Author name code: rouppevandervoort
ADS astronomy entries on 2022-09-14
=author:"Rouppe van der Voort, Luc H.M." OR =author:"Rouppe van der Voort, Luc" OR =author:"Rouppe van der Voort, L.H.M." OR =author:"Rouppe van der Voort, L." OR author:"van der Voort, L.R." 
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Title: Properties of ubiquitous magnetic reconnection events in the
    lower solar atmosphere
Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M.
Bibcode: 2022A&A...664A..72J
Altcode: 2022arXiv220308172J
  Context. Magnetic reconnection in the deep solar atmosphere can give
  rise to enhanced emission in the Balmer hydrogen lines, a phenomenon
  referred to as Ellerman bombs. Recent high-quality Hβ observations
  indicate that Ellerman bombs are more common than previously thought,
  and it was estimated that at any time, about half a million Ellerman
  bombs are present in the quiet Sun. <BR /> Aims: We performed an
  extensive statistical characterization of the quiet-Sun Ellerman
  bombs (QSEBs) in these new Hβ observations. <BR /> Methods: We
  analyzed a 1 h dataset of the quiet Sun observed with the Swedish
  1-m Solar Telescope that consists of spectral imaging in the Hβ
  and Hα lines as well as spectropolarimetric imaging in Fe I 6173
  Å. We used the k-means clustering and the 3D connected component
  labeling techniques to automatically detect QSEBs. <BR /> Results:
  We detected a total of 2809 QSEBs. The lifetimes vary between 9 s
  and 20.5 min, with a median of 1.14 min. The maximum area ranges
  between 0.0016 and 0.2603 Mm<SUP>2</SUP>, with a median of 0.018
  Mm<SUP>2</SUP>. The maximum brightness in the Hβ wing varies between
  1.06 and 2.76 with respect to the average wing intensity. A subset
  (14%) of the QSEBs displays enhancement of the Hβ line core. On
  average, the line core brightening appears 0.88 min after the
  onset of brightening in the wings, and the distance between these
  brightenings is 243 km. This gives rise to an apparent propagation
  speed ranging between −14.3 and +23.5 km s<SUP>−1</SUP>, with an
  average that is upward propagating at +4.4 km s<SUP>−1</SUP>. The
  average orientation is nearly parallel to the limbward direction. QSEBs
  are nearly uniformly distributed over the field of view, but we find
  empty areas with the size of mesogranulation. QSEBs are located more
  frequently near the magnetic network, where they are often larger,
  live longer, and are brighter. <BR /> Conclusions: We conclude
  that QSEBs are ubiquitous in the quiet Sun and appear everywhere,
  except in areas of mesogranular size with the weakest magnetic
  fields (B<SUB>LOS</SUB> ≲ 50 G). Our observations support the
  interpretation of reconnection along vertically extended current
  sheets. <P />Movies associated to Figs. 1 and 3 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243051/olm">https://www.aanda.org</A>

Title: A textbook example of magnetic flux emergence leading to EBs,
    UV bursts, surges and EUV signatures
Authors: Cabello, Iballa; Moreno-Insertis, . Fernando, , Prof; Rouppe
   van der Voort, Luc; Bose, Souvik; Nóbrega Siverio, Daniel
Bibcode: 2022cosp...44.2531C
Altcode:
  Small-scale eruptive phenomena (like Ellerman bombs (EBs), UV bursts,
  surges) constitute both a true challenge and an opportunity for
  progress in understanding the solar atmosphere since they involve very
  different layers from the photosphere to the low corona. In our work,
  we are aiming to characterize small-scale eruptive phenomena related to
  emerging flux regions. In particular, we use coordinated observations
  from the Swedish $1-$m Solar Telescope (SST), the Interface Region
  Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO,
  both HMI and AIA) to analyze an episode of magnetic flux emergence
  in an enhanced network that leads to an EB, a UV burst, a cool surge,
  and coronal signatures in the EUV. Through Milne-Eddington inversions
  of the {\ion{Fe}{I}} 6302 \AA\ line observed with SST/CRISP we
  obtain high-resolution (0.057"/pixel) magnetograms that allow us to
  reliably measure the magnetic field at the photosphere. A comparison
  with the corresponding SDO/HMI magnetograms reveals that this type
  of small-scale events are barely discernible in low-resolution (1")
  observations. During the emergence, a roundish dark bubble is visible
  in {\ion{Ca}{II} K} 3933 \AA\ at the location where the two opposite
  polarities of the emerging dipole are splitting apart. Several minutes
  later, indirect evidence of reconnection is found above the positive
  polarity of the dipole through the appearance of an EB in the wings
  of the {H$\alpha$} 6563 \AA\ and {\ion{Ca}{II} K} 3933 \AA\ lines
  from SST, and also in the SDO/AIA 1600 and 1700 \AA~data. Later,
  a surge shows up as an elongated structure visible in absorption in
  {H$\alpha$} and {\ion{Ca}{II} K}, extending over 12 Mm projected size
  on the disk. The shape of the surge is also apparent as an absorption
  feature in the SDO/AIA channels. Simultaneously with the surge (and
  at the location where the EB had appeared earlier on) a UV burst
  is clearly discernible as a strong and bright emission feature both
  in IRIS/SJI 1400 and 2796 \AA. Interestingly, this UV burst also has
  counterpart in SDO/AIA 94, 171, 193, 211, 304, and 335 \AA, meaning that
  we can find multi-thermal plasma up to a few MK in the reconnection
  site. This observation clearly shows the impact of the emergence of
  new magnetic field from the photosphere through the chromosphere and
  transition region and up into the corona. In addition, it provides an
  illustrative case to test new realistic simulations.

Title: Quantifying Properties of Photospheric Magnetic Cancellations
    in the Quiet Sun Internetwork
Authors: Ledvina, Vincent E.; Kazachenko, Maria D.; Criscuoli, Serena;
   Tilipman, Dennis; Ermolli, Ilaria; Falco, Mariachiara; Guglielmino,
   Salvatore; Jafarzadeh, Shahin; van der Voort, Luc Rouppe; Zuccarello,
   Francesca
Bibcode: 2022ApJ...934...38L
Altcode: 2022arXiv220604644L
  We analyzed spectropolarimetric data from the Swedish 1 m Solar
  Telescope to investigate the physical properties of small-scale magnetic
  cancellations in the quiet Sun photosphere. Specifically, we looked at
  the full Stokes polarization profiles along the Fe I 557.6 nm and of the
  Fe I 630.1 nm lines measured by the CRisp Imaging SpectroPolarimeter to
  study the temporal evolution of the line-of-sight magnetic field during
  42.5 minutes of quiet Sun evolution. From this magnetogram sequence, we
  visually identified 38 cancellation events. We then used the Yet Another
  Feature Tracking Algorithm to characterize the physical properties of
  these magnetic cancellations. We found on average 1.6 × 10<SUP>16</SUP>
  Mx of magnetic flux canceled in each event with an average cancellation
  rate of 3.8 × 10<SUP>14</SUP> Mx s<SUP>-1</SUP>. The derived canceled
  flux is associated with strong downflows, with an average speed of V
  <SUB>LOS</SUB> ≍ 1.1 km s<SUP>-1</SUP>. Our results show that the
  average lifetime of each event is 9.2 minutes with an average of 44.8%
  of initial magnetic flux being canceled. Our estimates of magnetic
  fluxes provide a lower limit since studied magnetic cancellation events
  have magnetic field values that are very close to the instrument noise
  level. We observed no horizontal magnetic fields at the cancellation
  sites and therefore cannot conclude whether the events are associated
  with structures that could cause magnetic reconnection.

Title: On the relationship between spicules and coronal bright points
Authors: Bose, Souvik; De Pontieu, Bart; Rouppe van der Voort, Luc;
   Nóbrega Siverio, Daniel
Bibcode: 2022cosp...44.2522B
Altcode:
  Coronal bright points (CBPs) are a set of small-scale, lower coronal
  loop systems connecting opposite magnetic polarities and are primarily
  characterized by enhanced emission in the extreme ultraviolet (EUV)
  wavelengths and X-rays. Being ubiquitous they are thought to play a
  definite role in heating the solar corona. This study aims to explore
  the chromospheric components associated with a CBP by focusing on
  spicules and small-scaled flux emergence. We used high-resolution
  observations in H$\beta$ and Fe I 617.3 nm spectral lines obtained
  from the Swedish 1-m Solar Telescope (SST) in coordination with the
  images acquired from the Atmospheric Imaging Assembly (AIA) instrument
  on-board the Solar Dynamics Observatory (SDO). On-disk spicules were
  automatically detected by employing advanced image processing techniques
  on the Dopplergrams derived from H$\beta$, and Mile-Eddington inversions
  of the Fe I 617.3 nm line provided the photospheric vector magnetic
  field. The AIA co-observations were co-aligned to SST with the latter
  serving as a reference. We find abundant occurrences of chromospheric
  spicules close to the "footpoints" of the CBP. The orientation of the
  spicules is predominantly aligned along with CBP loops which further
  indicates that they form a fundamental part of the same magnetic
  structure. Several examples of the spatio-temporal evolution indicate
  that much of the chromospheric plasma is heated to coronal temperatures
  implying that spicules potentially supply mass and energy to the CBP
  loops. Furthermore, we study chromospheric and corresponding coronal
  responses to two magnetic flux emergence events and their impact on the
  dynamics of the CBP. This study presents unique and unambiguous evidence
  that connects chromospheric spicular dynamics and flux emergence with
  a CBP for the very first time using high-resolution observations.

Title: Chromospheric emission from nanoflare heating in RADYN
    simulations
Authors: Bakke, H.; Carlsson, M.; Rouppe van der Voort, L.; Gudiksen,
   B. V.; Polito, V.; Testa, P.; De Pontieu, B.
Bibcode: 2022A&A...659A.186B
Altcode: 2022arXiv220111961B
  Context. Heating signatures from small-scale magnetic reconnection
  events in the solar atmosphere have proven to be difficult to
  detect through observations. Numerical models that reproduce flaring
  conditions are essential in understanding how nanoflares may act as a
  heating mechanism of the corona. <BR /> Aims: We study the effects of
  non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN
  simulations of nanoflare heated loops to investigate the diagnostic
  potential of chromospheric emission from small-scale events. <BR />
  Methods: The Mg II h and k, Ca II H and K, Ca II 854.2 nm, and Hα and
  Hβ chromospheric lines were synthesised from various RADYN models of
  coronal loops subject to electron beams of nanoflare energies. The
  contribution function to the line intensity was computed to better
  understand how the atmospheric response to the non-thermal electrons
  affects the formation of spectral lines and the detailed shape of
  their spectral profiles. <BR /> Results: The spectral line signatures
  arising from the electron beams highly depend on the density of the
  loop and the lower cutoff energy of the electrons. Low-energy (5 keV)
  electrons deposit their energy in the corona and transition region,
  producing strong plasma flows that cause both redshifts and blueshifts
  of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons
  deposit their energy in the lower transition region and chromosphere,
  resulting in increased emission from local heating. Our results indicate
  that effects from small-scale events can be observed with ground-based
  telescopes, expanding the list of possible diagnostics for the presence
  and properties of nanoflares.

Title: Evidence of the multi-thermal nature of spicular
    downflows. Impact on solar atmospheric heating
Authors: Bose, Souvik; Rouppe van der Voort, Luc; Joshi, Jayant;
   Henriques, Vasco M. J.; Nóbrega-Siverio, Daniel; Martínez-Sykora,
   Juan; De Pontieu, Bart
Bibcode: 2021A&A...654A..51B
Altcode: 2021arXiv210802153B
  Context. Spectroscopic observations of the emission lines formed in the
  solar transition region commonly show persistent downflows on the order
  of 10−15 km s<SUP>−1</SUP>. The cause of such downflows, however, is
  still not fully clear and has remained a matter of debate. <BR /> Aims:
  We aim to understand the cause of such downflows by studying the coronal
  and transition region responses to the recently reported chromospheric
  downflowing rapid redshifted excursions (RREs) and their impact on the
  heating of the solar atmosphere. <BR /> Methods: We have used two sets
  of coordinated data from the Swedish 1 m Solar Telescope, the Interface
  Region Imaging Spectrograph, and the Solar Dynamics Observatory for
  analyzing the response of the downflowing RREs in the transition
  region and corona. To provide theoretical support, we use an already
  existing 2.5D magnetohydrodynamic simulation of spicules performed
  with the Bifrost code. <BR /> Results: We find ample occurrences of
  downflowing RREs and show several examples of their spatio-temporal
  evolution, sampling multiple wavelength channels ranging from the cooler
  chromospheric to the hotter coronal channels. These downflowing features
  are thought to be likely associated with the returning components of
  the previously heated spicular plasma. Furthermore, the transition
  region Doppler shifts associated with them are close to the average
  redshifts observed in this region, which further implies that these
  flows could (partly) be responsible for the persistent downflows
  observed in the transition region. We also propose two mechanisms -
  (i) a typical upflow followed by a downflow and (ii) downflows along a
  loop -from the perspective of a numerical simulation that could explain
  the ubiquitous occurrence of such downflows. A detailed comparison
  between the synthetic and observed spectral characteristics reveals a
  distinctive match and further suggests an impact on the heating of the
  solar atmosphere. <BR /> Conclusions: We present evidence that suggests
  that at least some of the downflowing RREs are the chromospheric
  counterparts of the transition region and lower coronal downflows. <P
  />Movies associated to Figs. 1-3, 8, and 10 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141404/olm">https://www.aanda.org</A>

Title: Penumbral decay observed in active region NOAA 12585
Authors: Murabito, M.; Guglielmino, S. L.; Ermolli, I.; Romano, P.;
   Jafarzadeh, S.; Rouppe van der Voort, L. H. M.
Bibcode: 2021A&A...653A..93M
Altcode: 2021arXiv210604936M
  Context. The physical conditions leading the sunspot penumbra
  decay are poorly understood so far. <BR /> Aims: We investigate the
  photospheric magnetic and velocity properties of a sunspot penumbra
  during the decay phase to advance the current knowledge of the
  conditions leading to this process. <BR /> Methods: A penumbral
  decay was observed with the CRISP instrument at the Swedish 1 m
  Solar Telescope on 2016 September 4 and 5 in the active region NOAA
  12585. During these days, full-Stokes spectropolarimetric scans
  along the Fe I 630 nm line pair were acquired over more than one
  hour. We inverted these observations with the VFISV code to obtain
  the evolution of the magnetic and velocity properties. We complement
  the study with data from instruments on board the Solar Dynamics
  Observatory and Hinode space missions. <BR /> Results: The studied
  penumbra disappears progressively in time and space. The magnetic
  flux evolution seems to be linked to the presence of moving magnetic
  features (MMFs). Decreasing Stokes V signals are observed. Evershed
  flows and horizontal fields were detected even after the disappearance
  of the penumbral sector. <BR /> Conclusions: The analyzed penumbral
  decay seems to result from the interaction between opposite polarity
  fields in type III MMFs and penumbra, while the presence of overlying
  canopies regulates the evolution in the different penumbral
  sectors. <P />Movies associated with Fig. 6 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141034/olm">https://www.aanda.org</A>

Title: Signatures of ubiquitous magnetic reconnection in the deep
    atmosphere of sunspot penumbrae
Authors: Rouppe van der Voort, Luc H. M.; Joshi, Jayant; Henriques,
   Vasco M. J.; Bose, Souvik
Bibcode: 2021A&A...648A..54R
Altcode: 2021arXiv210111321R
  Context. Ellerman bombs are regions with enhanced Balmer line wing
  emission and mark magnetic reconnection in the deep solar atmosphere
  in active regions and the quiet Sun. They are often found in regions
  where opposite magnetic polarities are in close proximity. Recent
  high-resolution observations suggest that Ellerman bombs are more
  prevalent than previously thought. <BR /> Aims: We aim to determine
  the occurrence of Ellerman bombs in the penumbra of sunspots. <BR />
  Methods: We analyzed high spatial resolution observations of sunspots
  in the Balmer Hα and Hβ lines as well as auxiliary continuum channels
  obtained with the Swedish 1-m Solar Telescope and applied the k-means
  clustering technique to systematically detect and characterize Ellerman
  Bombs. <BR /> Results: Features with all the defining characteristics of
  Ellerman bombs are found in large numbers over the entire penumbra. The
  true prevalence of these events is only fully appreciated in the Hβ
  line due to the highest spatial resolution and lower chromospheric
  opacity. We find that the penumbra hosts some of the highest Ellerman
  bomb densities, surpassed only by the moat in the immediate surroundings
  of the sunspot. Some penumbral Ellerman bombs show flame morphology
  and rapid dynamical evolution. Many penumbral Ellerman bombs are fast
  moving with typical speed of 3.7 km s<SUP>−1</SUP> and sometimes more
  than 10 km s<SUP>−1</SUP>. Many penumbral Ellerman bombs migrate from
  the inner to the outer penumbra over hundreds of km, and some continue
  moving beyond the outer penumbral boundary into the moat. Many penumbral
  Ellerman bombs are found in the vicinity of regions with opposite
  magnetic polarity. <BR /> Conclusions: We conclude that reconnection
  is a near continuous process in the low atmosphere of the penumbra of
  sunspots that manifest in the form of penumbral Ellerman bombs. These
  are so prevalent that they may be a major sink of sunspot magnetic
  energy. <P />Movies associated to Figs. 1 and 6 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202040171/olm">https://www.aanda.org</A>

Title: Spicules and downflows in the solar chromosphere
Authors: Bose, Souvik; Joshi, Jayant; Henriques, Vasco M. J.; Rouppe
   van der Voort, Luc
Bibcode: 2021A&A...647A.147B
Altcode: 2021arXiv210107829B
  Context. High-speed downflows have been observed in the solar
  transition region (TR) and lower corona for many decades. Despite
  their abundance, it has been hard to find signatures of such downflows
  in the solar chromosphere. <BR /> Aims: In this work, we target
  an enhanced network region which shows ample occurrences of rapid
  spicular downflows in the Hα spectral line, which could potentially
  be linked to high-speed TR downflowing counterparts. <BR /> Methods:
  We used the k-means algorithm to classify the spectral profiles of
  on-disk spicules in Hα and Ca II K data observed from the Swedish
  1 m Solar Telescope and employed an automated detection method based
  on advanced morphological image processing operations to detect such
  downflowing features, in conjunction with rapid blue-shifted and
  red-shifted excursions (RBEs and RREs). <BR /> Results: We report
  the existence of a new category of RREs (termed as downflowing RRE)
  for the first time that, contrary to earlier interpretation, are
  associated with chromospheric field aligned downflows moving toward
  the strong magnetic field regions. Statistical analysis performed
  on nearly 20 000 RBEs and 15 000 RREs (including the downflowing
  counterparts), which were detected in our 97 min long dataset, shows
  that the downflowing RREs are very similar to RBEs and RREs except
  for their oppositely directed plane-of-sky motion. Furthermore, we
  also find that RBEs, RREs, and downflowing RREs can be represented
  by a wide range of spectral profiles with varying Doppler offsets,
  and Hα line core widths, both along and perpendicular to the spicule
  axis, that causes them to be associated with multiple substructures
  which evolve together. <BR /> Conclusions: We speculate that these
  rapid plasma downflows could well be the chromospheric counterparts
  of the commonly observed TR downflows. <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202040014/olm">https://www.aanda.org</A>

Title: High Resolution Observations of the Low Atmospheric Response
    to Small Heating Events in Active Regions
Authors: Testa, P.; Rouppe van der Voort, L.; De Pontieu, B.; Bakke, H.
Bibcode: 2020AGUFMSH004..03T
Altcode:
  We investigate the low atmospheric response to small heating events
  (nano/micro-flares) by analyzing coordinated active region observations
  with IRIS and SST, and also using the simultaneous SDO/AIA observations
  to study the coronal emission. The events we observe as intense
  brightenings in the chromosphere and transition region are clearly
  associated with heating of the overlying loops to high temperatures
  (5-10MK), as is clear from the strong Fe XVIII emission observed in
  the 94A SDO/AIA passband. Some of the chromospheric brightenings have
  been observed with the SST with the CRISP instrument in Ca II 8542 and
  H-alpha, as well as with the new CHROMIS instrument in Ca II K, which
  provides unprecedented resolution. We will present the rich spectral
  diagnostics offered by IRIS and SST for these events, including insights
  we obtained by applying a k-means clustering analysis to the SST and
  IRIS spectra during the heating events. Our observations will help
  provide more stringent constraints on the properties of non-thermal
  particles in nanoflares and microflares.

Title: Umbral chromospheric fine structure and umbral flashes modelled
as one: The corrugated umbra
Authors: Henriques, Vasco M. J.; Nelson, Chris J.; Rouppe van der
   Voort, Luc H. M.; Mathioudakis, Mihalis
Bibcode: 2020A&A...642A.215H
Altcode: 2020arXiv200805482H
  Context. The chromosphere of the umbra of sunspots features an
  assortment of dynamic fine structures that are poorly understood and
  often studied separately. Small-scale umbral brightenings (SSUBs),
  umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral
  dark fibrils are found in any observation of the chromosphere with
  sufficient spatial resolution performed at the correct umbral flash
  stage and passband. Understanding these features means understanding
  the dynamics of the umbral chromosphere. <BR /> Aims: We aim to fully
  understand the dynamics of umbral chromosphere through analysis of the
  relationships between distinct observed fine features and to produce
  complete models that explain both spectral profiles and the temporal
  evolution of the features. We seek to relate such understanding to
  umbral flashes. <BR /> Methods: We studied the spatial and spectral
  co-evolution of SDFs, SSUBs, and umbral flashes in Ca II 8542 Å
  spectral profiles. We produced models that generate the spectral
  profiles for all classes of features using non-local thermodynamic
  equilibrium radiative transfer with a recent version of the NICOLE
  inversion code. <BR /> Results: We find that both bright SSUBs and dark
  SDF structures are described with a continuous feature in the parameter
  space that is distinct from the surroundings even in pixel-by-pixel
  inversions. We find a phase difference between such features and
  umbral flashes in both inverted line-of-sight velocities and timing
  of the brightenings. For umbral flashes themselves we resolve,
  for the first time in inversion-based semi-empirical modelling,
  the pre-flash downflows, post-flash upflows, and the counter-flows
  present during the umbral flash phase. We further present a simple
  time-dependent cartoon model that explains the dynamics and spectral
  profiles of both fine structure, dark and bright, and umbral flashes
  in umbral chromospheres. <BR /> Conclusions: The similarity of the
  profiles between the brightenings and umbral flashes, the pattern of
  velocities obtained from the inversions, and the phase relationships
  between the structures all lead us to put forward that all dynamic
  umbral chromospheric structures observed to this date are a locally
  delayed or locally early portion of the oscillatory flow pattern that
  generates flashes, secondary to the steepening large-scale acoustic
  waves at its source. Essentially, SSUBs are part of the same shock or
  merely compression front responsible for the spatially larger umbral
  flash phenomenon, but out of phase with the broader oscillation.

Title: Signatures of ubiquitous magnetic reconnection in the lower
    solar atmosphere
Authors: Joshi, Jayant; Rouppe van der Voort, Luc H. M.; de la Cruz
   Rodríguez, Jaime
Bibcode: 2020A&A...641L...5J
Altcode: 2020arXiv200614975J
  Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in
  the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a
  signature of the fundamental process of magnetic reconnection at the
  smallest observable scale in the lower solar atmosphere. We analyze
  high spatial resolution observations (0<SUB>.</SUB><SUP>″</SUP>1)
  obtained with the Swedish 1-m Solar Telescope to explore signatures of
  QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly
  distributed throughout the quiet Sun, predominantly occurring in
  intergranular lanes. We find up to 120 QSEBs in the field of view for a
  single moment in time; this is more than an order of magnitude higher
  than the number of QSEBs found in earlier Hα observations. This
  suggests that about half a million QSEBs could be present in the
  lower solar atmosphere at any given time. The QSEB brightenings
  found in the Hβ line wings also persist in the line core with a
  temporal delay and spatial offset toward the nearest solar limb. Our
  results suggest that QSEBs emanate through magnetic reconnection along
  vertically extended current sheets in the lower solar atmosphere. The
  apparent omnipresence of small-scale magnetic reconnection may play
  an important role in the energy balance of the solar chromosphere. <P
  />Movies associated to Figs. 1-3, B1, and B2 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202038769/olm">https://www.aanda.org</A>

Title: High-resolution observations of the solar photosphere,
    chromosphere, and transition region. A database of coordinated IRIS
    and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
   M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
   Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
   V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
   Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
   Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
   Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
   M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
Bibcode: 2020A&A...641A.146R
Altcode: 2020arXiv200514175R
  NASA's Interface Region Imaging Spectrograph (IRIS) provides
  high-resolution observations of the solar atmosphere through ultraviolet
  spectroscopy and imaging. Since the launch of IRIS in June 2013, we
  have conducted systematic observation campaigns in coordination with
  the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
  complementary high-resolution observations of the photosphere and
  chromosphere. The SST observations include spectropolarimetric imaging
  in photospheric Fe I lines and spectrally resolved imaging in the
  chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
  a database of co-aligned IRIS and SST datasets that is open for
  analysis to the scientific community. The database covers a variety
  of targets including active regions, sunspots, plages, the quiet Sun,
  and coronal holes.

Title: A multi-diagnostic spectral analysis of penumbral microjets
Authors: Drews, Ainar; Rouppe van der Voort, Luc
Bibcode: 2020A&A...638A..63D
Altcode: 2020arXiv200502608D
  Context. Penumbral microjets (PMJs) are short-lived, jet-like objects
  found in the penumbra of sunspots. They were first discovered in
  chromospheric lines and have later also been shown to exhibit signals
  in transition region (TR) lines. Their origin and manner of evolution
  is not yet settled. <BR /> Aims: We perform a comprehensive analysis of
  PMJs through the use of spectral diagnostics that span from photospheric
  to TR temperatures to constrain PMJ properties. Methods We employed
  high-spatial-resolution Swedish 1-m Solar Telescope observations in the
  Ca II 8542 Å and H α lines, IRIS slit-jaw images, and IRIS spectral
  observations in the Mg II h &amp; k lines, the Mg II 2798.75 Å &amp;
  2798.82 Å triplet blend, the C II 1334 Å &amp; 1335 Å lines, and the
  Si IV 1394 Å &amp; 1403 Å lines. We derived a wide range of spectral
  diagnostics from these and investigated other secondary phenomena
  associated with PMJs. <BR /> Results: We find that PMJs exhibit varying
  degrees of signal in all of our studied spectral lines. We find low
  or negligible Doppler velocities and velocity gradients throughout
  our diagnostics and all layers of the solar atmosphere associated
  with these. Dark features in the inner wings of H α and Ca II 8542
  Å imply that PMJs form along pre-existing fibril structures. We find
  evidence for upper photospheric heating in a subset of PMJs through
  emission in the wings of the Mg II triplet lines. There is little
  evidence for ubiquitous twisting motion in PMJs. There is no marked
  difference in onset-times for PMJ brightenings in different spectral
  lines. <BR /> Conclusions: PMJs most likely exhibit only very modest
  mass-motions, contrary to earlier suggestions. We posit that PMJs form
  at upper photospheric or chromospheric heights at pre-existing fibril
  structures. <P />Movies associated to Appendix B are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202037911/olm">https://www.aanda.org</A>

Title: Characterization and formation of on-disk spicules in the Ca
    II K and Mg II k spectral lines (Corrigendum)
Authors: Bose, Souvik; Henriques, Vasco M. J.; Joshi, Jayant; Rouppe
   van der Voort, Luc
Bibcode: 2020A&A...637C...1B
Altcode:
  No abstract at ADS

Title: Ellerman bombs and UV bursts: reconnection at different
    atmospheric layers
Authors: Ortiz, Ada; Hansteen, Viggo H.; Nóbrega-Siverio, Daniel;
   Rouppe van der Voort, Luc
Bibcode: 2020A&A...633A..58O
Altcode: 2019arXiv191010736O
  The emergence of magnetic flux through the photosphere and into
  the outer solar atmosphere produces, amongst other dynamical
  phenomena, Ellerman bombs (EBs), which are observed in the wings of
  Hα and are due to magnetic reconnection in the photosphere below
  the chromospheric canopy. Signs of magnetic reconnection are also
  observed in other spectral lines, typical of the chromosphere or the
  transition region. An example are the ultraviolet (UV) bursts observed
  in the transition region lines of Si IV and the upper chromospheric
  lines of Mg II. In this work we analyze high-cadence, high-resolution
  coordinated observations between the Swedish 1m Solar Telescope (SST)
  and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα
  images from the SST provide us with the positions, timings, and
  trajectories of EBs in an emerging flux region. Simultaneous, co-aligned
  IRIS slit-jaw images at 133 (C II, transition region), 140 (Si IV,
  transition region), and 279.6 (Mg II k, core, upper chromosphere)
  nm as well as spectroscopy in the far- and near-ultraviolet from the
  fast spectrograph raster allow us to study the possible chromospheric
  and transition region counterparts of those EBs. Our main goal is
  to study the possible temporal and spatial relationship between
  several reconnection events at different layers in the atmosphere
  (namely EBs and UV bursts), the timing history between them, and the
  connection of these dynamical phenomena to the ejection of surges in
  the chromosphere. We also investigate the properties of an extended
  UV burst and their variations across the burst domain. Our results
  suggest a scenario where simultaneous and co-spatial EBs and UV bursts
  are part of the same reconnection system occurring sequentially along
  a vertical or nearly vertical current sheet. Heating and bidirectional
  jets trace the location where reconnection takes place. These results
  support and expand those obtained from recent numerical simulations
  of magnetic flux emergence. <P />The movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201936574/olm">https://www.aanda.org</A>

Title: Multi-scale observations of thermal non-equilibrium cycles
    in coronal loops
Authors: Froment, C.; Antolin, P.; Henriques, V. M. J.; Kohutova,
   P.; Rouppe van der Voort, L. H. M.
Bibcode: 2020A&A...633A..11F
Altcode: 2019arXiv191109710F
  Context. Thermal non-equilibrium (TNE) is a phenomenon that can
  occur in solar coronal loops when the heating is quasi-constant and
  highly-stratified. Under such heating conditions, coronal loops undergo
  cycles of evaporation and condensation. The recent observations of
  ubiquitous long-period intensity pulsations in coronal loops and their
  relationship with coronal rain have demonstrated that understanding the
  characteristics of TNE cycles is an essential step in constraining
  the circulation of mass and energy in the corona. <BR /> Aims:
  We report unique observations with the Solar Dynamics Observatory
  (SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured
  thermal properties across the extreme spatiotemporal scales covered by
  TNE processes. <BR /> Methods: Within the same coronal loop bundle,
  we captured 6 h period coronal intensity pulsations in SDO/AIA and
  coronal rain observed off-limb in the chromospheric Hα and Ca
  II K spectral lines with SST/CRISP and SST/CHROMIS. We combined
  a multi-thermal analysis of the cycles with AIA and an extensive
  spectral characterisation of the rain clumps with the SST. <BR />
  Results: We find clear evidence of evaporation-condensation cycles in
  the corona which are linked with periodic coronal rain showers. The
  high-resolution spectroscopic instruments at the SST reveal the
  fine-structured rain strands and allow us to probe the cooling
  phase of one of the cycles down to chromospheric temperatures. <BR />
  Conclusions: These observations reinforce the link between long-period
  intensity pulsations and coronal rain. They also demonstrate the
  capability of TNE to shape the dynamics of active regions on the large
  scales as well as on the smallest scales currently resolvable. <P
  />Movies associated to Figs. 3-5, and 8 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201936717/olm">https://www.aanda.org</A>

Title: Solar Hα features with hot onsets. IV. Network fibrils
Authors: Rutten, Robert J.; Rouppe van der Voort, Luc H. M.; De
   Pontieu, Bart
Bibcode: 2019A&A...632A..96R
Altcode: 2019arXiv190809315R
  Even in quiet areas underneath coronal holes the solar chromosphere
  contains ubiquitous heating events. They tend to be small scale and
  short lived, hence difficult to identify. Here we do not address
  their much-debated contribution to outer-atmosphere heating,
  but their aftermaths. We performed a statistical analysis of
  high-resolution observations in the Balmer Hα line to suggest
  that many slender dark Hα fibrils spreading out from network
  represent cooling gas that outlines tracks of preceding rapid
  type II spicule events or smaller similar but as yet unresolved
  heating agents in which the main gas constituent, hydrogen, ionizes
  at least partially. Subsequent recombination then causes dark Hα
  fibrils enhanced by nonequilibrium overopacity. We suggest that the
  extraordinary fibrilar appearance of the Hα chromosphere around network
  results from intermittent, frequent small-scale prior heating. <P
  />Movies associated to Fig. 3 and blinkers are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201936113/olm">https://www.aanda.org</A>

Title: Science Requirement Document (SRD) for the European Solar
    Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
   Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
   Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
   K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
   Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
   T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
   J.; Utz, D.; Zuccarello, F.
Bibcode: 2019arXiv191208650S
Altcode:
  The European Solar Telescope (EST) is a research infrastructure
  for solar physics. It is planned to be an on-axis solar telescope
  with an aperture of 4 m and equipped with an innovative suite of
  spectro-polarimetric and imaging post-focus instrumentation. The EST
  project was initiated and is driven by EAST, the European Association
  for Solar Telescopes. EAST was founded in 2006 as an association
  of 14 European countries. Today, as of December 2019, EAST consists
  of 26 European research institutes from 18 European countries. The
  Preliminary Design Phase of EST was accomplished between 2008 and
  2011. During this phase, in 2010, the first version of the EST Science
  Requirement Document (SRD) was published. After EST became a project
  on the ESFRI roadmap 2016, the preparatory phase started. The goal
  of the preparatory phase is to accomplish a final design for the
  telescope and the legal governance structure of EST. A major milestone
  on this path is to revisit and update the Science Requirement Document
  (SRD). The EST Science Advisory Group (SAG) has been constituted by
  EAST and the Board of the PRE-EST EU project in November 2017 and has
  been charged with the task of providing with a final statement on the
  science requirements for EST. Based on the conceptual design, the SRD
  update takes into account recent technical and scientific developments,
  to ensure that EST provides significant advancement beyond the current
  state-of-the-art. The present update of the EST SRD has been developed
  and discussed during a series of EST SAG meetings. The SRD develops
  the top-level science objectives of EST into individual science
  cases. Identifying critical science requirements is one of its main
  goals. Those requirements will define the capabilities of EST and the
  post-focus instrument suite. The technical requirements for the final
  design of EST will be derived from the SRD.

Title: Ellerman bombs and UV bursts: reconnection at different
    atmospheric layers
Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Nobrega, D. E.;
   Rouppe van der Voort, L.
Bibcode: 2019AGUFMSH13B..06H
Altcode:
  The emergence of magnetic flux through the photosphere and into the
  outer solar atmosphere produces, amongst many other dynamical phenomena,
  the appearance of Ellerman bombs (EBs) in the photosphere. EBs are
  observed in the wings of H(alpha) and are highly likely to be due to
  reconnection in the photosphere, below the chromospheric canopy. Signs
  of the reconnection process are also observed in several other spectral
  lines, typical of the chromosphere or the transition region. An example
  are the UV bursts observed in the transition region lines of Si IV and
  the upper chromospheric lines of Mg II. In this work we analyze high
  cadence, high resolution coordinated observations between the Swedish
  1-m Solar Telescope (SST) and the Interface Region Ima ging Spectrograph
  (IRIS) spacecraft in order to study the possible relationship between
  reconnection events at different layers in the atmosphere and, in
  particular, the timing history between them. H(alpha) images from the
  SST provide us with the positions, timings and trajectories of EBs in an
  emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at
  1330 (C II, transition region), 1400 (Si IV, transition region) and 2796
  (Mg II k, core, upper chromosphere) Ã…, as well as spectroscopy in
  the far and near ultraviolet from the fast spectrograph raster, allow
  us to study the possible chromospheric/transition region counterparts
  of those photospheric EBs. Our main goal is to study whether there is
  a temporal and spatial relationship between the appearance of an EB
  and the appearance of a UV burst and the connection of these dynamical
  phenomena to the appearance of surges in the chromosphere. We also
  investigate in detail the properties of an extended UV burst and their
  v ariations across the burst domain. Our results suggest a scenario
  where simultaneous and co-spatial EBs and UV bursts are part of the
  same reconnection system occurring sequentially along a vertical or
  nearly vertical current sheet. Heating and bidirectional jets trace the
  location where reconnection takes place. This scenario is in agreement
  with the most recent 3D numerical experiments modeling flux emergence.

Title: Characterization and formation of on-disk spicules in the Ca
    II K and Mg II k spectral lines
Authors: Bose, Souvik; Henriques, Vasco M. J.; Joshi, Jayant; Rouppe
   van der Voort, Luc
Bibcode: 2019A&A...631L...5B
Altcode: 2019arXiv191005533B
  We characterize, for the first time, type-II spicules in Ca II K 3934
  Å using the CHROMIS instrument at the Swedish 1 m Solar Telescope. We
  find that their line formation is dominated by opacity shifts with
  the K<SUB>3</SUB> minimum best representing the velocity of the
  spicules. The K<SUB>2</SUB> features are either suppressed by the
  Doppler-shifted K<SUB>3</SUB> or enhanced via increased contribution
  from the lower layers, leading to strongly enhanced but unshifted
  K<SUB>2</SUB> peaks, with widening towards the line core as consistent
  with upper-layer opacity removal via Doppler-shift. We identify spicule
  spectra in concurrent IRIS Mg II k 2796Å observations with very
  similar properties. Using our interpretation of spicule chromospheric
  line formation, we produce synthetic profiles that match observations.

Title: Semi-empirical model atmospheres for the chromosphere of the
    sunspot penumbra and umbral flashes
Authors: Bose, Souvik; Henriques, Vasco M. J.; Rouppe van der Voort,
   Luc; Pereira, Tiago M. D.
Bibcode: 2019A&A...627A..46B
Altcode: 2019arXiv190508264B
  Context. The solar chromosphere and the lower transition region
  are believed to play a crucial role in the heating of the solar
  corona. Models that describe the chromosphere (and the lower transition
  region), accounting for its highly dynamic and structured character
  are, so far, found to be lacking. This is partly due to the breakdown
  of complete frequency redistribution (CRD) in the chromospheric
  layers and also because of the difficulty in obtaining complete sets
  of observations that adequately constrain the solar atmosphere at
  all relevant heights. <BR /> Aims: We aim to obtain semi-empirical
  model atmospheres that reproduce the features of the Mg II h&amp;k
  line profiles that sample the middle chromosphere with focus on a
  sunspot. <BR /> Methods: We used spectropolarimetric observations
  of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar
  Telescope and used NICOLE inversions to obtain semi-empirical model
  atmospheres for different features in and around a sunspot. These
  were used to synthesize Mg II h&amp;k spectra using the RH1.5D
  code, which we compared with observations taken with the Interface
  Region Imaging Spectrograph (IRIS). <BR /> Results: Comparison of
  the synthetic profiles with IRIS observations reveals that there
  are several areas, especially in the penumbra of the sunspot,
  where most of the observed Mg II h&amp;k profiles are very well
  reproduced. In addition, we find that supersonic hot down-flows,
  present in our collection of models in the umbra, lead to synthetic
  profiles that agree well with the IRIS Mg II h&amp;k profiles, with
  the exception of the line core. <BR /> Conclusions: We put forward
  and make available four semi-empirical model atmospheres. Two for
  the penumbra, reflecting the range of temperatures obtained for the
  chromosphere, one for umbral flashes, and a model representative of
  the quiet surroundings of a sunspot. <P />Data of semi-empirical model
  atmospheres are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A46">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A46</A>.

Title: Dissecting bombs and bursts: non-LTE inversions of
    low-atmosphere reconnection in SST and IRIS observations
Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht,
   T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M.
Bibcode: 2019A&A...627A.101V
Altcode: 2019arXiv190502035V
  Ellerman bombs and UV bursts are transient brightenings that are
  ubiquitously observed in the lower atmospheres of active and emerging
  flux regions. As they are believed to pinpoint sites of magnetic
  reconnection in reconfiguring fields, understanding their occurrence
  and detailed evolution may provide useful insight into the overall
  evolution of active regions. Here we present results from inversions
  of SST/CRISP and CHROMIS, as well as IRIS data of such transient
  events. Combining information from the Mg II h &amp; k, Si IV, and Ca
  II 8542 Å and Ca II H &amp; K lines, we aim to characterise their
  temperature and velocity stratification, as well as their magnetic
  field configuration. We find average temperature enhancements of
  a few thousand kelvin, close to the classical temperature minimum
  and similar to previous studies, but localised peak temperatures
  of up to 10 000-15 000 K from Ca II inversions. Including Mg II
  appears to generally dampen these temperature enhancements to below
  8000 K, while Si IV requires temperatures in excess of 10 000 K at
  low heights, but may also be reproduced with secondary temperature
  enhancements of 35 000-60 000 K higher up. However, reproducing Si
  IV comes at the expense of overestimating the Mg II emission. The
  line-of-sight velocity maps show clear bi-directional jet signatures
  for some events and strong correlation with substructure in the
  intensity images in general. Absolute line-of-sight velocities range
  between 5 and 20 km s<SUP>-1</SUP> on average, with slightly larger
  velocities towards, rather than away from, the observer. The inverted
  magnetic field parameters show an enhancement of the horizontal
  field co-located with the brightenings at heights similar to that of
  the temperature increase. We are thus able to largely reproduce the
  observational properties of Ellerman bombs with the UV burst signature
  (e.g. intensities, profile asymmetries, morphology, and bi-directional
  jet signatures), with temperature stratifications peaking close
  to the classical temperature minimum. Correctly modelling the Si
  IV emission in agreement with all other diagnostics is however an
  outstanding issue and remains paramount in explaining its apparent
  coincidence with Hα emission. Fine-tuning the approach (accounting for
  resolution differences, fitting localised temperature enhancements,
  and/or performing spatially coupled inversions) is likely necessary
  in order to obtain better agreement between all considered diagnostics.

Title: VizieR Online Data Catalog: Sunspot penumbra and umbral
    flashes models (Bose+, 2019)
Authors: Bose, S.; Henriques, V. M. J.; Rouppe van der Voort, L.;
   Pereira, T. M. D.
Bibcode: 2019yCat..36270046B
Altcode:
  The tables included in this paper describes the stratification of
  atmospheric parameters for the cool and the hot penumbra, umbral flash
  and the quiet surrounding respectively as a function of geometric
  height and optical depth. <P />(4 data files).

Title: Automating Ellerman bomb detection in ultraviolet continua
Authors: Vissers, Gregal J. M.; Rouppe van der Voort, Luc H. M.;
   Rutten, Robert J.
Bibcode: 2019A&A...626A...4V
Altcode: 2019arXiv190107975V
  Ellerman bombs are transient brightenings in the wings of Hα 6563 Å
  that pinpoint photospheric sites of magnetic reconnection in solar
  active regions. Their partial visibility in the 1600 Å and 1700 Å
  continua registered routinely by the Atmospheric Imaging Assembly (AIA)
  onboard the Solar Dynamics Observatory (SDO) offers a unique opportunity
  to inventory such magnetic-field disruptions throughout the AIA database
  if a reliable recipe for their detection can be formulated. This is
  done here. We have improved and applied an Hα Ellerman bomb detection
  code to ten data sets spanning viewing angles from solar disc centre
  to the limb. They combine high-quality Hα imaging spectroscopy from
  the Swedish 1 m Solar Telescope with simultaneous AIA imaging around
  1600 Å and 1700 Å. A trial grid of brightness, lifetime and area
  constraints is imposed on the AIA images to define optimal recovery
  of the 1735 Ellerman bombs detected in Hα. The best results when
  optimising simultaneously for recovery fraction and reliability are
  obtained from 1700 Å images by requiring 5σ brightening above the
  average 1700 Å nearby quiet-Sun intensity, lifetime above one minute,
  area of 1-18 AIA pixels. With this recipe 27% of the AIA detections are
  Hα-detected Ellerman bombs while it recovers 19% of these (of which
  many are smaller than the AIA resolution). Better yet, among the top
  10% AIA 1700 Å detections selected with combined brightness, lifetime
  and area thresholds as many as 80% are Hα Ellerman bombs. Automated
  selection of the best 1700 Å candidates therefore opens the entire
  AIA database for detecting most of the more significant photospheric
  reconnection events. This proxy is applicable as a flux-dynamics
  tell-tale in studying any Earth-side solar active region since early
  2010 up to the present.

Title: Penumbral microjets at high spatial and temporal resolution
Authors: Rouppe van der Voort, Luc H. M.; Drews, Ainar
Bibcode: 2019A&A...626A..62R
Altcode: 2019arXiv190502981R
  Context. Sunspot observations in chromospheric spectral lines have
  revealed short-lived linear bright transients that are commonly
  referred to as penumbral microjets (PMJs). Details on the origin and
  physical nature of PMJs are to a large extend still unknown. <BR />
  Aims: We aim to characterize the dynamical nature of PMJs to provide
  guidance for future modeling efforts. <BR /> Methods: We analyzed
  high spatial (0.̋1) and temporal resolution (1 s) Ca II H filtergram
  (0.1 nm bandwidth) observations of a sunspot that were obtained on two
  consecutive days with the Swedish 1 m Solar Telescope. <BR /> Results:
  We find that PMJs appear to be the rapid brightening of an already
  existing (faint) fibril. The rapid brightening is the fast increase
  (typically less than 10 s) in intensity over significant length (several
  hundreds of kilometers) of the existing fibril. For most PMJs, no clear
  root or source from where the brightening appears to originate can be
  identified. After the fast onset, about half of the PMJs have tops that
  move with an apparent velocity of between 5 and 14 km s<SUP>-1</SUP>,
  most of them upward. No significant motion of the top is observed in
  the other PMJs. About one-third of the PMJs split into two parallel
  and coevolving linear features during the later phases of their
  lifetimes. <BR /> Conclusions: We conclude that mass flows can play only
  a limited role in the onset phase of PMJs. It is more likely that we
  see the effect of a fast heating front. <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201935343/olm">https://www.aanda.org</A>

Title: Energetics of magnetic transients in a solar active region
    plage
Authors: Chitta, L. P.; Sukarmadji, A. R. C.; Rouppe van der Voort,
   L.; Peter, H.
Bibcode: 2019A&A...623A.176C
Altcode: 2019arXiv190201650C
  Context. Densely packed coronal loops are rooted in photospheric
  plages in the vicinity of active regions on the Sun. The photospheric
  magnetic features underlying these plage areas are patches of mostly
  unidirectional magnetic field extending several arcsec on the solar
  surface. <BR /> Aims: We aim to explore the transient nature of
  the magnetic field, its mixed-polarity characteristics, and the
  associated energetics in the active region plage using high spatial
  resolution observations and numerical simulations. <BR /> Methods:
  We used photospheric Fe I 6173 Å spectropolarimetric observations of
  a decaying active region obtained from the Swedish 1-m Solar Telescope
  (SST). These data were inverted to retrieve the photospheric magnetic
  field underlying the plage as identified in the extreme-ultraviolet
  emission maps obtained from the Atmospheric Imaging Assembly (AIA)
  on board the Solar Dynamics Observatory (SDO). To obtain better
  insight into the evolution of extended unidirectional magnetic field
  patches on the Sun, we performed 3D radiation magnetohydrodynamic
  simulations of magnetoconvection using the MURaM code. <BR />
  Results: The observations show transient magnetic flux emergence
  and cancellation events within the extended predominantly unipolar
  patch on timescales of a few 100 s and on spatial scales comparable
  to granules. These transient events occur at the footpoints of active
  region plage loops. In one case the coronal response at the footpoints
  of these loops is clearly associated with the underlying transient. The
  numerical simulations also reveal similar magnetic flux emergence and
  cancellation events that extend to even smaller spatial and temporal
  scales. Individual simulated transient events transfer an energy
  flux in excess of 1 MW m<SUP>-2</SUP> through the photosphere. <BR
  /> Conclusions: We suggest that the magnetic transients could play
  an important role in the energetics of active region plage. Both in
  observations and simulations, the opposite-polarity magnetic field
  brought up by transient flux emergence cancels with the surrounding
  plage field. Magnetic reconnection associated with such transient events
  likely conduits magnetic energy to power the overlying chromosphere
  and coronal loops.

Title: Observationally Based Models of Penumbral Microjets
Authors: Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A.;
   Rouppe van der Voort, L.; Scharmer, G. B.; Carlsson, M.
Bibcode: 2019ApJ...870...88E
Altcode: 2018arXiv181107881E
  We study the polarization signals and physical parameters of penumbral
  microjets (PMJs) by using high spatial resolution data taken in the
  Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP
  and CHROMIS instruments at the Swedish 1 m Solar Telescope. We
  infer their physical parameters, such as physical observables in
  the photosphere and chromospheric velocity diagnostics, by different
  methods, including inversions of the observed Stokes profiles with
  the STiC code. PMJs harbor overall brighter Ca II K line profiles
  and conspicuous polarization signals in Ca II 854.2 nm, specifically
  in circular polarization that often shows multiple lobes mainly due
  to the shape of Stokes I. They usually overlap photospheric regions
  with a sheared magnetic field configuration, suggesting that magnetic
  reconnections could play an important role in the origin of PMJs. The
  discrepancy between their low LOS velocities and the high apparent
  speeds reported on earlier, as well as the existence of different
  vertical velocity gradients in the chromosphere, indicate that PMJs
  might not be entirely related to mass motions. Instead, PMJs could
  be due to perturbation fronts induced by magnetic reconnections
  occurring in the deep photosphere that propagate through the
  chromosphere. This reconnection may be associated with current heating
  that produces temperature enhancements from the temperature minimum
  region. Furthermore, enhanced collisions with electrons could also
  increase the coupling to the local conditions at higher layers during
  the PMJ phase, giving a possible explanation for the enhanced emission
  in the overall Ca II K profiles emerging from these transients.

Title: Solar Ultraviolet Bursts
Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.;
   Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal
   J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria
   S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark
   C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr
Bibcode: 2018SSRv..214..120Y
Altcode: 2018arXiv180505850Y
  The term "ultraviolet (UV) burst" is introduced to describe small,
  intense, transient brightenings in ultraviolet images of solar active
  regions. We inventorize their properties and provide a definition
  based on image sequences in transition-region lines. Coronal signatures
  are rare, and most bursts are associated with small-scale, canceling
  opposite-polarity fields in the photosphere that occur in emerging flux
  regions, moving magnetic features in sunspot moats, and sunspot light
  bridges. We also compare UV bursts with similar transition-region
  phenomena found previously in solar ultraviolet spectrometry and
  with similar phenomena at optical wavelengths, in particular Ellerman
  bombs. Akin to the latter, UV bursts are probably small-scale magnetic
  reconnection events occurring in the low atmosphere, at photospheric
  and/or chromospheric heights. Their intense emission in lines with
  optically thin formation gives unique diagnostic opportunities
  for studying the physics of magnetic reconnection in the low solar
  atmosphere. This paper is a review report from an International Space
  Science Institute team that met in 2016-2017.

Title: Spectropolarimetric Inversions of the Ca II 8542 Å Line in
    an M-class Solar Flare
Authors: Kuridze, D.; Henriques, V. M. J.; Mathioudakis, M.; Rouppe
   van der Voort, L.; de la Cruz Rodríguez, J.; Carlsson, M.
Bibcode: 2018ApJ...860...10K
Altcode: 2018arXiv180500487K
  We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using
  high-resolution full Stokes imaging spectropolarimetry of the Ca II
  8542 Å line obtained with the CRISP imaging spectropolarimeter at the
  Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the
  non-LTE code NICOLE are used to construct semiempirical models of the
  flaring atmosphere to investigate the structure and evolution of the
  flare temperature and magnetic field. A comparison of the temperature
  stratification in flaring and nonflaring areas reveals strong heating
  of the flare ribbon during the flare peak. The polarization signals
  of the ribbon in the chromosphere during the flare maximum become
  stronger when compared to its surroundings and to pre- and post-flare
  profiles. Furthermore, a comparison of the response functions to
  perturbations in the line-of-sight magnetic field and temperature in
  flaring and nonflaring atmospheres shows that during the flare, the
  Ca II 8542 Å line is more sensitive to the lower atmosphere where the
  magnetic field is expected to be stronger. The chromospheric magnetic
  field was also determined with the weak-field approximation, which
  led to results similar to those obtained with the NICOLE inversions.

Title: Chromospheric counterparts of solar transition region
    unresolved fine structure loops
Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Hansteen,
   Viggo H.; De Pontieu, Bart
Bibcode: 2018A&A...611L...6P
Altcode: 2018arXiv180304415P
  Low-lying loops have been discovered at the solar limb in transition
  region temperatures by the Interface Region Imaging Spectrograph
  (IRIS). They do not appear to reach coronal temperatures, and it
  has been suggested that they are the long-predicted unresolved fine
  structures (UFS). These loops are dynamic and believed to be visible
  during both heating and cooling phases. Making use of coordinated
  observations between IRIS and the Swedish 1-m Solar Telescope, we study
  how these loops impact the solar chromosphere. We show for the first
  time that there is indeed a chromospheric signal of these loops, seen
  mostly in the form of strong Doppler shifts and a conspicuous lack of
  chromospheric heating. In addition, we find that several instances have
  a inverse Y-shaped jet just above the loop, suggesting that magnetic
  reconnection is driving these events. Our observations add several
  puzzling details to the current knowledge of these newly discovered
  structures; this new information must be considered in theoretical
  models. <P />Two movies associated to Fig. 1 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201832762/olm">https://www.aanda.org</A>

Title: Ellerman bombs and UV bursts: reconnection at different
    atmospheric layers?
Authors: Hansteen, V. H.; Ortiz-Carbonell, A. N.; Rouppe van der
   Voort, L.
Bibcode: 2017AGUFMSH43A2801H
Altcode:
  The emergence of magnetic flux through the photosphere and into the
  outer solar atmosphere produces, amongst many other phenomena, the
  appearance of Ellerman bombs (EBs) in the photosphere. EBs are observed
  in the wings of H(alpha) and are highly likely to be due to reconnection
  in the photosphere, below the chromospheric canopy. However, signs of
  the reconnection process are also observed in several other spectral
  lines, typical of the chromosphere or transition region. An example
  are the UV bursts observed in the transition region lines of Si
  IV. In this work we analyze high cadence coordinated observations
  between the 1-m Swedish Solar Telescope and the IRIS spacecraft in
  order to study the possible relationship between reconnection events
  at different layers in the atmosphere, and in particular, the timing
  history between them. High cadence, high resolution H-alpha images
  from the SST provide us with the positions, timings and trajectories
  of Ellerman bombs in an emerging flux region. Simultaneous co-aligned
  IRIS slit-jaw images at 1400 and 1330 A and detailed Si IV spectra from
  the fast spectrograph raster allow us to study the transition region
  counterparts of those photospheric Ellerman bombs. Our main goal is to
  study whether there is a temporal relationship between the appearance
  of an EB and the appearance of a UV burst. Eventually we would like
  to investigate whether reconnection happens at discrete heights,
  or as a reconnection sheet spanning several layers at the same time.

Title: Surges and Si IV Bursts in the Solar Atmosphere: Understanding
    IRIS and SST Observations through RMHD Experiments
Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis,
   F.; Rouppe van der Voort, L.
Bibcode: 2017ApJ...850..153N
Altcode: 2017arXiv171008928N
  Surges often appear as a result of the emergence of magnetized
  plasma from the solar interior. Traditionally, they are observed
  in chromospheric lines such as Hα 6563 \mathringA and Ca II 8542
  \mathringA . However, whether there is a response to the surge
  appearance and evolution in the Si IV lines or, in fact, in many
  other transition region lines has not been studied. In this paper,
  we analyze a simultaneous episode of an Hα surge and a Si IV burst
  that occurred on 2016 September 03 in active region AR 12585. To that
  end, we use coordinated observations from the Interface Region Imaging
  Spectrograph and the Swedish 1-m Solar Telescope. For the first time,
  we report emission of Si IV within the surge, finding profiles that
  are brighter and broader than the average. Furthermore, the brightest
  Si IV patches within the domain of the surge are located mainly near
  its footpoints. To understand the relation between the surges and the
  emission in transition region lines like Si IV, we have carried out 2.5D
  radiative MHD (RMHD) experiments of magnetic flux emergence episodes
  using the Bifrost code and including the nonequilibrium ionization of
  silicon. Through spectral synthesis, we explain several features of
  the observations. We show that the presence of Si IV emission patches
  within the surge, their location near the surge footpoints and various
  observed spectral features are a natural consequence of the emergence of
  magnetized plasma from the interior to the atmosphere and the ensuing
  reconnection processes.

Title: Intermittent Reconnection and Plasmoids in UV Bursts in the
    Low Solar Atmosphere
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.;
   de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio,
   D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.;
   Carlsson, M.; Vissers, G.
Bibcode: 2017ApJ...851L...6R
Altcode: 2017arXiv171104581R
  Magnetic reconnection is thought to drive a wide variety of dynamic
  phenomena in the solar atmosphere. Yet, the detailed physical mechanisms
  driving reconnection are difficult to discern in the remote sensing
  observations that are used to study the solar atmosphere. In this
  Letter, we exploit the high-resolution instruments Interface Region
  Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at
  the Swedish 1-m Solar Telescope (SST) to identify the intermittency
  of magnetic reconnection and its association with the formation of
  plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV
  1403 Å UV burst spectra from the transition region show evidence of
  highly broadened line profiles with often non-Gaussian and triangular
  shapes, in addition to signatures of bidirectional flows. Such profiles
  had previously been linked, in idealized numerical simulations, to
  magnetic reconnection driven by the plasmoid instability. Simultaneous
  CHROMIS images in the chromospheric Ca II K 3934 Å line now provide
  compelling evidence for the presence of plasmoids by revealing highly
  dynamic and rapidly moving brightenings that are smaller than 0.″2 and
  that evolve on timescales of the order of seconds. Our interpretation
  of the observations is supported by detailed comparisons with synthetic
  observables from advanced numerical simulations of magnetic reconnection
  and associated plasmoids in the chromosphere. Our results highlight
  how subarcsecond imaging spectroscopy sensitive to a wide range of
  temperatures combined with advanced numerical simulations that are
  realistic enough to compare with observations can directly reveal the
  small-scale physical processes that drive the wide range of phenomena
  in the solar atmosphere.

Title: CRISPRED: CRISP imaging spectropolarimeter data reduction
    pipeline
Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.;
   Hillberg, T.; Rouppe van der Voort, L.
Bibcode: 2017ascl.soft08003D
Altcode:
  CRISPRED reduces data from the CRISP imaging spectropolarimeter at
  the Swedish 1 m Solar Telescope (SST). It performs fitting routines,
  corrects optical aberrations from atmospheric turbulence as well as
  from the optics, and compensates for inter-camera misalignments,
  field-dependent and time-varying instrumental polarization, and
  spatial variation in the detector gain and in the zero level offset
  (bias). It has an object-oriented IDL structure with computationally
  demanding routines performed in C subprograms called as dynamically
  loadable modules (DLMs).

Title: Beam electrons as a source of Hα flare ribbons
Authors: Druett, Malcolm; Scullion, Eamon; Zharkova, Valentina;
   Matthews, Sarah; Zharkov, Sergei; Rouppe van der Voort, Luc
Bibcode: 2017NatCo...815905D
Altcode:
  The observations of solar flare onsets show rapid increase of hard
  and soft X-rays, ultra-violet emission with large Doppler blue shifts
  associated with plasma upflows, and Hα hydrogen emission with red
  shifts up to 1-4 Å. Modern radiative hydrodynamic models account
  well for blue-shifted emission, but struggle to reproduce closely
  the red-shifted Hα lines. Here we present a joint hydrodynamic and
  radiative model showing that during the first seconds of beam injection
  the effects caused by beam electrons can reproduce Hα line profiles
  with large red-shifts closely matching those observed in a C1.5 flare
  by the Swedish Solar Telescope. The model also accounts closely for
  timing and magnitude of upward motion to the corona observed 29 s after
  the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar
  Dynamics Observatory.

Title: On the generation of solar spicules and Alfvénic waves
Authors: Martínez-Sykora, J.; De Pontieu, B.; Hansteen, V. H.;
   Rouppe van der Voort, L.; Carlsson, M.; Pereira, T. M. D.
Bibcode: 2017Sci...356.1269M
Altcode: 2017arXiv171007559M
  In the lower solar atmosphere, the chromosphere is permeated by jets
  known as spicules, in which plasma is propelled at speeds of 50 to
  150 kilometers per second into the corona. The origin of the spicules
  is poorly understood, although they are expected to play a role in
  heating the million-degree corona and are associated with Alfvénic
  waves that help drive the solar wind. We compare magnetohydrodynamic
  simulations of spicules with observations from the Interface Region
  Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules
  are shown to occur when magnetic tension is amplified and transported
  upward through interactions between ions and neutrals or ambipolar
  diffusion. The tension is impulsively released to drive flows, heat
  plasma (through ambipolar diffusion), and generate Alfvénic waves.

Title: Microjets in the penumbra of a sunspot
Authors: Drews, Ainar; Rouppe van der Voort, Luc
Bibcode: 2017A&A...602A..80D
Altcode: 2017arXiv170206078D
  Context. Penumbral microjets (PMJs) are short-lived jets found in
  the penumbra of sunspots, first observed in wide-band Ca II H line
  observations as localized brightenings, and are thought to be caused
  by magnetic reconnection. Earlier work on PMJs has focused on smaller
  samples of by-eye selected events and case studies. <BR /> Aims: It
  is our goal to present an automated study of a large sample of PMJs
  to place the basic statistics of PMJs on a sure footing and to study
  the PMJ Ca II 8542 Å spectral profile in detail. <BR /> Methods: High
  spatial resolution and spectrally well-sampled observations in the Ca II
  8542 Å line obtained from the Swedish 1-m Solar Telescope (SST) were
  reduced by a principle component analysis and subsequently used in the
  automated detection of PMJs using the simple machine learning algorithm
  k-nearest neighbour. PMJ detections were verified with co-temporal
  Ca II H line observations. <BR /> Results: We find a total of 453
  tracked PMJ events, 4253 PMJs detections tallied over all timeframes,
  and a detection rate of 21 events per timestep. From these, an average
  length, width and lifetime of 640 km, 210 km and 90 s are obtained. The
  average PMJ Ca II 8542 Å line profile is characterized by enhanced
  inner wings, often in the form of one or two distinct peaks, and a
  brighter line core as compared to the quiet-Sun average. Average blue
  and red peak positions are determined at - 10.4 km s<SUP>-1</SUP> and +
  10.2 km s<SUP>-1</SUP> offsets from the Ca II 8542 Å line core. We find
  several clusters of PMJ hot-spots within the sunspot penumbra, in which
  PMJ events occur in the same general area repeatedly over time. <BR />
  Conclusions: Our results indicate smaller average PMJs sizes and longer
  lifetimes compared to previously published values, but with statistics
  still in the same orders of magnitude. The investigation and analysis
  of the PMJ line profiles strengthens the proposed heating of PMJs to
  transition region temperatures. The presented statistics on PMJs form
  a solid basis for future investigations and numerical modelling of PMJs.

Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun
Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson,
   M.; Rouppe van der Voort, L.; Leenaarts, J.
Bibcode: 2017ApJ...839...22H
Altcode: 2017arXiv170402872H
  A spectacular manifestation of solar activity is the appearance of
  transient brightenings in the far wings of the Hα line, known as
  Ellerman bombs (EBs). Recent observations obtained by the Interface
  Region Imaging Spectrograph have revealed another type of plasma
  “bombs” (UV bursts) with high temperatures of perhaps up to 8 ×
  10<SUP>4</SUP> K within the cooler lower solar atmosphere. Realistic
  numerical modeling showing such events is needed to explain
  their nature. Here, we report on 3D radiative magnetohydrodynamic
  simulations of magnetic flux emergence in the solar atmosphere. We
  find that ubiquitous reconnection between emerging bipolar magnetic
  fields can trigger EBs in the photosphere, UV bursts in the mid/low
  chromosphere and small (nano-/micro-) flares (10<SUP>6</SUP> K) in
  the upper chromosphere. These results provide new insights into the
  emergence and build up of the coronal magnetic field and the dynamics
  and heating of the solar surface and lower atmosphere.

Title: Solar Hα features with hot onsets. II. A contrail fibril
Authors: Rutten, R. J.; Rouppe van der Voort, L. H. M.
Bibcode: 2017A&A...597A.138R
Altcode: 2016arXiv160907616R
  The solar chromosphere observed in Hα consists mostly of narrow
  fibrils. The longest typically originate in network or plage and arch
  far over adjacent internetwork. We use data from multiple telescopes
  to analyze one well-observed example in a quiet area. It resulted from
  the earlier passage of an accelerating disturbance in which the gas was
  heated to high temperature as in the spicule-II phenomenon. After this
  passage a dark Hα fibril appeared as a contrail. We use Saha-Boltzmann
  extinction estimation to gauge the onset and subsequent visibilities in
  various diagnostics and conclude that such Hα fibrils can indeed be
  contrail phenomena, not indicative of the thermodynamic and magnetic
  environment when they are observed but of more dynamic happenings
  before. They do not connect across internetwork cells but represent
  launch tracks of heating events and chart magnetic field during launch,
  not at present.

Title: Observing the Formation of Flare-driven Coronal Rain
Authors: Scullion, E.; Rouppe van der Voort, L.; Antolin, P.;
   Wedemeyer, S.; Vissers, G.; Kontar, E. P.; Gallagher, P. T.
Bibcode: 2016ApJ...833..184S
Altcode: 2016arXiv161009255S
  Flare-driven coronal rain can manifest from rapidly cooled plasma
  condensations near coronal loop tops in thermally unstable postflare
  arcades. We detect five phases that characterize the postflare decay:
  heating, evaporation, conductive cooling dominance for ∼120 s,
  radiative/enthalpy cooling dominance for ∼4700 s, and finally
  catastrophic cooling occurring within 35-124 s, leading to rain
  strands with a periodicity of 55-70 s. We find an excellent agreement
  between the observations and model predictions of the dominant
  cooling timescales and the onset of catastrophic cooling. At the
  rain-formation site, we detect comoving, multithermal rain clumps
  that undergo catastrophic cooling from ∼1 MK to ∼22,000 K. During
  catastrophic cooling, the plasma cools at a maximum rate of 22,700
  K s<SUP>-1</SUP> in multiple loop-top sources. We calculated the
  density of the extreme-ultraviolet (EUV) plasma from the differential
  emission measure of the multithermal source employing regularized
  inversion. Assuming a pressure balance, we estimate the density of
  the chromospheric component of rain to be 9.21 × 10<SUP>11</SUP>
  ± 1.76 × 10<SUP>11</SUP> cm<SUP>-3</SUP>, which is comparable with
  quiescent coronal rain densities. With up to eight parallel strands
  in the EUV loop cross section, we calculate the mass loss rate from
  the postflare arcade to be as much as 1.98 × 10<SUP>12</SUP> ±
  4.95 × 10<SUP>11</SUP> g s<SUP>-1</SUP>. Finally, we reveal a close
  proximity between the model predictions of {10}<SUP>5.8</SUP> K and the
  observed properties between {10}<SUP>5.9</SUP> and {10}<SUP>6.2</SUP>
  K, which defines the temperature onset of catastrophic cooling. The
  close correspondence between the observations and numerical models
  suggests that indeed acoustic waves (with a sound travel time of 68 s)
  could play an important role in redistributing energy and sustaining
  the enthalpy-based radiative cooling.

Title: Kinematics and Magnetic Properties of a Light Bridge in a
    Decaying Sunspot
Authors: Falco, M.; Borrero, J. M.; Guglielmino, S. L.; Romano, P.;
   Zuccarello, F.; Criscuoli, S.; Cristaldi, A.; Ermolli, I.; Jafarzadeh,
   S.; Rouppe van der Voort, L.
Bibcode: 2016SoPh..291.1939F
Altcode: 2016arXiv160607229F; 2016SoPh..tmp..107F
  We present the results obtained by analysing high spatial and spectral
  resolution data of the solar photosphere acquired by the CRisp Imaging
  SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011 of a
  large sunspot with a light bridge (LB) observed in NOAA AR 11263. These
  data are complemented by simultaneous Hinode Spectropolarimeter (SP)
  observation in the Fe I 630.15 nm and 630.25 nm lines. The continuum
  intensity map shows a discontinuity in the radial distribution of the
  penumbral filaments in correspondence with the LB, which shows a dark
  lane (≈0.3<SUP>″</SUP> wide and ≈8.0<SUP>″</SUP> long) along its
  main axis. The available data were inverted with the Stokes Inversion
  based on Response functions (SIR) code and physical parameters maps were
  obtained. The line-of-sight (LOS) velocity of the plasma along the LB
  derived from the Doppler effect shows motions towards and away from the
  observer up to 0.6 kms−<SUP>1</SUP> that are lower in value than the
  LOS velocities observed in the neighbouring penumbral filaments. The
  noteworthy result is that we find motions towards the observer of up to
  0.6 kms−<SUP>1</SUP> in the dark lane where the LB is located between
  two umbral cores, while the LOS velocity motion towards the observer
  is strongly reduced where the LB is located between an umbral core
  at one side and penumbral filaments on the other side. Statistically,
  the LOS velocities correspond to upflows or downflows, and comparing
  these results with Hinode/SP data, we conclude that the surrounding
  magnetic field configuration (whether more or less inclined) could have
  a role in maintaining the conditions for the process of plasma pile-up
  along the dark lane. The results obtained from our study support and
  confirm outcomes of recent magneto-hydrodynamic simulations showing
  upflows along the main axis of an LB.

Title: Reconnection brightenings in the quiet solar photosphere
Authors: Rouppe van der Voort, Luc H. M.; Rutten, Robert J.; Vissers,
   Gregal J. M.
Bibcode: 2016A&A...592A.100R
Altcode: 2016arXiv160603675R
  We describe a new quiet-Sun phenomenon which we call quiet-Sun
  Ellerman-like brightenings (QSEB). QSEBs are similar to Ellerman bombs
  (EB) in some respects but differ significantly in others. EBs are
  transient brightenings of the wings of the Balmer Hα line that mark
  strong-field photospheric reconnection in complex active regions. QSEBs
  are similar but smaller and less intense Balmer-wing brightenings
  that occur in quiet areas away from active regions. In the Hα wing,
  we measure typical lengths of less than 0.5 arcsec, widths of 0.23
  arcsec, and lifetimes of less than a minute. We discovered them
  using high-quality Hα imaging spectrometry from the Swedish 1-m
  Solar Telescope (SST) and show that, in lesser-quality data, they
  cannot be distinguished from more ubiquitous facular brightenings,
  nor in the UV diagnostics currently available from space platforms. We
  add evidence from concurrent SST spectropolarimetry that QSEBs also
  mark photospheric reconnection events, but in quiet regions on the
  solar surface. <P />The movies are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628889/olm">http://www.aanda.org</A>

Title: Emergence of Granular-sized Magnetic Bubbles Through the
    Solar Atmosphere. III. The Path to the Transition Region
Authors: Ortiz, Ada; Hansteen, Viggo H.; Bellot Rubio, Luis Ramón;
   de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Carlsson, Mats;
   Rouppe van der Voort, Luc
Bibcode: 2016ApJ...825...93O
Altcode: 2016arXiv160400302O
  We study, for the first time, the ascent of granular-sized magnetic
  bubbles from the solar photosphere through the chromosphere into the
  transition region and above. Such events occurred in a flux emerging
  region in NOAA 11850 on 2013 September 25. During that time, the
  first co-observing campaign between the Swedish 1-m Solar Telescope
  (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft
  was carried out. Simultaneous observations of the chromospheric Hα
  656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25
  nm line, were made with the CRISP spectropolarimeter at the Spitzer
  Space Telescope (SST) reaching a spatial resolution of 0.″14. At
  the same time, IRIS was performing a four-step dense raster of the
  emerging flux region, taking slit jaw images at 133 (C II, transition
  region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper
  chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy
  of several lines was performed by the IRIS spectrograph in the far-
  and near-ultraviolet, of which we have used the Si IV 140.3 and the
  Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging
  Assembly of the Solar Dynamics Observatory were used to investigate
  the possible coronal signatures of the flux emergence events. The
  photospheric and chromospheric properties of small-scale emerging
  magnetic bubbles have been described in detail in Ortiz et al. Here
  we are able to follow such structures up to the transition region. We
  describe the properties, including temporal delays, of the observed
  flux emergence in all layers. We believe this may be an important
  mechanism of transporting energy and magnetic flux from subsurface
  layers to the transition region and corona.

Title: The Appearance of Spicules in High Resolution Observations
    of Ca II H and Hα
Authors: Pereira, Tiago M. D.; Rouppe van der Voort, Luc; Carlsson,
   Mats
Bibcode: 2016ApJ...824...65P
Altcode: 2016arXiv160403116P
  Solar spicules are chromospheric fibrils that appear everywhere on
  the Sun, yet their origin is not understood. Using high resolution
  observations of spicules obtained with the Swedish 1 m Solar
  Telescope, we aim to understand how spicules appear in filtergrams
  and Dopplergrams, how they compare in Ca II H and Hα filtergrams,
  and what can make them appear and disappear. We find that spicules
  display a rich and detailed spatial structure, and show a distribution
  of transverse velocities that, when aligned with the line of sight,
  can make them appear at different Hα wing positions. They become more
  abundant at positions closer to the line core, reflecting a distribution
  of Doppler shifts and widths. In Hα width maps they stand out as
  bright features both on disk and off limb, reflecting their large
  Doppler motions and possibly higher temperatures than in the typical
  Hα formation region. Spicule lifetimes measured from narrowband images
  at only a few positions will be an underestimate because Doppler shifts
  can make them disappear prematurely from such images; for such cases,
  width maps are a more robust tool. In Hα and Ca II H filtergrams,
  off-limb spicules essentially have the same properties, appearance,
  and evolution. We find that the sudden appearance of spicules can be
  explained by Doppler shifts from their transverse motions, and does
  not require other convoluted explanations.

Title: Fan-shaped jets above the light bridge of a sunspot driven
    by reconnection
Authors: Robustini, Carolina; Leenaarts, Jorrit; de la Cruz Rodriguez,
   Jaime; Rouppe van der Voort, Luc
Bibcode: 2016A&A...590A..57R
Altcode: 2015arXiv150807927R
  We report on a fan-shaped set of high-speed jets above a strongly
  magnetized light bridge (LB) of a sunspot observed in the Hα line. We
  study the origin, dynamics, and thermal properties of the jets using
  high-resolution imaging spectroscopy in Hα from the Swedish 1m Solar
  Telescope and data from the Solar Dynamics Observatory and Hinode. The
  Hα jets have lengths of 7-38 Mm, are impulsively accelerated to a speed
  of ~100 km s<SUP>-1</SUP> close to photospheric footpoints in the LB,
  and exhibit a constant deceleration consistent with solar effective
  gravity. They are predominantly launched from one edge of the light
  bridge, and their footpoints appear bright in the Hα wings. Atmospheric
  Imaging Assembly data indicates elongated brightenings that are nearly
  co-spatial with the Hα jets. We interpret them as jets of transition
  region temperatures. The magnetic field in the light bridge has a
  strength of 0.8-2 kG and it is nearly horizontal. All jet properties
  are consistent with magnetic reconnection as the driver. <P />Movies
  associated to Figs. 1 and 2 are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201528022/olm">http://www.aanda.org</A>

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

Title: On the Active Region Bright Grains Observed in the Transition
    Region Imaging Channels of IRIS
Authors: Skogsrud, H.; Rouppe van der Voort, L.; De Pontieu, B.
Bibcode: 2016ApJ...817..124S
Altcode: 2015arXiv151205263S
  The Interface Region Imaging Spectrograph (IRIS) provides spectroscopy
  and narrow band slit-jaw (SJI) imaging of the solar chromosphere
  and transition region at unprecedented spatial and temporal
  resolutions. Combined with high-resolution context spectral imaging of
  the photosphere and chromosphere as provided by the Swedish 1 m Solar
  Telescope (SST), we can now effectively trace dynamic phenomena through
  large parts of the solar atmosphere in both space and time. IRIS SJI
  1400 images from active regions, which primarily sample the transition
  region with the Si IV 1394 and 1403 Å lines, reveal ubiquitous bright
  “grains” which are short-lived (two to five minute) bright roundish
  small patches of sizes 0.″5-1.″7 that generally move limbward with
  velocities up to about 30 km s<SUP>-1</SUP>. In this paper, we show
  that many bright grains are the result of chromospheric shocks impacting
  the transition region. These shocks are associated with dynamic fibrils
  (DFs), most commonly observed in Hα. We find that the grains show the
  strongest emission in the ascending phase of the DF, that the emission
  is strongest toward the top of the DF, and that the grains correspond
  to a blueshift and broadening of the Si IV lines. We note that the
  SJI 1400 grains can also be observed in the SJI 1330 channel which
  is dominated by C II lines. Our observations show that a significant
  part of the active region transition region dynamics is driven from
  the chromosphere below rather than from coronal activity above. We
  conclude that the shocks that drive DFs also play an important role
  in the heating of the upper chromosphere and lower transition region.

Title: Beyond spicule dynamics: spicule and fibril spectroscopy at
    high spatial and temporal resolution
Authors: Mendes Domingos Pereira, T.; Rouppe van der Voort, L.
Bibcode: 2015AGUFMSH31B2407M
Altcode:
  Solar spicules are chromospheric fibrils observed at the solar
  limb. They are observed everywhere in the Sun, but their origin is not
  yet understood. Much of our understanding of spicules has been obtained
  through filtergram observations and/or focused on the dynamics of
  spicules. Spectroscopic studies have been usually limited by spatial
  extent/resolution, temporal resolution, or variable seeing. In this
  work we make use of a unique time series of imaging spectroscopy
  at high spatial and temporal resolution, obtained with the Swedish
  Solar Telescope under excellent seeing and coordinated with the IRIS
  mission. With these data we characterize the evolution of spectra along
  quiet Sun fibrils and spicules, and discuss what makes them visible in
  filtergrams and sets them apart from other chromospheric fibrils. With
  combined H-alpha and Ca II H high-resolution observations we also
  discuss how spicules appear in these two lines, a long standing issue
  that has been interpreted in conflicting ways. Finally, using the wide
  range of IRIS diagnostics we put together the spectral evolution of
  spicules through the chromosphere and transition region.

Title: Observing Cascades of Solar Bullets at High Resolution. II.
Authors: Scullion, E.; Engvold, O.; Lin, Y.; Rouppe van der Voort, L.
Bibcode: 2015ApJ...814..123S
Altcode:
  High resolution observations from the Swedish 1-m Solar Telescope
  revealed bright, discrete, blob-like structures (which we refer to as
  solar bullets) in the Hα 656.28 nm line core that appear to propagate
  laterally across the solar atmosphere as clusters in active regions
  (ARs). These small-scale structures appear to be field aligned and
  many bullets become triggered simultaneously and traverse collectively
  as a cluster. Here, we conduct a follow-up study on these rapidly
  evolving structures with coincident observations from the Solar Dynamics
  Observatory/Atmospheric Imaging Assembly. With the co-aligned data sets,
  we reveal (a) an evolving multithermal structure in the bullet cluster
  ranging from chromospheric to at least transition region temperatures,
  (b) evidence for cascade-like behavior and corresponding bidirectional
  motions in bullets within the cluster, which indicate that there is a
  common source of the initial instability leading to bullet formation,
  and (c) a direct relationship between co-incident bullet velocities
  observed in Hα and He ii 30.4 nm and an inverse relationship with
  respect to bullet intensity in these channels. We find evidence
  supporting that bullets are typically composed of a cooler, higher
  density core detectable in Hα with a less dense, hotter, and fainter
  co-moving outer sheath. Bullets unequivocally demonstrate the finely
  structured nature of the AR corona. We have no clear evidence for
  bullets being associated with locally heated (or cooled), fast flowing
  plasma. Fast MHD pulses (such as solitons) could best describe the
  dynamic properties of bullets whereas the presence of a multithermal
  structure is new.

Title: Hα Line Profile Asymmetries and the Chromospheric Flare
    Velocity Field
Authors: Kuridze, D.; Mathioudakis, M.; Simões, P. J. A.; Rouppe van
   der Voort, L.; Carlsson, M.; Jafarzadeh, S.; Allred, J. C.; Kowalski,
   A. F.; Kennedy, M.; Fletcher, L.; Graham, D.; Keenan, F. P.
Bibcode: 2015ApJ...813..125K
Altcode: 2015arXiv151001877K
  The asymmetries observed in the line profiles of solar flares can
  provide important diagnostics of the properties and dynamics of the
  flaring atmosphere. In this paper the evolution of the Hα and Ca ii
  λ8542 lines are studied using high spatial, temporal, and spectral
  resolution ground-based observations of an M1.1 flare obtained with
  the Swedish 1 m Solar Telescope. The temporal evolution of the Hα
  line profiles from the flare kernel shows excess emission in the red
  wing (red asymmetry) before flare maximum and excess in the blue wing
  (blue asymmetry) after maximum. However, the Ca ii λ8542 line does
  not follow the same pattern, showing only a weak red asymmetry during
  the flare. RADYN simulations are used to synthesize spectral line
  profiles for the flaring atmosphere, and good agreement is found
  with the observations. We show that the red asymmetry observed in
  Hα is not necessarily associated with plasma downflows, and the blue
  asymmetry may not be related to plasma upflows. Indeed, we conclude
  that the steep velocity gradients in the flaring chromosphere modify
  the wavelength of the central reversal in the Hα line profile. The
  shift in the wavelength of maximum opacity to shorter and longer
  wavelengths generates the red and blue asymmetries, respectively.

Title: Evidence for a Transition Region Response to Penumbral
    Microjets in Sunspots
Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Carlsson,
   M.
Bibcode: 2015ApJ...811L..33V
Altcode: 2015arXiv150901402V
  Penumbral microjets (PMJs) are short-lived, fine-structured, and
  bright jets that are generally observed in chromospheric imaging of the
  penumbra of sunspots. Here we investigate their potential transition
  region signature by combining observations with the Swedish 1-m Solar
  Telescope in the Ca ii H and Ca ii 8542 Å lines with ultraviolet
  imaging and spectroscopy obtained with the Interface Region Imaging
  Spectrograph (IRIS), which includes the C ii 1334/1335 Å, Si iv
  1394/1403 Å, and Mg ii h &amp; k 2803/2796 Å lines. We find a clear
  corresponding signal in the IRIS Mg ii k, C ii, and Si iv slit-jaw
  images, typically offset spatially from the Ca ii signature in the
  direction along the jets: from base to top, the PMJs are predominantly
  visible in Ca ii, Mg ii k, and C ii/Si iv, suggesting progressive
  heating to transition region temperatures along the jet extent. Hence,
  these results support the suggestion from earlier studies that PMJs
  may heat to transition region temperatures.

Title: Ellerman Bombs at High Resolution. III. Simultaneous
    Observations with IRIS and SST
Authors: Vissers, G. J. M.; Rouppe van der Voort, L. H. M.; Rutten,
   R. J.; Carlsson, M.; De Pontieu, B.
Bibcode: 2015ApJ...812...11V
Altcode: 2015arXiv150700435V
  Ellerman bombs (EBs) are transient brightenings of the extended wings
  of the solar Balmer lines in emerging active regions. We describe
  their properties in the ultraviolet lines sampled by the Interface
  Region Imaging Spectrograph (IRIS), using simultaneous imaging
  spectroscopy in Hα with the Swedish 1-m Solar Telescope (SST) and
  ultraviolet images from the Solar Dynamics Observatory for Ellerman
  bomb detection and identification. We select multiple co-observed
  EBs for detailed analysis. The IRIS spectra strengthen the view that
  EBs mark reconnection between bipolar kilogauss fluxtubes with the
  reconnection and the resulting bi-directional jet located within the
  solar photosphere and shielded by overlying chromospheric fibrils in
  the cores of strong lines. The spectra suggest that the reconnecting
  photospheric gas underneath is heated sufficiently to momentarily reach
  stages of ionization normally assigned to the transition region and the
  corona. We also analyze similar outburst phenomena that we classify as
  small flaring arch filaments and ascribe to reconnection at a higher
  location. They have different morphologies and produce hot arches in
  million-Kelvin diagnostics.

Title: Magnetic Upflow Events in the Quiet-Sun
    Photosphere. I. Observations
Authors: Jafarzadeh, S.; Rouppe van der Voort, L.; de la Cruz
   Rodríguez, J.
Bibcode: 2015ApJ...810...54J
Altcode: 2015arXiv150707355J
  Rapid magnetic upflows in the quiet-Sun photosphere were recently
  uncovered from both Sunrise/IMaX and Hinode/SOT observations. Here, we
  study magnetic upflow events (MUEs) from high-quality, high- (spatial,
  temporal, and spectral) resolution, and full Stokes observations
  in four photospheric magnetically sensitive Fe i lines centered at
  5250.21, 6173.34, 6301.51, and 6302.50 Å acquired with the Swedish
  Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line
  Stokes V signals from those in the far blue wing whose signal-to-noise
  ratio (S/N) ≥slant 7. We find a larger number of MUEs at any given
  time (2.0× {10}<SUP>-2</SUP> arcsec<SUP>-2</SUP>), larger by one to
  two orders of magnitude, than previously reported. The MUEs appear
  to fall into four classes presenting different shapes of Stokes V
  profiles with (I) asymmetric double lobes, (II) single lobes, (III)
  double-humped (two same-polarity lobes), and (IV) three lobes (an
  extra blueshifted bump in addition to double lobes), of which less
  than half are single-lobed. We also find that MUEs are almost equally
  distributed in network and internetwork areas and they appear in the
  interior or at the edge of granules in both regions. Distributions
  of physical properties, except for horizontal velocity, of the MUEs
  (namely, Stokes V signal, size, line-of-sight velocity, and lifetime)
  are almost identical for the different spectral lines in our data. A
  bisector analysis of our spectrally resolved observations shows that
  these events host modest upflows and do not show a direct indication of
  the presence of supersonic upflows reported earlier. Our findings reveal
  that the numbers, types (classes), and properties determined for MUEs
  can strongly depend on the detection techniques used and the properties
  of the employed data, namely, S/Ns, resolutions, and wavelengths.

Title: Ellerman Bombs at High Resolution. IV. Visibility in Na I
    and Mg I
Authors: Rutten, R. J.; Rouppe van der Voort, L. H. M.; Vissers,
   G. J. M.
Bibcode: 2015ApJ...808..133R
Altcode: 2015arXiv150604426R
  Ellerman bombs are transient brightenings of the wings of the solar
  Balmer lines that mark reconnection in the photosphere. Ellerman noted
  in 1917 that he did not observe such brightenings in the Na i D and
  Mg i b lines. This non-visibility should constrain EB interpretation,
  but has not been addressed in published bomb modeling. We therefore
  test Ellerman’s observation and confirm it using high-quality imaging
  spectrometry with the Swedish 1-m Solar Telescope. However, we find
  a diffuse brightness in these lines that seems to result from prior
  EBs. We tentatively suggest this is a post-bomb hot-cloud phenomenon
  also found in recent EB spectroscopy in the ultraviolet.

Title: On the Temporal Evolution of Spicules Observed with IRIS,
    SDO, and Hinode
Authors: Skogsrud, H.; Rouppe van der Voort, L.; De Pontieu, B.;
   Pereira, T. M. D.
Bibcode: 2015ApJ...806..170S
Altcode: 2015arXiv150502525S
  Spicules are ubiquitous, fast moving jets observed off-limb in
  chromospheric spectral lines. Combining the recently launched Interface
  Region Imaging Spectrograph with the Solar Dynamics Observatory and
  Hinode, we have a unique opportunity to study spicules simultaneously
  in multiple passbands and from a seeing free environment. This makes
  it possible to study their thermal evolution over a large range of
  temperatures. A recent study showed that spicules appear in several
  chromospheric and transition region spectral lines, suggesting that
  spicules continue their evolution in hotter passbands after they
  fade from Ca ii H. In this follow-up paper, we answer some of the
  questions that were raised in the introductory study. In addition,
  we study spicules off-limb in C ii 1330 Å for the first time. We find
  that Ca ii H spicules are more similar to Mg ii 2976 Å spicules than
  initially reported. For a sample of 54 spicules, we find that 44% of
  Si iv 1400 Å spicules are brighter toward the top; 56% of the spicules
  show an increase in Si iv emission when the Ca ii H component fades. We
  find several examples of spicules that fade from passbands other than
  Ca ii H, and we observe that if a spicule fades from a passband,
  it also generally fades from the passbands with lower formation
  temperatures. We discuss what these new, multi-spectral results mean
  for the classification of type I and II spicules.

Title: The Multithermal and Multi-stranded Nature of Coronal Rain
Authors: Antolin, P.; Vissers, G.; Pereira, T. M. D.; Rouppe van der
   Voort, L.; Scullion, E.
Bibcode: 2015ApJ...806...81A
Altcode: 2015arXiv150404418A
  We analyze coordinated observations of coronal rain in loops,
  spanning chromospheric, transition region (TR), and coronal
  temperatures with sub-arcsecond spatial resolution. Coronal rain
  is found to be a highly multithermal phenomenon with a high degree
  of co-spatiality in the multi-wavelength emission. EUV darkening
  and quasi-periodic intensity variations are found to be strongly
  correlated with coronal rain showers. Progressive cooling of coronal
  rain is observed, leading to a height dependence of the emission. A
  fast-slow two-step catastrophic cooling progression is found, which
  may reflect the transition to optically thick plasma states. The
  intermittent and clumpy appearance of coronal rain at coronal heights
  becomes more continuous and persistent at chromospheric heights
  just before impact, mainly due to a funnel effect from the observed
  expansion of the magnetic field. Strong density inhomogeneities of
  0\buildrel{\prime\prime}\over{.} 2-0\buildrel{\prime\prime}\over{.} 5
  are found, in which a transition from temperatures of 10<SUP>5</SUP>
  to 10<SUP>4</SUP> K occurs. The 0\buildrel{\prime\prime}\over{.}
  2-0\buildrel{\prime\prime}\over{.} 8 width of the distribution
  of coronal rain is found to be independent of temperature. The
  sharp increase in the number of clumps at the coolest temperatures,
  especially at higher resolution, suggests that the bulk distribution
  of the rain remains undetected. Rain clumps appear organized in
  strands in both chromospheric and TR temperatures. We further find
  structure reminiscent of the magnetohydrodynamic (MHD) thermal mode
  (also known as entropy mode), thereby suggesting an important role of
  thermal instability in shaping the basic loop substructure. Rain core
  densities are estimated to vary between 2 × 10<SUP>10</SUP> and 2.5×
  {{10}<SUP>11</SUP>} cm<SUP>-3</SUP>, leading to significant downward
  mass fluxes per loop of 1-5 × 10<SUP>9</SUP> g s<SUP>-1</SUP>, thus
  suggesting a major role in the chromosphere-corona mass cycle.

Title: On Fibrils and Field Lines: the Nature of Hα Fibrils in the
    Solar Chromosphere
Authors: Leenaarts, Jorrit; Carlsson, Mats; Rouppe van der Voort, Luc
Bibcode: 2015ApJ...802..136L
Altcode: 2015arXiv150200295L
  Observations of the solar chromosphere in the line core of the Hα
  line show dark elongated structures called fibrils that show swaying
  motion. We performed a three-dimensional radiation-MHD simulation of a
  network region and computed synthetic Hα images from this simulation
  to investigate the relation between fibrils and the magnetic field
  lines in the chromosphere. The periods, amplitudes, and phase speeds
  of the simulated fibrils are consistent with observations. We find that
  some fibrils trace out the same field line along the fibril’s length,
  while other fibrils sample different field lines at different locations
  along their length. Fibrils sample the same field lines on a timescale
  of ∼200 s. This is shorter than their own lifetime. Fibril-threading
  field lines carry slow-mode waves, as well as transverse waves
  propagating with the Alfvén speed. Transverse waves propagating
  in opposite directions cause an interference pattern with complex
  apparent phase speeds. The relationship between fibrils and field lines
  is governed by constant migration and swaying of the field lines,
  their mass loading and draining, and their visibility in Hα. Field
  lines are visible where they lie close to the optical depth unity
  surface. The location of the latter is at a height at which the column
  mass reaches a certain fixed value. The visibility of the field line
  is thus determined by its own mass density and by the mass density of
  the material above it. Using the swaying motion of fibrils as a tracer
  of chromospheric transverse oscillations must be done with caution.

Title: Internetwork Chromospheric Bright Grains Observed With IRIS
    and SST
Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson,
   Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt,
   Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan;
   Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean;
   Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli,
   Sarah; Kankelborg, Charles
Bibcode: 2015ApJ...803...44M
Altcode: 2015arXiv150203490M
  The Interface Region Imaging Spectrograph (IRIS) reveals small-scale
  rapid brightenings in the form of bright grains all over coronal holes
  and the quiet Sun. These bright grains are seen with the IRIS 1330,
  1400, and 2796 Å slit-jaw filters. We combine coordinated observations
  with IRIS and from the ground with the Swedish 1 m Solar Telescope
  (SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H
  3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å,
  Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps
  in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.}
  33), temporal, and spectral resolution. We conclude that the IRIS
  slit-jaw grains are the counterpart of so-called acoustic grains,
  i.e., resulting from chromospheric acoustic waves in a non-magnetic
  environment. We compare slit-jaw images (SJIs) with spectra from the
  IRIS spectrograph. We conclude that the grain intensity in the 2796
  Å slit-jaw filter comes from both the Mg ii k core and wings. The
  signal in the C ii and Si iv lines is too weak to explain the presence
  of grains in the 1300 and 1400 Å SJIs and we conclude that the grain
  signal in these passbands comes mostly from the continuum. Although
  weak, the characteristic shock signatures of acoustic grains can often
  be detected in IRIS C ii spectra. For some grains, a spectral signature
  can be found in IRIS Si iv. This suggests that upward propagating
  acoustic waves sometimes reach all the way up to the transition region.

Title: CRISPRED: A data pipeline for the CRISP imaging
    spectropolarimeter
Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.;
   Hillberg, T.; Rouppe van der Voort, L.
Bibcode: 2015A&A...573A..40D
Altcode: 2014arXiv1406.0202D
  The production of science-ready data from major solar telescopes
  requires expertise beyond that of the typical observer. This is
  a consequence of the increasing complexity of instruments and
  observing sequences, which require calibrations and corrections
  for instrumental and seeing effects that are not only difficult to
  measure, but are also coupled in ways that require careful analysis
  in the design of the correction procedures. Modern space-based
  telescopes have data-processing pipelines capable of routinely
  producing well-characterized data products. High resolution imaging
  spectropolarimeters at ground-based telescopes need similar data
  pipelines.We present new methods for flat-fielding spectropolarimetric
  data acquired with telecentric Fabry-Perot instruments and a new
  approach for accurate camera co-alignment for image restoration. We
  document a procedure that forms the basis of current state-of-the-art
  processing of data from the CRISP imaging spectropolarimeter at the
  Swedish 1 m Solar Telescope (SST). By collecting, implementing, and
  testing a suite of computer programs, we have defined a data reduction
  pipeline for this instrument. This pipeline, CRISPRED, streamlines the
  process of making science-ready data.It is implemented and operated
  in IDL, with time-consuming steps delegated to C.CRISPRED will also be
  the basis for the data pipeline of the forthcoming CHROMIS instrument.

Title: Heating Signatures in the Disk Counterparts of Solar Spicules
    in Interface Region Imaging Spectrograph Observations
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Pereira, T. M. D.;
   Carlsson, M.; Hansteen, V.
Bibcode: 2015ApJ...799L...3R
Altcode: 2014arXiv1412.4531R
  We use coordinated observations with the Interface Region Imaging
  Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify
  the disk counterpart of type II spicules in upper-chromospheric and
  transition region (TR) diagnostics. These disk counterparts were
  earlier identified through short-lived asymmetries in chromospheric
  spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We
  find clear signatures of RBEs and RREs in Mg II h &amp; k, often with
  excursions of the central h3 and k3 absorption features in concert with
  asymmetries in co-temporal and co-spatial Hα spectral profiles. We find
  spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si
  IV 1394 and 1403 Å spectral lines and interpret this as a sign that
  type II spicules are heated to at least TR temperatures, supporting
  other recent work. These C II and Si IV spectral signals are weaker
  for a smaller network region than for more extended network regions in
  our data. A number of bright features around extended network regions
  observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified
  as network jets, can be clearly connected to Hα RBEs and/or RREs in
  our coordinated data. We speculate that at least part of the diffuse
  halo around network regions in the IRIS SJI 1330 and 1400 images can
  be attributed to type II spicules with insufficient opacity in the C
  II and Si IV lines to stand out as single features in these passbands.

Title: Unresolved Fine-scale Structure in Solar Coronal Loop-tops
Authors: Scullion, E.; Rouppe van der Voort, L.; Wedemeyer, S.;
   Antolin, P.
Bibcode: 2014ApJ...797...36S
Altcode: 2014arXiv1409.1920S
  New and advanced space-based observing facilities continue to lower
  the resolution limit and detect solar coronal loops in greater
  detail. We continue to discover even finer substructures within
  coronal loop cross-sections, in order to understand the nature of
  the solar corona. Here, we push this lower limit further to search
  for the finest coronal loop substructures, through taking advantage
  of the resolving power of the Swedish 1 m Solar Telescope/CRisp
  Imaging Spectro-Polarimeter (CRISP), together with co-observations
  from the Solar Dynamics Observatory/Atmospheric Image Assembly
  (AIA). High-resolution imaging of the chromospheric Hα 656.28 nm
  spectral line core and wings can, under certain circumstances, allow
  one to deduce the topology of the local magnetic environment of the
  solar atmosphere where its observed. Here, we study post-flare coronal
  loops, which become filled with evaporated chromosphere that rapidly
  condenses into chromospheric clumps of plasma (detectable in Hα)
  known as a coronal rain, to investigate their fine-scale structure. We
  identify, through analysis of three data sets, large-scale catastrophic
  cooling in coronal loop-tops and the existence of multi-thermal,
  multi-stranded substructures. Many cool strands even extend fully
  intact from loop-top to footpoint. We discover that coronal loop
  fine-scale strands can appear bunched with as many as eight parallel
  strands within an AIA coronal loop cross-section. The strand number
  density versus cross-sectional width distribution, as detected by CRISP
  within AIA-defined coronal loops, most likely peaks at well below 100
  km, and currently, 69% of the substructure strands are statistically
  unresolved in AIA coronal loops.

Title: On the Multi-threaded Nature of Solar Spicules
Authors: Skogsrud, H.; Rouppe van der Voort, L.; De Pontieu, B.
Bibcode: 2014ApJ...795L..23S
Altcode: 2014arXiv1410.1334S
  A dominant constituent in the dynamic chromosphere is spicules. Spicules
  at the limb appear as relatively small and dynamic jets that are
  observed to stick out everywhere. Many papers emphasize the important
  role spicules might play in the energy and mass balance of the
  chromosphere and corona. However, many aspects of spicules remain
  a mystery. In this Letter, we shed more light on the multi-threaded
  nature of spicules and their torsional component. We use high spatial,
  spectral, and temporal resolution observations from the Swedish
  1 m Solar Telescope in the Hα spectral line. The data target the
  limb, and we extract spectra from spicules far out from the limb
  to reduce the line-of-sight superposition effect. We discover that
  many spicules display very asymmetric spectra with some even showing
  multiple peaks. To quantify this asymmetry, we use a double-Gaussian
  fitting procedure and find an average velocity difference between the
  single-Gaussian components to be between 20 and 30 km s<SUP>-1</SUP>
  for a sample of 57 spicules. We observe that spicules show significant
  substructure where one spicule consists of many "threads." We interpret
  the asymmetric spectra as a line-of-sight superposition of threads
  in one spicule and therefore have a measure for a perpendicular flow
  inside spicules that will be important for future numerical models to
  reproduce. In addition, we show examples of λ - x slices perpendicular
  across spicules and find spectral tilts in individual threads, providing
  further evidence for the complex dynamical nature of spicules.

Title: On the prevalence of small-scale twist in the solar
    chromosphere and transition region
Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.;
   Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen,
   J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser,
   J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
   S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
   Martinez-Sykora, J.
Bibcode: 2014Sci...346D.315D
Altcode: 2014arXiv1410.6862D
  The solar chromosphere and transition region (TR) form an interface
  between the Sun’s surface and its hot outer atmosphere. There,
  most of the nonthermal energy that powers the solar atmosphere
  is transformed into heat, although the detailed mechanism remains
  elusive. High-resolution (0.33-arc second) observations with NASA’s
  Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere
  and TR that are replete with twist or torsional motions on sub-arc
  second scales, occurring in active regions, quiet Sun regions, and
  coronal holes alike. We coordinated observations with the Swedish
  1-meter Solar Telescope (SST) to quantify these twisting motions and
  their association with rapid heating to at least TR temperatures. This
  view of the interface region provides insight into what heats the low
  solar atmosphere.

Title: Dynamic Properties along the Neutral Line of a Delta Spot
    Inferred from High-resolution Observations
Authors: Cristaldi, A.; Guglielmino, S. L.; Zuccarello, F.; Romano,
   P.; Falco, M.; Rouppe van der Voort, L.; de la Cruz Rodríguez, J.;
   Ermolli, I.; Criscuoli, S.
Bibcode: 2014ApJ...789..162C
Altcode:
  Delta (δ) spots are complex magnetic configurations of sunspots
  characterized by umbrae of opposite polarity sharing a common
  penumbra. In order to investigate the fine structure of the region
  separating the two magnetic polarities of a δ spot, we studied the
  morphology, the magnetic configuration, and the velocity field in
  such a region using observations of active region (AR) NOAA 11267
  obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the
  Swedish Solar Telescope on 2011 August 6. The analysis of CRISP data
  shows upflows and downflows of ~ ± 3 km s<SUP>-1</SUP> in proximity
  of the δ spot polarity inversion line (PIL), and horizontal motions
  along the PIL of the order of ~1 km s<SUP>-1</SUP>. The results
  obtained from the SIR inversion of CRISP data also indicate that the
  transverse magnetic field in the brighter region separating the two
  opposite magnetic polarities of the δ spot is tilted about ~45°
  with respect to the PIL. Solar Dynamics Observatory/Helioseismic and
  Magnetic Imager observations confirm the presence of motions of ~
  ± 3 km s<SUP>-1</SUP> in proximity of the PIL, which were observed
  to last 15 hr. From the data analyzed, we conclude that the steady,
  persistent, and subsonic motions observed along the δ spot PIL can be
  interpreted as being due to Evershed flows occurring in the penumbral
  filaments that show a curved, wrapped configuration. The fluting of
  the penumbral filaments and their bending, continuously increased by
  the approaching motion of the negative umbra toward the positive one,
  give rise to the complex line-of-sight velocity maps that we observed.

Title: IRIS Observations of Twist in the Low Solar Atmosphere
Authors: De Pontieu, Bart; Rouppe van der Voort, Luc; Pereira,
   Tiago M. D.; Skogsrud, Haakon; McIntosh, Scott W.; Carlsson, Mats;
   Hansteen, Viggo
Bibcode: 2014AAS...22431302D
Altcode:
  The Interface Region Imaging Spectrograph (IRIS) small explorer
  was launched in June 2013. IRIS’s high-resolution (0.33 arcsec),
  high-cadence (2s) images and spectra reveal a solar chromosphere and
  transition region that is riddled with twist. This is evidenced by the
  presence of ubiquitous torsional motions on very small (subarcsec)
  spatial scales. These motions occur in active regions, quiet Sun
  and coronal holes on a variety of structures such as spicules at
  the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as
  low-lying loops. We use IRIS data and observations from the Swedish
  Solar Telescope (SST) in La Palma, Spain to describe these motions
  quantitatively, study their propagation, and illustrate how such
  strong twisting motions are often associated with significant and
  rapid heating to at least transition region temperatures.

Title: Emergence of Granular-sized Magnetic Bubbles through the
    Solar Atmosphere. I. Spectropolarimetric Observations and Simulations
Authors: Ortiz, Ada; Bellot Rubio, Luis R.; Hansteen, Viggo H.;
   de la Cruz Rodríguez, Jaime; Rouppe van der Voort, Luc
Bibcode: 2014ApJ...781..126O
Altcode: 2013arXiv1312.5735O
  We study a granular-sized magnetic flux emergence event that occurred
  in NOAA 11024 in 2009 July. The observations were made with the CRISP
  spectropolarimeter at the Swedish 1 m Solar Telescope achieving a
  spatial resolution of 0.''14. Simultaneous full Stokes observations of
  the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca
  II 854.2 nm line allow us to describe in detail the emergence process
  across the solar atmosphere. We report here on three-dimensional
  (3D) semi-spherical bubble events, where instead of simple magnetic
  footpoints, we observe complex semi-circular feet straddling a few
  granules. Several phenomena occur simultaneously, namely, abnormal
  granulation, separation of opposite-polarity legs, and brightenings at
  chromospheric heights. However, the most characteristic signature in
  these events is the observation of a dark bubble in filtergrams taken
  in the wings of the Ca II 854.2 nm line. There is a clear coincidence
  between the emergence of horizontal magnetic field patches and the
  formation of the dark bubble. We can infer how the bubble rises through
  the solar atmosphere as we see it progressing from the wings to the
  core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows
  mean upward Doppler velocities of 2 km s<SUP>-1</SUP> and expands at a
  horizontal speed of 4 km s<SUP>-1</SUP>. In about 3.5 minutes it travels
  some 1100 km to reach the mid chromosphere, implying an average ascent
  speed of 5.2 km s<SUP>-1</SUP>. The maximum separation attained by the
  magnetic legs is 6.''6. From an inversion of the observed Stokes spectra
  with the SIR code, we find maximum photospheric field strengths of 480 G
  and inclinations of nearly 90° in the magnetic bubble interior, along
  with temperature deficits of up to 250 K at log τ = -2 and above. To
  aid the interpretation of the observations, we carry out 3D numerical
  simulations of the evolution of a horizontal, untwisted magnetic flux
  sheet injected in the convection zone, using the Bifrost code. The
  computational domain spans from the upper convection zone to the lower
  corona. In the modeled chromosphere, the rising flux sheet produces a
  large, cool, magnetized bubble. We compare this bubble with the observed
  ones and find excellent agreement, including similar field strengths
  and velocity signals in the photosphere and chromosphere, temperature
  deficits, ascent speeds, expansion velocities, and lifetimes.

Title: IRIS observations of twist in the low solar atmosphere
Authors: De Pontieu, Bart; Pereira, Tiago; Rouppe van der Voort, Luc;
   Skogsrud, Hakon
Bibcode: 2014cosp...40E.654D
Altcode:
  The Interface Region Imaging Spectrograph (IRIS) small explorer
  was launched in June 2013. IRIS's high-resolution (0.33 arcsec),
  high-cadence (2s) images and spectra reveal a solar chromosphere and
  transition region that is riddled with twist. This is evidenced by the
  presence of ubiquitous torsional motions on very small (subarcsec)
  spatial scales. These motions occur in active regions, quiet Sun
  and coronal holes on a variety of structures such as spicules at
  the limb, rapid-blue/red-shifted events (RBEs and RREs) as well as
  low-lying loops. We use IRIS data and observations from the Swedish
  Solar Telescope (SST) in La Palma, Spain to describe these motions
  quantitatively and illustrate how such strong twisting motions are often
  associated with significant and rapid heating to at least transition
  region temperatures.

Title: Effects of flux emergence in the outer solar
    atmosphere. Observational advances
Authors: Ortiz Carbonell, Ada; De Pontieu, Bart; Bellot Rubio, L. R.;
   Hansteen, Viggo; Rouppe van der Voort, Luc; Carlsson, Mats
Bibcode: 2014cosp...40E2387O
Altcode:
  We study granular sized magnetic flux emergence events that occur in
  a flux emergence region in NOAA 11850 on September 25, 2013. During
  that time, the first co-observing campaign between the Swedish 1 m
  Solar Telescope and the IRIS spacecraft was carried out. Simultaneous
  observations of the Halpha 656.28 nm and Ca II 854.2 nm chromospheric
  lines, and the Fe I 630.25 nm photospheric line, were made with
  the CRISP/SST spectropolarimeter reaching a spatial resolution of
  0."14. At the same time, IRIS was performing a four-step dense raster
  of the said emerging flux region, taking slit-jaw images at 133 (C II
  transiti on region), 140 (Si IV, transition region), 279.6 (Mg II k,
  core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere),
  obtaining thus the highest resolution images ever taken of the upper
  chromosphere and transition region. The photospheric and chromospheric
  properties of the emerging magnetic flux bubbles have been described
  in detail in Ortiz et al. (2014). However, in the current work we are
  able to follow such lower atmosphere observations of flux emergence
  up to the transition region with unprecedented spatial and temporal
  resolution. We describe the properties (size, time delays, lifetime,
  velocities, temperature) of the observed signatures of flux emergence
  in the transition region. We believe this may be an important mechanism
  of transporting energy and magnetic flux to the upper layers of the
  solar atmosphere, namely the transition region and corona, at least
  in cases when active regions are formed by flux emerging through the
  photosphere. * Ortiz et al. (2014) ApJ 781, 126

Title: Short Dynamic Fibrils in Sunspot Chromospheres
Authors: Rouppe van der Voort, L.; de la Cruz Rodríguez, J.
Bibcode: 2013ApJ...776...56R
Altcode: 2013arXiv1308.3893R
  Sunspot chromospheres display vigorous oscillatory signatures when
  observed using chromospheric diagnostics such as the strong Ca II lines
  and Hα. New high-resolution sunspot observations from the Swedish
  1 m Solar Telescope show the ubiquitous presence of small-scale,
  periodic, jet-like features that move up and down. This phenomenon
  has not been described before. The typical width of these features is
  about 0.''3 and they display clear parabolic trajectories in space-time
  diagrams. The maximum extension of the top of the jets is lowest in
  the umbra, a few 100 km, and progressively longer further away from
  the umbra in the penumbra, with the longest extending more than 1000
  km. These jets resemble the dynamic fibrils found in plage regions but
  at smaller extensions. Local thermodynamic equilibrium inversion of
  spectropolarimetric Ca II 8542 observations enabled a comparison of the
  magnetic field inclination and properties of these short jets. We find
  that the most extended of these jets also have longer periods and tend
  to be located in regions with more horizontal magnetic fields. These
  results are direct observational confirmation of the mechanism of
  long-period waves propagating along inclined magnetic fields into the
  solar chromosphere. This mechanism was identified earlier as the driver
  of dynamic fibrils in plage, part of the mottles in the quiet Sun, and
  the type I spicules at the limb. The sunspot dynamic fibrils that we
  report here represent a new class of manifestation of this mechanism,
  distinct from the transient penumbral and umbral micro-jets reported
  earlier.

Title: Ellerman Bombs at High Resolution. II. Triggering, Visibility,
    and Effect on Upper Atmosphere
Authors: Vissers, Gregal J. M.; Rouppe van der Voort, Luc H. M.;
   Rutten, Robert J.
Bibcode: 2013ApJ...774...32V
Altcode: 2013arXiv1307.1547V
  We use high-resolution imaging spectroscopy with the Swedish 1-m Solar
  Telescope (SST) to study the transient brightenings of the wings
  of the Balmer Hα line in emerging active regions that are called
  Ellerman bombs. Simultaneous sampling of Ca II 8542 Å with the SST
  confirms that most Ellerman bombs also occur in the wings of this
  line, but with markedly different morphology. Simultaneous images
  from the Solar Dynamics Observatory (SDO) show that Ellerman bombs
  are also detectable in the photospheric 1700 Å continuum, again with
  differing morphology. They are also observable in 1600 Å SDO images,
  but with much contamination from C IV emission in transition-region
  features. Simultaneous SST spectropolarimetry in Fe I 6301 Å shows that
  Ellerman bombs occur at sites of strong-field magnetic flux cancellation
  between small bipolar strong-field patches that rapidly move together
  over the solar surface. Simultaneous SDO images in He II 304 Å, Fe IX
  171 Å, and Fe XIV 211 Å show no clear effect of the Ellerman bombs
  on the overlying transition region and corona. These results strengthen
  our earlier suggestion, based on Hα morphology alone, that the Ellerman
  bomb phenomenon is a purely photospheric reconnection phenomenon.

Title: Are Giant Tornadoes the Legs of Solar Prominences?
Authors: Wedemeyer, Sven; Scullion, Eamon; Rouppe van der Voort, Luc;
   Bosnjak, Antonija; Antolin, Patrick
Bibcode: 2013ApJ...774..123W
Altcode: 2013arXiv1306.2661W
  Observations in the 171 Å channel of the Atmospheric Imaging Assembly
  of the space-borne Solar Dynamics Observatory show tornado-like
  features in the atmosphere of the Sun. These giant tornadoes appear
  as dark, elongated, and apparently rotating structures in front of
  a brighter background. This phenomenon is thought to be produced
  by rotating magnetic field structures that extend throughout the
  atmosphere. We characterize giant tornadoes through a statistical
  analysis of properties such as spatial distribution, lifetimes,
  and sizes. A total number of 201 giant tornadoes are detected in a
  period of 25 days, suggesting that, on average, about 30 events are
  present across the whole Sun at a time close to solar maximum. Most
  tornadoes appear in groups and seem to form the legs of prominences,
  thus serving as plasma sources/sinks. Additional Hα observations with
  the Swedish 1 m Solar Telescope imply that giant tornadoes rotate as
  a structure, although they clearly exhibit a thread-like structure. We
  observe tornado groups that grow prior to the eruption of the connected
  prominence. The rotation of the tornadoes may progressively twist
  the magnetic structure of the prominence until it becomes unstable
  and erupts. Finally, we investigate the potential relation of giant
  tornadoes to other phenomena, which may also be produced by rotating
  magnetic field structures. A comparison to cyclones, magnetic tornadoes,
  and spicules implies that such events are more abundant and short-lived
  the smaller they are. This comparison might help to construct a power
  law for the effective atmospheric heating contribution as a function
  of spatial scale.

Title: Physical properties of a sunspot chromosphere with umbral
    flashes
Authors: de la Cruz Rodríguez, J.; Rouppe van der Voort, L.;
   Socas-Navarro, H.; van Noort, M.
Bibcode: 2013A&A...556A.115D
Altcode: 2013arXiv1304.0752D
  We present new high-resolution spectro-polarimetric Ca IIλ8542
  observations of umbral flashes in sunspots. At nearly 0.18 arcsec,
  and spanning about one hour of continuous observation, this is the
  most detailed dataset published thus far. Our study involves both LTE
  and non-LTE inversions (but includes also a weak field analysis as a
  sanity check) to quantify temperatures, mass flows and the full magnetic
  field vector geometry. We confirm earlier reports that UFs have very
  fine structure with hot and cool material intermixed at sub-arcsecond
  scales. The shock front is roughly 1000 K hotter than the surrounding
  material. We do not observe significant fluctuations of the field in the
  umbra. In the penumbra, however, the passage of the running penumbral
  waves alter the magnetic field strength by some 200 G (peak-to-peak
  amplitude) but it does not change the field orientation (at least not
  significantly within our sensitivity of a few degrees). From a fast
  Fourier transform analysis, we find a trend of decreasing power at high
  temporal frequencies at those locations with more horizontal magnetic
  fields, for the line-of-sight velocity and magnetic field strength. In
  the outer penumbra we find an absence of high frequency power while
  there is increasingly more power at high frequencies towards the
  umbra. <P />Movie and Appendices A and B are available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: A Detailed Comparison between the Observed and Synthesized
    Properties of a Simulated Type II Spicule
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit;
   Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie
   V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc
Bibcode: 2013ApJ...771...66M
Altcode: 2013arXiv1305.2397M
  We have performed a three-dimensional radiative MHD simulation of the
  solar atmosphere. This simulation shows a jet-like feature that shows
  similarities to the type II spicules observed for the first time with
  Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the
  solar disk are associated with these spicules. Observational results
  suggest they may contribute significantly in supplying the corona
  with hot plasma. We perform a detailed comparison of the properties
  of the simulated jet with those of type II spicules (observed with
  Hinode) and RBEs (with ground-based instruments). We analyze a wide
  variety of synthetic emission and absorption lines from the simulations
  including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition
  region and coronal temperatures (10,000 K to several million K). We
  compare their synthetic intensities, line profiles, Doppler shifts,
  line widths, and asymmetries with observations from Hinode/SOT and
  EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many
  properties of the synthetic observables resemble the observations,
  and we describe in detail the physical processes that lead to these
  observables. Detailed analysis of the synthetic observables provides
  insight into how observations should be analyzed to derive information
  about physical variables in such a dynamic event. For example, we
  find that line-of-sight superposition in the optically thin atmosphere
  requires the combination of Doppler shifts and spectral line asymmetry
  to determine the velocity in the jet. In our simulated type II spicule,
  the lifetime of the asymmetry of the transition region lines is shorter
  than that of the coronal lines. Other properties differ from the
  observations, especially in the chromospheric lines. The mass density
  of the part of the spicule with a chromospheric temperature is too low
  to produce significant opacity in chromospheric lines. The synthetic
  Ca II 8542 Å and Hα profiles therefore do not show signal resembling
  RBEs. These and other discrepancies are described in detail, and we
  discuss which mechanisms and physical processes may need to be included
  in the MHD simulations to mimic the thermodynamic processes of the
  chromosphere and corona, in particular to reproduce type II spicules.

Title: Magnetic tornadoes and chromospheric swirls - Definition
    and classification
Authors: Wedemeyer, Sven; Scullion, Eamon; Steiner, Oskar; de la Cruz
   Rodriguez, Jaime; Rouppe van der Voort, L. H. M.
Bibcode: 2013JPhCS.440a2005W
Altcode: 2013arXiv1303.0179W
  Chromospheric swirls are the observational signatures of rotating
  magnetic field structures in the solar atmosphere, also known as
  magnetic tornadoes. Swirls appear as dark rotating features in the core
  of the spectral line of singly ionized calcium at a wavelength of 854.2
  nm. This signature can be very subtle and difficult to detect given
  the dynamic changes in the solar chromosphere. Important steps towards
  a systematic and objective detection method are the compilation and
  characterization of a statistically significant sample of observed
  and simulated chromospheric swirls. Here, we provide a more exact
  definition of the chromospheric swirl phenomenon and also present a
  first morphological classification of swirls with three types: (I) Ring,
  (II) Split, (III) Spiral. We also discuss the nature of the magnetic
  field structures connected to tornadoes and the influence of limited
  spatial resolution on the appearance of their photospheric footpoints.

Title: Ellerman bombs: fallacies, fads, usage
Authors: Rutten, Robert J.; Vissers, Gregal J. M.; Rouppe van der
   Voort, Luc H. M.; Sütterlin, Peter; Vitas, Nikola
Bibcode: 2013JPhCS.440a2007R
Altcode: 2013arXiv1304.1364R
  Ellerman bombs are short-lived brightenings of the outer wings of Hα
  that occur in active regions with much flux emergence. We point out
  fads and fallacies in the extensive Ellerman bomb literature, discuss
  their appearance in various spectral diagnostics, and advocate their
  use as indicators of field reconfiguration in active-region topography
  using AIA 1700 Å images.

Title: Interplay of Three Kinds of Motion in the Disk Counterpart
of Type II Spicules: Upflow, Transversal, and Torsional Motions
Authors: Sekse, D. H.; Rouppe van der Voort, L.; De Pontieu, B.;
   Scullion, E.
Bibcode: 2013ApJ...769...44S
Altcode: 2013arXiv1304.2304S
  Recently, it was shown that the complex dynamical behavior of spicules
  has to be interpreted as the result of simultaneous action of three
  kinds of motion: (1) field aligned flows, (2) swaying motions, and
  (3) torsional motions. We use high-quality observations from the
  CRisp Imaging SpectroPolarimeter at the Swedish 1-m Solar Telescope
  to investigate signs of these different kinetic modes in spicules on
  the disk. Earlier, rapid blue-shifted excursions (RBEs), short-lived
  absorption features in the blue wing of chromospheric spectral lines,
  were identified as the disk counterpart of type II spicules. Here we
  report the existence of similar absorption features in the red wing of
  the Ca II 8542 and Hα lines: rapid redshifted excursions (RREs). RREs
  are found over the whole solar disk and are located in the same regions
  as RBEs: in the vicinity of magnetic field concentrations. RREs have
  similar characteristics as RBEs: they have similar lengths, widths,
  lifetimes, and average Doppler velocity. The striking similarity
  of RREs to RBEs implies that RREs are a manifestation of the same
  physical phenomenon and that spicules harbor motions that can result
  in a net redshift when observed on-disk. We find that RREs are less
  abundant than RBEs: the RRE/RBE detection count ratio is about 0.52
  at disk center and 0.74 near the limb. We interpret the higher number
  of RBEs and the decreased imbalance toward the limb as an indication
  that field-aligned upflows have a significant contribution to the
  net Dopplershift of the structure. Most RREs and RBEs are observed in
  isolation, but we find many examples of parallel and touching RRE/RBE
  pairs which appear to be part of the same spicule. We interpret the
  existence of these RRE/RBE pairs and the observation that many RREs
  and RBEs have varying Dopplershift along their width as signs that
  torsional motion is an important characteristic of spicules. The fact
  that most RBEs and RREs are observed in isolation agrees with the idea
  that transversal swaying motion is another important kinetic mode. We
  find examples of transitions from RRE to RBE and vice versa. These
  transitions sometimes appear to propagate along the structure with
  speeds between 18 and 108 km s<SUP>-1</SUP> and can be interpreted as
  the sign of a transverse (Alfvénic) wave.

Title: Heating of the Magnetic Chromosphere: Observational Constraints
    from Ca II λ8542 Spectra
Authors: de la Cruz Rodríguez, J.; De Pontieu, B.; Carlsson, M.;
   Rouppe van der Voort, L. H. M.
Bibcode: 2013ApJ...764L..11D
Altcode: 2013arXiv1301.3141D
  The heating of the Sun's chromosphere remains poorly understood. While
  progress has been made on understanding what drives the quiet-Sun
  internetwork chromosphere, chromospheric heating in strong magnetic
  field regions continues to present a difficult challenge, mostly
  because of a lack of observational constraints. We use high-resolution
  spectropolarimetric data from the Swedish 1 m Solar Telescope to
  identify the location and spatio-temporal properties of heating in
  the magnetic chromosphere. In particular, we report the existence of
  raised-core spectral line profiles in the Ca II λ8542 line. These
  profiles are characterized by the absence of an absorption line
  core, showing a quasi-flat profile between λ ≈ ±0.5 Å, and are
  abundant close to magnetic bright points and plage. Comparison with
  three-dimensional MHD simulations indicates that such profiles occur
  when the line of sight goes through an "elevated temperature canopy"
  associated with the expansion with height of the magnetic field of
  flux concentrations. This temperature canopy in the simulations is
  caused by ohmic dissipation where there are strong magnetic field
  gradients. The raised-core profiles are thus indicators of locations
  of increased chromospheric heating. We characterize the location and
  temporal and spatial properties of such profiles in our observations,
  thus providing much stricter constraints on theoretical models of
  chromospheric heating mechanisms than before.

Title: On the Temporal Evolution of the Disk Counterpart of Type II
    Spicules in the Quiet Sun
Authors: Sekse, D. H.; Rouppe van der Voort, L.; De Pontieu, B.
Bibcode: 2013ApJ...764..164S
Altcode: 2012arXiv1212.4988S
  The newly established type II spicule has been speculated to provide
  enough hot plasma to play an important role in the mass loading
  and heating of the solar corona. With the identification of rapid
  blueshifted excursions (RBEs) as the on-disk counterpart of type II
  spicules we have analyzed three different high-quality timeseries
  with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish
  Solar Telescope on La Palma and subjected to an automated detection
  routine to detect a large number of RBEs for statistical purposes. Our
  observations are of a quiet-Sun region at disk center and we find
  lower Doppler velocities, 15-40 km s<SUP>-1</SUP>, and Doppler widths,
  2-15 km s<SUP>-1</SUP>, of RBEs than in earlier coronal hole studies,
  30-50 km s<SUP>-1</SUP> and 7-23 km s<SUP>-1</SUP>, respectively. In
  addition, we examine the spatial dependence of Doppler velocities and
  widths along the RBE axis and conclude that there is no clear trend to
  this over the field of view or in individual RBEs in the quiet Sun at
  disk center. These differences with previous coronal hole studies are
  attributed to the more varying magnetic field configuration in quiet-Sun
  conditions. Using an extremely high-cadence data set has allowed us
  to improve greatly on the determination of lifetimes of RBEs, which we
  find to range from 5 to 60 s with an average lifetime of 30 s, as well
  as the transverse motions in RBEs, with transverse velocities up to
  55 km s<SUP>-1</SUP> and averaging 12 km s<SUP>-1</SUP>. Furthermore,
  our measurements of the recurrence rates of RBEs provide important
  new constraints on coronal heating by spicules. We also see many
  examples of a sinusoidal wave pattern in the transverse motion of
  RBEs with periods averaging 54 s and amplitudes from 21.5 to 129 km
  which agrees well with previous studies of wave motion in spicules at
  the limb. We interpret the appearance of RBEs over their full length
  within a few seconds as the result of a combination of three kinds
  of motions as is earlier reported for spicules. Finally, we look at
  the temporal connection between Hα and Ca II 8542 RBEs and find that
  Ca II 8542 RBEs in addition to being located closer to the footpoint
  also appear before the Hα RBEs. This connection between Ca II 8542
  and Hα supports the idea that heating is occurring in spicules and
  contributes more weight to the prominence of spicules as a source for
  heating and mass loading of the corona.

Title: Comparing Simultaneous Measurements of two High-Resolution
Imaging Spectropolarimeters: The `Göttingen' FPI@VTT and CRISP@SST
Authors: Bello González, N.; Bellot Rubio, L. R.; Ortiz, A.; Rezaei,
   R.; Rouppe van der Voort, L.; Schlichenmaier, R.
Bibcode: 2012ASPC..463..251B
Altcode: 2012arXiv1204.1023B
  In July 2009, the leading spot of the active region NOAA11024 was
  observed simultaneously and independently with the ‘Göttingen’
  FPI at VTT and CRISP at SST, i.e., at two different sites,
  telescopes, instruments and using different spectral lines. The data
  processing and data analysis have been carried out independently
  with different techniques. Maps of physical parameters retrieved
  from 2D spectro-polarimetric data observed with ‘Göttingen’
  FPI and CRISP show an impressive agreement. In addition, the
  ‘Göttingen’ FPI maps also exhibit a notable resemblance with
  simultaneous TIP (spectrographic) observations. The consistency in the
  results demonstrates the excellent capabilities of these observing
  facilities. Besides, it confirms the solar origin of the detected
  signals and the reliability of FPI-based spectro-polarimeters.

Title: On-Disk Coronal Rain
Authors: Antolin, Patrick; Vissers, Gregal; Rouppe van der Voort, Luc
Bibcode: 2012SoPh..280..457A
Altcode: 2012SoPh..tmp...78A; 2012arXiv1203.2077A
  Small and elongated, cool and dense blob-like structures are being
  reported with high resolution telescopes in physically different regions
  throughout the solar atmosphere. Their detection and the understanding
  of their formation, morphology, and thermodynamical characteristics can
  provide important information on their hosting environment, especially
  concerning the magnetic field, whose understanding constitutes a major
  problem in solar physics. An example of such blobs is coronal rain, a
  phenomenon of thermal non-equilibrium observed in active region loops,
  which consists of cool and dense chromospheric blobs falling along
  loop-like paths from coronal heights. So far, only off-limb coronal
  rain has been observed, and few reports on the phenomenon exist. In
  the present work, several data sets of on-disk Hα observations with
  the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar
  Telescope (SST) are analyzed. A special family of on-disk blobs is
  selected for each data set, and a statistical analysis is carried out
  on their dynamics, morphology, and temperature. All characteristics
  present distributions which are very similar to reported coronal rain
  statistics. We discuss possible interpretations considering other
  similar blob-like structures reported so far and show that a coronal
  rain interpretation is the most likely one. The chromospheric nature
  of the blobs and the projection effects (which eliminate all direct
  possibilities of height estimation) on one side, and their small sizes,
  fast dynamics, and especially their faint character (offering low
  contrast with the background intensity) on the other side, are found
  as the main causes for the absence until now of the detection of this
  on-disk coronal rain counterpart.

Title: Temporal Evolution of Velocity and Magnetic Field in and
    around Umbral Dots
Authors: Watanabe, Hiroko; Bellot Rubio, Luis R.; de la Cruz
   Rodríguez, Jaime; Rouppe van der Voort, Luc
Bibcode: 2012ApJ...757...49W
Altcode: 2012arXiv1207.6006W
  We study the temporal evolution of umbral dots (UDs) using measurements
  from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar
  Telescope. Scans of the magnetically sensitive 630 nm iron lines
  were performed under stable atmospheric conditions for 71 minutes
  with a cadence of 63 s. These observations allow us to investigate
  the magnetic field and velocity in and around UDs at a resolution
  approaching 0farcs13. From the analysis of 339 UDs, we draw the
  following conclusions: (1) UDs show clear hints of upflows, as predicted
  by magnetohydrodynamic simulations. By contrast, we could not find
  systematic downflow signals. Only in very deep layers, we detect
  localized downflows around UDs, but they do not persist in time. (2)
  We confirm that UDs exhibit weaker and more inclined fields than their
  surroundings, as reported previously. However, UDs that have strong
  fields above 2000 G or are in the decay phase show enhanced and more
  vertical fields. (3) There are enhanced fields at the migration front
  of UDs detached from penumbral grains, as if their motion were impeded
  by the ambient field. (4) Long-lived UDs travel longer distances with
  slower proper motions. Our results appear to confirm some aspects of
  recent numerical simulations of magnetoconvection in the umbra (e.g.,
  the existence of upflows in UDs), but not others (e.g., the systematic
  weakening of the magnetic field at the position of UDs).

Title: Implications for Coronal Heating from Coronal Rain
Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort,
   L.; Vissers, G.; Hansteen, V.
Bibcode: 2012ASPC..454..171A
Altcode:
  Coronal rain is a phenomenon above active regions in which cool plasma
  condensations fall down from coronal heights. Numerical simulations of
  loops have shown that such condensations can naturally form in the case
  of footpoint concentrated heating through the “catastrophic cooling”
  mechanism. In this work we analize high resolution limb observations in
  Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of
  SST and derive statistical properties. We further investigate the link
  between coronal rain and the coronal heating mechanisms by performing
  1.5-D MHD simulations of a loop subject to footpoint heating and to
  Alfvén waves generated in the photosphere. It is found that if a loop
  is heated predominantly from Alfvén waves coronal rain is inhibited
  due to the characteristic uniform heating they produce. Hence coronal
  rain can point both to the spatial distribution of the heating and to
  the agent of the heating itself, thus acting as a marker for coronal
  heating mechanisms.

Title: Velocities in Magnetoconvective Structures inside Sunspot
    Umbrae
Authors: Ortiz, A.; Rubio, L. B.; Rouppe van der Voort, L.
Bibcode: 2012ASPC..454..217O
Altcode:
  We study the velocity field of umbral dots at a resolution of
  0.″14. Our analysis is based on full Stokes spectropolarimetric
  measurements of a pore taken with the CRISP instrument at the Swedish
  1-m Solar Telescope. We determine the flow velocity at different heights
  in the photosphere from a bisector analysis of the Fe I 630 nm lines. We
  find that most umbral dots are associated with strong upflows in deep
  photospheric layers. Some of them also show concentrated patches of
  downflows at their edges with velocities of up to 1000 m s<SUP>-1</SUP>
  and enhanced net circular polarization signals. These results appear
  to validate numerical models of magnetoconvection in the presence of
  strong magnetic fields.

Title: Statistical Properties of the Disk Counterparts of Type
    II Spicules from Simultaneous Observations of Rapid Blueshifted
    Excursions in Ca II 8542 and Hα
Authors: Sekse, D. H.; Rouppe van der Voort, L.; De Pontieu, B.
Bibcode: 2012ApJ...752..108S
Altcode: 2012arXiv1204.2943S
  Spicules were recently found to exist as two different types when a new
  class of so-called type II spicules was discovered at the solar limb
  with the Solar Optical Telescope on board the Hinode spacecraft. These
  type II spicules have been linked with on-disk observations of rapid
  blueshifted excursions (RBEs) in the Hα and Ca II 8542 lines. Here
  we analyze observations optimized for the detection of RBEs in both
  Hα and Ca II 8542 lines simultaneously at a high temporal cadence
  taken with the Crisp Imaging Spectropolarimeter at the Swedish Solar
  Telescope on La Palma. In this study, we used a high-quality time
  sequence for RBEs at different blueshifts and employed an automated
  detection routine to detect a large number of RBEs in order to expand
  on the statistics of RBEs. We find that the number of detected RBEs
  is strongly dependent on the associated Doppler velocity of the images
  on which the search is performed. Automatic detection of RBEs at lower
  velocities increases the estimated number of RBEs to the same order of
  magnitude expected from limb spicules. This shows that RBEs and type
  II spicules are indeed exponents of the same phenomenon. Furthermore,
  we provide solid evidence that Ca II 8542 RBEs are connected to Hα
  RBEs and are located closer to the network regions with the Hα RBEs
  being a continuation of the Ca II 8542 RBEs. Our results show that
  RBEs have an average lifetime of 83.9 s when observed in both spectral
  lines and that the Doppler velocities of RBEs range from 10 to 25 km
  s<SUP>-1</SUP> in Ca II 8542 and 30 to 50 km s<SUP>-1</SUP> in Hα. In
  addition, we automatically determine the transverse motion of a much
  larger sample of RBEs than previous studies, and find that, just like
  type II spicules, RBEs undergo significant transverse motions of the
  order of 5-10 km s<SUP>-1</SUP>. Finally, we find that the intergranular
  jets discovered at Big Bear Solar Observatory are a subset of RBEs.

Title: Prominence seismology using ground- and space-based
    observations
Authors: Ballester, J. L.; Arregui, I.; Oliver, R.; Terradas, J.;
   Soler, R.; Lin, Y.; Engvold, O.; Langagen, O.; Rouppe van der Voort,
   L. H. M.
Bibcode: 2012EAS....55..169B
Altcode:
  Ground- and space-based observations have confirmed the presence of
  oscillatory motions in prominences and they have been interpreted in
  terms of magnetohydrodynamic (MHD) waves. This interpretation opens the
  door to perform prominence seismology, whose main aim is to determine
  physical parameters in magnetic and plasma structures (prominences)
  that are difficult to measure by direct means. Here, two prominence
  seismology applications are presented.

Title: Ubiquitous Torsional Motions in Type II Spicules
Authors: De Pontieu, B.; Carlsson, M.; Rouppe van der Voort, L. H. M.;
   Rutten, R. J.; Hansteen, V. H.; Watanabe, H.
Bibcode: 2012ApJ...752L..12D
Altcode: 2012arXiv1205.5006D
  Spicules are long, thin, highly dynamic features that jut out
  ubiquitously from the solar limb. They dominate the interface between
  the chromosphere and corona and may provide significant mass and energy
  to the corona. We use high-quality observations with the Swedish 1
  m Solar Telescope to establish that so-called type II spicules are
  characterized by the simultaneous action of three different types of
  motion: (1) field-aligned flows of order 50-100 km s<SUP>-1</SUP>,
  (2) swaying motions of order 15-20 km s<SUP>-1</SUP>, and (3) torsional
  motions of order 25-30 km s<SUP>-1</SUP>. The first two modes have been
  studied in detail before, but not the torsional motions. Our analysis
  of many near-limb and off-limb spectra and narrowband images using
  multiple spectral lines yields strong evidence that most, if not all,
  type II spicules undergo large torsional modulation and that these
  motions, like spicule swaying, represent Alfvénic waves propagating
  outward at several hundred km s<SUP>-1</SUP>. The combined action
  of the different motions explains the similar morphology of spicule
  bushes in the outer red and blue wings of chromospheric lines, and
  needs to be taken into account when interpreting Doppler motions to
  derive estimates for field-aligned flows in spicules and determining
  the Alfvénic wave energy in the solar atmosphere. Our results also
  suggest that large torsional motion is an ingredient in the production
  of type II spicules and that spicules play an important role in the
  transport of helicity through the solar atmosphere.

Title: Magnetic tornadoes as energy channels into the solar corona
Authors: Wedemeyer-Böhm, Sven; Scullion, Eamon; Steiner, Oskar;
   Rouppe van der Voort, Luc; de La Cruz Rodriguez, Jaime; Fedun, Viktor;
   Erdélyi, Robert
Bibcode: 2012Natur.486..505W
Altcode:
  Heating the outer layers of the magnetically quiet solar atmosphere to
  more than one million kelvin and accelerating the solar wind requires
  an energy flux of approximately 100 to 300 watts per square metre,
  but how this energy is transferred and dissipated there is a puzzle and
  several alternative solutions have been proposed. Braiding and twisting
  of magnetic field structures, which is caused by the convective flows
  at the solar surface, was suggested as an efficient mechanism for
  atmospheric heating. Convectively driven vortex flows that harbour
  magnetic fields are observed to be abundant in the photosphere
  (the visible surface of the Sun). Recently, corresponding swirling
  motions have been discovered in the chromosphere, the atmospheric
  layer sandwiched between the photosphere and the corona. Here we
  report the imprints of these chromospheric swirls in the transition
  region and low corona, and identify them as observational signatures
  of rapidly rotating magnetic structures. These ubiquitous structures,
  which resemble super-tornadoes under solar conditions, reach from
  the convection zone into the upper solar atmosphere and provide an
  alternative mechanism for channelling energy from the lower into the
  upper solar atmosphere.

Title: Dynamics of the Solar Magnetic Bright Points Derived from
    Their Horizontal Motions
Authors: Chitta, L. P.; van Ballegooijen, A. A.; Rouppe van der Voort,
   L.; DeLuca, E. E.; Kariyappa, R.
Bibcode: 2012ApJ...752...48C
Altcode: 2012arXiv1204.4362C
  The subarcsecond bright points (BPs) associated with the small-scale
  magnetic fields in the lower solar atmosphere are advected by
  the evolution of the photospheric granules. We measure various
  quantities related to the horizontal motions of the BPs observed in
  two wavelengths, including the velocity autocorrelation function. A
  1 hr time sequence of wideband Hα observations conducted at the
  Swedish 1 m Solar Telescope (SST) and a 4 hr Hinode G-band time
  sequence observed with the Solar Optical Telescope are used in this
  work. We follow 97 SST and 212 Hinode BPs with 3800 and 1950 individual
  velocity measurements, respectively. For its high cadence of 5 s as
  compared to 30 s for Hinode data, we emphasize more the results from
  SST data. The BP positional uncertainty achieved by SST is as low as 3
  km. The position errors contribute 0.75 km<SUP>2</SUP> s<SUP>-2</SUP>
  to the variance of the observed velocities. The raw and corrected
  velocity measurements in both directions, i.e., (v<SUB>x</SUB> ,
  v<SUB>y</SUB> ), have Gaussian distributions with standard deviations
  of (1.32, 1.22) and (1.00, 0.86) km s<SUP>-1</SUP>, respectively. The
  BP motions have correlation times of about 22-30 s. We construct the
  power spectrum of the horizontal motions as a function of frequency,
  a quantity that is useful and relevant to the studies of generation
  of Alfvén waves. Photospheric turbulent diffusion at timescales less
  than 200 s is found to satisfy a power law with an index of 1.59.

Title: A Sharp Look at Coronal Rain with Hinode/SOT and SST/CRISP
Authors: Antolin, P.; Carlsson, M.; Rouppe van der Voort, L.;
   Verwichte, E.; Vissers, G.
Bibcode: 2012ASPC..455..253A
Altcode: 2012arXiv1202.0787A
  The tropical wisdom that when it is hot and dense we can expect
  rain might also apply to the Sun. Indeed, observations and numerical
  simulations have showed that strong heating at footpoints of loops,
  as is the case for active regions, puts their coronae out of thermal
  equilibrium, which can lead to a phenomenon known as catastrophic
  cooling. Following local pressure loss in the corona, hot plasma
  locally condenses in these loops and dramatically cools down to
  chromospheric temperatures. These blobs become bright in Hα and
  Ca ii H in time scales of minutes, and their dynamics seem to be
  subject more to internal pressure changes in the loop rather than to
  gravity. They thus become trackers of the magnetic field, which results
  in the spectacular coronal rain that is observed falling down coronal
  loops. In this work we report on high resolution observations of coronal
  rain with the Solar Optical Telescope (SOT) on Hinode and CRISP at
  the Swedish Solar Telescope (SST). A statistical study is performed in
  which properties such as velocities and accelerations of coronal rain
  are derived. We show how this phenomenon can constitute a diagnostic
  tool for the internal physical conditions inside loops. Furthermore, we
  analyze transverse oscillations of strand-like condensations composing
  coronal rain falling in a loop, and discuss the possible nature of the
  wave. This points to the important role that coronal rain can play in
  the fields of coronal heating and coronal seismology.

Title: Flocculent Flows in the Chromospheric Canopy of a Sunspot
Authors: Vissers, Gregal; Rouppe van der Voort, Luc
Bibcode: 2012ApJ...750...22V
Altcode: 2012arXiv1202.5453V
  High-quality imaging spectroscopy in the Hα line, obtained with the
  CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m solar
  Telescope (SST) at La Palma and covering a small sunspot and its
  surroundings, is studied. They exhibit ubiquitous flows both along
  fibrils making up the chromospheric canopy away from the spot and
  in the superpenumbra. We term these flows "flocculent" to describe
  their intermittent character, that is, morphologically reminiscent
  of coronal rain. The flocculent flows are investigated further in
  order to determine their dynamic and morphological properties. For
  the measurement of their characteristic velocities, accelerations,
  and sizes, we employ a new versatile analysis tool, the CRisp
  SPectral EXplorer (CRISPEX), which we describe in detail. Absolute
  velocities on the order of 7.2-82.4 km s<SUP>-1</SUP> are found,
  with an average value of 36.5 ± 5.9 km s<SUP>-1</SUP> and slightly
  higher typical velocities for features moving toward the sunspot than
  away. These velocities are much higher than those determined from the
  shift of the line core, which shows patches around the sunspot with
  velocity enhancements of up to 10-15 km s<SUP>-1</SUP> (both red- and
  blueshifted). Accelerations are determined for a subsample of features
  that show clear accelerating or decelerating behavior, yielding an
  average of 270 ± 63 m s<SUP>-2</SUP> and 149 ± 63 m s<SUP>-2</SUP>
  for the accelerating and decelerating features, respectively. Typical
  flocculent features measure 627 ± 44 km in length and 304 ± 30 km
  in width. On average, 68 features are detected per minute, with an
  average lifetime of 67.7 ± 8.8 s. The dynamics and phenomenology
  of the flocculent flows suggest they may be driven by a siphon flow,
  where the flocculence could arise from a density perturbation close
  to one of the footpoints or along the loop structure.

Title: Dynamics of the Photospheric Bright Points Observed With SST
    and Hinode
Authors: Chitta, Lakshmi Pradeep; van Ballegooijen, A.; Rouppe van
   der Voort, L.; DeLuca, E.; Kariyappa, R.
Bibcode: 2012AAS...22020614C
Altcode: 2012AAS...22020614P
  The horizontal motions of the solar magnetic bright points (BPs)
  observed in two wavelengths (SST Halpha and Hinode/SOT G-band)
  is studied in detail. With emphasis on SST results: the velocity
  distribution of horizontal motions is found to be a Gaussian. The
  auto-correlations of observed velocities is also obtained. An
  empirical fit to the observed auto-correlation gives us a positional
  uncertainty of 3 km and the error in the velocity measurements to be
  0.87 km s$^{-1}$. Due to the non-Lorentzian, cusp-like nature of the
  auto-correlation, the power spectrum of the BP motions shows enhanced
  power at frequencies exceeding 0.02 Hz. The diffusion of magnetic field
  due to granular evolution at short timescales is found to satisfy a
  power law with a slope of 1.59.

Title: The Formation of the Hα Line in the Solar Chromosphere
Authors: Leenaarts, J.; Carlsson, M.; Rouppe van der Voort, L.
Bibcode: 2012ApJ...749..136L
Altcode: 2012arXiv1202.1926L
  We use state-of-the-art radiation-MHD simulations and three-dimensional
  (3D) non-LTE radiative transfer computations to investigate Hα
  line formation in the solar chromosphere and apply the results of
  this investigation to develop the potential of Hα as a diagnostic
  of the chromosphere. We show that one can accurately model Hα line
  formation assuming statistical equilibrium and complete frequency
  redistribution provided the computation of the model atmosphere
  included non-equilibrium ionization of hydrogen and the Lyα and
  Lyβ line profiles are described by Doppler profiles. We find that
  3D radiative transfer is essential in modeling hydrogen lines due
  to the low photon destruction probability in Hα. The Hα opacity
  in the upper chromosphere is mainly sensitive to the mass density
  and only weakly sensitive to the temperature. We find that the Hα
  line-core intensity is correlated with the average formation height:
  The larger the average formation height is, the lower the intensity
  will be. The line-core width is a measure of the gas temperature
  in the line-forming region. The fibril-like dark structures seen in
  Hα line-core images computed from our model atmosphere are tracing
  magnetic field lines. These structures are caused by field-aligned
  ridges of enhanced chromospheric mass density that raise their average
  formation height, and therefore make them appear dark against their
  deeper-formed surroundings. We compare with observations, and find that
  the simulated line-core widths are very similar to the observed ones,
  without the need for additional microturbulence.

Title: High-resolution observations of type II spicules
Authors: Rouppe van der Voort, Luc
Bibcode: 2012decs.confE..41R
Altcode:
  Type II spicules are a class of spicules that is connected with
  mass-loading and heating of the solar corona. Type II spicules are
  characterized by short lifetimes and high apparent velocities. Combined
  with their narrow spatial widths, type II spicules are challenging to
  observe. We use the CRISP imaging spectropolarimeter at the Swedish
  1-m Solar Telescope on La Palma to observe type II spicules at the
  limb and their counterparts on the solar disk, the so-called "Rapid
  Blue-shifted Excursions" (RBEs). The combination of adaptive optics
  and image post-processing allows CRISP to attain high resolution
  simultaneously in the spatial, temporal and spectral domains. Here we
  present results from the analysis of several high-quality data sets
  which allow to constrain the physical properties of type II spicules.

Title: Small-scale rotating magnetic flux structures as alternative
    energy channels into the low corona
Authors: Wedemeyer-Böhm; , Sven; Scullion; , Eamon; Steiner; , Oskar;
   Rouppe van der Voort, Luc; de la Cruz Rodriguez, Jaime; Erdelyi,
   Robertus; Fedun, Viktor
Bibcode: 2012decs.confE..67W
Altcode:
  Vortex flows are frequently observed in the downflow areas in the lanes
  between granules. The magnetic field is advected and trapped by these
  flows in the low photosphere. Consequently, the rotation of a vortex
  flow is transferred to the atmospheric layers above by means of the
  magnetic flux structure. This effect results in so-called swirls, which
  are observed in the chromosphere. New simultaneous observations with
  the Swedish Solar Telescope and the Solar Dynamics Observatory reveal
  that chromospheric swirls can have a coronal counterpart. This finding
  implies that the rotating flux structure couples the layers of the solar
  atmosphere from the photosphere to the (low) corona. Three-dimensional
  numerical simulations confirm this picture and reproduce the swirl
  signature. A combined analysis of the simulations and observations
  implies that such small-scale rotating flux structures could provide
  an alternative mechanism for channeling substantial energy from the
  photosphere into the upper solar atmosphere.

Title: Type-II spicules: Heating and magnetic field properties from
    aligned CRISP/SST and SDO observations
Authors: Scullion, E.; Rouppe van der Voort, L.; de la Cruz Rodriguez,
   J.
Bibcode: 2012decs.confE..44S
Altcode:
  Over the past decade there has been a resurgence in the study of
  small-scale chromospheric jets known, classically, as spicules. Recent
  observations have lead us to conclude that there are two distinct
  varieties of spicule, namely, slower type-I (i.e. mottles, dynamic
  fibrils, H-alpha spicules etc.) and faster type-II (RBEs: Rapid
  Blue-shift Excursions on-disk). Such events dominate the dynamics of
  the chromosphere. Joint SDO (Solar Dynamics Observatory) and Hinode
  observations have revealed that fast spicules are the source of hot
  plasma channelling into the corona. Here we report on the properties
  of this widespread heating with observations from the high resolution
  CRISP (CRisp Imaging SpectroPolarimeter) instrument at the SST (1-m
  Swedish Solar Telescope, La Palma) and co-aligned SDO data. Furthermore,
  we reveal new insight into the formation of type-II spicules through
  considering the distribution of RBEs with respect to the photospheric
  magnetic field (via CRISP).

Title: Small-scale, Dynamic Bright Blobs in Solar Filaments and
    Active Regions
Authors: Lin, Y.; Engvold, O.; Rouppe van der Voort, L. H. M.
Bibcode: 2012ApJ...747..129L
Altcode: 2012arXiv1201.0406L
  High-cadence high spatial resolution observations in Hα with the
  Swedish 1 m Solar Telescope on La Palma have revealed the existence
  of small-scale highly dynamic bright blobs. A fast wavelength
  tuning spectropolarimeter provides spectral information of these
  structures. The blobs slide along thin magnetic threads at speeds
  in the range from 45 km s<SUP>-1</SUP> to 111 km s<SUP>-1</SUP>. The
  blobs have a slightly elongated shape and their lengths increase by a
  factor of three from close to half an arcsecond when they first appear
  until they disappear one to two minutes later. The brightest blobs
  show the highest speed. The widths of the Hα line emission of the
  blobs correspond to non-thermal velocities in the plasma less than
  10 km s<SUP>-1</SUP>, which implies that they are not the result of
  shock-driven heating. The dynamic character of the bright blobs is
  similar to what can be expected from an MHD fast-mode pulse.

Title: Observing the Fine Structure of Loops through High-resolution
    Spectroscopic Observations of Coronal Rain with the CRISP Instrument
    at the Swedish Solar Telescope
Authors: Antolin, P.; Rouppe van der Voort, L.
Bibcode: 2012ApJ...745..152A
Altcode: 2011arXiv1112.0656A
  Observed in cool chromospheric lines, such as Hα or Ca II H, coronal
  rain corresponds to cool and dense plasma falling from coronal
  heights. Considered as a peculiar sporadic phenomenon of active
  regions, it has not received much attention since its discovery
  more than 40 years ago. Yet, it has been shown recently that a
  close relationship exists between this phenomenon and the coronal
  heating mechanism. Indeed, numerical simulations have shown that
  this phenomenon is most likely due to a loss of thermal equilibrium
  ensuing from a heating mechanism acting mostly toward the footpoints of
  loops. We present here one of the first high-resolution spectroscopic
  observations of coronal rain, performed with the CRisp Imaging Spectro
  Polarimeter (CRISP) instrument at the Swedish Solar Telescope. This
  work constitutes the first attempt to assess the importance of coronal
  rain in the understanding of the coronal magnetic field in active
  regions. With the present resolution, coronal rain is observed to
  literally invade the entire field of view. A large statistical set is
  obtained in which dynamics (total velocities and accelerations), shapes
  (lengths and widths), trajectories (angles of fall of the blobs),
  and thermodynamic properties (temperatures) of the condensations
  are derived. Specifically, we find that coronal rain is composed of
  small and dense chromospheric cores with average widths and lengths of
  ~310 km and ~710 km, respectively, average temperatures below 7000 K,
  displaying a broad distribution of falling speeds with an average of
  ~70 km s<SUP>-1</SUP>, and accelerations largely below the effective
  gravity along loops. Through estimates of the ion-neutral coupling in
  the blobs we show that coronal rain acts as a tracer of the coronal
  magnetic field, thus supporting the multi-strand loop scenario, and
  acts as a probe of the local thermodynamic conditions in loops. We
  further elucidate its potential in coronal heating. We find that
  the cooling in neighboring strands occurs simultaneously in general
  suggesting a similar thermodynamic evolution among strands, which can
  be explained by a common footpoint heating process. Constraints for
  coronal heating models of loops are thus provided. Estimates of the
  fraction of coronal volume with coronal rain give values between 7%
  and 30%. Estimates of the occurrence time of the phenomenon in loops
  set times between 5 and 20 hr, implying that coronal rain may be a
  common phenomenon, in agreement with the frequent observations of cool
  downflows in extreme-ultraviolet lines. The coronal mass drain rate
  in the form of coronal rain is estimated to be on the order of 5 ×
  10<SUP>9</SUP> g s<SUP>-1</SUP>, a significant quantity compared to
  the estimate of mass flux into the corona from spicules.

Title: Generation and propagation of Alfvenic waves in spicules
Authors: De Pontieu, B.; Okamoto, T. J.; Rouppe van der Voort, L.;
   Hansteen, V. H.; Carlsson, M.
Bibcode: 2011AGUFMSH13B1956D
Altcode:
  Both spicules and Alfven waves have recently been implicated in
  playing a role in the heating of the outer atmosphere. Yet we do
  not know how spicules or Alfven waves are generated. Here we focus
  on the properties of Alfvenic waves in spicules and their role in
  forming spicules. We use high-resolution observations taken with the
  Solar Optical Telescope onboard Hinode, and with the CRISP Fabry-Perot
  Interferometer at the Swedish Solar Telescope (SST) in La Palma to study
  the generation and propagation of Alfvenic waves in spicules and their
  disk counterparts. Using automated detection algorithms to identify
  propagating waves in limb spicules, we find evidence for both up-
  and downward propagating as well as standing waves. Our data suggests
  significant reflection of waves in and around spicules and provides
  constraints for theoretical models of spicules and wave propagation
  through the chromosphere. We also show observational evidence (using
  SST data) of the generation of Alfven waves and the role they play in
  forming spicules.

Title: Ellerman Bombs at High Resolution. I. Morphological Evidence
    for Photospheric Reconnection
Authors: Watanabe, Hiroko; Vissers, Gregal; Kitai, Reizaburo; Rouppe
   van der Voort, Luc; Rutten, Robert J.
Bibcode: 2011ApJ...736...71W
Altcode: 2011arXiv1105.4008W
  High-resolution imaging-spectroscopy movies of solar active region NOAA
  10998 obtained with the Crisp Imaging Spectropolarimeter at the Swedish
  1-m Solar Telescope show very bright, rapidly flickering, flame-like
  features that appear intermittently in the wings of the Balmer Hα line
  in a region with moat flows and likely some flux emergence. They show
  up at regular Hα blue-wing bright points that outline the magnetic
  network, but flare upward with much larger brightness and distinct "jet"
  morphology seen from aside in the limbward view of these movies. We
  classify these features as Ellerman bombs and present a morphological
  study of their appearance at the unprecedented spatial, temporal, and
  spectral resolution of these observations. The bombs appear along the
  magnetic network with footpoint extents up to 900 km. They show apparent
  travel away from the spot along the pre-existing network at speeds
  of about 1 km s<SUP>-1</SUP>. The bombs flare repetitively with much
  rapid variation at timescales of seconds only, in the form of upward
  jet-shaped brightness features. These reach heights of 600-1200 km and
  tend to show blueshifts; some show bi-directional Doppler signature
  and some seem accompanied with an Hα surge. They are not seen in the
  core of Hα due to shielding by overlying chromospheric fibrils. The
  network where they originate has normal properties. The morphology of
  these jets strongly supports deep-seated photospheric reconnection of
  emergent or moat-driven magnetic flux with pre-existing strong vertical
  network fields as the mechanism underlying the Ellerman bomb phenomenon.

Title: Quiet-Sun imaging asymmetries in Na I D<SUB>1</SUB> compared
    with other strong Fraunhofer lines
Authors: Rutten, R. J.; Leenaarts, J.; Rouppe van der Voort, L. H. M.;
   de Wijn, A. G.; Carlsson, M.; Hansteen, V.
Bibcode: 2011A&A...531A..17R
Altcode: 2011arXiv1104.4307R
  Imaging spectroscopy of the solar atmosphere using the Na I
  D<SUB>1</SUB> line yields marked asymmetry between the blue and
  red line wings: sampling a quiet-Sun area in the blue wing displays
  reversed granulation, whereas sampling in the red wing displays normal
  granulation. The Mg I b<SUB>2</SUB> line of comparable strength does
  not show this asymmetry, nor does the stronger Ca II 8542 Å line. We
  demonstrate the phenomenon with near-simultaneous spectral images in
  Na I D<SUB>1</SUB>, Mg I b<SUB>2</SUB>, and Ca II 8542 Å from the
  Swedish 1-m Solar Telescope. We then explain it with line-formation
  insights from classical 1D modeling and with a 3D magnetohydrodynamical
  simulation combined with NLTE spectral line synthesis that permits
  detailed comparison with the observations in a common format. The
  cause of the imaging asymmetry is the combination of correlations
  between intensity and Dopplershift modulation in granular overshoot
  and the sensitivity to these of the steep profile flanks of the Na
  I D<SUB>1</SUB> line. The Mg I b<SUB>2</SUB> line has similar core
  formation but much wider wings due to larger opacity buildup and
  damping in the photosphere. Both lines obtain marked core asymmetry
  from photospheric shocks in or near strong magnetic concentrations,
  less from higher-up internetwork shocks that produce similar asymmetry
  in the spatially averaged Ca II 8542 Å profile.

Title: Upflows in the Central Dark Lane of Sunspot Light Bridges
Authors: Rouppe van der Voort, L.; Bellot Rubio, L. R.; Ortiz, A.
Bibcode: 2010ApJ...718L..78R
Altcode: 2010arXiv1006.4578R
  We use high spatial and spectral resolution observations obtained with
  the CRisp Imaging SpectroPolarimeter at the Swedish 1 m Solar Telescope
  to analyze the velocity profile of granular light bridges (LBs) in
  a sunspot. We find upflows associated with the central dark lanes of
  the LBs. From bisectors in the Fe I 630.15 nm line we find that the
  magnitude of the upflows varies with height, with the strongest upflows
  being deeper in the atmosphere. Typical upflow velocities measured from
  the 70% bisector are around 500 m s<SUP>-1</SUP> with peaks above 1 km
  s<SUP>-1</SUP>. The upflows in the central dark lane are surrounded by
  downflows of weaker magnitude, sometimes concentrated in patches with
  enhanced velocities reaching up to 1.1 km s<SUP>-1</SUP>. A small
  spatial offset between the upflows and the continuum dark lane is
  interpreted as a line-of-sight effect due to the elevated nature of the
  dark lane and the LB above the umbral surroundings. Our observations
  show that the central dark lane in granular LBs is not equivalent to
  the intergranular lanes of normal photospheric granulation that host
  convective downflows. These results support recent MHD simulations of
  magneto-convection in sunspot atmospheres.

Title: Downflows in Sunspot Umbral Dots
Authors: Ortiz, A.; Bellot Rubio, L. R.; Rouppe van der Voort, L.
Bibcode: 2010ApJ...713.1282O
Altcode: 2010arXiv1003.1897O
  We study the velocity field of umbral dots (UDs) at a resolution
  of 0farcs14. Our analysis is based on full Stokes measurements of a
  pore taken with the Crisp Imaging Spectro-Polarimeter at the Swedish
  1 m Solar Telescope. We determine the flow velocity at different
  heights in the photosphere from a bisector analysis of the Fe I 630 nm
  lines. In addition, we use the observed Stokes Q, U, and V profiles
  to characterize the magnetic properties of these structures. We find
  that most UDs are associated with strong upflows in deep photospheric
  layers. Some of them also show concentrated patches of downflows at
  their edges, with sizes of about 0farcs25, velocities of up to 1000
  m s<SUP>-1</SUP>, and enhanced net circular polarization signals. The
  downflows evolve rapidly and have lifetimes of only a few minutes. These
  results appear to validate numerical models of magnetoconvection in
  the presence of strong magnetic fields.

Title: Observation and analysis of chromospheric magnetic fields .
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; van Noort,
   M.; Rouppe van der Voort, L.
Bibcode: 2010MmSAI..81..716D
Altcode: 2010arXiv1004.0698D
  The solar chromosphere is a vigorously dynamic region of the sun,
  where waves and magnetic fields play an important role. To improve
  chromospheric diagnostics, we present new observations in Ca II 8542
  carried out with the SST/CRISP on La Palma, working in full-Stokes
  mode. We measured Stokes line profiles in active regions. The line
  profiles observed close to the solar limb show signals in all four
  Stokes parameters, while profiles observed close to disk center only
  show signals above the noise level in Stokes I and V. We used the NLTE
  inversion code 'NICOLE' to derive atmospheric parameters in umbral
  flashes present in a small round sunspot without penumbra.

Title: Spectropolarimetry with CRISP at the Swedish 1-m Solar
    Telescope
Authors: Ortiz, A.; Rouppe van der Voort, L. H. M.
Bibcode: 2010ASSP...19..150O
Altcode: 2009arXiv0902.4150O; 2010mcia.conf..150O
  CRISP (Crisp Imaging Spectro-polarimeter), the new spectropolarimeter
  at the Swedish 1-m Solar Telescope, opens a new perspective in solar
  polarimetry. With better spatial resolution (0.13″) than Hinode in
  the Fe I 6302 Å lines and similar polarimetric sensitivity reached
  through postprocessing, CRISP complements the SP spectropolarimeter
  onboard Hinode. We present some of the data that we obtained in our
  June 2008 campaign and preliminary results from LTE inversions of a
  pore containing umbral dots.

Title: On-disk Counterparts of Type II Spicules in the Ca II 854.2
    nm and Hα Lines
Authors: Rouppe van der Voort, L.; Leenaarts, J.; de Pontieu, B.;
   Carlsson, M.; Vissers, G.
Bibcode: 2009ApJ...705..272R
Altcode: 2009arXiv0909.2115R
  Recently, a second type of spicules was discovered at the solar
  limb with the Solar Optical Telescope onboard the Japanese Hinode
  spacecraft. These previously unrecognized type II spicules are thin
  chromospheric jets that are shorter lived (10-60 s) and that show much
  higher apparent upward velocities (of order 50-100 km s<SUP>-1</SUP>)
  than the classical spicules. Since they have been implicated in
  providing hot plasma to coronal loops, their formation, evolution,
  and properties are important ingredients for a better understanding
  of the mass and energy balance of the low solar atmosphere. Here, we
  report on the discovery of the disk counterparts of type II spicules
  using spectral imaging data in the Ca II 854.2 nm and Hα lines with
  the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope in
  La Palma. We find rapid blueward excursions in the line profiles of
  both chromospheric lines that correspond to thin, jet-like features
  that show apparent velocities of order 50 km s<SUP>-1</SUP>. These
  blueward excursions seem to form a separate absorbing component with
  Doppler shifts of order 20 and 50 km s<SUP>-1</SUP> for the Ca II 854.2
  nm and Hα line, respectively. We show that the appearance, lifetimes,
  longitudinal and transverse velocities, and occurrence rate of these
  rapid blue excursions on the disk are very similar to those of the type
  II spicules at the limb. A detailed study of the spectral line profiles
  in these events suggests that plasma is accelerated along the jet,
  and plasma is being heated throughout the short lifetime of the event.

Title: Small-scale swirl events in the quiet Sun chromosphere
Authors: Wedemeyer-Böhm, S.; Rouppe van der Voort, L.
Bibcode: 2009A&A...507L...9W
Altcode: 2009arXiv0910.2226W
  Context: Recent progress in instrumentation enables solar observations
  with high resolution simultaneously in the spatial, temporal, and
  spectral domains. <BR />Aims: We use such high-resolution observations
  to study small-scale structures and dynamics in the chromosphere
  of the quiet Sun. <BR />Methods: We analyse time series of spectral
  scans through the Ca ii 854.2 nm spectral line obtained with the CRISP
  instrument at the Swedish 1-m Solar Telescope. The targets are quiet
  Sun regions inside coronal holes close to disc-centre. <BR />Results:
  The line core maps exhibit relatively few fibrils compared to what
  is normally observed in quiet Sun regions outside coronal holes. The
  time series show a chaotic and dynamic scene that includes spatially
  confined “swirl” events. These events feature dark and bright
  rotating patches, which can consist of arcs, spiral arms, rings or
  ring fragments. The width of the fragments typically appears to be of
  the order of only 0.2 arcsec, which is close to the effective spatial
  resolution. They exhibit Doppler shifts of -2 to -4 km s<SUP>-1</SUP>
  but sometimes up to -7 km s<SUP>-1</SUP>, indicating fast upflows. The
  diameter of a swirl is usually of the order of 2´´. At the location
  of these swirls, the line wing and wide-band maps show close groups of
  photospheric bright points that move with respect to each other. <BR
  />Conclusions: A likely explanation is that the relative motion of the
  bright points twists the associated magnetic field in the chromosphere
  above. Plasma or propagating waves may then spiral upwards guided by
  the magnetic flux structure, thereby producing the observed intensity
  signature of Doppler-shifted ring fragments. <P />The movie is only
  available in electronic form at http://www.aanda.org <P />Marie Curie
  Intra-European Fellow of the European Commission.

Title: Swaying Threads of a Solar Filament
Authors: Lin, Y.; Soler, R.; Engvold, O.; Ballester, J. L.; Langangen,
   Ø.; Oliver, R.; Rouppe van der Voort, L. H. M.
Bibcode: 2009ApJ...704..870L
Altcode: 2009arXiv0909.2792L
  From recent high-resolution observations obtained with the Swedish 1
  m Solar Telescope in La Palma, we detect swaying motions of individual
  filament threads in the plane of the sky. The oscillatory characters of
  these motions are comparable with oscillatory Doppler signals obtained
  from corresponding filament threads. Simultaneous recordings of motions
  in the line of sight and in the plane of the sky give information
  about the orientation of the oscillatory plane. These oscillations
  are interpreted in the context of the magnetohydrodynamic (MHD)
  theory. Kink MHD waves supported by the thread body are proposed as
  an explanation of the observed thread oscillations. On the basis of
  this interpretation and by means of seismological arguments, we give
  an estimation of the thread Alfvén speed and magnetic field strength
  by means of seismological arguments.

Title: On the continuum intensity distribution of the solar
    photosphere
Authors: Wedemeyer-Böhm, S.; Rouppe van der Voort, L.
Bibcode: 2009A&A...503..225W
Altcode: 2009arXiv0905.0705W
  Context: For many years, there seemed to be significant differences
  between the continuum intensity distributions derived from observations
  and simulations of the solar photosphere. <BR />Aims: In order to
  settle the discussion on these apparent discrepancies, we present a
  detailed comparison between simulations and seeing-free observations
  that takes into account the crucial influence of instrumental image
  degradation. <BR />Methods: We use a set of images of quiet Sun
  granulation taken in the blue, green and red continuum bands of the
  Broadband Filter Imager of the Solar Optical Telescope (SOT) onboard
  Hinode. The images are deconvolved with point spread functions (PSF)
  that account for non-ideal contributions due to instrumental stray-light
  and imperfections. In addition, synthetic intensity images are degraded
  with the corresponding PSFs. The results are compared with respect to
  spatial power spectra, intensity histograms, and the centre-to-limb
  variation of the intensity contrast. <BR />Results: The intensity
  distribution of SOT granulation images is broadest for the blue
  continuum at disc-centre and narrows towards the limb and for longer
  wavelengths. The distributions are relatively symmetric close to the
  limb but exhibit a growing asymmetry towards disc-centre. The intensity
  contrast, which is connected to the width of the distribution, is found
  to be (12.8 ± 0.5)%, (8.3 ± 0.4)%, and (6.2 ± 0.2)% at disc-centre
  for blue, green, and red continuum, respectively. Removing the influence
  of the PSF unveils much broader intensity distributions with a secondary
  component that is otherwise only visible as an asymmetry between
  the darker and brighter than average part of the distribution. The
  contrast values increase to (26.7 ± 1.3)%, (19.4 ± 1.4)%, and
  (16.6 ± 0.7)% for blue, green, and red continuum, respectively. The
  power spectral density of the images exhibits a pronounced peak at
  spatial scales characteristic for the granulation pattern and a steep
  decrease towards smaller scales. The observational findings like the
  absolute values and centre-to-limb variation of the intensity contrast,
  intensity histograms, and power spectral density are well matched with
  corresponding synthetic observables from three-dimensional radiation
  (magneto-)hydrodynamic simulations. <BR />Conclusions: We conclude that
  the intensity contrast of the solar continuum intensity is higher than
  usually derived from ground-based observations and is well reproduced
  by modern radiation (magneto-)hydrodynamic models. Properly accounting
  for image degradation effects is of crucial importance for comparisons
  between observations and numerical models.

Title: Spectropolarimetric Diagnostics at the Solar Photosphere near
    the Limb
Authors: Yelles Chaouche, L.; Solanki, S. K.; Rouppe van der Voort,
   L.; van Noort, M.
Bibcode: 2009ASPC..405..189Y
Altcode: 2009arXiv0910.2885Y
  In the present work, we investigate the formation of Stokes profiles
  and spectro-polarimetric diagnostics in an active region plage near
  the limb. We use 3-D radiation-MHD simulations with unipolar fields
  of an average strength of 400 G, which is largely concentrated in
  flux tubes in which the field reaches typical kilo-Gauss values. We
  generate synthetic Stokes spectra by radiative transfer calculations,
  then we degrade the simulated Stokes signal to account for observational
  conditions. The synthetic data treated in this manner are compared with
  and found to roughly reproduce spectro-polarimetric high-resolution
  observations at μ=0.39 obtained by the SOUP instrument with the
  Swedish 1-m Solar Telescope at the beginning of 2006.

Title: Three-Dimensional Non-LTE Radiative Transfer Computation of
    the CA 8542 Infrared Line From a Radiation-MHD Simulation
Authors: Leenaarts, J.; Carlsson, M.; Hansteen, V.; Rouppe van der
   Voort, L.
Bibcode: 2009ApJ...694L.128L
Altcode: 2009arXiv0903.0791L
  The interpretation of imagery of the solar chromosphere in the
  widely used Ca II 854.2 nm infrared line is hampered by its complex,
  three-dimensional, and non-LTE formation. Forward modeling is
  required to aid understanding. We use a three-dimensional non-LTE
  radiative transfer code to compute synthetic Ca II 854.2 nm images
  from a radiation-MHD simulation of the solar atmosphere spanning
  from the convection zone to the corona. We compare the simulation
  with observations obtained with the CRISP filter at the Swedish 1 m
  Solar Telescope. We find that the simulation reproduces dark patches
  in the blue line wing caused by Doppler shifts, brightenings in the
  line core caused by upward-propagating shocks, and thin dark elongated
  structures in the line core that form the interface between upward and
  downward gas motion in the chromosphere. The synthetic line core is
  narrower than the observed one, indicating that the Sun exhibits both
  more vigorous large-scale dynamics as well as small scale motions that
  are not resolved within the simulation, presumably owing to a lack of
  spatial resolution.

Title: The solar continuum intensity distribution. Settling the
    conflict between observations and simulations
Authors: Wedemeyer-Böhm, S.; Rouppe van der Voort, L.
Bibcode: 2009MmSAI..80..635W
Altcode: 2009arXiv0908.3758W
  For many years, there seemed to be significant differences between
  the continuum intensity distributions derived from observations and
  simulations of the solar photosphere. In order to settle the discussion
  on these apparent discrepancies, we present a detailed comparison
  between simulations and seeing-free observations that takes into
  account the crucial influence of instrumental image degradation. We
  use a set of images of quiet Sun granulation taken in the blue, green
  and red continuum bands of the Broadband Filter Imager of the Solar
  Optical Telescope (SOT) onboard Hinode. The images are deconvolved with
  Point Spread Functions (PSF) that account for non-ideal contributions
  due to instrumental stray-light and imperfections. In addition,
  synthetic intensity images are degraded with the corresponding
  PSFs. The results are compared with respect to spatial power spectra,
  intensity histograms, and the centre-to-limb variation of the
  intensity contrast. The observational findings are well matched with
  corresponding synthetic observables from three-dimensional radiation
  (magneto-)hydrodynamic simulations. We conclude that the intensity
  contrast of the solar continuum intensity is higher than usually
  derived from ground-based observations and is well reproduced by
  modern numerical simulations. Properly accounting for image degradation
  effects is of crucial importance for comparisons between observations
  and numerical models. It finally settles the traditionally perceived
  conflict between observations and simulations.

Title: Stokes imaging polarimetry using image restoration at the
    Swedish 1-m solar telescope
Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M.
Bibcode: 2008A&A...489..429V
Altcode: 2008arXiv0805.4296V
  Aims: We aim to achieve both high spatial resolution and high
  polarimetric sensitivity, using an earth-based 1m-class solar
  telescope, for the study of magnetic fine structure on the surface of
  the Sun. <BR />Methods: We use a setup with 3 high-speed, low-noise
  cameras to construct datasets with interleaved polarimetric states,
  particularly suitable for Multi-Object Multi-Frame Blind Deconvolution
  image restorations. We discuss the polarimetric calibration routine
  and various potential sources of error in the results. <BR />Results:
  We obtained near diffraction limited images, which have a noise level
  of ≈ 10<SUP>-3</SUP> I_cont. We confirm that dark cores have a weaker
  magnetic field and a lower inclination angle with respect to the solar
  surface than the edges of the penumbral filament. We demonstrate that
  the magnetic field strength in faculae-striations is significantly
  lower than in other nearby parts of the faculae.

Title: On small active region filaments, fibrils and surges
Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort,
   L. H. M.; van Noort, M.
Bibcode: 2008AdSpR..42..803L
Altcode:
  High resolution Hα images and magnetograms (0.2 arc s) of an active
  region were obtained in alternating time series at 42 s cadences
  using the Swedish 1-m Solar Telescope on 2004 August 21. The Hα
  filtergrams reveal an active region filament and surges consisting
  of thread-like structures which have widths similar to the widths of
  chromospheric fibrils, both recorded down to the resolution limit in
  the best images. All observed structures in the active region appear
  highly dynamic. Fibrils show counterstreaming strongly resembling the
  counterstreaming threads in filaments. Streaming, along the threads
  of surges extending more than 10 arc s, is higher in speed (∼20
  km s<SUP>-1</SUP>) than in the filament and fibrils and appears to
  flow independently over and above the chromospheric fibrils. Blue
  shifts seen in the Hα Dopplergrams confirm the outward mass motion
  of the surges. However, in at least one case, we also see simultaneous
  downflows from the same site but in the opposite direction and downward
  toward the chromosphere. We suggest that the site between these
  two outward and downward flows identifies the place where magnetic
  reconnection could occur and thereby cause of the surge. This appears
  to imply that the reconnection site is in the high chromosphere or
  low corona.

Title: Moat Flow in the Vicinity of Sunspots for Various Penumbral
    Configurations
Authors: Vargas Domínguez, S.; Rouppe van der Voort, L.; Bonet,
   J. A.; Martínez Pillet, V.; Van Noort, M.; Katsukawa, Y.
Bibcode: 2008ApJ...679..900V
Altcode: 2008arXiv0802.1457V
  High-resolution time series of sunspots have been obtained with
  the Swedish 1 m Solar Telescope between 2003 and 2006 at different
  locations on the solar disk. Proper motions in seven different active
  regions have been studied. The analysis was performed by applying local
  correlation tracking to every series of sunspots, each of them more than
  40 minutes long. The sunspots' shapes include a different variety of
  penumbral configurations. We report on the systematic behavior of the
  large-scale outflows surrounding the sunspots, commonly known as moat
  flows, that are essentially present only when preceded by a penumbra
  not tangential but perpendicular to the sunspot border. We present
  one case for which this rule appears not to be confirmed. We speculate
  that the magnetic neutral line, which is located in the vicinity of the
  anomalous region, might be responsible for blocking the outflow. These
  new results confirm the systematic and strong relation between the
  moat flows and the existence of penumbrae. A comparative statistical
  study between moats and standard granulation is also performed.

Title: Measurements of Plasma Motions in Dynamic Fibrils
Authors: Langangen, Øystein; Rouppe van der Voort, Luc; Lin, Yong
Bibcode: 2008ApJ...673.1201L
Altcode: 2007arXiv0710.1011L
  We present a 40 minute time series of filtergrams from the red and
  blue wings of the Hα line in an active region near the solar disk
  center. From these filtergrams we construct both Dopplergrams and summed
  "line center" images. Several dynamic fibrils (DFs) are identified
  in the summed images. The data are used to simultaneously measure the
  proper motion and the Doppler signals in DFs. For calibration of the
  Doppler signals, we use spatially resolved spectrograms of a similar
  active region. Significant variations in the calibration constant for
  different solar features are observed, and only regions containing
  DFs have been used in order to reduce calibration errors. We find
  a coherent behavior of the Doppler velocity and the proper motion
  that clearly demonstrates that the evolution of DFs involves plasma
  motion. The Doppler velocities are found to be a factor of 2-3 smaller
  than velocities derived from proper motions in the image plane. The
  difference can be explained by the radiative processes involved, as
  the Doppler velocity is a result of the local atmospheric velocity
  weighted with the response function. As a result, the Doppler velocity
  originates from a wide range of heights in the atmosphere. This is
  contrasted with the proper-motion velocity, which is measured from the
  sharply defined bright tops of the DFs and is therefore a very local
  velocity measure. The Doppler signal originates from well below the
  top of the DF. Finally, we discuss how this difference, together with
  the lower spatial resolution of older observations, has contributed to
  some of the confusion about the identity of DFs, spicules, and mottles.

Title: Spectroscopic Measurements of Dynamic Fibrils in the Ca II
    λ8662 Line
Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort,
   Luc; Hansteen, Viggo; De Pontieu, Bart
Bibcode: 2008ApJ...673.1194L
Altcode: 2007arXiv0710.0247L
  We present high spatial resolution spectroscopic measurements of dynamic
  fibrils (DFs) in the Ca II λ8662 line. These data show clear Doppler
  shifts in the identified DFs, which demonstrates that at least a subset
  of DFs are actual mass motions in the chromosphere. A statistical
  analysis of 26 DFs reveals a strong and statistically significant
  correlation between the maximal velocity and the deceleration. The
  range of the velocities and the decelerations are substantially lower,
  about a factor 2, in our spectroscopic observations compared to the
  earlier results based on proper motion in narrowband images. There
  are fundamental differences in the different observational methods;
  when DFs are observed spectroscopically, the measured Doppler shifts
  are a result of the atmospheric velocity, weighted with the response
  function to velocity over an extended height. When the proper motion
  of DFs is observed in narrowband images, the movement of the top
  of the DF is observed. This point is sharply defined because of the
  high contrast between the DF and the surroundings. The observational
  differences between the two methods are examined by several numerical
  experiments using both numerical simulations and a time series of
  narrowband Hα images. With basis in the simulations we conclude that
  the lower maximal velocity is explained by the low formation height of
  the Ca IR line. We conclude that the present observations support the
  earlier result that DFs are driven by magnetoacoustic shocks excited
  by convective flows and p-modes.

Title: Evidence of Traveling Waves in Filament Threads
Authors: Lin, Y.; Engvold, O.; Rouppe van der Voort, L. H. M.; van
   Noort, M.
Bibcode: 2007SoPh..246...65L
Altcode:
  High-resolution Hα filtergrams (0.2″) obtained with the Swedish 1-m
  Solar Telescope resolve numerous very thin, thread-like structures in
  solar filaments. The threads are believed to represent thin magnetic
  flux tubes that must be longer than the observable threads. We report
  on evidence for small-amplitude (1 - 2 km s<SUP>−1</SUP>) waves
  propagating along a number of threads with an average phase velocity of
  12 km s<SUP>−1</SUP> and a wavelength of 4″. The oscillatory period
  of individual threads vary from 3 to 9 minutes. Temporal variation
  of the Doppler velocities averaged over a small area containing a
  number of individual threads shows a short-period (3.6 minutes) wave
  pattern. These short-period oscillations could possibly represent fast
  modes in accordance with numerical fibril models proposed by Díaz et
  al. (Astron. Astrophys.379, 1083, 2001). In some cases, it is clear
  that the propagating waves are moving in the same direction as the
  mass flows.

Title: What are 'Faculae'?
Authors: Berger, T. E.; Title, A. M.; Tarbell, T.; Rouppe van der
   Voort, L.; Löfdahl, M. G.; Scharmer, G. B.
Bibcode: 2007ASPC..369..103B
Altcode:
  We present very high resolution filtergram and magnetogram observations
  of solar faculae taken at the Swedish 1-meter Solar Telescope (SST)
  on La Palma. Three datasets with average line-of-sight angles of 16,
  34, and 53 degrees are analyzed. The average radial extent of faculae is
  at least 400~km. In addition we find that contrast versus magnetic flux
  density is nearly constant for faculae at a given disk position. These
  facts and the high resolution images and movies reveal that faculae are
  not the interiors of small flux tubes - they are granules seen through
  the transparency caused by groups of magnetic elements or micropores
  ``in front of'' the granules. Previous results which show a strong
  dependency of facular contrast on magnetic flux density were caused
  by bin-averaging of lower resolution data leading to a mixture of
  the signal from bright facular walls and the associated intergranular
  lanes and micropores. The findings are relevant to studies of total
  solar irradiance (TSI) that use facular contrast as a function of disk
  position and magnetic field in order to model the increase in TSI with
  increasing sunspot activity.

Title: Relationships between magnetic foot points and G-band bright
    structures
Authors: Ishikawa, R.; Tsuneta, S.; Kitakoshi, Y.; Katsukawa, Y.;
   Bonet, J. A.; Vargas Domínguez, S.; Rouppe van der Voort, L. H. M.;
   Sakamoto, Y.; Ebisuzaki, T.
Bibcode: 2007A&A...472..911I
Altcode: 2008arXiv0802.1765I
  Aims:Magnetic elements are thought to be described by flux tube models,
  and are well reproduced by MHD simulations. However, these simulations
  are only partially constrained by observations. We observationally
  investigate the relationship between G-band bright points and magnetic
  structures to clarify conditions, which make magnetic structures
  bright in G-band. <BR />Methods: The G-band filtergrams together with
  magnetograms and dopplergrams were taken for a plage region covered
  by abnormal granules as well as ubiquitous G-band bright points,
  using the Swedish 1-m Solar Telescope (SST) under very good seeing
  conditions. <BR />Results: High magnetic flux density regions are
  not necessarily associated with G-band bright points. We refer to the
  observed extended areas with high magnetic flux density as magnetic
  islands to separate them from magnetic elements. We discover that G-band
  bright points tend to be located near the boundary of such magnetic
  islands. The concentration of G-band bright points decreases with inward
  distance from the boundary of the magnetic islands. Moreover, G-band
  bright points are preferentially located where magnetic flux density is
  higher, given the same distance from the boundary. There are some bright
  points located far inside the magnetic islands. Such bright points have
  higher minimum magnetic flux density at the larger inward distance from
  the boundary. Convective velocity is apparently reduced for such high
  magnetic flux density regions regardless of whether they are populated
  by G-band bright points or not. The magnetic islands are surrounded by
  downflows. <BR />Conclusions: These results suggest that high magnetic
  flux density, as well as efficient heat transport from the sides or
  beneath, are required to make magnetic elements bright in G-band.

Title: Contrast Analysis of Solar Faculae and Magnetic Bright Points
Authors: Berger, T. E.; Rouppe van der Voort, L.; Löfdahl, M.
Bibcode: 2007ApJ...661.1272B
Altcode:
  The morphology and contrast of small-scale solar magnetic elements at
  four disk positions is analyzed. The data were obtained at the Swedish
  1 m Solar Telescope (SST) over 3 yr (2003-2005). Two of the data sets
  have disk positions near disk center (average μ=cosθ&gt;0.8) and show
  numerous ``magnetic bright points'' (MBPs), and two are sufficiently
  limbward to show prominent ``faculae'' (average μ&lt;=0.6). The
  filtergrams are obtained in the 430.5 nm G band and 436.4 nm
  ``continuum'' bandpasses; the magnetograms are Fe I 630.25 nm Stokes
  V images taken with the Solar Optical Universal Polarimeter (SOUP)
  tunable filter. In all images we achieve nearly diffraction-limited
  resolution (~100 km in the G band). Analysis shows that MBPs and faculae
  are distinct radiative signatures of the magnetic field: MBPs have a
  constant or slightly decreasing contrast with increasing magnetogram
  signal, while facular contrast increases linearly with magnetogram
  signal. Faculae are much larger than MBPs, with an average radial
  width of 400 km. The observations support recent modeling showing that
  faculae are granules seen through the opacity reduction provided by
  magnetic elements (or groups thereof), while MBPs are caused by lateral
  radiation leakage scattering from deeper layers of the magnetic element.

Title: High-Resolution Observations and Numerical Simulations of
    Chromospheric Fibrils and Mottles
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
   van Noort, M.; Carlsson, M.
Bibcode: 2007ASPC..368...65D
Altcode:
  With the recent advent of the Swedish 1-m Solar Telescope (SST),
  advanced image processing techniques, as well as numerical simulations
  that provide a more realistic view of the chromosphere, a comprehensive
  understanding of chromospheric jets such as spicules, mottles and
  fibrils is now within reach. In this paper, we briefly summarize results
  from a recent analysis of dynamic fibrils, short-lived jet-like features
  that dominate the chromosphere (as imaged in Hα) above and about active
  region plage. Using extremely high-resolution observations obtained
  at the SST, and advanced numerical 2D radiative MHD simulations, we
  show that fibrils are most likely formed by chromospheric shock waves
  that occur when convective flows and global oscillations leak into the
  chromosphere along the field lines of magnetic flux concentrations. <P
  />In addition, we present some preliminary observations of quiet Sun
  jets or mottles. We find that the mechanism that produces fibrils
  in active regions is most likely also at work in quiet Sun regions,
  although it is modified by the weaker magnetic field and the presence of
  more mixed-polarity. A comparison with numerical simulations suggests
  that the weaker magnetic field in quiet Sun allows for significantly
  stronger (than in active regions) transverse motions that are
  superposed on the field-aligned, shock-driven motions. This leads
  to a more dynamic, and much more complex environment than in active
  region plage. In addition, our observations of the mixed polarity
  environment in quiet Sun regions suggest that other mechanisms, such
  as reconnection, may well play a significant role in the formation of
  some quiet Sun jets. Simultaneous high-resolution magnetograms (such
  as those provided by Hinode), as well as numerical simulations that
  take into account a whole variety of different magnetic configurations,
  will be necessary to determine the relative importance in quiet Sun of,
  respectively, the fibril-mechanism and reconnection.

Title: Chromospheric Spectrometry at High Spatial Resolution
Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L.;
   Hansteen, V. H.; de Pontieu, B.
Bibcode: 2007ASPC..368..145L
Altcode:
  In this summary we present spectrograms and images of the
  chromosphere obtained in a co-observation campaign with the SST and
  the DOT. The data are used to identify and measure the Doppler shifts
  of dynamic fibrils. Quantitative comparison with the results of <P
  />\citep{ol-2006ApJ...647L..73H} requires compensation for several
  observational issues.

Title: On the Moat-Penumbra Relation
Authors: Vargas Domínguez, S.; Bonet, J. A.; Martínez Pillet, V.;
   Katsukawa, Y.; Kitakoshi, Y.; Rouppe van der Voort, L.
Bibcode: 2007ApJ...660L.165V
Altcode: 2007astro.ph..2713V
  Proper motions in a sunspot group with a δ-configuration and close to
  the solar disk center have been studied by employing local correlation
  tracking techniques. The analysis is based on a more than 1 hr time
  series of G-band images. Radial outflows with a mean speed of 0.67
  km s<SUP>-1</SUP> have been detected around the spots, the well-known
  sunspots moats. However, these outflows are not found in those umbral
  core sides without penumbra. Moreover, moat flows are only found
  in those sides of penumbrae located in the direction marked by the
  penumbral filaments. Penumbral sides perpendicular to them show no
  moat flow. These results strongly suggest a relation between the
  moat flow and the well-known, filament-aligned Evershed flow. The
  standard picture of a moat flow originating from a blocking of the
  upward propagation of heat is discussed in some detail.

Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen,
   V. H.; Carlsson, M.; van Noort, M.
Bibcode: 2007ApJ...660L.169R
Altcode: 2007astro.ph..3535R
  We present high spatial and high temporal resolution observations of
  the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on
  La Palma. We observe that many mottles, jetlike features in the quiet
  Sun, display clear up- and downward motions along their main axis. In
  addition, many mottles show vigorous transverse displacements. Unique
  identification of the mottles throughout their lifetime is much harder
  than for their active region counterpart, dynamic fibrils. This is
  because many seem to lack a sharply defined edge at their top, and
  significant fading often occurs throughout their lifetime. For those
  mottles that can be reliably tracked, we find that the mottle tops
  often undergo parabolic paths. We find a linear correlation between
  the deceleration these mottles undergo and the maximum velocity they
  reach, similar to what was found earlier for dynamic fibrils. Combined
  with an analysis of oscillatory properties, we conclude that at least
  part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In
  addition, the mixed-polarity environment and vigorous dynamics suggest
  that reconnection may play a significant role in the formation of some
  quiet-Sun jets.

Title: Observations and Simulations of Fibrils and Mottles
Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort,
   Luc; van Noort, Michiel; Carlsson, Mats
Bibcode: 2007astro.ph..2081D
Altcode:
  With the recent advent of the Swedish 1-m Solar Telescope (SST),
  advanced image processing techniques, as well as numerical simulations
  that provide a more realistic view of the chromosphere, a comprehensive
  understanding of chromospheric jets such as spicules, mottles and
  fibrils is now within reach. In this paper, we briefly summarize
  results from a recent analysis of dynamic fibrils, short-lived
  jet-like features that dominate the chromosphere (as imaged in
  H-alpha) above and about active region plage. Using extremely
  high-resolution observations obtained at the SST, and advanced
  numerical 2D radiative MHD simulations, we show that fibrils are most
  likely formed by chromospheric shock waves that occur when convective
  flows and global oscillations leak into the chromosphere along the
  field lines of magnetic flux concentrations. In addition, we present
  some preliminary observations of quiet Sun jets or mottles. We find
  that the mechanism that produces fibrils in active regions is most
  likely also at work in quiet Sun regions, although it is modified by
  the weaker magnetic field and the presence of more mixed-polarity. A
  comparison with numerical simulations suggests that the weaker magnetic
  field in quiet Sun allows for significantly stronger (than in active
  regions) transverse motions that are superposed on the field-aligned,
  shock-driven motions. This leads to a more dynamic, and much more
  complex environment than in active region plage. In addition, our
  observations of the mixed polarity environment in quiet Sun regions
  suggest that other mechanisms, such as reconnection, may well play a
  significant role in the formation of some quiet Sun jets.

Title: Velocities Measured in Small-Scale Solar Magnetic Elements
Authors: Langangen, Øystein; Carlsson, Mats; Rouppe van der Voort,
   Luc; Stein, R. F.
Bibcode: 2007ApJ...655..615L
Altcode: 2006astro.ph.11741L
  We have obtained high-resolution spectrograms of small-scale magnetic
  structures with the Swedish 1-m Solar Telescope. We present Doppler
  measurements at 0.2" spatial resolution of bright points, ribbons,
  and flowers, and their immediate surroundings, in the C I λ5380.3 line
  (formed in the deep photosphere) and the two Fe I lines at 5379.6 and
  5386.3 Å. The velocity inside the flowers and ribbons are measured to
  be almost zero, while we observe downflows at the edges. These downflows
  are increasing with decreasing height. We also analyze realistic
  magnetoconvective simulations to obtain a better understanding of the
  interpretation of the observed signal. We calculate how the Doppler
  signal depends on the velocity field in various structures. Both the
  smearing effect of the nonnegligible width of this velocity response
  function along the line of sight and of the smearing from the telescope
  and atmospheric point-spread function are discussed. These studies lead
  us to the conclusion that the velocity inside the magnetic elements
  is really upflow of order 1-2 km s<SUP>-1</SUP>, while the downflows
  at the edges really are much stronger than observed, of order 1.5-3.3
  km s<SUP>-1</SUP>.

Title: High-Resolution Observations and Modeling of Dynamic Fibrils
Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
   van Noort, M.; Carlsson, M.
Bibcode: 2007ApJ...655..624D
Altcode: 2007astro.ph..1786D
  We present unprecedented high-resolution Hα observations, obtained
  with the Swedish 1 m Solar Telescope, that, for the first time,
  spatially and temporally resolve dynamic fibrils in active regions on
  the Sun. These jetlike features are similar to mottles or spicules in
  quiet Sun. We find that most of these fibrils follow almost perfect
  parabolic paths in their ascent and descent. We measure the properties
  of the parabolic paths taken by 257 fibrils and present an overview
  of the deceleration, maximum velocity, maximum length, and duration,
  as well as their widths and the thickness of a bright ring that often
  occurs above dynamic fibrils. We find that the observed deceleration
  of the projected path is typically only a fraction of solar gravity
  and incompatible with a ballistic path at solar gravity. We report on
  significant differences of fibril properties between those occurring
  above a dense plage region and those above a less dense plage region
  where the magnetic field seems more inclined from the vertical. We
  compare these findings to advanced numerical two-dimensional radiative
  MHD simulations and find that fibrils are most likely formed by
  chromospheric shock waves that occur when convective flows and global
  oscillations leak into the chromosphere along the field lines of
  magnetic flux concentrations. Detailed comparison of observed and
  simulated fibril properties shows striking similarities of the values
  for deceleration, maximum velocity, maximum length, and duration. We
  compare our results with observations of mottles and find that a
  similar mechanism is most likely at work in the quiet Sun.

Title: High Spatial Resolution Observations of Solar Magnetic
    Structures
Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.;
   Hansteen, V.
Bibcode: 2006ASPC..354...37R
Altcode:
  We present observations of the dynamic evolution of photospheric
  magnetic structures in the G-band, continuum, magnetograms and
  Dopplergrams. The observations were obtained with the Swedish one-m
  Solar Telescope on La Palma. Using adaptive optics and the Multi-Object
  Multi-Frame Blind Deconvolution image restoration technique, we obtained
  several datasets at close to the diffraction limit of the telescope
  (0.1 arcsec) over long periods of time. We show examples of the
  dynamical evolution of different magnetic structures: the advection
  of individual bright points by the granular flow, the formation and
  fragmentation of flux sheets, and the continuous transition between
  micro-pores, elongated ribbons and more circular ``flowers''. Narrow
  sheets with downdrafts are found right at the edges of magnetic field
  concentrations.

Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
   van Noort, M.; Carlsson, M.
Bibcode: 2006AGUFMSH23B0359D
Altcode:
  With the recent advent of the Swedish 1-m Solar Telescope (SST),
  advanced image processing techniques, as well as numerical simulations
  that provide a more realistic view of the chromosphere, a comprehensive
  understanding of chromospheric jets such as spicules, mottles and
  fibrils is now within reach. We will present results from a recent
  analysis of dynamic fibrils, short-lived jet-like features that
  dominate the chromosphere (as imaged in Hα) above and about active
  region plage. These jets are similar to mottles and spicules in quiet
  Sun. Our analysis is based on a time series of extremely high-resolution
  (120 km) images taken in Hα linecenter at 1 second cadence, obtained by
  the Oslo group at the SST in October 2005. The 78 min long time series
  for the first time, spatially and temporally resolves dynamic fibrils
  in active regions. Our analysis shows that most of the fibrils follow
  almost perfect parabolic paths in their ascent and descent. We measure
  the properties of the parabolic paths taken by 257 different dynamic
  fibrils, and find that the observed deceleration of the projected
  path is typically only a fraction of solar gravity, and incompatible
  with a ballistic path at solar gravity. We report on significant
  differences of measured fibril properties between those occurring in
  association with a dense plage region, and those above a less dense
  plage region where the magnetic field seems more inclined away from
  the vertical. We compare these observational findings to advanced
  numerical 2D radiative MHD simulations, and find that fibrils are most
  likely formed by chromospheric shock waves that occur when convective
  flows and global oscillations leak into the chromosphere along the
  field lines of magnetic flux concentrations. Detailed comparison
  of the properties of fibrils found in our observations and those in
  our numerical simulations shows striking similarities of the values
  for deceleration, maximum velocity, maximum length and duration. The
  numerical simulations also reproduce the correlations we observe between
  various fibrils properties, as well as the regional differences, taking
  into account the different magnetic configuration for the various
  regions. We compare our results with observations of mottles and find
  that a similar mechanism is most likely at work in the quiet Sun.

Title: Solar Image Restoration by use of Multi-Object Multi-Frame
    Blind Deconvolution
Authors: van Noort, M.; Rouppe van der Voort, L.; Löfdahl, M.
Bibcode: 2006ASPC..354...55V
Altcode:
  We present examples of the application of the image restoration
  method of Multi-Object Multi-Frame Blind Deconvolution to observations
  obtained with the Swedish one-meter Solar Telescope on La Palma. This
  restoration method is an extension of Joint Phase Diverse Speckle
  image restoration. Multiple realizations of multiple objects
  can now be restored jointly, facilitating near-perfect alignment
  between different objects. This greatly reduces false signals in the
  determination of derived quantities, such as magnetograms, Dopplergrams
  and G-band-continuum difference images.

Title: High-Resolution Observations of Fast Events in the Solar
    Chromosphere
Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M.
Bibcode: 2006ApJ...648L..67V
Altcode:
  We present new, high spatial and high temporal resolution observations
  of the Sun in Hα obtained with the Swedish 1 m Solar Telescope on
  La Palma. The combined use of adaptive optics and image restoration
  techniques yielded a near-diffraction-limited time series with a cadence
  of 3 frames s<SUP>-1</SUP> of two different active regions. The unique
  combination of high temporal and spatial resolution reveals to us the
  existence of highly dynamic structures, moving at velocities of up to
  240 km s<SUP>-1</SUP>, and high-velocity waves in the chromosphere. The
  rapid motions appear to be common, as they are observed in two data sets
  recorded in succession at different locations on the solar disk. The
  dynamic events are probably associated with reconfigurations of the
  magnetic field.

Title: Rapid Temporal Variability of Faculae: High-Resolution
    Observations and Modeling
Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort,
   L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G.
Bibcode: 2006ApJ...646.1405D
Altcode:
  We present high-resolution G-band observations (obtained with the
  Swedish 1 m Solar Telescope) of the rapid temporal variability of
  faculae, which occurs on granular timescales. By combining these
  observations with magnetoconvection simulations of a plage region, we
  show that much of this variability is not intrinsic to the magnetic
  field concentrations that are associated with faculae, but rather
  a phenomenon associated with the normal evolution and splitting of
  granules. We also show examples of facular variability caused by
  changes in the magnetic field, with movies of dynamic behavior of
  the striations that dominate much of the facular appearance at 0.1"
  resolution. Examples of these dynamics include merging, splitting,
  rapid motion, apparent fluting, and possibly swaying.

Title: High resolution observations of solar magnetic fields
Authors: Rouppe van der Voort, L.
Bibcode: 2006IAUJD...3E..77R
Altcode:
  With the development of adaptive optics systems and image restoration
  techniques, high spatial resolution observations of small scale magnetic
  fields have become more frequent. I will discuss some of the recent
  studies that reveal the intricate structure and dynamics of solar
  magnetic fields with increasing clarity.

Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.;
   van Noort, M.; Carlsson, M.
Bibcode: 2006ApJ...647L..73H
Altcode: 2006astro.ph..7332H
  The formation of jets such as dynamic fibrils, mottles, and spicules
  in the solar chromosphere is one of the most important, but also
  most poorly understood, phenomena of the Sun's magnetized outer
  atmosphere. We use extremely high resolution observations from the
  Swedish 1 m Solar Telescope combined with advanced numerical modeling
  to show that in active regions these jets are a natural consequence of
  upwardly propagating slow-mode magnetoacoustic shocks. These shocks
  form when waves generated by convective flows and global p-mode
  oscillations in the lower lying photosphere leak upward into the
  magnetized chromosphere. We find excellent agreement between observed
  and simulated jet velocities, decelerations, lifetimes, and lengths. Our
  findings suggest that previous observations of quiet-Sun spicules and
  mottles may also be interpreted in light of a shock-driven mechanism.

Title: High Resolution Spectropolarimetry of Penumbral Formation
    with IBIS
Authors: Reardon, Kevin; Casini, R.; Cavallini, F.; Tomczyk, S.;
   Rouppe van der Voort, L.; Van Noort, M.; Woeger, F.; Socas Navarro,
   H.; IBIS Team
Bibcode: 2006SPD....37.3503R
Altcode: 2006BAAS...38..260R
  We present the results of first spectropolarimetric observations
  made with the Interferometric Bidimensional Spectrometer (IBIS)
  at the NSO/Dunn Solar Telescope. The use of narrowband imaging and
  post-facto reconstruction techniques allows for observations close
  to the diffraction limit of the vector magnetic field. We will show
  observations of the the formation of an individual penumbral filament
  around a small pore. We measure the magnetic field and velocity field
  of the forming penumbral filament. The spectropolarimetric mode of
  IBIS will be available to the community in the fall of 2006.

Title: Dynamics of an active region filament, fibrils and surges in
    high resolution
Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort,
   L. H. M.; van Noort, M.
Bibcode: 2006cosp...36.3193L
Altcode: 2006cosp.meet.3193L
  High resolution H alpha and magnetograms 0 2 arc sec of an active region
  were obtained in alternating time series at 42 sec cadences using the
  Swedish 1-m Solar Telescope on 2004 August 21 The 79 times 52 arcsec
  field of view was centered at N11 W5 The H alpha filtergrams reveal an
  active region filament and surges consisting of thread-like structures
  which have widths similar to the widths of chromospheric fibrils both
  recorded down to the resolution limit in the best images All observed
  structures in the active region are highly dynamic The flow speeds in
  some active filament threads 25 km s -1 are higher than typical speeds
  in quiescent filament threads Fibrils show counterstreaming strongly
  resembling the counterstreaming threads in filaments The transverse
  speeds of mass within fibrils are comparable to the typical speeds
  of mass flows in quiescent filament threads sim 10 km s -1 Streaming
  along the threads of surges extending more than 10 arc sec is higher
  in speed sim 20 km s -1 than in the filament and fibrils and appears
  to flow over the chromospheric fibrils Small surges near one end of
  the filament appear to flow into the filament thereby mass seems to
  be added to the filament Blue shifts seen in the H alpha Dopplergrams
  confirm the outward mass motion of the surges However in at least one
  case we also see simultaneous red shifts from the same site in the
  opposite direction toward the chromosphere We suggest that the site
  between these two opposite motions identifies the place where magnetic

Title: The Diagnostic Potential of the MG i 4571.1 Å Line
Authors: Langangen, Ø.; Carlsson, M.; Rouppe van der Voort, L.
Bibcode: 2005ESASP.600E..65L
Altcode: 2005ESPM...11...65L; 2005dysu.confE..65L
  No abstract at ADS

Title: The wings of Ca II H and K as solar fluxtube diagnostics
Authors: Sheminova, V. A.; Rutten, R. J.; Rouppe van der Voort,
   L. H. M.
Bibcode: 2005A&A...437.1069S
Altcode:
  We combine high-resolution Ca II H and K spectrograms from the
  Swedish Vacuum Solar Telescope with standard fluxtube modeling to
  derive photospheric temperature and velocity stratifications within
  individual magnetic elements in plage near a sunspot. We find that 1D
  on-axis modeling gives better consistency than spatial averaging over
  flaring-fluxtube geometry. Our best-fit temperature stratifications
  suggest that magnetic elements are close to radiative equilibrium
  throughout their photospheres. Their brightness excess throughout the
  H and K wings compared with the quiet photosphere is primarily due to
  low density, not to mechanical heating. We conclude that the extended
  H and K wings provide excellent fine-structure diagnostics for both
  high-resolution observations and simulations of the solar photosphere.

Title: The temperature gradient in and around solar magnetic fluxtubes
Authors: Sheminova, V. A.; Rutten, R. J.; Rouppe van der Voort,
   L. H. M.
Bibcode: 2005KFNTS...5..110S
Altcode:
  We use spectra covering the outer part of the extended wing of the solar
  Ca II K line observed at high angular resolution with the Swedish Vacuum
  Solar Telescope to test standard solar fluxtube models. The wings of the
  Ca II resonance lines are formed in LTE both with regard to excitation
  (source function) and to ionization (opacity) and, therefore, sample
  temperature stratifications in relatively straightforward fashion. We
  obtain best fits by combining steeper temperature gradients than
  those in the standard models for both the tube inside and the tube
  environment. Similarly steep gradients are also determined from a
  numerical magnetoconvection simulation by the late A. S. Gadun. It
  is found that the energy balance in the individual magnetic elements
  appears to be close to radiative equilibrium throughout the photosphere.

Title: High resolution magnetogram measurements of solar faculae
Authors: Berger, T. E.; Rouppe van der Voort, L.; Lofdahl, M. G.
Bibcode: 2005AGUSMSP31A..02B
Altcode:
  We present new images of magnetic elements near the solar limb
  ("faculae") along with magnetogram measurements and contrast
  profiles. Imaging magnetogram observations were made of AR 10377 at μ
  = cosθ = 0.6 on 06-June-2003 using the Swedish 1-m Solar Telescope;
  the data are the highest spatial resolution magnetogram measurements of
  faculae to date. Contrary to previous lower resolution measurements,
  we find no correlation between facular contrast and magnetic flux
  density at a given disk position. Increasing magnetic flux density
  in a region implies an increasing prevalence of micropores. Previous
  observations which lacked sufficient spatial resolution to discern
  dark micropore "floors" from bright facular walls find a strong
  non-linear dependence of facular contrast on magnetic flux density,
  with decreasing contrast beyond a certain flux density. We show instead
  that the observed contrast of bright facular walls is independent of
  magnetic flux density when properly segmented from dark micropores. The
  observations are useful for examining the detailed structure of faculae
  including the dark lanes found on the disk-center side of many faculae
  (explained by several recent 3D MHD numerical simulations). The average
  radial profile for 678 faculae segmented from the dataset is very
  nearly gaussian with a FWHM radial extent of 265 km and an extended
  tail on the limbward side, as predicted by current MHD simulations.

Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical
    evolution
Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson,
   M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E.
Bibcode: 2005A&A...435..327R
Altcode:
  Small magnetic structures can be seen in G-band filtergrams as isolated
  bright points, strings of bright points and dark micro-pores. At a
  resolution of 0.1 arcsec, new forms of magnetic structures are found
  in strong field areas: elongated “ribbons” and more circular
  “flowers”. In this paper we study the temporal evolution of
  these small scale magnetic structures. In strong-field regions the
  time-evolution is more that of a magnetic fluid than that of collections
  of flux-tubes that keep their identity. We find that the granular flow
  concentrates the magnetic field into flux sheets that are visible as
  thin bright features in the filtergrams. Weak upflows are found in
  the flux sheets and downflows in the immediate surroundings. The flux
  sheets often become unstable to a fluting instability and the edges
  buckle. The sheets tend to break up into strings of bright points,
  still with weak upflows in the magnetic elements and zero velocity or
  downflows between them. Where there are larger flux concentrations
  we find ribbons, flowers and micro-pores. There is a continuous
  transition between these forms and they evolve from one form to
  another. The appearance is mostly determined by the horizontal size
  - larger structures are dark (micro-pores), narrower structures are
  ribbon shaped and the flowers are the smallest in extent. All these
  structures have darker inner parts and a bright edge. The plasma is
  found to be at rest in the ribbons, with small concentrations of weak
  upflow sites. Narrow sheets with downdrafts are found right at the
  edges of the magnetic field concentrations.

Title: Solar Filaments and Photospheric Network
Authors: Lin, Yong; Wiik, Jun Elin; Engvold, Oddbjørn; Van Der Voort,
   Luc Rouppe; Frank, Zoe A.
Bibcode: 2005SoPh..227..283L
Altcode:
  The locations of barbs of quiescent solar filaments are compared with
  the photospheric/chromospheric network, which thereby serves as a
  proxy of regions with enhanced concentrations of magnetic flux. The
  study covers quiet regions, where also the photospheric network as
  represented by flow converging regions, i.e., supergranular cell
  boundaries, contain largely weak magnetic fields. It is shown that
  close to 65% of the observed end points of barbs falls within the
  network boundaries. The remaining fraction points into the inner areas
  of the network cells. This confirms earlier findings (Lin et al.,
  Solar Physics, 2004) that quiescent filaments are basically connected
  with weaker magnetic fields in the photosphere below.

Title: Solar magnetic elements at 0.1 arcsec resolution. General
    appearance and magnetic structure
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl,
   M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
   Title, A.; Scharmer, G.
Bibcode: 2004A&A...428..613B
Altcode:
  New observations of solar magnetic elements in a remnant active region
  plage near disk center are presented. The observations were obtained at
  the recently commissioned Swedish 1-m Solar Telescope on La Palma. We
  examine a single 430.5 nm G-band filtergram that resolves ∼70 km
  (0.1 arcsec) structures and find new forms of magnetic structures
  in this particular region. A cotemporal Ca II H-line image is used
  to examine the low-chromosphere of network elements. A cotemporal Fe
  I 630.25 nm magnetogram that resolves structures as small as 120 km
  (0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx
  cm<SUP>-2</SUP> quantifies the magnetic structure of the region. A
  Ni I 676.8 nm Dopplergram establishes relative velocity patterns
  associated with the network features with an accuracy of about 300 m
  s<SUP>-1</SUP>. We find that magnetic flux in this region as seen in
  both the magnetogram and the G-band image is typically structured into
  larger, amorphous, ``ribbons'' which are not resolved into individual
  flux tubes. The measured magnetic flux density in the ribbon structures
  ranges from 300 to 1500 Mx cm<SUP>-2</SUP>, the higher values occurring
  at localized concentrations embedded within the ribbons. The Dopplergram
  indicates relative downflows associated with all magnetic elements
  with some indication that higher downflows occur adjacent to the peak
  magnetic flux location. The mean absolute magnetic flux density of the
  remnant plage network is about 130 Mx cm<SUP>-2</SUP>; in the lowest
  flux regions of the field-of-view, the mean absolute flux density is
  approximately 60 Mx cm<SUP>-2</SUP>. Within these quiet regions we do
  not find evidence of pervasive kilo-gauss strength magnetic elements
  as seen in recent high resolution internetwork studies. In general,
  the observations confirm recent 3-dimensional numerical simulations
  which show that the magnetic field in high-density regions such as
  plage is concentrated in complex structures that are not generally
  composed of discrete magnetic flux tubes. <P />Appendices are only
  available in electronic form at http://www.edpsciences.org

Title: Observations of solar magnetic elements with 0.1" resolution
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl,
   M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
   Title, A. M.; Scharmer, G.
Bibcode: 2004AAS...204.2005B
Altcode: 2004BAAS...36..686B
  New observations of solar magnetic elements in a remnant active region
  plage near disk center are presented. The observations were taken with
  the Swedish 1-meter Solar Telescope on La Palma. We examine a single
  430.5 nm G-band filtergram that resolves ∼70 km (0.''1) structures
  and find new forms of magnetic structures in this particular region. A
  simultaneous Ca II H-line image is used to examine the low-chromosphere
  of network elements. A simultaneous Fe I 630.25 nm magnetogram
  that resolves structures as small as 120 km (0.''18) FWHM with a
  flux sensitivity of approximately 130 Mx cm<SUP>-2</SUP> quantifies
  the magnetic structure of the region. A Ni I 676.8 nm Dopplergram
  establishes relative velocity patterns associated with the network
  features with an accuracy of about 300 m s<SUP>-1</SUP>. Magnetic flux
  in this region as seen in both the magnetogram and the G-band image
  is typically structured into larger, amorphous, ``ribbons'' with a
  wide range of flux density values, rather than isolated kilogauss
  flux tubes. We also present filtergrams and magnetograms of magnetic
  elements at the solar limb showing that solar faculae are resolved
  into bright granular walls that appear to project 350 to 500 km above
  the photosphere.

Title: Penumbral structure at 0.1 arcsec resolution. I. General
    appearance and power spectra
Authors: Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Kiselman,
   D.; Scharmer, G. B.
Bibcode: 2004A&A...414..717R
Altcode:
  We analyse sunspot filtergrams of unprecedented quality obtained by
  \citet{scharmer02dark} with the Swedish 1-m Solar Telescope on La
  Palma. The observations comprise images in three different wavelength
  bands: 488, 436, and 430 nm (G-band). We find that there are still
  unresolved penumbral filaments which must have widths smaller than
  80 km. The fine structuring along the filaments is limited. Penumbral
  grains have internal structure and look like they are split or crossed
  by narrow dark structures. We calculate intensity power spectra of the
  penumbra from images that are corrected for seeing using the Phase
  Diversity technique. The effects of high order aberrations that are
  not corrected for are estimated to be too low to be consistent with
  a flat power spectrum. The penumbral power spectra do not show any
  signs of bumps or peaks that could correspond to a preferred scale
  at 0\farcs35 for the width of penumbral filaments. We argue that the
  power spectrum is not a very reliable source of information concerning
  preferred scales.

Title: Observations at 0{.  ^{primeprime}}1 Resolution of the Dynamic
    Evolution of Magnetic Elements
Authors: Carlsson, Mats; Rouppe van der Voort, Luc; Hansteen, Viggo
Bibcode: 2004IAUS..223..207C
Altcode: 2005IAUS..223..207C
  We present observations of the dynamic evolution of photospheric
  magnetic elements in the G-band, magnetograms and Dopplergrams. The
  observations were obtained with the Swedish 1m Solar Telescope on La
  Palma at close to the diffraction limit of 0{. ^{primeprime}}1. In
  the most quiet regions we observe individual bright points in the
  G-band with corresponding magnetic signal in the magnetograms. Where
  the filling factor of the magnetic field is larger, the bright points
  interact when advected by the granular and super-granular flow-fields,
  flux sheets form and fragment. The plage region of the decaying active
  region is filled with more complex topologies like ribbon structures
  with darker interior and bright, knotted edges. These change into
  flower-like shape when small in extent and into micro-pores when
  the flux region is larger in extent. The magnetic elements in the
  plage region are associated with upflows with strong downflows in the
  immediate vicinity in the low-field region.

Title: Observations of Umbral Flashes
Authors: Rouppe van der Voort, L. H. M.; Krijger, J. M.
Bibcode: 2003csss...12..607R
Altcode:
  We present observations of oscillations in the chromosphere of the
  umbra of sunspots. The observations were obtained with the Swedish
  Vacuum Solar Telescope (SVST) and the Dutch Open Telescope (DOT)
  on La Palma, comprising spectrograms and filtergrams in the Ca II H
  line. The sawtooth pattern in the spectroscopic time evolution of the Ca
  II H core is shown as well as evidence for a connection between umbral
  flashes and running penumbral waves from image sequences. Running waves,
  coherent over a large fraction of the penumbra, seem to be excited by
  flashes that occur close to the umbra-penumbral boundary. Comparing
  the intensity oscillations in the Ca II H line with TRACE observations
  in the 1600 Å passband, we find a phase difference of approximately
  25 ° with 1600 Å leading the Ca II H intensity oscillation which we
  attribute to complex dynamical behaviour.

Title: La Palma observations of umbral flashes
Authors: Rouppe van der Voort, L. H. M.; Rutten, R. J.; Sütterlin,
   P.; Sloover, P. J.; Krijger, J. M.
Bibcode: 2003A&A...403..277R
Altcode:
  We present high-quality Ca II H &amp; K data showing chromospheric
  flashes in sunspot umbrae collected with the Swedish Vacuum Solar
  Telescope, the Dutch Open Telescope, and the Swedish 1-m Solar Telescope
  at the Roque de los Muchachos Observatory on La Palma. Differential
  movies, time slices, spectrograms, and Fourier power maps demonstrate
  that umbral flashes and running penumbral waves are closely related
  oscillatory phenomena, combining upward shock propagation with coherent
  wave spreading over the entire spot. We attribute the flash brightening
  to large redshift by post-shock material higher up. We find no obvious
  relation between umbral dots and umbral flashes.

Title: On the time variability of the Evershed effect
Authors: Rouppe van der Voort, L. H. M.
Bibcode: 2003A&A...397..757R
Altcode:
  The time variability of the Evershed effect in the penumbra of a sunspot
  is studied from a time series of high-spatial resolution spectrograms
  covering the wings of the Ca Ii K line accompanied by Ca Ii K slit-jaw
  images obtained with the Swedish Vacuum Solar Telescope. Simultaneous
  G-band observations from the Dutch Open Telescope are used to
  study the penumbral scene around the spectrograph slit on the
  photospheric level. Four spectral lines are selected to probe the
  Evershed effect at different heights in the penumbral atmosphere. The
  Evershed flow is found to show variations on a time scale of 8-14 min
  which is a confirmation of earlier observations from tunable filter
  instruments. The largest amplitudes in the variation are found where
  the Evershed flow is strongest. No systematic height dependence is
  found for the propagation of the velocity variations. On several
  occasions, an increase in the Evershed signal is associated with an
  intensity increase. In the weak Evershed signal detected in bright
  filaments, no significant variability is found. As an alternative
  to the interpretation of temporal enhancements in the flow speed,
  it is speculated that temporal height variations of a flow channel
  hosting a steady Evershed flow can explain the observed variations in
  the Evershed effect.

Title: Sunspot structure and dynamics
Authors: Rouppe van der Voort, Luc
Bibcode: 2003PhDT.......159R
Altcode:
  Fundamental physical processes occur on small spatial scales in the
  solar atmosphere and high-spatial-resolution observations are necessary
  to test theoretical models. This thesis presents high-spatial-resolution
  observations of sunspots obtained with the Swedish solar telescopes on
  La Palma: the Swedish Vacuum Solar Telescope (SVST) and its successor
  the Swedish 1-m Solar Telescope (SST). Spectroscopic observations are
  used to derive semi-empirical models of the temperature stratification
  of fine-structure elements in the sunspot penumbra by inversion of the
  radiation temperatures in the damping wings of the Ca II K line. These
  models are used to investigate the line formation of spectral line
  blends in the Ca II K wings. Line-core Dopplershift measurements of a
  selection of these spectral line blends are used to investigate the
  height dependence of bulk velocities in the penumbral atmosphere,
  including the Evershed flow. Examples are found of concentrations of
  the Evershed flow in the form of channels that tend to coincide with
  dark filaments. A weak correlation is found between reduced intensity
  and enhanced velocity but there exist examples of strong flows in
  bright structures. Numerical tests of flow channels in the penumbral
  atmosphere suggest that the bulk of the Evershed flow is concentrated
  in the lower atmosphere and has a velocity slower than 6 km s1. No
  indication is found of the Evershed flow being elevated above the
  continuum. A time series of spectrograms with the spectrograph slit
  positioned at a quasi-fixed position in the penumbra shows that the
  Evershed flow is variable on a time scale of 8-14 minutes. The time
  variability does not display any significant height dependence. SST
  observations of sunspots at unprecedented resolution (better than 0.12
  arcseconds) reveal bright penumbral filaments with dark cores. The dark
  cores have apparent widths of 90 km but are not resolved. Other examples
  of very narrow dark features in the magnetic-field dominated regions
  outside sunspots are also found. Observations from the SVST, SST and
  the Dutch Open Telescope are used to study chromospheric oscillations
  in the sunspot umbra and penumbra. These observations comprise time
  series of spectrograms and narrow-band filtergrams covering the
  line-cores of the Ca II H and K lines. Umbral flashes and running
  penumbral waves are found to be closely related oscillatory phenomena,
  combining upward shock propagation with coherent wave spreading over
  the entire sunspot. Flash brightening is attributed to large redshift
  by post-shock material higher in the atmosphere.

Title: First Results from the Swedish 1-m Solar Telescope
Authors: Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Rouppe van
   der Voort, L. H. M.
Bibcode: 2003ASPC..307....3S
Altcode:
  No abstract at ADS

Title: Dark cores in sunspot penumbral filaments
Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan;
   Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M.
Bibcode: 2002Natur.420..151S
Altcode:
  Sunspot umbrae-the dark central regions of the spots-are surrounded
  by brighter filamentary penumbrae, the existence of which remains
  largely inexplicable. The penumbral filaments contain magnetic fields
  with varying inclinations and are associated with flowing gas, but
  discriminating between theoretical models has been difficult because
  the structure of the filaments has not hitherto been resolved. Here
  we report observations of penumbral filaments that reveal dark cores
  inside them. We cannot determine the nature of these dark cores,
  but their very existence provides a crucial test for any model of
  penumbrae. Our images also reveal other very small structures, in line
  with the view that many of the fundamental physical processes in the
  solar photosphere occur on scales smaller than 100km.

Title: Penumbral structure and kinematics from high-spatial-resolution
    observations of Ca II K
Authors: Rouppe van der Voort, L. H. M.
Bibcode: 2002A&A...389.1020R
Altcode:
  High-spatial-resolution spectra of the Ca Ii K line obtained with the
  Swedish Vacuum Solar Telescope (SVST) on La Palma are used to study
  the penumbra of a sunspot. The observed radiation temperatures in
  the Ca Ii K wing are used to derive the temperature stratification
  of fine-structure elements in the penumbra. It is found that in
  general, over the observed atmospheric depth range, penumbral
  structures keep their relative brightness identity with respect
  to their local surroundings, i.e., bright (dark) structures in the
  lower photosphere remain bright (dark) in the upper photosphere. Hot
  structures have a larger temperature difference between the bottom and
  the top of the photosphere than cool structures. Three semi-empirical
  atmosphere models, a cool, hot and mean model, are presented as
  being representative for the temperature stratification of penumbral
  fine structure. The mean temperature distribution of the centre-side
  penumbra is found to be up to 50 K hotter in the higher photosphere as
  compared to the limb-side penumbra. Hot structures being more numerous
  in the centre-side penumbra can account for this difference. These are
  primarily found near the outer penumbral boundary. It is suggested that
  the asymmetry can be explained by a differential line-of-sight effect
  that is caused by isotherms in bright structures having in the higher
  photosphere a tilt angle of approximately 7° with the horizontal,
  pointing downward towards the outer boundary. Line blends in the
  extended Ca Ii K wing are selected to study the Evershed effect and
  its height dependence. At a number of locations, the Evershed effect
  is found to be concentrated in channels which have a tendency to
  coincide with dark filaments. A weak correlation between brightness and
  velocity signal is found but also a number of bright structures with a
  significant Evershed signal. Simple numerical tests of flow channels in
  the penumbral atmosphere are performed to confront existing theoretical
  models with the observations. From these experiments it is found that
  the bulk of the flow must be concentrated in the lower atmospheric
  layers, i.e., below 200 km, and must have a velocity not higher than
  6 km s<SUP>-1</SUP>. A channel width of 200 km is found to give the
  best reproduction of the observed velocities, so that the flow is
  either concentrated in a single channel or in a bundle of narrower
  channels. No direct indication is found of the Evershed channels
  being elevated above the continuum, and it is estimated that the flow
  channels reach down to at least 50 km above the continuum. Based on
  observations made with the Swedish Vacuum Solar Telescope which was
  operated on the island of La Palma by the Royal Swedish Academy of
  Sciences in the Spanish Observatorio del Roque de los Muchachos of
  the Instituto de Astrofisica de Canarias.

Title: Study of the Structure and Dynamics of the Penumbra of Sunspots
(CD-ROM Directory: contribs/rouppe)
Authors: Rouppe van der Voort, L.
Bibcode: 2001ASPC..223..744R
Altcode: 2001csss...11..744R
  No abstract at ADS

Title: Proxy Magnetometry of the Photosphere: Why are G-Band Bright
    Points so Bright?
Authors: Rutten, R. J.; Kiselman, D.; Rouppe van der Voort, L.;
   Plez, B.
Bibcode: 2001ASPC..236..445R
Altcode: 2001aspt.conf..445R
  No abstract at ADS

