = 0.5. The results obtained
  demonstrate that plasma inhomogeneities enhance the efficiency of the
  non-linear wave coupling.

Title: Wide-spectrum slow magnetoacoustic waves in coronal loops
Authors: Tsiklauri, D.; Nakariakov, V. M.
Bibcode: 2001A&A...379.1106T
Altcode: 2001astro.ph..7579T
  A model interpreting variations of EUV brightness upward propagating
  in solar coronal loops as slow magnetoacoustic waves is developed. A
  loop is considered to have a non-zero plane inclination angle and
  offset of the circular loop centre from the baseline. The model also
  incorporates effects of dissipation and gravitational stratification. A
  linear evolutionary equation is derived and applied to investigations of
  slow wave dynamics. Both the non-zero plane inclination and the offset
  decrease the growth of the wave amplitude due to stratification. It
  is shown that wide-spectrum slow magnetoacoustic waves, consistent
  with currently available observations in the low frequency part of
  the spectrum, can provide a rate of heat deposition sufficient to heat
  the loop. In this scenario, the heat would be deposited near the loop
  footpoints, which agrees with the current observational data.

Title: A weakly nonlinear Alfvénic pulse in a transversely
    inhomogeneous medium
Authors: Tsiklauri, D.; Arber, T. D.; Nakariakov, V. M.
Bibcode: 2001A&A...379.1098T
Altcode: 2001astro.ph..7580T
  The interaction of a weakly nonlinear Alfvénic pulse with an Alfvén
  speed inhomogeneity in the direction perpendicular to the magnetic
  field is investigated. Identical to the phase mixing experienced by
  a harmonic Alfvén wave, sharp transverse gradients are generated in
  the pulse by the inhomogeneity. In the initial stage of the evolution
  of an initially plane Alfvénic pulse, the transverse gradients
  efficiently generate transversely propagating fast magnetoacoustic
  waves. However, high resolution full MHD numerical simulations of
  the developed stage of the pulse evolution show that the generation
  saturates due to destructive wave interference. It is shown that the
  weakly non-linear description of the generated fast magnetoacoustic
  wave is well described by the driven wave equation proposed in
  Nakariakov et al. (\cite{Nakariakov97}), and a simple numerical code
  (2D MacCromack), which solves it with minimal CPU resources, produces
  identical results to those obtained from the full MHD code (Lare2d,
  Arber et al. \cite{Arber01}). A parametric study of the phenomenon
  is undertaken, showing that, contrary to one's expectations, steeper
  inhomogeneities of the Alfvén speed do not produce higher saturation
  levels of the fast wave generation. There is a certain optimal gradient
  of the inhomogeneity that ensures the maximal efficiency of the fast
  wave generation.

Title: Non-resonant nonlinear coupling of magnetohydrodynamic waves
    in inhomogeneous media
Authors: Nakariakov, V. M.; Tsiklauri, D.; Arber, T. D.
Bibcode: 2001astro.ph.10493N
Altcode:
  A new mechanism for the enhanced generation of compressible fluctuations
  by Alfven waves is presented. A strongly nonlinear regime of Alfven
  wave phase-mixing is numerically simulated in a one-dimensionally
  inhomogeneous plasma of finite temperature. It is found that the
  inhomogeneity of the medium determines the efficiency of nonlinear
  excitation of magnetoacoustic waves. The level of the compressible
  fluctuations is found to be higher (up to the factor of two) in
  inhomogeneous regions. The amplitude of the generated magnetoacoustic
  wave can reach up to 30% of the source Alfven wave amplitude, and
  this value is practically independent of the Alfven wave amplitude
  and the steepness of Alfven speed profile. The highest amplitudes of
  compressible disturbances are reached in plasmas with beta of about
  0.5. The further growth of the amplitude of compressible fluctuations
  is depressed by saturation.

Title: Nonlinear magneto-acoustic waves in the solar atmosphere
Authors: Mendoza-Briceño, César A.; Ibáñez, Miguel H.; Nakariakov,
   Valery M.
Bibcode: 2001DyAtO..34..399M
Altcode:
  The weakly nonlinear evolution of quasi-isentropic magnetoacoustic
  waves in the solar atmosphere is analyzed. The plasma is assumed to
  be initially homogeneous, in thermal equilibrium and with a straight
  and homogeneous magnetic field frozen in. Additionally, the plasma is
  assumed to be cooled by the standard cooling function in the range of
  10 <SUP>4</SUP> K&lt;T&lt;10 <SUP>7</SUP> K and heated by a mechanism
  which is proportional to the density and temperature. The range of
  temperature where the plasma is isentropically unstable, the e-folding
  time for thermal instability and the corresponding time and length-scale
  for wave breaking are found.

Title: Slow magneto-acoustic waves in coronal loops
Authors: Verwichte, E.; Nakariakov, V. M.; Berghmans, D.; Hochedez,
   J. -F.
Bibcode: 2001ESASP.493..395V
Altcode: 2001sefs.work..395V
  No abstract at ADS

Title: Determination of the coronal magnetic field by coronal loop
    oscillations
Authors: Nakariakov, V. M.; Ofman, L.
Bibcode: 2001A&A...372L..53N
Altcode:
  We develop a new method for the determination of the absolute value
  of the magnetic field strength in coronal closed magnetic structures,
  based on the analysis of flare-generated oscillations of coronal
  loops. Interpretation of the oscillations observed in terms of global
  standing kink waves allows to connect the period of the oscillations and
  the loops length with the magnetic field strength in the loops. For loop
  oscillations observed with TRACE on 14th July 1998 and 4th July 1999,
  we estimate the magnetic field strength as 4-30 G. Using TRACE 171 Å
  and 195 Å images of the loop, taken on 4th July 1999 to determine the
  plasma density, we estimate the magnetic field in the loop as 13+/-
  9 G. Improved diagnostic of the loop length, the oscillation period,
  and the plasma density in the loop will significantly improve the
  method's precision.

Title: Slow magnetoacoustic waves in coronal loops: EIT and TRACE
Authors: Robbrecht, E.; Verwichte, E.; Berghmans, D.; Hochedez, J. F.;
   Poedts, S.; Nakariakov, V. M.
Bibcode: 2001A&A...370..591R
Altcode:
  On May 13, 1998 the EIT (Extreme ultraviolet Imaging Telescope) on board
  of SoHO (Solar and Heliospheric Observatory) and TRACE (Transition
  Region And Coronal Explorer) instruments produced simultaneous high
  cadence image sequences of the same active region (AR 8218). TRACE
  achieved a 25 s cadence in the Fe Ix (171 Å) bandpass while EIT
  achieved a 15 s cadence (operating in ``shutterless mode'', SoHO JOP
  80) in the Fe Xii (195 Å) bandpass. These high cadence observations
  in two complementary wavelengths have revealed the existence of weak
  transient disturbances in an extended coronal loop system. These
  propagating disturbances (PDs) seem to be a common phenomenon in
  this part of the active region. The disturbances originate from small
  scale brightenings at the footpoints of the loops and propagate along
  the loops. The projected propagation speeds roughly vary between 65
  and 150 km s<SUP>-1</SUP> for both instruments which is close to and
  below the expected sound speed in the coronal loops. The measured slow
  magnetoacoustic propagation speeds seem to suggest that the transients
  are sound (or slow) wave disturbances. This work differs from previous
  studies in the sense that it is based on a multi-wavelength observation
  of an entire loop bundle at high cadence by two EUV imagers. The
  observation of sound waves along the same path shows that they propagate
  along the same loop, suggesting that loops contain sharp temperature
  gradients and consist of either concentric shells or thin loop threads,
  at different temperatures.

Title: Fast magnetoacoustic waves in a randomly structured solar
    corona
Authors: Murawski, K.; Nakariakov, V. M.; Pelinovsky, E. N.
Bibcode: 2001A&A...366..306M
Altcode:
  The propagation of fast magnetoacoustic waves in a randomly structured
  solar corona is considered in the linear and cold plasma limits. The
  random field is assumed to be static and associated with plasma density
  inhomogeneities only. A transcendental dispersion relation for the
  fast magnetoacoustic waves which propagate perpendicularly to the
  magnetic field is derived in the weak random field approximation. It
  is shown analytically that the fast magnetosonic waves experience
  acceleration, attenuation, and dispersion in comparison to the
  homogeneous case. These analytical findings are essentially confirmed
  by numerical simulations for a wide-spectrum pulse, except that the
  waves were found decelerated. It is concluded that the coronal Moreton
  waves can be applied to MHD seismology of the solar corona.

Title: MHD Seismology of the Solar Cornoa with SOHO and TRACE
Authors: Nakariakov, V. M.
Bibcode: 2001IAUS..203..353N
Altcode:
  Recent discoveries of MHD wave motions in the solar corona (slow
  magnetoacoustic waves in polar plumes and long loops, oscillations of
  coronal loops, coronal Moreton waves) done with EUV imaging telescopes
  onboard SOHO and TRACE provide an observational basis for the MHD
  seismology of the corona. Measuring the properties of MHD waves and
  oscillations (periods, wavelengths, amplitudes, temporal and spatial
  signatures), combined with theoretical modelling of the wave phenomena,
  allow us to determine values of the mean parameters of the corona
  (the magnetic field strength, transport coefficients, etc.). As
  an example, we consider post-flare decaying oscillations of loops,
  observed with TRACE (14th July 1998 at 12:55 UT). An analysis of the
  oscillations shows that they are quasi-harmonic, with a period of
  about 265 s, and quickly decaying with the decay time of about 14.5
  min. We interpret these oscillations as a standing kink global modes
  of the loops. The period of oscillations allows us to determine the
  Alfvén speed in the oscillating loop about 770 km/s. This value can
  be used for deduction of the value of the magnetic field in the loop
  (giving 15-25 G). The decay time, together with the assumption that
  the decay is caused by viscous (or resistive) dissipation, gives us
  the Reynolds number of 10<SUP>5.3-6.1</SUP> (or the Lundquist number
  of 10<SUP>5.0-5.8</SUP>). Other possible means for the oscillation
  decay, such as wave leakage and linear and nonlinear wave coupling
  are estimated.

Title: A developed stage of Alfvén wave phase mixing
Authors: Botha, G. J. J.; Arber, T. D.; Nakariakov, V. M.; Keenan,
   F. P.
Bibcode: 2000A&A...363.1186B
Altcode:
  Alfvén wave phase mixing is an extensively studied mechanism for
  dissipating wave energy in an inhomogeneous medium. It is common in the
  vast majority of phase mixing papers to assume that even though short
  scale lengths and steep gradients develop as a result of phase mixing,
  nonlinear wave coupling does not occur. However, weakly nonlinear
  studies have shown that phase mixing generates magnetoacoustic
  modes. Numerical results are presented which show the nonlinear
  generation of magnetosonic waves by Alfvén wave phase mixing. The
  efficiency of the effect is determined by the wave amplitude, the
  frequency of the Alfvén waves and the gradient in the background
  Alfvén speed. Weakly nonlinear theory has shown that the amplitude
  of the fast magnetosonic wave grows linearly in time. The simulations
  presented in this paper extend this result to later times and show
  saturation of the fast magnetosonic component at amplitudes much lower
  than that of the Alfvén wave. For the case where Alfvén waves are
  driven at the boundary, simulating photospheric footpoint motion,
  a clear modulation of the saturated amplitude is observed. All the
  results in this paper are for a low amplitude (&lt;= 0.1), single
  frequency Alfvén wave and a uniform background magnetic field in
  a two dimensional domain. For this simplified geometry, and with a
  monochromatic driver, we concluded that the nonlinear generation of
  fast modes has little effect on classical phase mixing.

Title: Slow magnetoacoustic waves in coronal loops
Authors: Nakariakov, V. M.; Verwichte, E.; Berghmans, D.; Robbrecht, E.
Bibcode: 2000A&A...362.1151N
Altcode:
  A theoretical model interpreting propagating disturbances of EUV
  emission intensity, recently observed in coronal loops, is constructed
  in terms of slow magnetoacoustic waves. The model is one-dimensional
  and incorporates effects of nonlinearity, dissipation due to finite
  viscosity and thermal conduction, and gravitational stratification of
  plasma in the loop. It has been found that, for the observationally
  detected parameters of the waves, the main factors influencing the
  wave evolution are dissipation and stratification. The upwardly
  propagating waves of observed periods (5-20 min) are found to decay
  significantly in the vicinity of the loop apex, explaining the rarity
  of observational detection of downwardly propagating waves. The model
  provides a theoretical basis for development of MHD seismology of the
  coronal loops.

Title: Flare-generated coronal loop oscillations: A tool for MHD
    coronal seismology
Authors: Nakariakov, Valery M.
Bibcode: 2000AIPC..537..264N
Altcode: 2000wdss.conf..264N
  Quasi-periodical oscillations of positions of coronal loops were
  observed in the extreme-ultraviolet band (171A, FeIX) with the imaging
  telescope onboard the TRACE spacecraft. Oscillating transversal
  displacements (swinging) of a long (~130 Mm) thin (diameter ~2 Mm)
  bright loop, were detected. The oscillations were excited by a flare
  (14th July 1998 at about 12:55 UT) at distance of about 60 Mm from the
  loop. The frequency of the oscillations was about 4 mHz (the period
  about 265 s) and was determined by the wavelet analysis. Neighboring
  perpendicular slits show synphase temporal behavior, suggesting that the
  oscillations are produced by a kink global fast magnetoacoustic mode
  of the loop. With this interpretation, we determine the kink speed in
  the loop as 1040 km/s, which gives the Alfvén speed of about 770+/-40
  km/s. The detection and analysis of post-flare oscillations of coronal
  loops provides us with an efficient tool for indirect determination of
  coronal parameters, e.g. the magnetic field found to be of about 20 G. .

Title: Propagational Aspects of Sunquake Waves
Authors: Medrek, M.; Murawski, K.; Nakariakov, V.
Bibcode: 2000AcA....50..405M
Altcode:
  We present the results of numerical simulations of impulsively generated
  seismic waves excited by a spatially localized impulse source which is
  connected with a nearby solar flare. The solar atmosphere is modeled
  as a two layer medium with constant temperature over the photosphere
  and linearly growing temperature below the photosphere. Effects
  of magnetic fields are neglected. Only two dimensional effects are
  considered. The source is localized slightly below the photosphere. The
  numerical results show that the initial pulse of enhanced pressure,
  which can be connected with the thermal energy release by interaction
  of flare-generated particles with the sub-photospheric medium in the
  flare-loop footpoint, generates an acoustic (seismic) wave. Interaction
  of the wave with the solar surface produces perturbations registered as
  sunquakes. Typical observationally registered features of the sunquakes,
  such as characteristic wave signatures and acceleration of the wave
  with the distance from the epicenter, are well reproduced with the
  model developed. It is found that the seismic waves are essentially
  dispersive and non-linear. The proposed model provides us with a
  theoretical basis for sunquake seismology of the solar interior.

Title: Dissipation of Slow Magnetosonic Waves in Coronal Plumes
Authors: Ofman, L.; Nakariakov, V. M.; Sehgal, N.
Bibcode: 2000ApJ...533.1071O
Altcode:
  Recently, slow magnetosonic waves were identified in polar plumes,
  at heights up to about 1.2 R<SUB>solar</SUB> using the Extreme
  Ultraviolet Imaging Telescope (EIT) observations of quasi-periodic EUV
  intensity fluctuations, and higher in the corona using the Ultraviolet
  Coronagraph Spectrometer (UVCS) white-light channel. First, we derive
  the linear dispersion relation for the slow waves in the viscous
  plasma. Next, we derive and solve an evolutionary equation of the
  Burgers type for the slow waves, incorporating the effects of radial
  stratification, quadratic nonlinearity, and viscosity. Finally, we
  model the propagation and dissipation of slow magnetosonic waves in
  polar plumes using one-dimensional and two-dimensional MHD codes in
  spherical geometry. The waves are launched at the base of the corona
  with a monochromatic source. We find that the slow waves nonlinearly
  steepen as they propagate away from the Sun into the solar wind. The
  nonlinear steepening of the waves leads to enhanced dissipation owing to
  compressive viscosity at the wave fronts. The efficient dissipation of
  the slow wave by compressive viscosity leads to damping of the waves
  within the first solar radii above the surface. We investigate the
  parametric dependence of the wave properties.

Title: Nonlinear dissipative spherical Alfvén waves in solar
    coronal holes
Authors: Nakariakov, V. M.; Ofman, L.; Arber, T. D.
Bibcode: 2000A&A...353..741N
Altcode:
  The weakly nonlinear dynamics of linearly polarized, spherical Alfvén
  waves in coronal holes is investigated. An evolutionary equation,
  combining the effects of spherical stratification, nonlinear steepening
  and dissipation due to shear viscosity is derived. The equation is a
  spherical analog of the scalar Cohen-Kulsrud-Burgers equation. Three
  main stages of the wave evolution are distinguished: geometrical
  amplification, wave breaking and enhanced dissipation. The wave
  dissipation is dramatically increased by the nonlinear transfer of
  energy to smaller scales. The scenario of the nonlinear dissipation is
  practically independent of viscosity. The dissipation rate is stronger
  for highest amplitudes, and depends weakly on the wave period and the
  temperature of the atmosphere. Waves with periods less than 300 s and
  initial amplitudes about 2-3% of the Alfvén speed at the base of the
  corona are subject to the nonlinear steepening and dissipation in less
  than 10 solar radii. For the Alfvén waves with amplitudes less than
  25 km s<SUP>-1</SUP> at the base of the corona, the maximum amplitude
  of up to 200 km s<SUP>-1</SUP> is reached at several solar radii. The
  nonlinear distortion of the wave shape is accompanied by the generation
  of longitudinal motions and density perturbations.

Title: Magnetoacoustic Waves of Small Amplitude in Optically Thin
    Quasi-isentropic Plasmas
Authors: Nakariakov, Valery M.; Mendoza-Briceño, César A.; Ibáñez
   S., Miguel H.
Bibcode: 2000ApJ...528..767N
Altcode: 1999astro.ph..9318N
  The evolution of quasi-isentropic magnetohydrodynamic waves of small
  but finite amplitude in an optically thin plasma is analyzed. The
  plasma is assumed to be initially homogeneous, in thermal equilibrium
  and with a straight and homogeneous magnetic field frozen in. Depending
  on the particular form of the heating/cooling function, the plasma may
  act as a dissipative or active medium for magnetoacoustic waves, while
  Alfvén waves are not affected directly. An evolutionary equation for
  fast and slow magnetoacoustic waves in the single wave limit has been
  derived and solved that allows us to analyze the wave modification
  by competition of weakly nonlinear and quasi-isentropic effects. It
  was shown that the sign of the quasi-isentropic term determines
  the scenario of the evolution, either dissipative or active. In the
  dissipative case, when the plasma is first-order isentropically stable,
  the magnetoacoustic waves are damped and the time for shock wave
  formation is delayed. However, in the active case when the plasma is
  isentropically overstable, the wave amplitude grows, the strength of
  the shock increases, and the breaking time decreases. The magnitude
  of the above effects depends on the angle between the wavevector and
  the magnetic field. For hot (T&gt;10<SUP>4</SUP> K) atomic plasmas
  with solar abundances either in the interstellar medium or in the
  solar atmosphere, as well as for the cold (T&lt;10<SUP>3</SUP> K)
  ISM molecular gas, the range of temperatures in which the plasma is
  isentropically unstable and the corresponding timescale and length
  scale for wave breaking have been found.

Title: Slow Magnetoacoustic Waves in Coronal Loops?
Authors: Robbrecht, E.; Berghmans, D.; Nakariakov, V.; Poedts, S.
Bibcode: 1999ESASP.446..575R
Altcode: 1999soho....8..575R
  On May 13, 1998 the EIT and TRACE instruments produced simultaneous
  high cadence image sequences of the same active region (AR 8218). TRACE
  achieved a 25 sec cadence in the 171 deg passband while EIT achieved
  a 15 sec cadence (operating in 'shutterless mode', SOHO JOP 80) in the
  195 deg passband. These high cadence observations in two complementary
  wavelengths have revealed the existence of weak disturbances in an
  extended coronal loop system. The disturbances originate from small
  scale brightenings at the footpoints of the loops and propagate along
  the loops at an apparant speed of the order of 150 km/s which is close
  to the expected sound speed. To conclude whether these propagating
  disturbances should be interpreted as slow magnetoacoustic waves or as
  mass motions ('microflows'), we compare our observational findings with
  theoretical models. Our results suggest that the recent discovery of
  DeForest and Gurman (1998) of slow MHD waves in polar plumes, are in
  fact not typical of polar plumes but occur also in extended coronal
  structures elsewhere.

Title: MHD Waves in Open Magnetic Structures
Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K.
Bibcode: 1999ASPC..184..243N
Altcode:
  The dynamics of Alfvén and magnetoacoustic waves of finite amplitude in
  open magnetic structures is considered. A transversal inhomogeneity in
  the magnetic structure in the density and/or in a field-aligned steady
  flow gives rise to Alfvén wave phase mixing, which, in the almost
  collisionless and dissipationless plasma of the corona, nonlinearly
  generates obliquely propagating fast magnetoacoustic waves. The
  generated fast waves are more effectively damped than the Alfvén
  waves and so the process of heating by phase mixing is enhanced. This
  "indirect" heating leads to the spreading of the heated plasma across
  the magnetic field. We point out also that an inhomogeneous flow gives
  rise to the secular generation of longitudinal motions through Alfvén
  wave phase mixing, accelerating the plasma along the field.

Title: TRACE observation of damped coronal loop oscillations:
    Implications for coronal heating
Authors: Nakariakov, V. M.; Ofman, L.; Deluca, E. E.; Roberts, B.;
   Davila, J. M.
Bibcode: 1999Sci...285..862N
Altcode:
  The imaging telescope on board the Transition Region and Coronal
  Explorer (TRACE) spacecraft observed the decaying transversal
  oscillations of a long [(130 ± 6) × 10<SUP>6</SUP> meters], thin
  [diameter (2.0 ± 0.36) × 10<SUP>6</SUP> meters], bright coronal
  loop in the 171 angstrom FeIX emission line. The oscillations were
  excited by a solar flare in the adjacent active region. The decay
  time of the oscillations is 14.5 ± 2.7 minutes for an oscillation
  with a frequency 3.90 ± 0.13 millihertz. The coronal dissipation
  coefficient is estimated to be eight to nine orders of magnitude
  larger than the theoretically predicted classical value. The larger
  dissipation coefficient may solve existing difficulties with wave
  heating and reconnection theories.

Title: Determination of the Reynolds number from TRACE Observation
    of Damped Coronal Loop Oscillations Induced by a Flare
Authors: Ofman, L.; Nakariakov, V. M.; Deluca, E.; Roberts, B.;
   Davila, J. M.
Bibcode: 1999AAS...194.7909O
Altcode: 1999BAAS...31..964O
  The Transition Region and Coronal Expolorer (TRACE) observes the solar
  corona with unprecedented spatial and temporal resolution. We analyzed
  active region loop observation in the 171 Angstroms Fe IX emission line,
  and report the direct observations of damped transverse oscillations
  of a long (130+/-6 Mm) thin (diameter 2+/-0.36 Mm) bright active region
  loop. The oscillations were detected following a flare in the adjacent
  active region. We determined the oscillation frequency and the decay
  time by the least-square fit of an exponentially decaying sinusoidal
  function. Using the dispersion relation for the transverse oscillations,
  and the observed loop geometry we estimated the Alfven crossing time
  in the loop. The Alfven time can be used to determine the magnetic
  field strength in the loop if the density is known. All parts of the
  loop were observed to oscillate transversly in-phase, implying that the
  ocillation is a global mode of the loop. Using dissipative MHD model for
  resonant absorption of global mode oscillations for the coronal loop
  we determined the Reynolds number that produces the observed damping
  rate of the observed global mode. The value of the Reynolds number is
  in the 10(5-10^6) range, which is eight to nine orders of magnitude
  smaller than the classical coronal value. We discuss the important
  implication of the small Reynolds number on coronal heating theories.

Title: Slow Magnetosonic Waves in Coronal Plumes
Authors: Ofman, L.; Nakariakov, V. M.; DeForest, C. E.
Bibcode: 1999ApJ...514..441O
Altcode:
  Recent observations of polar plumes in the southern solar coronal
  hole by the Extreme-Ultraviolet Imaging Telescope (EIT) on board
  the SOHO spacecraft show signatures of quasi-periodic compressional
  waves with periods of 10-15 minutes. The relative wave amplitude
  was found to increase with height in the plumes up to about 1.2
  R<SUB>solar</SUB>. Using a one-dimensional linear wave equation for
  the magnetosonic wave, we show that the waves are propagating and
  that their amplitude increases with height. The observed propagation
  velocity agrees well with the expected sound velocity inside the
  plumes. We present the results of the first nonlinear, two-dimensional,
  magnetohydrodynamic (MHD) simulation of the magnetosonic waves in
  plumes for typical coronal conditions consistent with observations
  and gravitationally stratified solar corona. We find numerically
  that outward-propagating slow magnetosonic waves are trapped, and
  nonlinearly steepen in the polar plumes. The nonlinear steepening of
  the magnetosonic waves may contribute significantly to the heating of
  the lower corona by compressive dissipation.

Title: Excitation of magnetospheric waveguide modes by magnetosheath
    flows
Authors: Mann, Ian R.; Wright, Andrew N.; Mills, Katharine J.;
   Nakariakov, Valery M.
Bibcode: 1999JGR...104..333M
Altcode:
  Standard models of the Earth's outer magnetospheric waveguide assume
  that a perfectly reflecting magnetopause can trap energy inside the
  waveguide. In contrast, we show that the near-noon magnetopause often
  acts as a leaky boundary, wave trapping only being possible for large
  magnetosheath flow speeds. Moreover, for sufficiently fast flow speeds,
  we show how waveguide modes may be energized by magnetosheath flows via
  the overreflection mechanism. Unbounded simulations of the growth of
  surface waves via the development of a Kelvin-Helmholtz instability
  (KHI) vortex sheet show growth rates which increase without limit
  proportional to wavenumber (k<SUB>y</SUB>), until the assumption of a
  thin boundary is no longer valid. For a bounded magnetosphere, however,
  overreflected body type waveguide modes can introduce wavenumber
  selection, that is, generate modes with maximum linear growth rates
  at finite k<SUB>y</SUB>. A necessary condition is that the wave is
  propagating in the magnetosphere, that is, the wave's turning point
  lies inside the magnetosphere. By developing a new description of
  both KHI and waveguide mode growth in terms of overreflection and
  the propagation of negative energy waves, we show how the maximum
  growth rate can be understood in terms of the reflection coefficient
  of waves incident upon the magnetopause. Our model can also explain
  the observed local time dependence of Pc5 field line resonance wave
  power, and can explain the observed correlation between high solar wind
  speeds and Pc5 wave power. Finally, we show how a waveguide with a free
  magnetopause boundary supports quarter-wavelength modes. These modes
  have lower frequencies than the standard (magnetopause velocity node)
  half-wavelength modes, perhaps generating the millihertz waveguide
  mode eigenfrequencies which appear to drive field line resonances in
  HF radar data.

Title: Alfvén wave phase mixing in two-dimensional open magnetic
    configurations.
Authors: Ruderman, M. S.; Nakariakov, V. M.; Roberts, B.
Bibcode: 1999joso.proc..143R
Altcode:
  The phase mixing of Alfvén waves in planar two-dimensional open
  magnetic configurations is considered. It is assumed that the
  characteristic vertical spatial scale of the configuration is much
  larger than the horizontal scale, and that the latter is of the order
  of a wavelength. The WKB method is used to derive the governing
  equation for the wave amplitude, which in appropriate coordinates
  is the diffusion equation with the diffusion coefficient being
  spatially dependent. The dependency of the energy flux on the vertical
  coordinate is obtained for monochromatic waves, and illustrated for
  three particular cases. The theory is applied to Alfvén wave damping
  in coronal holes.

Title: Alfven wave phase mixing in two-dimensional open magnetic
    configurations.
Authors: Ruderman, Michael S.; Nakariakov, Valery M.; Roberts, Bernard
Bibcode: 1998A&A...338.1118R
Altcode:
  The phase mixing of Alfven waves in planar two-dimensional open
  magnetic plasma configurations is considered. It is assumed that
  the characteristic vertical spatial scale of the configuration is
  much larger than the horizontal scale, and that the latter is of
  the order of a wavelength. The WKB method is used to derive the
  governing equation for the wave amplitude, which in appropriate
  coordinates is the diffusion equation with the diffusion coefficient
  being spatially dependent. The dependency of the energy flux
  on the vertical coordinate is obtained for monochromatic waves,
  and illustrated for three particular cases. In all three cases,
  at low heights phase-mixed Alfven waves damp at the same rate as
  in a one-dimensional configuration. However, in the first and third
  cases phase mixing operates only at low and intermediate heights and
  practically stops at heights larger than a few characteristic vertical
  length scales. Only a part of the energy flux is damped due to phase
  mixing. In the second case the situation is reversed: the damping of
  the energy flux with height is much faster that in one-dimensional
  configurations. The rate of damping of the energy flux with height
  due to phase mixing in two-dimensional configurations thus depends
  strongly on the particular form of the configuration. The theory is
  applied to Alfven wave damping in coronal holes.

Title: Nonlinear coupling of MHD waves in inhomogeneous steady flows
Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K.
Bibcode: 1998A&A...332..795N
Altcode:
  The nonlinear coupling of MHD waves in a cold (beta =3D 0) compressible
  plasma with a smoothly inhomogeneous low-speed steady flow directed
  along the magnetic field is considered. The effect is similar to Alfven
  wave phase mixing in a static, inhomogeneous medium and leads to the
  production of steep transversal gradients in the plasma parameters,
  which increases dissipation. Transversal gradients in the total
  pressure, produced by phase mixing, lead to the secular generation of
  obliquely propagating fast magnetosonic waves, at double the frequency
  and the wavenumber of the source Alfven waves. The efficiency of the
  generation is defined by the Alfven wave amplitude and the transversal
  spatial scale of the flow inhomogeneity. The secular growth of density
  perturbations, connected with fast waves, takes place for flow speeds
  that are considerably below the thresholds of the Kelvin - Helmholtz and
  negative energy wave instabilities. The initial stage of the nonlinear
  generation of the fast waves is considered analytically and illustrated
  by numerical simulations.

Title: Nonlinear Waves in Force-Free Fibrils
Authors: Zhugzhda, Y. D.; Nakariakov, V. M.
Bibcode: 1998ASPC..150..151Z
Altcode: 1998npsp.conf..151Z; 1998IAUCo.167..151Z
  No abstract at ADS

Title: A Manifestation of Negative Energy Waves in the Solar
    Atmosphere
Authors: Joarder, P. S.; Nakariakov, V. M.; Roberts, B.
Bibcode: 1997SoPh..176..285J
Altcode:
  Magnetosonic modes of magnetic structures of the solar atmosphere in
  the presence of inhomogeneous steady flows are considered. It is shown
  that, when the speed of the steady flow exceeds the phase speed of
  one of the modes, the mode has negative energy, and can be subject to
  an over-stability due to the negative energy wave instabilities. It is
  shown that registered steady flows in the solar atmosphere, with speeds
  below the threshold of the Kelvin-Helmholtz instability, can provide
  the existence of the magnetosonic negative energy wave phenomena. In
  particular, in isolated photospheric magnetic flux tubes, there are kink
  surface modes with negative energy, produced by the external granulation
  downflows. Dissipative instability of these modes due to finite thermal
  conductivity and explosive instability due to nonlinear coupling of
  these modes with Alfvén waves are discussed. For coronal loops, it is
  found that only very high-speed flows (&gt;300 km s<SUP>-1</SUP>) can
  produce negative energy slow body modes. In solar wind flow structures,
  both slow and fast body modes have negative energy and are unstable.

Title: Latent Heating of Coronal Loops
Authors: Zhugzhda, Y. D.; Nakariakov, V. M.
Bibcode: 1997SoPh..175..107Z
Altcode:
  The theory of weakly nonlinear slow magnetosonic waves in thin flux
  tubes is developed. A Korteweg-de Vries equation is derived for
  slow body waves. The analysis of solitary and periodic solutions
  shows that the nonlinear waves produce running narrowings of the
  tube. The similarity of the shallow-water theory and the thin-flux-tube
  approximation helps to outline the peaking and breaking of the nonlinear
  slow body waves. It appears that the running narrowing of the tube
  works as the de Laval nozzle in generating a cool supersonic jet
  and a subsequent shock that heats the plasma. The presented theory of
  nonlinear slow body waves suggests a mechanism of coronal loop heating,
  which meets many of the observational constraints.

Title: Alfvén Wave Phase Mixing as a Source of Fast Magnetosonic
    Waves
Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K.
Bibcode: 1997SoPh..175...93N
Altcode:
  The nonlinear excitation of fast magnetosonic waves by phase mixing
  Alfvén waves in a cold plasma with a smooth inhomogeneity of
  density across a uniform magnetic field is considered. If initially
  fast waves are absent from the system, then nonlinearity leads to
  their excitation by transversal gradients in the Alfvén wave. The
  efficiency of the nonlinear Alfvén-fast magnetosonic wave coupling is
  strongly increased by the inhomogeneity of the medium. The fast waves,
  permanently generated by Alfvén wave phase mixing, are refracted
  from the region with transversal gradients of the Alfvén speed. This
  nonlinear process suggests a mechanism of indirect plasma heating by
  phase mixing through the excitation of obliquely propagating fast waves.

Title: Oscillations in Prominence Fine-Structures
Authors: Joarder, P. S.; Nakariakov, V. M.; Roberts, B.
Bibcode: 1997SoPh..173...81J
Altcode:
  Oscillatory spectra of solar quiescent prominences highlight the
  importance of incorporating the effect of prominence fine-structure
  in the theory of prominence oscillations. We determine the
  magnetohydrodynamic modes of oscillation of an elementary, zero-β
  model of a prominence fibril, arguing that the fast body kink modes,
  namely, the string and the internal magnetic Love modes, produce the
  observed short periodicities in prominence fine-structures. Estimates
  for the periods of these modes are presented: the modes are subject
  to testing in future high-resolution observations.

Title: On nonlinear dynamics of twisted magnetic flux tubes.
Authors: Zhugzhda, Yu. D.; Nakariakov, V. M.
Bibcode: 1997joso.proc...95Z
Altcode:
  No abstract at ADS

Title: Negative energy wave instabilities in magnetic flux tubes.
Authors: Nakariakov, V. M.; Roberts, B.; Joarder, P. S.
Bibcode: 1997joso.proc...64N
Altcode:
  No abstract at ADS

Title: Nonlinearly Selected Frequencies in Coronal Loops
Authors: Nakariakov, V. M.; Roberts, B.
Bibcode: 1996SoPh..168..273N
Altcode:
  A nonlinear process for the resonant generation of low-frequency fast
  magnetosonic kink waves in coronal loops is discussed. The efficiency of
  the process is strongly enhanced due to the existence of a nonlinearly
  selected frequency produced by a constant frequency difference in the
  dispersion curves in the short wavelength limit. The kink wave with
  the selected frequency interacts with high-frequency kink and sausage
  waves. The efficiency of such interaction does not require coherence in
  the interactive waves. In a loop of width 2 × 10<SUP>3</SUP> km, field
  strength 50 G and number density 5 × 10<SUP>15</SUP> m<SUP>−3</SUP>,
  the nonlinearly selected frequency is of order 46 mHz (period 21.8 s),
  but this may range through 11 mHz to 184 mHz (periods 86.5 s to 5.4 s)
  for typical coronal conditions.

Title: On the interaction of longitudinal and transversal waves in
    thin magnetic flux tubes.
Authors: Nakariakov, V. M.; Zhugzhda, Y. D.; Ulmschneider, P.
Bibcode: 1996A&A...312..691N
Altcode:
  An analytical investigation of the nonlinear interaction of longitudinal
  and transversal waves in thin magnetic flux tubes is presented and
  the nonlinear terms which give rise to wave generation of other modes
  and to shock formation are isolated. The nonlinear resonant three-wave
  interaction of longitudinal and transversal waves is studied together
  with the growth and decay behaviour of these waves. This analytical
  study clarifies our previous numerical computations of nonlinear wave
  generation and of the steepening of longitudinal as well as transversal
  wave profiles.

Title: MHD modes of solar wind flow tubes.
Authors: Nakariakov, V. M.; Roberts, B.; Mann, G.
Bibcode: 1996A&A...311..311N
Altcode:
  The properties of linear magnetosonic waves trapped within solar wind
  flow tubes are considered. The trapping is due to reflection of the
  waves from the tube boundary, which may correspond to either a jump
  in plasma density or magnetic field, or a jump in the steady flow
  velocity. The structure of the waves transverse to the magnetic field
  is defined by the boundary conditions. The phase and group speeds of
  the waves depend upon the wave frequency and wavelength. For realistic
  conditions in a flow tube, corresponding to a "spaghetti structure",
  there are two types of trapped waves propagating away from the Sun, fast
  and slow waves. These waves are "backward", in the sense that they would
  propagate towards the Sun in the absence of any difference in the steady
  flow velocity inside and outside the tube. The "forward" magnetosonic
  waves are not trapped under solar wind conditions. Both slow and fast
  backward waves have phase velocities that are sub-Alfvenic. There are
  no MHD waves propagating towards the Sun.

Title: Resonant interactions of modes in coronal magnetic flux tubes
Authors: Nakariakov, V. M.; Oraevsky, V. N.
Bibcode: 1995SoPh..160..289N
Altcode:
  Nonlinear resonant interactions of different kinds of fast magnetosonic
  (FMS) waves trapped in the inhomogeneity of a low-β plasma density,
  stretched along a magnetic field (as, for example, in coronal loops)
  are investigated. A set of equations describing the amplitudes of
  interactive modes is derived for an arbitrary density profile. The
  quantitative characteristics of such interactions are found. The
  decay instability of the wave with highest frequency is possible in
  the system. If amplitudes of interactive modes have close values,
  the long-period temporal and spatial oscillations are in the system.

Title: Effects of plasma temperature anisotropy on the
    Kelvin-Helmholtz instability of the heliopause
Authors: Kruznetsov, V. D.; Nakaryakov, V. M.; Tsyganov, P. V.
Bibcode: 1995AstL...21..710K
Altcode: 1995PAZh...21..793K
  No abstract at ADS

Title: Magnetosonic Waves in Structured Atmospheres with Steady
    Flows, I
Authors: Nakariakov, V. M.; Roberts, B.
Bibcode: 1995SoPh..159..213N
Altcode:
  The magnetosonic modes of magnetic plasma structures in the solar
  atmosphere are considered taking into account steady flows of plasma
  in the internal and external media and using a slab geometry. The
  investigation brings nearer the theory of magnetosonic waveguides,
  in such structures as coronal loops and photospheric flux tubes, to
  realistic conditions of the solar atmosphere. The general dispersion
  relation for the magnetosonic modes of a magnetic slab in magnetic
  surroundings is derived, allowing for field-aligned steady flows in
  either region. It is shown that flows change both qualitatively and
  quantitatively the characteristics of magnetosonic modes. The flow may
  lead to the appearance of a new type of trapped mode, namelybackward
  waves. These waves are the usual slab modes propagating in the
  direction opposite to the internal flow, but advected with the flow. The
  disappearance of some modes due to the flow is also demonstrated.

Title: On Fast Magnetosonic Coronal Pulsations
Authors: Nakariakov, V. M.; Roberts, B.
Bibcode: 1995SoPh..159..399N
Altcode:
  The linear properties of the fast magnetosonic modes of a coronal loop
  modelled as a smooth density inhomogeneity in a uniform magnetic field
  are compared with the case of a step function slab. It is shown that
  the group velocityC<SUB>g</SUB>of the modes, important in determining
  the structure of impulsively excited wave packets, possesses a minimum
  for a wide class of profile including the slab, with the exception of
  the Epstein profile for which the minimum inC<SUB>g</SUB>moves out
  to infinity. Results for the simple step profile are thus of wider
  validity, and likely to be applicable to coronal loops.

Title: Waves in Cosmic Magnetic Structures Taking into Account the
    Anisotropic Plasma Pressure
Authors: Nakariakov, V. M.; Oraevsky, V. N.
Bibcode: 1995SoPh..158...29N
Altcode:
  This paper provides an analysis of magneto-sonic eigenwaves travelling
  in magnetic plasma structures based on the Chew-Goldberger-Low
  approximation, for which the plasma kinetic pressure is different
  along and across the magnetic field. The anisotropy does not lead to
  the emergence of new modes. The dependence of phase velocities of the
  waves, trapped by a single magnetic surface, on the pressure anisotropy
  is investigated. For a magnetic slab with field-free surroundings,
  the dispersion relations for the eigenwaves are obtained. The
  pressure anisotropy may change dispersion relations of such modes
  significantly. In particular, backward waves are possible in the
  case of strong anisotropy. The dependences of the thresholds for the
  mirror and hose instabilities on the system parameters are obtained. In
  particular, hose and mirror instabilities of such waves are absent for
  some wave number regions. The results are used to obtain the eigenwave
  characteristics in coronal loops and chromospheric flux tubes.

Title: Superreflection of fast magnetosonic waves from a tangential
    discontinuity of velocity
Authors: Nakaryakov, V. M.; Stepanyants, Yu. A.
Bibcode: 1994AstL...20..763N
Altcode: 1994PAZh...20..873N
  No abstract at ADS

Title: Parametric modulation of magnetoacoustic waves in a magnetic
    field tube by an external wave
Authors: Nakaryakov, V. M.; Petrukhin, N. S.
Bibcode: 1994AstL...20..365N
Altcode: 1994PAZh...20..431N
  No abstract at ADS

Title: Effect of pressure anisotropy on the magnetosonic wave
    propagation in a plasma sheet
Authors: Gromov, E. M.; Nakariakov, V. M.; Oraevskii, V. N.
Bibcode: 1992SvAL...18..341G
Altcode: 1992PAZh...18..841G
  The propagation of magnetosonic waves localized within a sheet of
  magnetized plasma is studied, with the anisotropy of plasma kinetic
  pressure in the magnetic field taken into account. Dispersion
  dependences of wave phase velocities on the ratio of the sheet size
  to the wavelength are derived for various plasma parameters within and
  outside the sheet. The thresholds of the appearance of mirror and hose
  instabilities are investigated. The ranges of the parameters of the
  system under consideration for which anisotropy must be taken into
  account are indicated. Changes due to anisotropy are estimated for
  various magnetic structures in the solar atmosphere.

Title: Generation of Low-Frequency Pulsations in Magnetic Waveguides
    in the Sun's Atmosphere
Authors: Nakaryakov, V. M.; Petrukhin, N. S.; Fainshtein, S. M.
Bibcode: 1991SvAL...17..423N
Altcode:
  No abstract at ADS

Title: Generation of low-frequency pulsations in magnetic waveguides
    in the solar atmosphere
Authors: Nakariakov, V. M.; Petrukhin, N. S.; Feinstein, S. M.
Bibcode: 1991PAZh...17.1008N
Altcode:
  An analytical conclusion is presented of Schroedinger's nonlinear
  equation describing the nonlinear dynamics of the envelope of the
  eigenmodes of the magnetic flux tube in the solar atmosphere within the
  framework of the ideal magnetic layer approximation. The generation of
  LF pulsations arising as a result of the instability of the packets
  of captured modes is investigated, and estimates of the period of LF
  pulsations for photospheric magnetic tubes are made.

Title: Generation of the Alfven-wave second harmonic in a plane
    plasma waveguide
Authors: Nakariakov, V. N.; Fainshtein, S. M.
Bibcode: 1991RaF....34..211N
Altcode: 1991R&QE...34..211N
  The interaction and self-defocusing of different types of waves in
  unbounded media are studied. A plane waveguide filled with 'cold'
  plasma with a constant magnetic field guided along the waveguide walls
  is considered. It is shown that efficient excitation of the Alfven
  wave harmonic is possible in this plasma system.

