Author name code: hoyng
ADS astronomy entries on 2022-09-14
author:"Hoyng, Peter" 
------------------------------------------------------------------------  

Title: Mode analysis of numerical geodynamo models
Authors: Schrinner, M.; Schmitt, D.; Hoyng, P.
Bibcode: 2011PEPI..188..185S
Altcode: 2011arXiv1101.3022S
  It has been suggested in Hoyng (2009) that dynamo action can be analysed
  by expansion of the magnetic field into dynamo modes and statistical
  evaluation of the mode coefficients. We here validate this method by
  analysing a numerical geodynamo model and comparing the numerically
  derived mean mode coefficients with the theoretical predictions. The
  model belongs to the class of kinematically stable dynamos with a
  dominating axisymmetric, dipolar and non-periodic fundamental dynamo
  mode. Our present study supports that contributions from higher
  order modes to the magnetic field result from the deformation of the
  fundamental mode by the turbulent flow. The analysis requires a number
  of steps: the computation of the so-called dynamo coefficients, the
  derivation of the temporally and azimuthally averaged dynamo eigenmodes
  and the decomposition of the magnetic field of the numerical geodynamo
  model into the eigenmodes. For the determination of the theoretical mode
  excitation levels the turbulent velocity field needs to be projected
  on the dynamo eigenmodes. We compare the theoretically and numerically
  derived mean mode coefficients and find reasonably good agreement
  for most of the modes. Some deviation might be attributable to the
  approximation involved in the theory. Since the dynamo eigenmodes
  are not self-adjoint, a spectral interpretation of the eigenmodes is
  not possible.

Title: An efficient method for computing the eigenfunctions of the
    dynamo equation
Authors: Schrinner, M.; Schmitt, D.; Jiang, J.; Hoyng, P.
Bibcode: 2010A&A...519A..80S
Altcode: 2009arXiv0911.3279S
  <BR /> Aims: We present an elegant method of determining the
  eigensolutions of the induction and dynamo equations in a fluid
  embedded in a vacuum. <BR /> Methods: The magnetic field is expanded
  in a complete set of functions. The new method is based on the
  biorthogonality of the adjoint electric current and the vector
  potential with an inner product defined by a volume integral over
  the fluid domain. The advantage of this method is that the velocity
  and the dynamo coefficients of the induction and the dynamo equation
  do not have to be differentiated and thus even numerically determined
  tabulated values of the coefficients produce reasonable results. <BR />
  Results: We provide test calculations and compare with published results
  obtained by the classical treatment based on the biorthogonality of the
  magnetic field and its adjoint. We especially consider dynamos with
  mean-field coefficients determined from direct numerical simulations
  of the geodynamo and compare with initial value calculations and the
  full MHD simulations.

Title: Saturation and time dependence of geodynamo models
Authors: Schrinner, M.; Schmitt, D.; Cameron, R.; Hoyng, P.
Bibcode: 2010GeoJI.182..675S
Altcode: 2009arXiv0909.2181S
  In this study we address the question under which conditions a
  saturated velocity field stemming from geodynamo simulations leads
  to an exponential growth of the magnetic field in a corresponding
  kinematic calculation. We perform global self-consistent geodynamo
  simulations and calculate the evolution of a kinematically advanced
  tracer field. The self-consistent velocity field enters the induction
  equation in each time step, but the tracer field does not contribute to
  the Lorentz force. This experiment has been performed by Cattaneo and
  Tobias and is closely related to the test field method by Schrinner
  et al. We find two dynamo regimes in which the tracer field either
  grows exponentially or approaches a state aligned with the actual
  self-consistent magnetic field after an initial transition period. Both
  regimes can be distinguished by the Rossby number and coincide with
  the dipolar and multipolar dynamo regimes identified by Christensen and
  Aubert. Dipolar dynamos with low Rossby number are kinematically stable
  whereas the tracer field grows exponentially in the multipolar dynamo
  regime. This difference in the saturation process for dynamos in both
  regimes comes along with differences in their time variability. Within
  our sample of 20 models, solely kinematically unstable dynamos show
  dipole reversals and large excursions. The complicated time behaviour
  of these dynamos presumably relates to the alternating growth of
  several competing dynamo modes. On the other hand, dynamos in the low
  Rossby number regime exhibit a rather simple time dependence and their
  saturation merely results in a fluctuation of the fundamental dynamo
  mode about its critical state.

Title: Statistical dynamo theory: Mode excitation
Authors: Hoyng, P.
Bibcode: 2009PhRvE..79d6320H
Altcode: 2009arXiv0903.3844H
  We compute statistical properties of the lowest-order multipole
  coefficients of the magnetic field generated by a dynamo of arbitrary
  shape. To this end we expand the field in a complete biorthogonal set of
  base functions, viz. B=∑<SUB>k</SUB>a<SUP>k</SUP>(t)b<SUP>k</SUP>(r)
  . The properties of these biorthogonal function sets are treated
  in detail. We consider a linear problem and the statistical
  properties of the fluid flow are supposed to be given. The turbulent
  convection may have an arbitrary distribution of spatial scales. The
  time evolution of the expansion coefficients a<SUP>k</SUP> is
  governed by a stochastic differential equation from which we infer
  their averages ⟨a<SUP>k</SUP>⟩ , autocorrelation functions
  ⟨a<SUP>k</SUP>(t)a<SUP>k∗</SUP>(t+τ)⟩ , and an equation for
  the cross correlations ⟨a<SUP>k</SUP>a<SUP>ℓ∗</SUP>⟩ . The
  eigenfunctions of the dynamo equation (with eigenvalues λ<SUB>k</SUB>
  ) turn out to be a preferred set in terms of which our results assume
  their simplest form. The magnetic field of the dynamo is shown
  to consist of transiently excited eigenmodes whose frequency and
  coherence time is given by Iλ<SUB>k</SUB> and -1/Rλ<SUB>k</SUB> ,
  respectively. The relative rms excitation level of the eigenmodes,
  and hence the distribution of magnetic energy over spatial scales,
  is determined by linear theory. An expression is derived for
  ⟨|a<SUP>k</SUP>|<SUP>2</SUP>⟩/⟨|a<SUP>0</SUP>|<SUP>2</SUP>⟩ in
  case the fundamental mode b<SUP>0</SUP> has a dominant amplitude, and
  we outline how this expression may be evaluated. It is estimated that
  ⟨|a<SUP>k</SUP>|<SUP>2</SUP>⟩/⟨|a<SUP>0</SUP>|<SUP>2</SUP>⟩∼1/N
  , where N is the number of convective cells in the dynamo. We show that
  the old problem of a short correlation time (or first-order smoothing
  approximation) has been partially eliminated. Finally we prove that
  for a simple statistically steady dynamo with finite resistivity all
  eigenvalues obey Rλ<SUB>k</SUB>&lt;0 .

Title: Analysis of the variability of the axial dipole moment of a
    numerical geodynamo model
Authors: Kuipers, J.; Hoyng, P.; Wicht, J.; Barkema, G. T.
Bibcode: 2009PEPI..173..228K
Altcode:
  We have analysed the time evolution of the axial dipole moments
  (ADMs) from three numerical geodynamo models by relating it to
  the Fokker-Planck equation governing the systematic and random ADM
  motion. We have determined the effective growth rate of the ADM and
  the diffusion coefficient D characterising its random fluctuations. We
  find that the numerical ADM data exhibit a nonlinear quenching that
  is not significantly different from that of the Sint-2000 data. The
  quenching is only partly due to a reduction of the r.m.s. convective
  flow speed with increasing ADM. Our results suggest that in these
  numerical models similar mechanisms may be at work as in the earth's
  core, and that the results of Brendel et al. [Brendel, K., Kuipers,
  J., Barkema, G.T., Hoyng, P., 2007. An analysis of the fluctuations
  of the geomagnetic dipole. Phys. Earth Planet. Inter. 162, 249-255]
  are unlikely to be an artifact caused by the restricted length of
  the dataset. They also suggest that the dynamics of the ADM is that
  of a Brownian particle (i.e. driven by additive noise) in a bistable
  potential, and we illustrate some consequences of this idea.

Title: An analysis of the fluctuations of the geomagnetic dipole
Authors: Brendel, K.; Kuipers, J.; Barkema, G. T.; Hoyng, P.
Bibcode: 2007PEPI..162..249B
Altcode: 2007arXiv0707.0623B
  The time evolution of the strength of the Earth's virtual axial
  dipole moment (VADM) is analyzed by relating it to the Fokker-Planck
  equation, which describes a random walk with VADM-dependent drift
  and diffusion coefficients. We demonstrate first that our method
  is able to retrieve the correct shape of the drift and diffusion
  coefficients from a time series generated by a test model. Analysis
  of the Sint-2000 data shows that the geomagnetic dipole mode has a
  linear growth time of 20-7+13 kyear, and that the nonlinear quenching
  of the growth rate follows a quadratic function of the type [1-(]. On
  theoretical grounds, the diffusive motion of the VADM is expected to
  be driven by multiplicative noise, and the corresponding diffusion
  coefficient to scale quadratically with dipole strength. However,
  analysis of the Sint-2000 VADM data reveals a diffusion which depends
  only very weakly on the dipole strength. This may indicate that the
  magnetic field quenches the amplitude of the turbulent velocity in
  the Earth's outer core.

Title: Relativistic astrophysics and cosmology : a primer
Authors: Hoyng, Peter
Bibcode: 2006racp.book.....H
Altcode:
  No abstract at ADS

Title: Relativistic Astrophysics and Cosmology
Authors: Hoyng, Peter
Bibcode: 2006rac..book.....H
Altcode:
  No abstract at ADS

Title: End-to-end simulations for the LISA Technology Package
Authors: Hannen, V. M.; Smit, M.; Hoyng, P.; Selig, A.; Schleicher, A.
Bibcode: 2003CQGra..20S.261H
Altcode:
  We present an end-to-end simulation facility which has been developed in
  the framework of the LISA Technology Package (LTP) architect study for
  SMART-2, the technology demonstration mission that precedes LISA. The
  simulator evolves positions and orientations of the spacecraft and two
  test masses contained in the inertial sensors of LTP under the influence
  of external and internal forces and torques and under the influence of
  control loops for satellite drag-free control (DFC) and electrostatic
  test mass suspension. Altogether, a coupled system with 18 degrees of
  freedom is solved numerically. A series of test runs has been performed
  to verify the correct functioning of the various models contained in
  the end-to-end simulator and to provide a preliminary assessment of the
  performance of DFC algorithms and control laws for test mass suspension,
  which are currently foreseen for use in the basic operation mode of LTP.

Title: The field, the mean and the meaning
Authors: Hoyng, Peter
Bibcode: 2003and..book....1H
Altcode: 2003eclm.book....1H
  No abstract at ADS

Title: Solar Cycle
Authors: Ossendrijver, M.; Hoyng, P.
Bibcode: 2002eaa..bookE1980O
Altcode:
  The term solar cycle refers to a quasi-periodic variation with a period
  of about 11 years, visible in many of the Sun's observables. The
  solar cycle is most easily observed in indices directly related to
  the Sun's magnetic field, such as SUNSPOTS. During the last decades,
  solar-cycle variations have also been found in many other aspects of
  the Sun (irradiance, surface flows, coronal shape, oscil...

Title: A theoretical analysis of the observed variability of the
    geomagnetic dipole field
Authors: Hoyng, P.; Schmitt, D.; Ossendrijver, M. A. J. H.
Bibcode: 2002PEPI..130..143H
Altcode:
  We present a detailed analysis of the Sint-800 virtual axial dipole
  moment (VADM) data in terms of an αΩ mean field model of the geodynamo
  that features a non-steady generation of poloidal from toroidal magnetic
  field. The result is a variable excitation of the dipole mode and the
  overtones, and there are occasional dipole reversals. The model permits
  a theoretical evaluation of the statistical properties of the dipole
  mode. We show that the model correctly predicts the distribution of
  the VADM and the autocorrelation function inferred from the Sint-800
  data. The autocorrelation technique allows us to determine the turbulent
  diffusion time τ<SUB>d</SUB>= R<SUP>2</SUP>/ β of the geodynamo. We
  find that τ<SUB>d</SUB> is about 10-15 kyr. The model is able to
  reproduce the observed secular variation of the dipole mode, and the
  mean time between successive dipole reversals. On the other hand, the
  duration of a reversal is a factor ∼2 too long. This could be due to
  imperfections in the model or to unknown systematics in the Sint-800
  data. The use of mean field theory is shown to be selfconsistent.

Title: Magnetic field reversals and secular variation in a bistable
    geodynamo model
Authors: Schmitt, D.; Ossendrijver, M. A. J. H.; Hoyng, P.
Bibcode: 2001PEPI..125..119S
Altcode:
  A method is described which enables the calculation of statistical
  properties of geodynamo models. The result is a Fokker-Planck equation
  for the probability distribution of the dipole moment. The method
  only requires a non-oscillatory, predominantly dipolar magnetic field
  and helicity fluctuations. The fluctuations perturb the fundamental
  dynamo mode and lead to the excitation of higher modes. This results in
  stochastic oscillations of the dipole field amplitude in a bistable
  potential with minima representing normal and reversed polarity,
  and occasional jumps between them. The shape of the potential is
  determined by supercritical dynamo excitation and nonlinear limitation
  of field growth. Application of the method to a mean-field dynamo model
  with random fluctuations of the helicity parameter α reproduces the
  observed relation between the secular variation and the reversal rate
  of the geomagnetic field, as well as the amplitude distribution of
  the dipole field inferred from the Sint-800 record.

Title: The geodynamo as a bistable oscillator
Authors: Hoyng, P.; Ossendrijver, M. A. J. H.; Schmitt, D.
Bibcode: 2001GApFD..94..263H
Altcode:
  Our intent is to provide a simple and quantitative understanding of the
  variability of the axial dipole component of the geomagnetic field on
  both short and long time scales. To this end we study the statistical
  properties of a prototype nonlinear mean field model. An azimuthal
  average is employed, so that (1) we address only the axisymmetric
  component of the field, and (2) the dynamo parameters have a
  random component that fluctuates on the (fast) eddy turnover time
  scale. Numerical solutions with a rapidly fluctuating alpha reproduce
  several features of the geomagnetic field: (1) a variable, dominantly
  dipolar field with additional fine structure due to excited overtones,
  and sudden reversals during which the field becomes almost quadrupolar,
  (2) aborted reversals and excursions, (3) intervals between reversals
  having a Poisson distribution. These properties are robust, and appear
  regardless of the type of nonlinearity and the model parameters. A
  technique is presented for analysing the statistical properties of
  dynamo models of this type. The Fokker-Planck equation for the amplitude
  a of the fundamental dipole mode shows that a behaves as the position
  of a heavily damped particle in a bistable potential ~(1-a^2)^2,
  subject to random forcing. The dipole amplitude oscillates near the
  bottom of one well and makes occasional jumps to the other. These
  reversals are induced solely by the overtones. Theoretical expressions
  are derived for the statistical distribution of the dipole amplitude,
  the variance of the dipole amplitude between reversals, and the mean
  reversal rate. The model explains why the reversal rate increases
  with increasing secular variation, as observed. Moreover, the present
  reversal rate of the geodynamo, once per (2-3)x10^5years, is shown
  to imply a secular variation of the dipole moment of ~15% (about the
  current value). The theoretical dipole amplitude distribution agrees
  well with the Sint-800 data.

Title: Cyclic Evolution of Sunspots: Gleaning New Results from
    Old Data
Authors: Solanki, S. K.; Fligge, M.; Pulkkinen, P.; Hoyng, P.
Bibcode: 2000JApA...21..163S
Altcode:
  No abstract at ADS

Title: Book Review: The little book of the big bang : a cosmic primer
    / Copernicus, 1998/Springer, 1998
Authors: Hoyng, P.
Bibcode: 1999SSRv...88..608H
Altcode:
  No abstract at ADS

Title: The geodynamo as a bistable oscillator
Authors: Ossendrijver, M. A. J. H.; Hoyng, P.; Schmitt, D.
Bibcode: 1999AGAb...15...73O
Altcode: 1999AGM....15..J13O
  Paleomagnetism has convincingly shown that the Earth has reversed
  the polarity of its almost dipolar magnetic field many times in the
  past. The origin of the field is generally ascribed to dynamo action
  in the liquid outer core. Numerical simulation of a mean-field model
  demonstrates that random fluctuations of the dynamo alpha-effect
  cause occasional rapid magnetic reversals, like those of the
  Earth's field. The fluctuations perturb the fundamental stationary
  dipolar mode and lead to the excitation of higher oscillatory dynamo
  modes. This results in stochastic oscillations of the dipole field
  amplitude in a bistable potential with minima representing normal and
  reversed polarity, and occasional jumps between them. The shape of
  the potential is determined by supercritical dynamo excitation and
  nonlinear limitation of field growth. A statistical analysis shows
  that the model reproduces the essential features of the geomagnetic
  secular variation and reversal record.

Title: Op zoek naar de grens van het heelal.
Authors: Hoyng, P.
Bibcode: 1998Zenit..25..340H
Altcode:
  No abstract at ADS

Title: Book Review: SCORe '96 / Kluwer, 1997 &amp; 1998
Authors: Hoyng, P.
Bibcode: 1998SSRv...85..552H
Altcode:
  No abstract at ADS

Title: Chromospheric and Photospheric Activity of the Sun as a Star
    Using SOLSTICE Data
Authors: Catalano, S.; Lanza, A. F.; Brekke, P.; Rottman, Gary J.;
   Hoyng, P.
Bibcode: 1998ASPC..154..584C
Altcode: 1998csss...10..584C
  The distribution of photospheric and chromospheric active regions
  (spots and plages) on the surface of stars is mainly recovered from
  the deconvolution of light or line flux curves. Here we check the
  reliability of the maximum entropy technique to derive surface maps from
  the rotational modulation of the continuum and line fluxes observed
  in active stars. We compare the solar chromospheric map obtained from
  the deconvolution of daily average of the C 2 line flux at 133.5 nm
  measured by SOLSTICE, and solar Hα images contemporaneously obtained
  at Catania observatory. Hα plages may not exactly overlay C 2 plages,
  however the agreement between the reconstructed and the direct images
  is strikingly good. Only small (&lt;20<SUP>circ</SUP>-30<SUP>circ</SUP>
  in diameter) or low contrast (Delta I/I &lt; 0.3) structures are not
  recovered. We use also the Wolf sunspot number to study the longitude
  distribution of the active regions in photosphere and chromosphere,
  contrasting the solar behaviour with that of other active stars.

Title: A Model for a Thin Magnetised Disc in LMC X-3
Authors: Hoyng, P.
Bibcode: 1998IAUS..188..379H
Altcode:
  Dynamo action in turbulent accretion discs will generate randomly
  oriented magnetic fields. The mean field &lt;! !B ! !&gt; can be
  assumed to be zero. We have determined the radial distribution of the
  r.m.s. magnetic field (i.e. the six quantities &lt; ! !B<SUB></SUB>
  theta <SUP>2</SUP> ! !&gt;, &lt; ! !B<SUB></SUB> theta B<SUB>r</SUB>
  ! !&gt;, etc.). The disc structure is determined with the help of
  the standard thin disc equations by allowing for the influence of the
  magnetic field on temperature and angular momentum. This work is an
  extension of that of Schramkowski et al. (1996) who considered only
  vertical turbulent transport of magnetic energy. We include also radial
  transport, energetically unimportant, but essential in that it fixes
  the radial distribution of the r.m.s. field. Our approach is still
  kinematic. It is found that a magnetic disc is thinner and hotter than
  its non-magnetised counterpart. If the escaping magnetic energy heats a
  disc corona, the radial distribution of the r.m.s. magnetic field may
  be inferred from the `ratio' of the thermal emission of the disc and
  the nonthermal emission of the corona. From the X-ray spectrum of black
  hole candidate LMC X-3 we confirm values in the range of 10<SUP>6</SUP>
  - 10<SUP>7</SUP>G found by Schramkowski et al. (1996). Schramkowski
  G.P. et al.: 1996, A&amp;A 315, 638.

Title: Mean magnetic field and energy balance of Parker's surface-wave
    dynamo.
Authors: Ossendrijver, A. J. H.; Hoyng, P.
Bibcode: 1997A&A...324..329O
Altcode:
  We study the surface-wave dynamo proposed by Parker (1993) as a model
  for the solar dynamo, by solving equations for the mean magnetic
  field {vec}(B)_0_, as well as for the mean 'magnetic energy tensor'
  ((T))=&lt;{vec}(BB)&gt;/8π. This tensor provides information about the
  energy balance, rms field strengths and correlation coefficients between
  field components. The main goal of this paper is to check whether the
  equations for {vec}(B)_0_ and ((T)) are compatible, i.e. whether both
  have "reasonable" solutions for a set of "reasonable" parameters. We
  apply the following constraints: {vec}(B)_0_ has a period of 22 years
  and, taking into account the effect of period variations, a decay time
  of 10 dynamo periods, and ((T)) is marginally stable. We find that
  under these constraints, the equations for {vec}(B)_0_ and ((T)) are
  compatible only if, apart from turbulent transport out of the dynamo
  region, an additional energy sink is introduced. If this extra term is
  omitted, then marginal stability of ((T)) requires a turbulent diffusion
  in the convection zone of the order β_2_&gt;3x10^14^cm^2^/s, whereas
  the conditions on {vec}(B)_0_ require β_2_=~10^12^cm^2^/s. Furthermore,
  the rms surface field strength, the maximum rms field strength and the
  magnetic energy flux through the upper surface of the convection zone
  cannot simultaneously assume solar values. We explore the possibility
  that the extra energy sink is provided by resistive dissipation,
  hitherto not accounted for in the equation for ((T)), by considering
  various cases. We demonstrate that with a heuristically modified
  equation for ((T)), the inconsistencies can be removed. Our results
  suggest that resistive dissipation is the dominant sink of magnetic
  energy, and that resistive heating may amount to several percent of
  the solar luminosity.

Title: GRB 960720 and GRB 970111
Authors: in 't Zand, J.; Heise, J.; Hoyng, P.; Jager, R.; Piro, L.;
   Costa, E.; Feroci, M.; Frontera, F.; dal Fiume, D.; Orlandini, M.;
   Pizzichini, G.; Nicastro, L.; Muller, J.
Bibcode: 1997IAUC.6569....1I
Altcode: 1997IAUC.6569Q...1I
  J. in 't Zand, J. Heise, P. Hoyng, and R. Jager, Space Research
  Organization of the Netherlands (SRON), Utrecht; L. Piro, E. Costa, and
  M. Feroci, Istituto Astrofisica Spaziale, CNR, Frascati; F. Frontera,
  D. Dal Fiume, M. Orlandini, G. Pizzichini, and L. Nicastro, Istituto
  Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, Bologna; and
  J. Muller, BeppoSAX, Rome, and SRON, Utrecht, write: "The positions of
  the x-ray counterparts to GRB 960720 and GRB 970111 as determined with
  the BeppoSAX Wide Field Cameras (cf. IAUC 6467 and 6533, respectively)
  have been refined through an improvement of the spatial calibration of
  the instruments. The calibration concerns small misalignments between
  both cameras and between the cameras and the satellite coordinate
  system, the image scales of both WFCs, and the modeling of the point
  spread function. The calibration was tested with 123 detections of
  catalogued x-ray sources with accurately known positions, as found in 15
  SAX WFC observations with a variety of orientation angles. These sources
  have a representative range of intensities and off-axis angles. None of
  the detections has a difference between catalogue and measured source
  positions &gt; 3'.9, and 97 percent have a difference of &lt; 3'. In a
  subset of fourteen sources having significance levels that are within
  a factor of 2 of the levels of GRB 960720 and GRB 970111, one source
  shows a difference of &gt; 3' from the catalogue position. For GRB
  960720, we find a 3'-radius error circle centered on R.A. = 17h30m37s,
  Decl. = +49o05'.8 (equinox 2000.0). This position is 10'.5 from the
  previously reported position, for which a 10' error circle radius was
  reported. Neither the previously reported SAX NFI source (IAUC 6480)
  nor the optical source (IAUC 6526) lies within the error circle. The
  new error circle does include the radio-loud quasar 4C 49.29 (z =
  1.038). The error circle for GRB 970111 is centered on R.A. = 15h28m15s,
  Decl. = +19o36'.3. This center is 4'.2 from the previously reported
  position, and the circle overlaps the IPN annulus reported by Hurley
  et al. (IAUC 6545). None of the previously reported SAX NFI sources
  (Butler et al., IAUC 6539), RASS sources (Voges et al., IAUC 6539),
  radio source (Frail et al., IAUC 6545), and ROSAT HRI source (IAUC 6567)
  lies within the 3'- radius error circle."

Title: Is the Solar Cycle Timed by a Clock?
Authors: Hoyng, P.
Bibcode: 1996SoPh..169..253H
Altcode:
  Dicke (1978) has argued that the phase of the solar cycle appears to
  be coupled to an internal clock: shorter cycles are usually followed
  by longer ones, as if the Sun remembers the correct phase. The data
  set is really too short to demonstrate the presence of a phase memory,
  but phase and amplitude of the cycle are strongly correlated for 300
  yr or more. It is shown that this memory effect can be explained
  by mean field theory in terms of fluctuations in α, which induce
  coherent changes in the frequency and amplitude of a dynamo wave. It
  is concluded that there is neither a strong observational indication
  nor a theoretical need for an extra timing device, in addition to the
  one provided by dynamo wave physics.

Title: Does the sun have a memory?
Authors: Ossendrijver, A. J. H.; Hoyng, P.; Schmitt, D.
Bibcode: 1996NAWG.1996..247O
Altcode:
  The authors study the effect of stochastic fluctuations in α using
  a simple axisymmetric mean field dynamo model. There are two main
  features: Firstly, the excitation of higher eigenmodes of the dynamo
  equation, providing a possible explanation of asymmetries between the
  two hemispheres and, close to the equator, the formation of small
  regions of the "wrong" polarity. Secondly, a correlation between
  fluctuations in phase and amplitude of the dipole field. Phase and
  amplitude are not locked onto one another, i.e. the dynamo has
  no "memory". The authors use the phase-amplitude correlation to
  compare the model with sunspot data, where a similar correlation is
  observed. This provides a rough estimate of the required strength
  of the α-fluctuations. It is found that α, defined as an azimuthal
  average, has fluctuations of the order δα/α<SUB>0</SUB> ≍ 3 at
  mid latitudes.

Title: Flow and distribution of magnetic energy in accretion disks:
    a kinematic approach.
Authors: Schramkowski, G. P.; van Niekerk, E. C. M.; Hoyng, P.;
   Achterberg, A.
Bibcode: 1996A&A...315..638S
Altcode:
  By taking account of the statistical nature of kinematic mean field
  dynamo theory, it is pointed out that the mean magnetic field &lt;B&gt;
  should always vanish. A more fruitful approach therefore seems to be to
  study the dynamics of the second moment &lt;BB&gt; of magnetic field
  which contains information on mean magnetic energy and statistical
  correlations between the various components of the magnetic field. The
  general dynamics of this tensor are discussed. As an application
  a stationary solution for zero helicity is presented in a local
  approximation. It is found that such a dynamo can only be maintained by
  compressible turbulence. The implications of this non-helical solution
  for the structure and emission of stationary thin accretion disks
  are discussed. The limits of applicability of the kinematic approach
  are briefly discussed. We apply our model to the inner region of the
  accretion disk around the black hole candidate LMC X-3. We find typical
  rms magnetic field strength values in the range 4x10^6^-4x10^7^Gauss.

Title: Book reviews
Authors: Shore, Steven N.; van der Kruit, P. C.; Achterberg, A.;
   Hoyng, P.; de Jager, Cornelis
Bibcode: 1996SSRv...78..557S
Altcode:
  No abstract at ADS

Title: Stochastic and nonlinear fluctuations in a mean field dynamo.
Authors: Ossendrijver, A. J. H.; Hoyng, P.
Bibcode: 1996A&A...313..959O
Altcode:
  We study the effect of rapid stochastic fluctuations in the kinetic
  helicity in a plane parallel mean field dynamo model for the
  Sun. The α-parameter has a fluctuating component δα=α-α_0_,
  which is modelled as a random forcing term. The fluctuations give
  rise to variations in the amplitude and phase of the dynamo wave,
  such that shorter cycles have higher amplitudes, as is observed in
  the solar cycle. By making a second order expansion close to the
  unperturbed marginally stable dynamo wave we are able to go beyond
  the weak forcing limit studied by Hoyng. We show that with increasing
  strength of the forcing the effective dynamo frequency decreases. We
  introduce a simple non-linearity to model α-quenching and derive a
  set of linear equations for the mean field, valid in the weak forcing
  case. With α-quenching, phase and amplitude fluctuations are bounded,
  but still correlated. The strength of the α-quenching is measured by a
  parameter q=-(T_e_/α_0_)(dα/dT)|_T_e__, where T_e_ is the equilibrium
  value of the toroidal field. We make a comparison with sunspot data, and
  conclude that these are well explained by the model if δα/α_0_=~2.2
  and q=~0.7. Finally we briefly consider the alternative possibility of
  fluctuations caused by nonlinear dynamics, without external forcing
  (δα=0). We show that the resulting phase-amplitude diagram does
  not agree with observations. Although this is no proof that the
  phase-amplitude correlation cannot be reproduced by nonlinear chaos,
  we conclude that stochastic noise provides a more natural explanation.

Title: Stochastic excitation and memory of the solar dynamo.
Authors: Ossendrijver, A. J. H.; Hoyng, P.; Schmitt, D.
Bibcode: 1996A&A...313..938O
Altcode:
  We consider a simple axisymmetric mean field dynamo model for the
  Sun in the α{OMEGA}-limit and study the effect of rapid, latitude
  dependent stochastic fluctuations in α. The fluctuations excite
  overtones of the fundamental mode of the mean magnetic field. We
  decompose the mean field into eigenmodes and derive an equation for
  the mode coefficients. Transient mode excitation gives rise to a
  mean field with spatial and temporal variability, and may provide an
  explanation for grand minima, the observed phase-amplitude correlation,
  North-South asymmetries and, close to the equator, reverse polarity
  regions in the solar butterfly diagram. We find that the North-South
  asymmetry often peaks near the activity minimum, in agreement with
  the observations. The most likely candidate for the origin of the
  fluctuations are giant cells. Sunspot data are well reproduced if α,
  defined as an azimuthal average, has fluctuations δα of the order
  δα/α_0_=~3 at colatitude θ=60deg, assuming that there are 20 giant
  cells from pole to pole, with a coherence time of 1 month. The model
  predicts that the resulting phase and amplitude fluctuations of the
  fundamental mode are correlated for about 90 dynamo periods.

Title: Book reviews
Authors: Humphreys, R. M.; Kemp, S.; Savonije, G.; van der Hucht,
   K. A.; van der Kruit, P. C.; Miley, G.; Bumba, V.; van Nieuwkoop,
   J.; van Hoolst, T.; Cox, A.; Rutten, R. J.; Kleczek, J.; de Jager,
   Cornelis; Jerzykiewicz, M.; Zwaan, C.; Poedts, S.; Sakai, Jun-Ichi;
   Pecker, J. -C.; Heikkila, W.; de Jong, T.; Wilson, P. R.; Müller,
   E. A.; Hoyng, P.; Icke, V.; Shore, S. N.; Achterberg, A.; Lucchin, F.;
   Butcher, H.; Ne'Eman, Y.; Heidmann, J.; Belton, M. J. S.; de Graauw,
   Th.; Waters, L. B. F. M.; Pacini, F.; Hultqvist, B.; Akasofu, S. -I.;
   Vial, J. -C.; Schatzman, E.; van der Laan, H.; Cole, K. D.; Vanbeveren,
   D.; Southwood, D.; van der Klis, M.; Katgert, Peter
Bibcode: 1996SSRv...76..339H
Altcode:
  No abstract at ADS

Title: STARS - Seismic Telescope for Astrophysical Research from
    Space. Report on the phase A study.
Authors: Badiali, M.; Catala, C.; Favata, F.; Fridlund, M.; Frandsen,
   S.; Gough, D. O.; Hoyng, P.; Pace, O.; Roca-Cortés, T.; Roxburgh,
   I. W.; Sterken, C.; Volonté, S.
Bibcode: 1996star.book.....B
Altcode:
  STARS is an asteroseismology mission, which, through the acquisition of
  very accurate light curves, will detect oscillations in a wide variety
  of stars. This will allow, for the first time, the internal structure
  of stars of different age, composition and spectral type, to be studied
  directly, bringing the spectacular successes of helioseismology within
  reach for a large number of other stars spreading over most of the
  Hertzsprung-Russell diagram, including solar-type stars in open
  clusters. The results of the phase A study are presented in this report.

Title: Solar and stellar dynamos
Authors: Hoyng, P.
Bibcode: 1996ASPC..109...59H
Altcode: 1996csss....9...59H
  No abstract at ADS

Title: STARS: Seismic Telescope for Astrophysical Research from Space
Authors: Badiali, M.; Catala, C.; Favata, F.; Fridlund, M.; Frandsen,
   S.; Gough, D. O.; Hoyng, P.; Pace, O.; Roca-Cortes, T.; Roxburgh,
   I. W.; Sterken, C.; Volonte, A.
Bibcode: 1996ESADS...4....1B
Altcode:
  No abstract at ADS

Title: Does the Sun Have a Memory?
Authors: Ossendrijver, A. J. H.; Hoyng, P.; Schmitt, D.
Bibcode: 1996sgmf.proc..247O
Altcode:
  We study the effect of stochastic fluctuations in alpha using
  a simple axisymmetric mean feld dynamo model. There are two main
  features. Firstly, the excitation of higher eigenmodes of the dynamo
  equation, providing a possible explanation of asymmetries between the
  two hemispheres and, close to the equator, the formation of small
  regions of the 'wrong' polarity. Secondly, a correlation between
  fluctuations in phase and amplitude of the dipole field. Phase and
  amplitude are not locked onto one another, i.e. the dynamo has no
  'memory'. We use the phase-amplitude correlation to compare the
  findings of our model with sunspot data, where a similar correlation
  is observed. This provides a rough estimate of the required strength
  of the alpha-fluctuations. It is found that alpha, defined as an
  azimuthal average, has fluctuations of the order delta alpha / alpha ~
  3 at mid latitudes.

Title: Book Review: Lectures on solar and planetary dynamos [invited
    papers] / Cambridge U Press, 1995
Authors: Hoyng, P.; Somov, B. V.; Hoyng, P.
Bibcode: 1995SoPh..160...61H
Altcode: 1995SoPh..160...61P
  No abstract at ADS

Title: Grail: a proposal for a gravitational Radiation Antenna in
    the Netherlands
Authors: Frossati, G.; Steel, D.; de Zeeuw, T.; Gaemers, K.; Linde,
   F.; van den Heuvel, E.; van den Klis, M.; Sloot, P.; Hoekstra, A.;
   de Ronde, J.; van den Graaf, H.; Hartjes, F.; van Holten, J. W.;
   Nooren, G. J.; Oberski, J.; de Waele, A.; van den Heijden, R. W.;
   Rogalla, H.; Flokstra, J.; Ter Brake, H.; Veldhuis, D.; Fluitman,
   J. H.; Bereschot, J. W.; Eldersboek, M.; Lodder, J. C.; de Haab, P.;
   Hoyng, P.; Spallicci, A.
Bibcode: 1995grai.rept....1F
Altcode:
  No abstract at ADS

Title: Dynamo spectroscopy.
Authors: Hoyng, P.; Schutgens, N. A. J.
Bibcode: 1995A&A...293..777H
Altcode:
  We study the properties of the magnetic field {vec}(B)
  of a dynamo by expanding the field in a special set of
  base functions, namely the eigenfunctions of its dynamo equation
  [{vec}(B)={SIGMA}<SUB>sigma</SUB>_c<SUP>sigma</SUP>^(t){vec}(b)<SUP>sigma</SUP>^({vec}(r))].
  We prove that the time evolution of the mean &lt;c<SUP>sigma</SUP>^&gt;
  and the frequency spectrum of c<SUP>sigma</SUP>^(t) are
  completely determined by the eigenvalue λ<SUB>sigma</SUB>_ of
  the dynamo equation. The spectrum is a Lorentzian with central
  frequency ω<SUB>sigma</SUB>_=Imλ<SUB>sigma</SUB>_ and width
  γ<SUB>sigma</SUB>_=-Reλ<SUB>sigma</SUB>_. This property had been
  conjectured by Hoyng (1988), and is now shown to hold for arbitrary
  dynamos. The proof is relatively straightforward, and we point out the
  error in the analysis of Hoyng (1988). Finally, we illustrate how this
  property sheds light on the physical meaning of the dynamo equation,
  and we compare the predicted spectra qualitatively with recent studies
  of the global properties of the solar magnetic field.

Title: The effect of random alpha-fluctuations and the global
    properties of the solar magnetic field.
Authors: Hoyng, P.; Schmitt, D.; Teuben, L. J. W.
Bibcode: 1994A&A...289..265H
Altcode:
  We study the effect of rapid random fluctuations in the dynamo parameter
  α in a simple axisymmetric mean-field dynamo. The model is 1D; it is
  a shell with latitude-dependent fields. Radial turbulent diffusion is
  modeled by a prescribed factor exp(imagkr)/r in the field potentials. We
  consider mainly linear models. The fluctuations excite overtones of
  the fundamental mode which are otherwise damped. Butterfly diagrams
  and frequency spectra S_l_(ν) of the Legendre expansion coefficients
  c_l_(t) of the toroidal mean field [B(θ,t)={SIGMA}_l_c_l_(t)P_l_(cos
  θ)] are compared with observations of the solar magnetic field. The
  results are: (1). The model accounts for the observed relative phases
  of the coefficients c_l_(t) for odd l at the frequency 1/(22yr)
  of the fundamental mode, and potentially also for their relative
  amplitudes. (2). The spectra S_l_(ν) are broad and featureless for
  even l, while for odd l the frequency 1/(22yr) of the fundamental
  mode dominates. They agree qualitatively with the results of Gokhale
  and coworkers. It is unlikely that the modal structure in the surface
  field for even l found by Stenflo and colleagues can be produced by mean
  field models with random α-fluctuations. (3). Butterfly diagrams have
  a solar-type structure for 1&lt;~kR&lt;~5 (R = position of the bottom
  of the convection zone). The fluctuations generate many phenomena also
  seen in the solar cycle, such as slow activity modulations due to mode
  beating, accidental disappearances of the activity, transequatorial
  activity, and sometimes quite persistent North-South asymmetries in
  the butterfly diagram. The character of the latter depends sensitively
  on the value of kR. (4). The amplitudes of the eigenmodes are shown
  to behave as randomly excited coupled oscillators, which allows us
  to explain the excitation levels of the overtones, their frequency
  spectra and the structure of the North-South asymmetries. (5). In the
  latitude region where the dynamo operates the local fluctuations in
  ({vec}(u).∇ x {vec}(u))τ_c_ are ~60 to 70 times larger than the
  mean value of ({vec}(u).∇ x {vec}(u))τ_c_, and the fluctuations in
  α are 6 to 7 times larger than the mean value of α.

Title: Distribution and flow of magnetic energy in an accretion disk
Authors: Schramkowski, G. P.; van Niekerk, E. C. M.; Hoyng, P.;
   Achterberg, A.
Bibcode: 1994SSRv...68..329S
Altcode:
  No abstract at ADS

Title: The solar dynamo
Authors: Hoyng, P.
Bibcode: 1994ASIC..433..387H
Altcode:
  No abstract at ADS

Title: Random forcing and the mode structure of the solar magnetic
    field
Authors: Schmitt, D.; Hoyng, P.; Teuben, L. J. W.
Bibcode: 1994smf..conf..110S
Altcode:
  No abstract at ADS

Title: Helicity fluctuations in mean field theory: an explanation
    for the variability of the solar cycle?
Authors: Hoyng, P.
Bibcode: 1993A&A...272..321H
Altcode:
  We consider the effect of fluctuations δα(t) in the
  mean helicity α<SUB>0</SUB> (both assumed independent of
  position) on a plane dynamo wave. The time scale τ<SUB>c</SUB>
  of the fluctuations is much shorter than the diffusion time
  1/β<SUB>0</SUB>κ<SUP>2</SUP>(β<SUB>0</SUB>κ<SUP>2</SUP>τ<SUB>c</SUB>
  ≪ 1; β<SUB>0</SUB> = turbulent diffusion
  coefficient; κ = wave number). We distinguish
  weak and strong random forcing, according to whether
  (δα<SUB>r.m.s.</SUB>/α<SUB>0</SUB>)√β<SUB>0</SUB>κ<SUP>2</SUP>τ<SUB>c</SUB>
  is small or large with respect to 1, and we present a detailed analysis
  of the weak forcing case. <P />Simple equations are derived for the
  phases and the logarithmic amplitudes of the poloidal and toroidal
  mean field, in which the forcing terms appear as additive noise. Phase
  difference and amplitude ratio of the poloidal and toroidal (mean) field
  are subject to small fluctuations of constant r.m.s. magnitude. Simple
  expressions are derived for the r.m.s. phase shift, amplitude
  drift and quality factor of the toroidal (mean) field. These
  depend on the fluctuations only through the quantity D = ¼(
  δα<SUB>r.m.s.</SUB>/α<SUB>0</SUB>)<SUP>2</SUP>β<SUB>0</SUB>κ<SUP>2</SUP>τ<SUB>c</SUB>
  which plays the role of a diffusion coefficient. The results are:
  (1). Phase shift Δ and logarithmic amplitude Λ each perform a
  random walk; (2). In the α<SUP>2</SUP>-limit these random walks are
  uncorrelated; the phase is very stable but the amplitude is completely
  irregular; (3). In the αω-limit there exists a correlation: Λ +
  Δ ≃ 0, which persists for many dynamo periods. The quality factor
  is then given by Q = 1/D. <P />The model is then applied to the solar
  dynamo. The predicted correlation Λ + Δ ≃ 0 implies that weaker
  (stronger) cycles last longer (shorter) than average, which is a
  well-known observed feature of the solar cycle. We define Λ and Δ
  using the epochs of solar maxima and the sunspot numbers, and show that
  Λ + Δ ≃ 0 is obeyed rather well. This indicates that fluctuations
  in the mean helicity are an important mechanism causing the observed
  phase and amplitude variations of the solar cycle. Simulations show
  many features also seen in the solar cycle, such as quasi-periodicity,
  intermittency and long periods of low activity. Further inferences are:
  (1). The sunspot numbers appear to be proportional to the strength of
  the toroidal field; (2). The quality factor Q is about 10, which makes
  the solar dynamo a border-line case between weak and strong forcing;
  (3). The solar data indicate that it is necessary to allow for nonlinear
  effects; (4). The mean helicity fluctuations δα(t) are caused by very
  large spatial fluctuations in the local helicity. This could explain
  the discrepancy between theoretical estimates for α<SUB>0</SUB>
  and values derived from mean field models.

Title: Prisma - the First Space Mission to See Inside the Stars
Authors: Appourchaux, T.; Gough, D. O.; Catala, C.; Frandsen, S.;
   Frohlich, C.; Hoyng, P.; Jones, A.; Lemaire, P.; Tondello, G.; Weiss,
   W. W.
Bibcode: 1993ASPC...42..411A
Altcode: 1993gong.conf..411A
  No abstract at ADS

Title: Can Mean Helicity Fluctuations Explain the Variability of
    the Solar Cycle?
Authors: Hoyng, P.
Bibcode: 1993IAUS..157...71H
Altcode:
  No abstract at ADS

Title: Distribution of magnetic energy in -dynamos, I: The method
Authors: van Geffen, J. H. G. M.; Hoyng, P.
Bibcode: 1993GApFD..71..187V
Altcode:
  In this paper a method for solving the equation for the mean magnetic
  energy &lt;BB&gt; of a solar type dynamo with an axisymmetric convection
  zone geometry is developed and the main features of the method are
  described. This method is referred to as the finite magnetic energy
  method since it is based on the idea that the real magnetic field B of
  the dynamo remains finite only if &lt;BB&gt; remains finite. Ensemble
  averaging is used, which implies that fields of all spatial scales are
  included, small-scale as well as large-scale fields. The method yields
  an energy balance for the mean energy density B2/8 of the dynamo, from
  which the relative energy production rates by the different dynamo
  processes can be inferred. An estimate for the r.m.s. field strength
  at the surface and at the base of the convection zone can be found by
  comparing the magnetic energy density and the outgoing flux at the
  surface with the observed values. We neglect resistive effects and
  present arguments indicating that this is a fair assumption for the
  solar convection zone. The model considerations and examples presented
  indicate that (1) the energy loss at the solar surface is almost
  instantaneous; (2) the convection in the convection zone takes place
  in the form of giant cells; (3) the r.m.s. field strength at the base
  of the solar convection zone is no more than a few hundred gauss; (4)
  the turbulent diffusion coefficient within the bulk of the convection
  zone is about 1014cm2s-1, which is an order of magnitude larger than
  usually adopted in solar mean field models.

Title: PRISMA: Probing Stars from Core to Corona
Authors: Appourchaux, T.; Catala, C.; Catalano, S.; Fransden, S.;
   Fröhlich, C.; Gough, D. O.; Hoyng, P.; Jones, A.; Lemaire, P.;
   Tondello, G.; Weiss, W.
Bibcode: 1993ASSL..183..505A
Altcode: 1993pssc.symp..505A
  No abstract at ADS

Title: Stochastic excitation of global magnetic fields by fluctuations
    in the mean helicity
Authors: Hoyng, P.; van Geffen, J. H. G. M.
Bibcode: 1993GApFD..68..203H
Altcode:
  The mean field B in the dynamo equation can be interpreted as a
  longitude-averaged field, and this implies that there are fluctuations
  in the mean parameters characterizing the turbulent flow. In this paper
  we study the effect of fluctuations in the mean helicity numerically
  and analytically in a very simple spherical 2-dynamo: there is no
  differential rotation and the non-fluctuating parts of and do not depend
  on position (we call this the 2-sphere dynamo). The dynamo equation then
  contains a term × (t)B, which describes the effect of the fluctuations
  in the mean helicity. We show that this type of random forcing implies
  that the dynamo has to operate (slightly) subcritically, and that in
  addition many eigenmodes are excited, rather than only the fundamental
  mode. The advantage of this simple α2-dynamo model is that we can
  support the numerical results with analytical estimates, for instance,
  for the value of the dynamo number at which the dynamo operates,
  the relative excitation levels of the modes, and their spectra. This
  is achieved with the help of an expansion technique: B is expanded
  in terms of a complete, orthogonal set of eigenfunctions. We have
  taken the mean helicity fluctuations to be position-independent for
  simplicity. This, however, renders the dynamo model so simple that only
  dipole fields are excited and magnetic field reversals are absent. We
  also briefly study the effect of non-linearities, in particular of
  α-effect quenching. Non-linearities provide a reference level to the
  fundamental mode, but do not affect the relative excitation levels of
  the modes.

Title: Book review
Authors: Hoyng, P.
Bibcode: 1992SoPh..137..203H
Altcode:
  No abstract at ADS

Title: Book-Review - Helioseismology from Space
Authors: Foing, B. H.; Hoyng, P.
Bibcode: 1992SoPh..137..203F
Altcode:
  No abstract at ADS

Title: Mean Field Dynamo Theory
Authors: Hoyng, P.
Bibcode: 1992ASIC..373...99H
Altcode: 1992sla..conf...99H
  No abstract at ADS

Title: The velocity sensitivity of resonant scattering spectrometers
    employing a piezoelastic modulator
Authors: Hoyng, P.
Bibcode: 1991SoPh..133...43H
Altcode:
  The resonant scattering spectrometers of the IRIS ground-based
  network for measuring whole-disc solar velocity oscillations make
  use of a piezoelastic modulator. The velocity noise generated by
  this optical component is analysed with particular emphasis on the
  required stability of the amplitude of oscillation, a. The product
  of the absolute stability ¦ a − a<SUB>m</SUB>¦/a<SUB>m</SUB>and
  the relative stability δa<SUB>r.m.s.</SUB>/a<SUB>m</SUB> may not be
  larger than 10 <SUP>−4</SUP> to 10 <SUP>−5</SUP> (depending on
  specific wishes), where a<SUB>m</SUB>is the optimum amplitude. The
  velocity noise due to photon statistics is slightly enhanced, but
  other instrumental sources of velocity noise remain unaffected.

Title: Non-Steady Global Magnetic Fields in Kinematic Theory
Authors: van Geffen, J. H. G. M.; Hoyng, P.; Zwaan, C.
Bibcode: 1991LNP...380..129V
Altcode: 1991IAUCo.130..129V; 1991sacs.coll..129V
  The dynamo equation for the mean field (B) contains a random forcing
  term of unknown magnitude, which is therefore always omitted. The
  influence of this term is potentially large. To evaluate its effect,
  we employ ensemble averaging. If an ensemble average is used, there
  is no random forcing term in the dynamo equation. The effect of
  fluctuations is that the ensemble members get out of phase, so that
  (B) 0. The damping time of (B) can be found by requiring that the mean
  energy (BB) remains finite. The eigenvalues of the dynamo equation then
  all have negative real parts. Im determines the period, and -Re/Im the
  relative period stability of the dynamo. We have developed a code to
  solve the equation for (BB) in a spherical shell (the convection zone),
  assuming axisymmetry. We report our first results, which do not yet
  include differential rotation. Using spherically symmetric boundary
  conditions, we reproduce the well known 2-dynamo, whose behaviour is
  known analytically. For instance, for an 2-dynamo located in a shell
  with inner boundary at R/2, we find that (BB) remains finite for
  R2/ = 1.48, where 3 represents turbulent diffusion and .y turbulent
  vorticity. Taking = 1/4(βy)1/2 — a factor of four below maximum
  helicity — implies that we have a dynamo number Ca ≡ Rα/β =
  0.30. Using this value we find a damping time of 6 X 10-2 R 2/β
  for (B), which is a measure for the coherence time of B in a single
  ensemble member. This result implies that the large-scale field of
  this particular α2-dynamo reorganizes its structure completely on
  a time scale of only about one year (for solar values of R and β),
  and it shows the enormous influence of random forcing in general.

Title: Book reviews
Authors: Haubold, Hans Joachim; Snijders, M. A. J.; van Rensbergen,
   W.; de Jager, Cornelis; van de Weygaert, R.; Achterberg, A.; Linssen,
   P. F. J.; Hoyng, P.; Kleczek, J.; Bauer, Siegfried J.; Hayakawa, S.
Bibcode: 1990SSRv...54..187H
Altcode:
  No abstract at ADS

Title: Mechanisms for Dynamo Mode Excitation
Authors: Hoyng, P.
Bibcode: 1990IAUS..142...45H
Altcode:
  No abstract at ADS

Title: Book Review: Instabilities in space and laboratory plasmas. /
    CUP, 1986.
Authors: Hoyng, P.
Bibcode: 1990SSRv...54..192H
Altcode: 1990SSRv...54..192M
  No abstract at ADS

Title: Excitation of Dynamo Modes
Authors: Hoyng, P.
Bibcode: 1990IAUS..138..359H
Altcode:
  No abstract at ADS

Title: Turbulent transport of magnetic fields. V. Distribution of
    magnetic energy in a simple 2-dynamo
Authors: van Geffen, J. H. G. M.; Hoyng, P.
Bibcode: 1990GApFD..53..109V
Altcode:
  In this paper we analyse the stationary mean energy density tensor
  Tij = BiBj for the x2-sphere. This model is one of the simplest
  possible turbulent dynamos, originally due to Krause and Steenbeck
  (1967): a conducting sphere of radius R with homogeneous, isotropic and
  stationary turbulent convection, no differential rotation and negligible
  resistivity. The stationary solution of the (linear) equation for Tij
  is found analytically. Only Trr, Tθθ and Tφ φ are unequal to zero,
  and we present their dependence on the radial distance r. <BR /><BR
  />The stationary solution depends on two coefficients describing the
  turbulent state: the diffusion coefficient β≈ &lt; u2 &gt; τ c/3
  and the vorticity coefficient γ≈ &lt;|dtri ×u|2&gt;τc/3 where
  u(r, t) is the turbulent velocity and c its correlation time. But
  the solution is independent of the dynamo coefficient α≈-&lt;
  u· dtri ×u&gt; τc/3 although α does occur in the equation for
  Tij. This result confirms earlier conclusions that helicity is not
  required for magnetic field generation. In the stationary state,
  magnetic energy is generated by the vorticity and transported to the
  boundary, where it escapes at the same rate. The solution presented
  contains one free parameter that is connected with the distribution
  of B over spatial scales at the boundary, about which Tij gives no
  information. We regard this investigation as a first step towards the
  analysis of more complicated, solar-type dynamos.

Title: On the Sensitivity of Resonant Scattering Spectrometers for
    Whole-Disk Solar Velocity Oscillation Measurements
Authors: Hoyng, P.
Bibcode: 1989ApJ...345.1088H
Altcode:
  The sensitivity of resonant scattering spectrometers for measuring
  solar velocity oscillations in integrated light is analyzed. It is
  shown that, with the help of simple analytical tools and qualitative
  arguments, most of the contributions to the instrumental sensitivity
  can be estimated. Although the frequency dependence of the individual
  sources of instrument noise cannot always be predicted reliably in
  this way, it is possible to derive useful estimates of instrumental
  parameters via comparison with a desired noise level.

Title: Turbulent transport of magnetic fields. IV - Damping of the
    mean field (B) in alpha-squared dynamos with alpha proportional to
    COS Theta
Authors: van Geffen, J. H. G. M.; Hoyng, P.
Bibcode: 1989A&A...213..429V
Altcode:
  It is proven that the dynamo equation for the mean field (B) of the
  spherical alpha-squared dynamo with alpha proportional to cos Theta
  and constant beta have only exponentially damped solutions, if the
  mean energy density (B-squared) is nonzero and finite. The origin and
  interpretation of this phenomenon is reviewed.

Title: Book Review: The internal solar angular velocity. / Reidel,
    1987
Authors: Hoyng, P.
Bibcode: 1989SSRv...49..438H
Altcode: 1989SSRv...49..438D
  No abstract at ADS

Title: Turbulent Transport of Magnetic Fields. III. Stochastic
    Excitation of Global Magnetic Modes
Authors: Hoyng, P.
Bibcode: 1988ApJ...332..857H
Altcode:
  Studies outlining how the role of fluctuation in the turbulent
  convection can be analyzed with ensemble averages are reviewed and the
  relation between the observed magnetic field and the ensemble average
  magnetic field is solved by expanding the observed field in terms of
  eigenfunctions of the dynamo equation. The effect of the fluctuations
  is shown to take the form of stochastic excitation of all eigenmodes
  of the dynamo equation, rather than steady excitation of only the
  fundamental eigenmode, as in traditional dynamo models. The solar
  dynamo is considered and the occurrence of coronal holes and of field
  reversals in the earth's dynamo are discussed. A comparison is made
  between this problem and the problem of scalar transport.

Title: Book review
Authors: Hoyng, P.
Bibcode: 1988SoPh..117..203H
Altcode:
  No abstract at ADS

Title: Book-Review - Advances in - and Asteroseismology
Authors: Christensen-Dalsgaard, J.; Frandsen, S.; Hoyng, P.
Bibcode: 1988SoPh..117..203C
Altcode:
  No abstract at ADS

Title: Turbulent transport of magnetic fields. I - A simple mechanical
    model. II - The role of fluctuations in kinematic theory
Authors: Hoyng, P.
Bibcode: 1987A&A...171..348H
Altcode:
  In the present paper, a simple mechanical model is analysed that
  reproduces a few basic features of the average magnetic field in the Sun
  and the Earth. It provides an intuitive picture of how the α-effect
  operates in combination with differential rotation. The model serves
  three purposes: (1) it enhances an intuitive understanding of the
  behaviour of a magnetic field in turbulent fluids; (2) it shows that
  kinematic models (and ultimately, non-linear models also) must allow for
  fluctuations as one is essentially dealing with a stochastic process;
  (3) it serves as a starting point for a re-analysis of the kinematic
  dynamo problem.

Title: Turbulent Transport of Magnetic Fields - Part Two - the Role
    of Fluctuations in Kinematic Theory
Authors: Hoyng, P.
Bibcode: 1987A&A...171..357H
Altcode:
  It is argued that due to fluctuations in the turbulent convection,
  the ensemble averaged field &lt; B &gt; of a dynamo must damp
  out to zero if &lt;BB&gt;, in particular &lt; B<SUP>2</SUP> &gt;,
  is to remain finite. This idea is supported by two case studies:
  a simple α<SUP>2</SUP> dynamo and the αΩ dynamo in the local
  approximation. Apart from the well-known constants α and β, the
  equation for &lt; BB &gt; contains the parameter γ ≡ &lt; ∣
  V × u<SUB>1</SUB>∣<SUP>2</SUP> &gt; τ<SUB>c</SUB>/3, that also
  occurred in the model analysed the preceding paper. These results lead
  to a new statistical formulation of kinematic dynamo theory, which
  must eventually be extended to nonlinear theory. New aspects are:
  1) Turbulent transport of B is a stochastic process and fluctuations
  have a large influence; 2) B in the dynamo fluctuates on all spatial
  scales and bears only a statistical relation to &lt; B &gt;; 3)
  The frequency stability δω/ω of αΩ dynamos and the average time
  between spontaneous reversals in α<SUP>2</SUP> dynamos are determined
  by kinematic theory; for the solar dynamo δω/ω is estimated to
  be larger than observed, but nonlinear effects may cure this; 4) The
  average magnetic stress tensor (i. e. energy density, Lorentz force and
  field cross-correlation coefficients) as well as γ are determinable
  quantities (for the Sun γ ≍ 10<SUP>-10</SUP> s<SUP>-1</SUP>); 5) In
  principle, helical turbulence (α ≠ 0) is not necessary for dynamo
  action; 6) The global resonances found by Stenflo and Vogel, and the
  appearance of coronal holes may correspond to stochastic excitations of
  overtones of the dynamo equation. The meaning of the ensemble average,
  the energy balance of the dynamo and the physics of turbulent mixing
  are discussed in detail. The limitations are: zero resistivity, and
  homogeneous, isotropic turbulence with a short correlation time.

Title: Generalized Compton-Getting transformation for nonrelativistic
    particles.
Authors: Stevens, G. A.; Hoyng, P.
Bibcode: 1986A&A...168..354S
Altcode:
  The problem of how to construct the particle distribution function in
  a frame of reference that moves with respect to the frame in which
  the observations were made is considered. The starting point of the
  analysis is to expand the velocity distribution functions in both
  frames of reference in spherical harmonics. A solution of this so-called
  transformation problem is given in integral form, together with a method
  that allows practical application. The (optimum) data acquisition and
  data reduction method is discussed in detail. Six examples are given
  that illustrate various aspects of the transformation problem. Finally,
  the transformation method is tested and conditions for a reliable
  construction of the three-dimensional distribution function in the
  moving frame are investigated. The statistical aspects of the problem
  are briefly discussed.

Title: Book-Review - Localisation and Orientation in Biology and
    Engineering
Authors: Ubbels, G. A.; Hovenier, J. W.; Russell, C. T.;
   Nieuwenhuijzen, H.; Heintze, J. R. W.; Linssen, P. F. J.; van der
   Kruit, P. C.; Thé, P. S.; Somogyi, Antal J.; Cuperman, S.; Rephaeli,
   Y.; Stirpe, G. M.; Kleczek, J.; Kresák, L.; de Kool, M.; Wapstra,
   A. H.; Swanenburg, B. N.; Hoyng, P.; Reijnen, G. C. M.; Somov,
   B. V.; van der Hucht, K. A.; Sehnal, L.; Namba, O.; Schwartz, A. W.;
   v. D. Stadt, H.; McNally, D.; de Hoop, D.
Bibcode: 1986SSRv...44..393U
Altcode:
  No abstract at ADS

Title: Book-Review - Seismology of the Sun and the Distant Stars
Authors: Gough, D. O.; Hoyng, P.
Bibcode: 1986SSRv...44..401G
Altcode:
  No abstract at ADS

Title: Observations of Steady Anomalous Magnetic Heating in Thin
    Current Sheets
Authors: Martens, P. C. H.; van den Oord, G. H. J.; Hoyng, P.
Bibcode: 1985SoPh...96..253M
Altcode:
  A faint steadily emitting loop-like structure has been observed
  by HXIS in its low energy channels (3.5-8.0 keV) on November 5/6,
  1980. These HXIS observations have permitted us to follow the thermal
  evolution of this loop for a period of about 15 hr and from this study
  we conclude that only a fraction of 0.1% of the volume of the loop is
  steadily heated at the rather large rate of 0.6 erg cm<SUP>-3</SUP>
  s<SUP>-1</SUP>. We interpret this heating as the dissipation of magnetic
  fields in thin current sheets and we find that the dissipation with
  classical resistivity is very unlikely, while ion-kinetic tearing, as
  proposed by Galeev et al. (1981), suits the observations very well. The
  enhancement of the resistivity over the classical resistivity then
  turns out to be a factor 4 × 10<SUP>4</SUP>. Dissipation in extremely
  thin sheets via the ion-acoustic instability (Duijveman et al., 1981)
  cannot be completely excluded when the cross-field heat conductivity
  is anomalously enhanced by a factor 400.

Title: Microwave and X-ray observations of delayed brightenings at
    sites remote from the primary flare locations
Authors: Nakajima, H.; Dennis, B. R.; Hoyng, P.; Nelson, G.; Kosugi,
   T.; Kai, K.
Bibcode: 1985ApJ...288..806N
Altcode:
  Five examples of solar flares observed with the 17-GHz interferometer
  at Nobeyama in which a secondary microwave burst occurred at a
  distance of 100,000 km to 1,000,000 km from the primary flare site
  are presented. The secondary microwave burst in all five cases had
  a similar time profile to the primary burst with a delay of 2 to
  25 s. The velocity of a triggering agent inferred from this delay
  and spatial separation is 10,000 km to 100,000 km/s. The intensity
  of the secondary burst was a factor of 3 to 25 smaller than that of
  the primary burst in all events except for one case in which it was
  a factor of 2 larger. The polarization degree of the secondary burst
  at 17 GHz was 35 percent, significantly higher than the average value
  for typical impulsive bursts. Two of the events were accompanied by
  meterwave type III/V bursts located high in the corona between the
  primary and secondary sites. For two of the other events, X-ray images
  of the secondary source were obtained with the hard-X-ray imaging
  spectrometer on the Solar Maximum Mission. These observations strongly
  suggest that the distant microwave bursts were produced by electrons
  with energies of 10 keV to 100 keV which were channeled along a huge
  loop from the main flare site to the remote location.

Title: Kinematic dynamo theory for an arbitrary mean flow.
Authors: Hoyng, P.
Bibcode: 1984ESASP.220...97H
Altcode: 1984ESPM....4...97H
  The problem of an arbitrary, incompressible mean flow v<SUB>0</SUB> in
  kinematic dynamo theory is amenable to a systematic treatment leading
  to an intuitively simple result. The author's approach is based on
  the theory of stochastic differential equations.

Title: Microwave and X-ray observations of delayed brightenings at
    sites remote from the primary flare locations
Authors: Nakajima, H.; Dennis, B. R.; Hoyng, P.; Nelson, G.; Kosugi,
   T.; Kai, K.
Bibcode: 1984STIN...8433326N
Altcode:
  Five examples of solar flares observed with the 17-GHz interferometer
  at Nobeyama in which a secondary microwave burst occurred at a
  distance of 100,000 km to 1,000,000 km from the primary flare site
  are presented. The secondary microwave burst in all five cases had
  a similar time profile to the primary burst with a delay of 2 to 25
  s. The velocity of a triggering agent inferred from this delay and
  spatial separation is 10,000 km to 100,000 km/s. The intensity of the
  secondary burst was a factor of 3 to 25 smaller than that of the primary
  burst in all events except for one case in which it was a factor of 2
  larger. The polarization degree of the secondary burst at 17 GHz was
  35%, significantly higher than the average value for typical impulsive
  bursts. Two of the events were accompanied by meterwave type III/V
  bursts located high in the corona between the primary and secondary
  sites. For two of the other events, X-ray images of he secondary source
  were obtained with the hard-X-ray imaging spectrometer on the Solar
  Maximum Mission. These observations strongly suggest that the distant
  microwave bursts were produced by electrons with energies of 10 keV
  to 100 keV which were channeled along a huge loop from the main flare
  site to the remote location.

Title: Microwave and X-ray Observations of Delayed Brightenings at
    Sites Remote from the Primary Flare Locations
Authors: Nakajima, H.; Dennis, B. R.; Hoyng, P.; Nelson, G.; Kosugi,
   T.; Kai, K.
Bibcode: 1984BAAS...16..524N
Altcode:
  No abstract at ADS

Title: Report of ESA's topical team on solar and heliospheric physics.
Authors: Christensen-Dalsgaard, J.; Delache, P.; Hoyng, P.; Priest,
   E. R.; Schwenn, R.; Stenflo, J. O.
Bibcode: 1984ESASP1070...26C
Altcode:
  No abstract at ADS

Title: Hard X-Ray Studies of the Large Coronal Feature on June
    29, 1980
Authors: Harrison, R. A.; Simnett, G. M.; Hoyng, P.; Lafleur, H.;
   van Beek, H. F.
Bibcode: 1984sii..conf..287H
Altcode:
  No abstract at ADS

Title: Evidence for Extensive Magnetic Structures Between Two Active
    Regions from Studies of Flares on June 24, 1980
Authors: Simnett, G. M.; Harrison, R. A.; Hoyng, P.; van Beek, H. F.
Bibcode: 1984sii..conf..273S
Altcode:
  No abstract at ADS

Title: Analysis of the 1980 November 18 limb flare observed by the
    hard X-ray imaging spectrometer (HXIS)
Authors: Haug, E.; Elwert, G.; Hoyng, P.
Bibcode: 1984AdSpR...4g.211H
Altcode: 1984AdSpR...4..211H
  X-ray images of the 18 November 1980 limb flare taken by the HXIS
  instrument aboard SMM were analysed. The hard X-rays originated from
  three spots on the SW limb of the solar disk with different altitudes
  and time evolution. The locations of the brightest spots in hard and
  soft X-rays are compared with the predictions of flare models. The
  X-ray spctra from the pixels with highest count rates can be fitted by
  power laws. The spatial variation of the spectral index is in agreement
  with the existence of a non-thermal electron component.

Title: Imaging of Impulsive Solar Flare Phenomena
Authors: Duijveman, A.; Hoyng, P.
Bibcode: 1983SoPh...86..279D
Altcode:
  We review some recent advances in our understanding of impulsive solar
  flare phenomena obtained through new hard X-ray and radio imaging
  instruments (the Solar Maximum Mission and Hinotori satellites, the
  VLA and VLBI).

Title: Microwave and hard X-ray imaging of a solar flare on 1980
    November 5
Authors: Hoyng, P.; Marsh, K. A.; Zirin, H.; Dennis, B. R.
Bibcode: 1983ApJ...268..865H
Altcode:
  VLA and SMM hard X ray data on the solar flares of November 5, 1980 are
  analyzed and compared with data from other sources. The VLA provided
  measurements at 15 GHz at 10 sec intervals, using left and right
  circular polarizations with a 0.6 arcsec resolution. The hard X ray
  imaging spectrometer on the SMM obtained data in six bands from 3.5-30
  keV, with 8 x 8 arcsec resolution and 1.5 sec separation. The data were
  examined for a possible nonthermal source for the microwave component
  of the emissions detected, the origin of 16-30 keV excess fluxes,
  the relation between the X ray and microwave sources, the magnetic
  connection between observed loops, and the physical characteristics
  of the radiating loop. The data were consistent with a model that
  assumes fast electrons are accelerated to a single power-law energy
  distribution and freely stream along the magnetic field. The data also
  agreed with a thick-target model for solar flare X ray emission.

Title: The structure and evolution of a solar flare as observed in
    3.5 30 keV X-rays
Authors: Harrison, R. A.; Simnett, G. M.; Hoyng, P.; Lafleur, H.;
   van Beek, H. F.
Bibcode: 1983SoPh...84..237H
Altcode:
  On July 5, 1980 the Hard X-Ray Imaging Spectrometer on board the Solar
  Maximum Mission observed a complex flare event starting at 22 : 32 UT
  from AR 2559 (Hale 16955), then at N 28 W 29, which developed finally
  into a 2-ribbon flare. In this paper we compare the X-ray images with
  Hα photographs taken at the Big Bear Solar Observatory and identify
  the site of the most energetic flare phenomena. During the early
  phases of the event the hard X-rays (&gt;16 keV) came from a compact
  source located near one of the two bright Hα kernels; we believe the
  latter are at the footpoints of a compact magnetic loop. The kernel
  identified with the X-ray source is immediately adjacent to one of the
  principal sunspots and in fact appears to `rotate' around the sunspot
  over 90° in the early phase of the flare. Two intense X-ray bursts
  occur at the site of the rotating kernel, and following each burst
  the loop fills with hot, X-ray emitting plasma. If the first burst is
  interpreted as bremsstrahlung from a beam of electrons impinging on a
  collisionally dominated medium, the energy in such electrons, &gt;16
  keV, is ∼ 5 × 10<SUP>30</SUP> erg. The altitude of the looptop is
  7-10 × 10<SUP>3</SUP> km. The temperature structure of the flare is
  extremely non-homogeneous, and the highest temperatures are found in
  the top of the loop.

Title: X-Ray Imaging of Three Flares during the Impulsive Phase
Authors: Duijveman, A.; Hoyng, P.; Machado, M. E.
Bibcode: 1982SoPh...81..137D
Altcode:
  The impulsive phases of three flares that occurred on April 10, May 21,
  and November 5, 1980 are discussed. Observations were obtained with the
  Hard X-ray Imaging Spectrometer (HXIS) and other instruments aboard
  SMM, and have been supplemented with Hα data and magnetograms. The
  flares show hard X-ray brightenings (16-30 keV) at widely separated
  locations that spatially coincide with bright Hα patches. The bulk of
  the soft X-ray emission (3.5-5.5 keV) originates from in between the
  hard X-ray brightenings. The latter are located at different sides
  of the neutral line and start to brighten simultaneously to within
  the time resolution of HXIS. Concluded is that: The bright hard X-ray
  patches coincide with the footpoints of loops.

Title: Impulsive Acceleration and Heating in Flares
Authors: Hoyng, P.
Bibcode: 1982Obs...102..119H
Altcode:
  No abstract at ADS

Title: Observations of a POST Flare Radio Burst in X-Rays
Authors: Svestka, Z.; Hoyng, P.; van Tend, W.; Boelee, A.; de Jager,
   C.; Stewart, R. T.; Acton, L. W.; Bruner, E. C.; Gabriel, A. H.;
   Rapley, C. G.; de Jager, C.; LaFleur, H.; Nelson, G.; Simnett, G. M.;
   van Beek, H. F.; Wagner, W. J.
Bibcode: 1982SoPh...75..305S
Altcode:
  More than six hours after the two-ribbon flare of 21 May 1980, the
  hard X-ray spectrometer aboard the SMM imaged an extensive arch above
  the flare region which proved to be the lowest part of a stationary
  post-flare noise storm recorded at the same time at Culgoora. The X-ray
  arch extended over 3 or more arc minutes to a projected distance of
  95 000 km, and its real altitude was most probably between 110 000
  and 180 000 km. The mean electron density in the cloud was close to
  10<SUP>9</SUP> cm<SUP>−3</SUP> and its temperature stayed for many
  hours at a fairly constant value of about 6.5 × 10<SUP>6</SUP> K. The
  bent crystal spectrometer aboard the SMM confirms that the arch emission
  was basically thermal. Variations in brightness and energy spectrum at
  one of the supposed footpoints of the arch seem to correlate in time
  with radio brightness suggesting that suprathermal particles from
  the radio noise regions dumped in variable quantities into the low
  corona and transition layer; these particles may have contributed to
  the population of the arch, after being trapped and thermalized. The
  arch extended along the H<SUB>∥</SUB> = 0 line thus apparently
  hindering any upward movement of the upper loops reconnected in the
  flare process. There is evidence from Culgoora that this obstacle may
  have been present above the flare since 15-30 min after its onset.

Title: Origin and Location of the Hard X-Ray Emission in a Two-Ribbon
    Flare
Authors: Hoyng, P.; Duijveman, A.; Machado, M. E.; Rust, D. M.;
   Svestka, Z.; Boelee, A.; de Jager, C.; Frost, K. T.; Lafleur, H.;
   Simnett, G. M.; van Beek, H. F.; Woodgate, B. E.
Bibcode: 1981ApJ...246L.155H
Altcode:
  No abstract at ADS

Title: Fast plasma heating by anomalous and inertial resistivity
    effects in the solar atmosphere
Authors: Duijveman, A.; Hoyng, P.; Ionson, J. A.
Bibcode: 1981ApJ...245..721D
Altcode:
  A simple model is presented to describe fast plasma heating by anomalous
  and inertial resistivity effects. It is noted that a small fraction of
  the plasma contains strong currents that run parallel to the magnetic
  field and are driven by an exponentiating electric field. The anomalous
  character of the current dissipation derives from the excitation of
  electrostatic ion-cyclotron and/or ion-acoustic waves. The possible
  role of resistivity deriving from geometrical effects ('inertial
  resistivity') is also considered. Using a marginal stability analysis,
  equations for the average electron and ion temperatures are derived
  and numerically solved. No loss mechanisms are taken into account. The
  evolution of the plasma is described as a path in the drift velocity
  diagram, where the drift velocity is plotted as a function of the
  electron to ion temperature ratio.

Title: The Relationship Between the Microwave and Hard X-Ray Sources
    in a Solar Flare
Authors: Marsh, K. A.; Zirin, H.; Hoyng, P.; Dennis, B. R.
Bibcode: 1981BAAS...13..889M
Altcode:
  No abstract at ADS

Title: The limb flare of 1980 April 30 as seen by the hard X-ray
    imaging spectrometer
Authors: van Beek, H. F.; de Jager, C.; Schadee, A.; Svestka, Z.;
   Boelee, A.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.; Fryer,
   R.; Simnett, G. M.; Imhof, J. P.; LaFleur, H.; Maseland, H. V. A. M.;
   Mels, W. M.; Schrijver, J.; van der Laan, J. J. M.; van Rens, P.; van
   Tend, W.; Werkhoven, F.; Willmore, A. P.; Wilson, J. W. G.; Machado,
   M. E.; Zandee, W.
Bibcode: 1981ApJ...244L.157V
Altcode:
  X-ray imaging of the limb event of 1980 April 30 shows that the flaring
  involved two distinct components: a pointlike component, which was the
  source of the initial hard X-ray burst and an extensive tongue reaching
  some 30,000 km above the limb. The tongue had a higher temperature
  than the other parts of the structure and seemed to be enhanced by
  energetic electrons that derived their energy from the initial source.

Title: Hard X-ray imaging of two flares in active region 2372
Authors: Hoyng, P.; Duijveman, A.; Boelee, A.; de Jager, C.; Galama,
   M.; Hoekstra, R.; Imhof, J.; Lafleur, H.; Machado, M. E.; Fryer, R.
Bibcode: 1981ApJ...244L.153H
Altcode:
  Hard X-ray images of two flares observed by the Hard X-ray Imaging
  Spectrometer (HXIS) aboard SMM on 1980 April 7 and 10 are discussed. A
  comparison with H-alpha images and the photospheric magnetic field
  maps shows that the emission originates in (arcades of) loops which
  differ greatly in the hardness of the X-ray spectra. On April 7 the
  hardest X-ray emission coincided with the brightest H-alpha patch. On
  April 10 the most intense X-ray emission appeared to be concentrated
  in a looplike structure with a softer spectrum at the top and a
  harder spectrum in the legs. Temperature estimates from flux ratios
  in different energy bands tend to confirm that small, hot components
  are embedded in more extensive, cooler flaring regions.

Title: Structural development of the X-ray limb flare of 30 April 1980
Authors: de Jager, C.; Hoyng, P.; Lafleur, H.; Schadee, A.; Svestka,
   Z.; van Beek, H. F.; van Tend, W.; Fryer, R.; Simnett, G. M.
Bibcode: 1981AdSpR...1m.251D
Altcode: 1981AdSpR...1..251D
  We describe the development of the limb flare of 30 April 1980, 20:20
  UT, as observed by the Hard X-ray Imaging Spectrometer (HXIS) aboard
  the Solar Maximum Mission (SMM). It consisted of a short-lived bright
  nucleus (FWHM &lt; 10,000 km), just inside the Sun's limb; a longer
  lasting tongue, extending to a height of ~ 30,000 km, and a more
  complicated feature, approximately situated at the Sun's limb. The
  tongue was a pre-existing magnetic structure that started emitting
  X-rays only a few seconds after the bright nucleus, and which had a
  slightly higher temperature than the nucleus; its X-ray emission may
  be caused by electrons escaped from the nucleus.

Title: Solar maximum mission experiment: Early results of the hard
    X-ray imaging experiment
Authors: Boelee, A.; de Jager, C.; Duijveman, A.; Galama, M.; Hoekstra,
   R.; Hoyng, P.; Imhof, J. P.; Lafleur, H.; Maseland, H. V. A. M.; Mels,
   W. A.; Schadee, A.; Schrijver, J.; Svestka, Z.; van Beek, H. F.;
   van Rens, P.; van der Laan, J. J. M.; van Tend, W.; Werkhoven, F.;
   Wiersma, G.; Zandee, W.; Simnett, G. M.; Charlton, C. P.; Fryer, R.;
   Willmore, A. P.; Wilson, J. W. G.; Machado, M. E.
Bibcode: 1981AdSpR...1m.255B
Altcode: 1981AdSpR...1R.255B
  We have selected four widely different flares from the early
  period of operations of the Hard X-Ray Imaging Spectrometer (HXIS)
  on SMM to illustrate the characteristic imaging properties of this
  experiment. For the small flare of April 4, 1980, we demonstrate the
  instrument's capability for locating a compact source. In the weak,
  but extensive, flare of April 6 we show how well the instrument can
  display spatial structure, and also the low level of the instrument
  background. In the 1B flare of April 7 we are able to locate positions
  of the X-ray emission in the soft and hard channels, and estimate
  the positional variations of the emission patches. Finally, in the IN
  flare of April 10, which produced the strongest hard X-ray burst we
  have seen so far, we repeat some of the studies made for the April 7
  event, and also demonstrate the capability of the HXIS instrument to
  study the development, with high time resolution, of individual 8''
  × 8'' elements of the flare.

Title: Solar Maximum Mission experiment - Early results of the hard
    X-ray imaging experiment
Authors: Simnett, G. M.; Charlton, C. P.; Fryer, R.; Boelee, A.;
   de Jager, C.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.;
   Imhof, J. P.
Bibcode: 1981AdSpR...1m.255S
Altcode: 1981AdSpR...1..255S
  Four widely different flares from the early period of operations
  of the Hard X-ray Imaging Spectrometer (HXIS) on SMM have been
  selected to illustrate the characteristic imaging properties of this
  experiment. For the small flare of April 4, 1980, the instrument's
  capability for locating a compact source is demonstrated. In the weak,
  but extensive, flare of April 6, the ability of the instrument to
  display spatial structure, and also the low level of the instrument
  background, are shown. In the 1B flare of April 7, positions of the
  X-ray emission in the soft and hard channels are capable of being
  located, and the positional variations of the emission patches can
  be estimated. Finally, in the 1N flare of April 10, which produced
  the strongest hard X-ray burst seen so far, some of the studies made
  for the April 7 event are repeated, and the capability of the HXIS
  instrument to study the development, with high time resolution, of
  individual 8 x 8 arcsec elements of the flare is also demonstrated.

Title: Structural development of the X-ray limb flare of 30 April
    1980.
Authors: de Jager, C.; Fryer, R.; Hoyng, P.; Lafleur, H.; Schadee,
   A.; Simnett, G. M.; Svestka, Z.; van Beek, H. F.; van Tend, W.
Bibcode: 1981hea..conf..251D
Altcode:
  No abstract at ADS

Title: Solar Maximum Mission experiment: early results of the hard
    X-ray imaging experiment.
Authors: Simnett, G. M.; Boelee, A.; Charlton, C. P.; de Jager, C.;
   Duijveman, A.; Fryer, R.; Galama, M.; Hoekstra, R.; Hoyng, P.; Imhof,
   J. P.; Lafleur, H.; Machado, M. E.; Maseland, H. V. A. M.; Mels,
   W. A.; Schadee, A.; Schrijver, J.; Svestka, Z.; van Beek, H. F.;
   van Rens, P.; van der Laan, J. J. M.; van Tend, W.; Werkhoven, F.;
   Wiersma, G.; Willmore, A. P.; Wilson, J. W. G.; Zandee, W.
Bibcode: 1981hea..conf..255S
Altcode:
  No abstract at ADS

Title: Strong Langmuir wave turbulence - Some results with
    selfconsistent Landau damping
Authors: van Grunsven, T. F. J.; Hoyng, P.; Nicholson, D. R.
Bibcode: 1980A&A....91....7V
Altcode:
  An initial value problem of the high-frequency Langmuir wave electric
  field in two spatial dimensions expressed by the split-timestep Fourier
  method is solved for the case when two equally strong wave vector pumps
  are initialized in the k-space. One case corresponds to weak Langmuir
  turbulence indicating that time evolution agrees with weak turbulence
  concepts of Landau damping and three-wave decay interactions. In the
  case of strong Langmuir turbulence, the soliton collapse to the smallest
  length scales is prevented by Landau damping. It is concluded that the
  shape of electron velocity distribution is approximately constant up to
  a specific maximum velocity, but rapidly decreases at larger velocities.

Title: Impulsive electron acceleration to energies of tens of kT/e/
    by Langmuir wave turbulence
Authors: Hoyng, P.; Duijveman, A.; van Grunsven, T. F. J.; Nicholson,
   D. R.
Bibcode: 1980A&A....91...17H
Altcode:
  An expression which describes the evolution of the tail of electron
  distribution determined by the spatial inhomogeneity of the Langmuir
  wave and escape of fast electrons from the acceleration region is
  applied to specific situations. One case analyzed a many-current-layer
  model with properties for accelerating electron streams which generate
  type III solar radio bursts; the model shows that it can emit an
  electron flux of 10 to the 32nd/s, with a switch-on time of 0.05 s,
  and a maximum acceleration energy of 50 keV. Another problem involves
  a single current layer (shock wave) accelerating electrons to energies
  of 10 kT(e); the electrons can be additionally accelerated by a second
  stage process. The first (fast stage) electron acceleration to 100
  to 1000 kT(e) can only result from high frequency electrostatic wave
  turbulence if the shock has a complex structure.

Title: Recent Results from the Solar Maximum Mission: Observational,
    Interpretational and Theoretical Aspects
Authors: Hoyng, P.
Bibcode: 1980BAAS...12..903H
Altcode:
  No abstract at ADS

Title: Multifarious Spatial Structure in a Compact Hard X-Ray Flare
Authors: Harrison, R. A.; Vanbeek, H. F.; Hoyng, P.; Lafleur, H.;
   Simnett, G. M.
Bibcode: 1980BAAS...12..911H
Altcode:
  No abstract at ADS

Title: A Study of the Impulsive Flares and Homologous Flares From
    AR2372 From April 6-13, Using Hard X-Ray Images
Authors: Simnett, G. M.; Machado, M.; Harrison, R. A.; Hoyng, P.;
   Lafleur, H.; Svestka, Z.; Vanbeek, H. F.
Bibcode: 1980BAAS...12S.899S
Altcode:
  No abstract at ADS

Title: Hard X-Ray Images of the Continuously Active Region 2478 from
    June 3-6, 1980
Authors: Fryer, R. J.; Hoyng, P.; Lafleur, H.; Simnett, G. M.; van
   Beek, H. V.
Bibcode: 1980BAAS...12..892F
Altcode:
  No abstract at ADS

Title: Brief Report of Meeting - IAU / Solar Maximum Year
Authors: Švestka, Zdenk; Van Hoven, Gerard; Hoyng, Peter; Kuperus, Max
Bibcode: 1980SoPh...67..379S
Altcode:
  No abstract at ADS

Title: The hard X-ray imaging spectrometer (HXIS).
Authors: van Beek, H. F.; Hoyng, P.; Lafleur, B.; Simnett, G. M.
Bibcode: 1980SoPh...65...39V
Altcode:
  The HXIS, a joint instrument of the Space Research Laboratory at
  Utrecht, The Netherlands, and the Department of Space Research of
  the University of Birmingham, U.K., images the Sun in hard X-rays: Six
  energy bands in energy range 3.5-30 keV, spatial resolution 8″ over Ø
  2'40″ and 32″ over Ø 6'24″ field of view, and time resolution of
  0.5-7 s depending on the mode of operation. By means of a `flare flag'
  it alerts all the other SMM instruments when a flare sets in and informs
  them about the location of the X-ray emission. The experiment should
  yield information about the position, extension and spectrum of the hard
  X-ray bursts in flares, their relation to the magnetic field structure
  and to the quasi-thermal soft X-rays, and about the characteristics
  and development of `type IV' electron clouds above flare regions.

Title: A Model for Impulsive Electron Acceleration to Energies of
    Tens of Kt/e
Authors: Hoyng, P.; Duijveman, A.; van Grunsven, T. F. J.; Nicholson,
   D. R.
Bibcode: 1980IAUS...91..299H
Altcode:
  No abstract at ADS

Title: Energetic particles in solar flares
Authors: Ramaty, R.; Paizis, C.; Colgate, S. A.; Dulk, G. A.; Hoyng,
   P.; Knight, J. W.; Lin, R. P.; Melrose, D. B.; Orrall, F.; Shapiro,
   P. R.
Bibcode: 1980sfsl.work..117R
Altcode: 1980sofl.symp..117R
  The various manifestations of energetic particles in solar
  flares are examined, and possible mechanisms for the acceleration
  of these particles are considered. Hard X-ray observations and
  possible mechanisms for the production of the dominant form of solar
  energetic particles, electrons with energies between 10 and 100 keV,
  are discussed, with consideration of thin-target models, thick-target
  models and thick-target models with reverse currents, and first-phase
  acceleration mechanisms for energetic electrons emitting impulsive
  microwave and fast-drift Type III radio bursts as well as impulsive hard
  X rays, which are detected themselves 20 min after the flare at 1 AU
  are considered. Radio evidence on the number, energy and pitch-angle
  distributions of energetic particles produced during solar flares
  is summarized, and observations at 1 AU of proton and electron energy
  spectra, the proton/electron ratio and energetic particle events rich in
  He-3 from solar flares are discussed. Finally, consideration is given to
  gamma-ray evidence of nuclear reactions in flares and white-light flares

Title: Relaxation and bremsstrahlung of thick-target electron streams:
    a simple application of the Legendre expansion method.
Authors: Hoyng, P.; Melrose, D. B.; Adams, J. C.
Bibcode: 1979ApJ...230..950H
Altcode:
  The Legendre series expansion of the three-dimensional equations for
  electron velocity and Langmuir wave distributions provides a numerical
  treatment for the relaxation of a stationary, axisymmetric electron
  stream in a homogeneous and fully ionized hydrogen plasma. Both
  quasilinear and Coulomb interactions figure in the treatment. The
  electrons are represented by a Fokker-Planck system of equations,
  which allows elimination of the distinction between background and
  stream electrons. A computation of the bremsstrahlung associated with
  the isotropic component of the electron distribution yields a simple
  model for radiation from thick-target electron streams, for example
  in impulsive hard X-ray emission from solar flares. Results of the
  study suggest power law-like bremsstrahlung spectra as they are often
  observed can be produced quite readily.

Title: Practical use of the Legendre-expanded quasi-linear equations
    for anisotropic particles and Langmuir waves.
Authors: Hoyng, P.; Stevens, G. A.; Melrose, D. B.
Bibcode: 1979ApJ...230..961H
Altcode:
  The Legendre expansion for the three-dimensional quasilinear
  equations for axisymmetric streams of particles and Langmuir waves
  is discussed. In particular, it is shown that a well-posed numerical
  problem based on the Legendre expansion can be defined for conditions
  equivalent to a decomposition into an arbitrary number of mutually
  coinciding one-dimensional streams of particles and waves. The
  difficulty of negative distribution functions occurring in the Legendre
  expansion has also proven tractable. The Legendre expansion method
  is applicable to the study of the suprathermal streams of electrons
  which occur frequently in astrophysical plasmas.

Title: Energetic particles in solar flares. Chapter 4 in the
    proceedings of the 2nd SKYLAB Workshop on Solar Flares
Authors: Ramaty, R.; Colgate, S. A.; Dulk, G. A.; Hoyng, P.; Knight,
   J. W., III; Lin, R. P.; Melrose, D. B.; Paizis, C.; Orrall, F.;
   Shapiro, P. R.
Bibcode: 1978epsf.proc.....R
Altcode:
  The recent direct observational evidence for the acceleration of
  particles in solar flares, i.e. radio emission, bremsstrahlung
  X-ray emission, gamma-ray line and continuum emission, as well as
  direct observations of energetic electrons and ions, are discussed
  and intercorrelated. At least two distinct phases of acceleration of
  solar particles exist that can be distinguished in terms of temporal
  behavior, type and energy of particles accelerated and the acceleration
  mechanism. Bulk energization seems the likely acceleration mechanism
  for the first phase while Fermi mechanism is a viable candidate for
  the second one.

Title: Nonlinear Langmuir waves during type III solar radio bursts.
Authors: Nicholson, D. R.; Goldman, M. V.; Hoyng, P.; Weatherall, J. C.
Bibcode: 1978ApJ...223..605N
Altcode:
  Type III solar radio bursts are thought to be associated with
  intense levels of electron-beam-excited Langmuir waves. The nonlinear
  evolution of these waves, in time and in two spatial dimensions, due to
  their coupling to other waves is studied numerically. For parameters
  appropriate to one-half the earth-sun distance, nonlinear effects are
  found to be important, as in previous one-dimensional work. However, a
  new and important phenomenon, two-dimensional soliton collapse, is found
  to occur. This collapse, induced directly by the wave-packet nature of
  the beam-excited waves, produces two-dimensional wave spectra extending
  over a much broader range of wave numbers than has been predicted by
  inhomogeneous quasi-linear theory. The results compare favorably with
  certain aspects of recent observations. The background magnetic field
  is neglected; while substantially justified for the present parameters,
  this neglect may require reexamination at locations closer to the sun.

Title: Diagnostics of solar flare hard X-ray sources.
Authors: Hoyng, P.; Knight, J. W.; Spicer, D. S.
Bibcode: 1978SoPh...58..139H
Altcode:
  The dynamics of hard X-ray producing electron beams in solar flares
  can be strongly affected by the occurrence of a reverse current. The
  parameter diagram for a beam can be divided into three regimes, one
  of which is the usual thick target case, the two others being due to
  two different possible consequences of the reverse current. The use
  of this parameter diagram as a possible diagnostic tool for solar
  flare hard X-ray sources is discussed, together with the necessary
  observations and their interpretation.

Title: Legendre expansion of the quasi-linear equations for
    anisotropic particles and Langmuir waves.
Authors: Hoyng, P.; Melrose, D. B.
Bibcode: 1977ApJ...218..866H
Altcode:
  The quasi-linear diffusion and friction coefficients for axisymmetric
  electron distributions interacting with Langmuir waves are evaluated
  explicitly by expanding the distribution of waves in Legendre
  polynomials. The quasi-linear equations are then reduced to a
  form in which both the distributions of waves and of particles are
  simultaneously expanded in Legendre polynomials, and all coefficients
  are evaluated explicitly. It is argued that such expansions are likely
  to be justified in practice and that the results obtained should prove
  useful in discussing quasi-linear relaxation under various conditions in
  three dimensions rather than one dimension. New results are anticipated
  for the problem of the propagation of electron streams causing type
  III solar radio bursts. The influence of the magnetic field on the
  Langmuir waves is neglected.

Title: Book reviews
Authors: Reijnen, G. C. M.; Ness, Norman F.; Kliore, Arvydas J.;
   Sonnenschein, F. J.; Hoogenboom, A. M.; Hack, Margherita; Johnson,
   F. S.; Reijnen, G. C. M.; Kleczek, J.; van Bueren, H. G.; de Graaff,
   W.; Hoyng, P.; Swider, W.; Grevesse, N.
Bibcode: 1977SSRv...20..677R
Altcode:
  No abstract at ADS

Title: A two-component model of impulsive microwave burst emission
    consistent with soft and hard X rays.
Authors: Boehme, A.; Fuerstenberg, F.; Hildebrandt, J.; Saal, O.;
   Krueger, A.; Hoyng, P.; Stevens, G. A.
Bibcode: 1977SoPh...53..139B
Altcode:
  A two-component (core-halo) emission model has been applied
  reconciling hard and soft X-ray burst emissions with the microwave
  burst radiation. The core region is represented by a nonthermal
  energy distribution (Maxwellian+power law tail) and assumed to be
  surrounded by a thermal halo. Parameters characterizing the energy
  distribution and emission measures have been derived numerically from
  soft and hard X-ray measurements. Using an artificial magnetic field
  model the microwave flux spectrum has been calculated on the basis of
  gyro-synchrotron emission and absorption by solving the equation of
  radiation transfer along the ray trajectories. Open parameters were
  used to adapt the spectrum to the radio measurements.

Title: Radiation from a source in a cold magnetoactive plasma,
    re-examined. Application to cyclotron and multipole radiation
Authors: Hoyng, P.; Stevens, G. A.
Bibcode: 1977PhFl...20..520H
Altcode:
  An analysis is presented of the radiation emitted by an arbitrary
  source embedded in a cold, magnetoactive plasma and physically
  distinct from the latter. The plasma is supposed to be infinite and
  homogeneous; its dielectric properties are described by a dielectric
  tensor ɛ. Expressions for the radiation fields are derived using
  the technique of Fourier decomposition. An expression for the vector
  potential is constructed and elaborated as far as possible for an
  arbitrary current source. The approach differs from that in previous
  work on technical points, the main one being the sequence in which the
  various integrations are carried out. The radiation flux is defined on
  the basis of Poynting's vector S; a distinction is made between current
  sources behaving as a given function of time and randomly fluctuating
  sources. In the latter case an ensemble average is preferred over
  a time average. A comparison is made with existing treatments in the
  literature, and a variety of defects is pointed out. The general result
  for the radiation flux is then specified for cyclotron radiation from a
  stationary ensemble of electrons and for multipole radiation. Throughout
  the paper a compact notation is used based on the work of Bremmer.

Title: Book reviews
Authors: Reijnen, G. C. M.; Kleczek, J.; Millman, Peter M.; Vesseur,
   H. J. A.; Bar-Nun, Akiva; de Jager, C.; van Albada, T. S.; Rawer,
   K.; Hinze, J. O.; Trümper, J.; de Jager, Cornelis; Müller, O.;
   Kovalevsky, J.; Hammerschlag, R. H.; Hoyng, Peter
Bibcode: 1977SSRv...20..235R
Altcode:
  No abstract at ADS

Title: On the Nature of Impulsive Electron Acceleration in Solar
    Hard X-ray Flares. II. A Theory
Authors: Hoyng, P.
Bibcode: 1977A&A....55...31H
Altcode:
  Summary. The suggestion is elaborated that shock wave generated
  Langmuir waves accelerate electrons in the adjoining plasma. Langmuir
  wave generation can be achieved in ion- acoustic unstable shocks by
  induced bremsstrahlung from electrons. A crude model analysis shows the
  Langmuir waves to have short wavelengths, k kD/4, while propagating
  almost parallel to the shock plane. It is possible that sufficient
  power in Langmuir waves is generated to explain the observed scale of
  electron acceleration. The evolution of the Langmuir wave distribution
  emerging from the shock is analyzed including the effects of spatial
  gradients. The initial stage is dominated by nonlinear Landau damping,
  which isotropizes the waves and increases their wavelengths, leading
  eventually to the onset of the modulational instability. Virtually
  all wave energy is now converted into fast electrons by Landau
  damping. Depending on parameters, these processes are completed within
  a distance of the order of 100 m from the shock. Next follows an
  extended nonthermal region in which fast electrons (+ selfconsistent
  level of Langmuir waves) slowly relax to thermal equilibrium. The
  length of this region is t0 -t0 km and depends among other factors,
  on the amount of fast electron scattering. The resulting fast electron
  velocity distribution is not discussed. However, there is hope for
  a universal distribution because strong wave-wave interactions take
  place before acceleration, while redistribution of energy between fast
  electrons occurs afterwards. Key words: solar flare hard X-rays shock
  wave stochastic acceleration parametric instability

Title: On the nature of impulsive electron acceleration in solar
    hard X-ray flares. I. Inferences from observations.
Authors: Hoyng, P.
Bibcode: 1977A&A....55...23H
Altcode:
  The nature of the impulsive electron acceleration regularly observed
  in solar flares is investigated on the basis of relevant observations,
  primarily in hard X-rays. Essential observational data are reviewed
  for flares that most clearly exhibit electron acceleration, the
  bremsstrahlung interpretation is adopted, and acceleration by a dc
  field or electron runaway is ruled out. Acceleration by collective
  wave-particle interactions is considered, with attention focused on
  Langmuir waves. It is proposed that Langmuir waves are most likely
  generated in the slow shock waves of Petschek's (1964) model of
  magnetic-field-line merging, after which they propagate outward and
  accelerate electrons in a relatively large volume. The generation
  mechanism of such waves in ion-acoustic turbulent shocks is then
  analyzed, and the question of electron acceleration is examined in
  some detail. The evolution of the Langmuir-wave distribution resulting
  in electron acceleration is computed along with the relaxation of the
  fast electrons back to thermal equilibrium.

Title: High time resolution analysis of solar hard X-ray flares
    observed on board the ESRO TD-1A satellite.
Authors: Hoyng, Peter; Brown, John C.; van Beek, H. Frank
Bibcode: 1976SoPh...48..197H
Altcode:
  The Utrecht solar hard X-ray spectrometer S-100 on board the ESRO TD-1A
  satellite covers the energy range above 25 keV with 12 logarithmically
  spaced channels. Continuous sun-pointing is combined with high time
  resolution: 1.2 s for the four low energy channels (25-90 keV) and
  4.8 s for the others. It is emphasized that the instrument design and
  calibration yield data virtually free of pile-up and other instrumental
  defects.

Title: An Error Analysis of Power Spectra
Authors: Hoyng, P.
Bibcode: 1976A&A....47..449H
Altcode:
  Summary. An expression is derived for the uncertainty in the Fourier
  power spectrum due to noise present in the data. We consider both
  series with Poisson-distributed noise as well as with normal-distributed
  noise. The expressions for the errors involve only the power spectrum
  itself and they can be readily generalized to arbitrarily distributed
  noise. A working example is discussed. Key words: error analysis -
  power spectra - Fourier transform

Title: Betatron acceleration in a large solar hard X-ray burst.
Authors: Brown, J. C.; Hoyng, P.
Bibcode: 1975ApJ...200..734B
Altcode:
  The problem of diagnosing flare particle acceleration mechanisms from
  hard X-ray bursts is discussed, and it is argued that the electron
  trap model of bursts is more amenable to observational investigation
  at present than models of thick-target type. It is then shown that
  data for the large X-ray burst of 1972 August 4 are consistent with
  the source electrons being trapped in a very large vibrating coronal
  magnetic bottle. Furthermore, the observations show that the burst time
  profile is not dominated by collisional losses. It is proposed instead
  that the entire profile is essentially determined by betatron action
  of the varying trap field on the electrons. This betatron model is
  then analyzed in detail and shown to predict very well the observed
  correlation of electron flux and spectral index in this event when
  it is supposed that the electrons are initially produced by runaway
  in a direct electric field. Comparison of the model with observations
  permits inference of the approximate form of magnetic field evolution
  in the trap. Finally the physics behind this field evolution is briefly
  considered. Subject headings: flares, solar - X-rays, solar

Title: Hard X-rays from the sun.
Authors: van Beek, H. F.; Hoyng, P.; de Jager, C.; Stevens, G. A.
Bibcode: 1975NTNA...41..101V
Altcode:
  No abstract at ADS

Title: High Time Resolution Analysis of Solar Flares Observed on
    the ESRO Td-Ia Satellite
Authors: Hoyng, P.; Brown, J. C.; Stevens, G.; van Beek, H. F.
Bibcode: 1975IAUS...68..233H
Altcode:
  No abstract at ADS

Title: Studies on hard X-ray emission from solar flares and on
    cyclotron radiation from a cold magnetoplasma
Authors: Hoyng, P.
Bibcode: 1975PhDT.........1H
Altcode:
  This thesis proposes an interpretation of hard X-ray emission from solar
  flares and presents a theoretical study on the generation of cyclotron
  radiation by a source in a cold magnetically active plasma. Hard
  X-ray observations by the ESRO TD-1A satellite are analyzed, the
  solar flares of May 18 and August 4, 1972, are examined in detail,
  and a set of consistent parameters are derived for the flare of
  May 18 from observations of hard-X rays and simultaneously emitted
  centimeter radio waves. A model for the flare of August 4 is suggested
  in which a coronal magnetic trap is filled with fast electrons by
  some acceleration process, trap eigenmode oscillations are excited,
  and expansion occurs. It is proposed that electrons in ordinary solar
  flares are accelerated by resonant interactions with Langmuir waves
  generated in thin current sheets and propagating laterally from the
  sheets. In the cyclotron study, the radiation flux is defined on the
  basis of Poynting's vector, and an expression for the harmonic frequency
  is found which differs nontrivially from the one commonly used.

Title: High time resolution analysis of solar flares observed on
    the ESRO TD-1A satellite.
Authors: Hoyng, P.; Brown, J. C.; Stevens, G.; van Beek, H. F.
Bibcode: 1975IAUS...68Q.233H
Altcode:
  No abstract at ADS

Title: High time resolution analysis of solar flares observed on
    the ESRO TD-1A satellite.
Authors: Hoyng, P.; Brown, J. C.; Stevens, G.; van Beek, H. F.
Bibcode: 1975IAUS...68R.233H
Altcode:
  No abstract at ADS

Title: Studies on hard X-ray emission from solar flares and on
    cyclotron radiation from a cold magnetoplasma.
Authors: Hoyng, P.
Bibcode: 1975UtrOv.311.....H
Altcode:
  No abstract at ADS

Title: On the Determination of the Photospheric Velocity Distribution
    from Profiles of Weak Fraunhofer Lines
Authors: Rutten, R. J.; Hoyng, P.; de Jager, C.
Bibcode: 1974SoPh...38..321R
Altcode:
  We derive the conditions under which the profile of a weak Fraunhofer
  line can be described as the convolution of the separate profiles
  of damping, thermal and non-thermal motions at the average depth of
  formation of the line. The average velocity distribution along the line
  of sight, rather than its customary chosen macro- and micro-turbulent
  components, is then found from the deconvolution of the observed profile
  with the known other contributions. Reversely, the observed profiles
  can be compared to predicted profiles on the basis of De Jager's (1974)
  theoretical turbulence broadening curves.

Title: On the Formation and Unfolding of Pulse Height Distributions
Authors: Hoyng, P.; Stevens, G. A.
Bibcode: 1974Ap&SS..27..307H
Altcode:
  Spacecraft measurements of X-ray or particle pulse height distributions
  have become increasingly accurate during the last fifteen years,
  and they will continue to do so. The present paper deals with the
  question how one can reconstruct original photon or particle spectra
  from measured pulse height distributions. The statistical aspects
  of the formation of pulse height distributions are investigated. A
  method is presented that allows for a reliable reconstruction of the
  original spectrum. Its essentials are the formulation and subsequent
  solution of a matrix equation connecting pulse height distribution
  with photon/particle spectrum; an error analysis of the reconstructed
  spectrum is given. The present method has two advantages over the
  usualχ <SUP>2</SUP>-minimum method: It is able to recover more
  spectral detail and it requires less computing time. Finally, a
  numerical example is given.

Title: Interpretation of a Hard Solar X-Ray Burst
Authors: Hoyng, P.; Stevens, G. A.
Bibcode: 1973sari.conf...97H
Altcode:
  No abstract at ADS

