rouppevandervoort

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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.
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. Aims: We performed an
extensive statistical characterization of the quiet-Sun Ellerman
bombs (QSEBs) in these new Hβ observations. 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. 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 Mm2, with a median of 0.018
Mm2. 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−1, with an
average that is upward propagating at +4.4 km s−1. 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. 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 (BLOS ≲ 50 G). Our observations support the
interpretation of reconnection along vertically extended current
sheets. 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>

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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
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.

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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
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 × 1016
Mx of magnetic flux canceled in each event with an average cancellation
rate of 3.8 × 1014 Mx s-1. The derived canceled
flux is associated with strong downflows, with an average speed of V
LOS ≍ 1.1 km s-1. 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.

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Title: On the relationship between spicules and coronal bright points
Authors: Bose, Souvik; De Pontieu, Bart; Rouppe van der Voort, Luc;
Nóbrega Siverio, Daniel
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.

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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.
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. 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. 
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. 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.

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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
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−1. The cause of such downflows, however, is
still not fully clear and has remained a matter of debate. 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. 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. 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. 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. 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>

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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.
2021A&A...653A..93M Altcode: 2021arXiv210604936M
Context. The physical conditions leading the sunspot penumbra
decay are poorly understood so far. 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. 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. 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. 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. 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>

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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
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. Aims: We aim to determine
the occurrence of Ellerman bombs in the penumbra of sunspots. 
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. 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−1 and sometimes more
than 10 km s−1. 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. 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. 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>

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Title: Spicules and downflows in the solar chromosphere
Authors: Bose, Souvik; Joshi, Jayant; Henriques, Vasco M. J.; Rouppe
van der Voort, Luc
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. 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. 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). 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. Conclusions: We speculate that these
rapid plasma downflows could well be the chromospheric counterparts
of the commonly observed TR downflows. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202040014/olm">https://www.aanda.org</A>

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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.
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.

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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
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. 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. 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. 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. 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.

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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
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.″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. 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>

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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.
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.

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Title: A multi-diagnostic spectral analysis of penumbral microjets
Authors: Drews, Ainar; Rouppe van der Voort, Luc
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. 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. 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. 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. 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>

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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
2020A&A...637C...1B Altcode:
No abstract at ADS

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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
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. 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.
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. 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. 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. 
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. 
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. 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
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. 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.
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.
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
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 K3 minimum best representing the velocity of the
spicules. The K2 features are either suppressed by the
Doppler-shifted K3 or enhanced via increased contribution
from the lower layers, leading to strongly enhanced but unshifted
K2 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.
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. 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. 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). 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. 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. 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.
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-1 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.
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. (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.
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
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. 
Aims: We aim to characterize the dynamical nature of PMJs to provide
guidance for future modeling efforts. 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. 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-1,
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. 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. 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.
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. 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. 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. 
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-2 through the photosphere. 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.
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
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.
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
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. 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.
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.
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.
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.
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
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.
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
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. 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. 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. 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-1 and +
10.2 km s-1 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. 
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.
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 ×
104 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 (106 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.
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.
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-1 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 × 1011
± 1.76 × 1011 cm-3, 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 × 1012 ±
4.95 × 1011 g s-1. Finally, we reveal a close
proximity between the model predictions of {10}5.8 K and the
observed properties between {10}5.9 and {10}6.2
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.
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″ wide and ≈8.0″ 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−1 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−1 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.
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. 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
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
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
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-1 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. 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.
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.
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-1. 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.
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.
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.
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.
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.
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.
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}-2 arcsec-2), 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.
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.
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.
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 105
to 104 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 × 1010 and 2.5×
{{10}11} cm-3, leading to significant downward
mass fluxes per loop of 1-5 × 109 g s-1, 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
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
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.
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.
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.
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.
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-1
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.
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.
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-1 in proximity
of the δ spot polarity inversion line (PIL), and horizontal motions
along the PIL of the order of ~1 km s-1. 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-1 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
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
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-1 and expands at a
horizontal speed of 4 km s-1. 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-1. 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
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
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.
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.
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
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.
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. 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
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.
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
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.
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-1 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.
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.
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-1, and Doppler widths,
2-15 km s-1, of RBEs than in earlier coronal hole studies,
30-50 km s-1 and 7-23 km s-1, 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-1 and averaging 12 km s-1. 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.
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
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
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.
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.
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-1
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.
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-1 in Ca II 8542 and 30 to 50 km s-1 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-1. 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.
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.
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-1,
(2) swaying motions of order 15-20 km s-1, and (3) torsional
motions of order 25-30 km s-1. 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-1. 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
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.
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 km2 s-2
to the variance of the observed velocities. The raw and corrected
velocity measurements in both directions, i.e., (vx ,
vy ), have Gaussian distributions with standard deviations
of (1.32, 1.22) and (1.00, 0.86) km s-1, 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.
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
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-1 are found,
with an average value of 36.5 ± 5.9 km s-1 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-1 (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-2 and 149 ± 63 m s-2
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.
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.
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
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
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.
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.
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-1 to 111 km s-1. 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-1, 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.
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-1, 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 ×
109 g s-1, 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.
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.
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-1. 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 D1 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.
2011A&A...531A..17R Altcode: 2011arXiv1104.4307R
Imaging spectroscopy of the solar atmosphere using the Na I
D1 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 b2 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 D1, Mg I b2, 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 D1 line. The Mg I b2 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.
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-1 with peaks above 1 km
s-1. 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-1. 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.
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-1, 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.
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.
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.
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-1)
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-1. These
blueward excursions seem to form a separate absorbing component with
Doppler shifts of order 20 and 50 km s-1 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.
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. Aims: We use such high-resolution observations
to study small-scale structures and dynamics in the chromosphere
of the quiet Sun. 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. 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-1
but sometimes up to -7 km s-1, 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. 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. The movie is only
available in electronic form at http://www.aanda.org 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.
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.
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. 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. 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. 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. 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.
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.
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.
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.
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. 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. Results:
We obtained near diffraction limited images, which have a noise level
of ≈ 10-3 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.
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-1) 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.
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
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
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.
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−1) waves
propagating along a number of threads with an average phase velocity of
12 km s−1 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.
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.
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. 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. 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. 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.
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.
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. 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.
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 \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.
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-1 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.
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
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.
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-1, while the downflows
at the edges really are much stronger than observed, of order 1.5-3.3
km s-1.

---------------------------------------------------------
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.
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.
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.
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.
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.
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-1 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-1, 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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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-2 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-1. 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-2, 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-2; in the lowest
flux regions of the field-of-view, the mean absolute flux density is
approximately 60 Mx cm-2. 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. 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.
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-2 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-1. 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.
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
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.
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.
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.
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
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.
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.
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.
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-1. 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.
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.
2001ASPC..236..445R Altcode: 2001aspt.conf..445R
No abstract at ADS