explanation blue bibcodes open ADS page with paths to full text
Author name code: vissers
ADS astronomy entries on 2022-09-14
author:"Vissers, Gregal J.M."
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Title: Active region chromospheric magnetic fields. Observational
inference versus magnetohydrostatic modelling
Authors: Vissers, G. J. M.; Danilovic, S.; Zhu, X.; Leenaarts, J.;
Díaz Baso, C. J.; da Silva Santos, J. M.; de la Cruz Rodríguez,
J.; Wiegelmann, T.
2022A&A...662A..88V Altcode: 2021arXiv210902943V
Context. A proper estimate of the chromospheric magnetic fields is
thought to improve modelling of both active region and coronal mass
ejection evolution. However, because the chromospheric field is not
regularly obtained for sufficiently large fields of view, estimates
thereof are commonly obtained through data-driven models or field
extrapolations, based on photospheric boundary conditions alone and
involving pre-processing that may reduce details and dynamic range in
the magnetograms. <BR /> Aims: We investigate the similarity between
the chromospheric magnetic field that is directly inferred from
observations and the field obtained from a magnetohydrostatic (MHS)
extrapolation based on a high-resolution photospheric magnetogram. <BR
/> Methods: Based on Swedish 1-m Solar Telescope Fe I 6173 Å and
Ca II 8542 Å observations of NOAA active region 12723, we employed
the spatially regularised weak-field approximation (WFA) to derive
the vector magnetic field in the chromosphere from Ca II, as well as
non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca
II to infer a model atmosphere for selected regions. Milne-Eddington
inversions of Fe I serve as photospheric boundary conditions for the
MHS model that delivers the three-dimensional field, gas pressure,
and density self-consistently. <BR /> Results: For the line-of-sight
component, the MHS chromospheric field generally agrees with the
non-LTE inversions and WFA, but tends to be weaker by 16% on average
than these when larger in magnitude than 300 G. The observationally
inferred transverse component is systematically stronger, up to an
order of magnitude in magnetically weaker regions, but the qualitative
distribution with height is similar to the MHS results. For either
field component, the MHS chromospheric field lacks the fine structure
derived from the inversions. Furthermore, the MHS model does not
recover the magnetic imprint from a set of high fibrils connecting
the main polarities. <BR /> Conclusions: The MHS extrapolation and
WFA provide a qualitatively similar chromospheric field, where the
azimuth of the former is better aligned with Ca II 8542 Å fibrils than
that of the WFA, especially outside strong-field concentrations. The
amount of structure as well as the transverse field strengths are,
however, underestimated by the MHS extrapolation. This underscores the
importance of considering a chromospheric magnetic field constraint in
data-driven modelling of active regions, particularly in the context
of space weather predictions.
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Title: Heating of the solar chromosphere through current dissipation
Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de
la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.;
Rempel, M.
2022A&A...661A..59D Altcode: 2022arXiv220203955D
Context. The solar chromosphere is heated to temperatures higher than
predicted by radiative equilibrium. This excess heating is greater
in active regions where the magnetic field is stronger. <BR />
Aims: We aim to investigate the magnetic topology associated with an
area of enhanced millimeter (mm) brightness temperatures in a solar
active region mapped by the Atacama Large Millimeter/submillimeter
Array (ALMA) using spectropolarimetric co-observations with the 1-m
Swedish Solar Telescope (SST). <BR /> Methods: We used Milne-Eddington
inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions,
and a magnetohydrostatic extrapolation to obtain constraints on the
three-dimensional (3D) stratification of temperature, magnetic field,
and radiative energy losses. We compared the observations to a snapshot
of a magnetohydrodynamics simulation and investigate the formation
of the thermal continuum at 3 mm using contribution functions. <BR />
Results: We find enhanced heating rates in the upper chromosphere of up
to ∼5 kW m<SUP>−2</SUP>, where small-scale emerging loops interact
with the overlying magnetic canopy leading to current sheets as shown
by the magnetic field extrapolation. Our estimates are about a factor
of two higher than canonical values, but they are limited by the ALMA
spatial resolution (∼1.2″). Band 3 brightness temperatures reach
about ∼10<SUP>4</SUP> K in the region, and the transverse magnetic
field strength inferred from the non-LTE inversions is on the order
of ∼500 G in the chromosphere. <BR /> Conclusions: We are able to
quantitatively reproduce many of the observed features including the
integrated radiative losses in our numerical simulation. We conclude
that the heating is caused by dissipation in current sheets. However,
the simulation shows a complex stratification in the flux emergence
region where distinct layers may contribute significantly to the
emission in the mm continuum. <P />The movie is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243191/olm">https://www.aanda.org</A>
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Title: COCOPLOT: COlor COllapsed PLOTting software Using colour to
view 3D data as a 2D image
Authors: Druett, Malcolm K.; Pietrow, Alexander G. M.; Vissers,
Gregal J. M.; Robustini, Carolina; Calvo, Flavio
2022RASTI...1...29D Altcode: 2021arXiv211110786D
Most modern solar observatories deliver data products formatted as 3D
spatio-temporal data cubes, that contain additional, higher dimensions
with spectral and/or polarimetric information. This multi-dimensional
complexity presents a major challenge when browsing for features of
interest in several dimensions simultaneously. We developed the COlor
COllapsed PLOTting (COCOPLOT) software as a quick-look and context image
software, to convey spectral profile or time evolution from all the
spatial pixels ($x,y$) in a 3D [$n_x,n_y,n_\lambda$] or [$n_x,n_y,n_t$]
data cube as a single image, using color. This can avoid the need to
scan through many wavelengths, creating difference and composite images
when searching for signals satisfying multiple criteria. Filters are
generated for the red, green, and blue channels by selecting values
of interest to highlight in each channel, and their weightings. These
filters are combined with the data cube over the third dimension axis
to produce an $n_x \times n_y \times 3$ cube displayed as one true
color image. Some use cases are presented for data from the Swedish 1-m
Solar Telescope (SST) and IRIS, including H$\alpha$ solar flare data,
a comparison with $k$-means clustering for identifying asymmetries
in the Ca II K line and off-limb coronal rain in IRIS C II slit-jaw
images. These illustrate identification by color alone using COCOPLOT
of locations including line wing or central enhancement, broadening,
wing absorption, and sites with intermittent flows or time-persistent
features. COCOPLOT is publicly available in both IDL and Python.
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Title: COCOPLOT: COlor COllapsed PLOTting software
Authors: Druett, Malcolm K.; Pietrow, Alexander G. M.; Vissers,
Gregal J. M.; Robustini, Carolina
2021ascl.soft11008D Altcode:
The COCOPLOT (COlor COllapsed PLOTting) quick-look and context image
code conveys spectral profile information from all of the spatial
pixels in a 3D datacube as a single image using color. It can also
identify and expose temporal behavior and display and highlight
solar features. COCOPLOT thus aids in identifying regions of interest
quickly. The software is available in Python and IDL, and can be used
as a standalone package or integrated into other software.
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Title: ISP-SST/ISPy: ISPy release v0.2.0
Authors: Díaz Baso, Carlos José; Vissers, Gregal; Calvo, Flavio;
Pietrow, Alexander; Yadav, Rahul; De La Cruz Rodríguez, Jaime;
Zivadinovic, Lazar
2021zndo...5608441D Altcode: 2021zndo...5608441J
This new version includes all previous features (basic functionality
for SST data I/O and calibration, image alignment and inversion
result visualisations) and the following new ones: neuralDenoising,
potentialExtrapolation, field-dependent crosstalk, mapping to CEA
coordinates, neuralEstimation, cube visualisation and routines for
creating SST spectral line programs.
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Title: SSTRED: Data- and metadata-processing pipeline for CHROMIS
and CRISP
Authors: Löfdahl, Mats G.; Hillberg, Tomas; de la Cruz Rodríguez,
Jaime; Vissers, Gregal; Andriienko, Oleksii; Scharmer, Göran B.;
Haugan, Stein V. H.; Fredvik, Terje
2021A&A...653A..68L Altcode: 2018arXiv180403030L
Context. Data from ground-based, high-resolution solar telescopes
can only be used for science with calibrations and processing, which
requires detailed knowledge about the instrumentation. Space-based
solar telescopes provide science-ready data, which are easier to
work with for researchers whose expertise is in the interpretation of
data. Recently, data-processing pipelines for ground-based instruments
have been constructed. <BR /> Aims: We aim to provide observers
with a user-friendly data pipeline for data from the Swedish 1-meter
Solar Telescope (SST) that delivers science-ready data together with
the metadata needed for proper interpretation and archiving. <BR />
Methods: We briefly describe the CHROMospheric Imaging Spectrometer
(CHROMIS) instrument, including its (pre)filters, as well as recent
upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and
polarization optics. We summarize the processing steps from raw data
to science-ready data cubes in FITS files. We report calibrations
and compensations for data imperfections in detail. Misalignment
of Ca II data due to wavelength-dependent dispersion is identified,
characterized, and compensated for. We describe intensity calibrations
that remove or reduce the effects of filter transmission profiles
as well as solar elevation changes. We present REDUX, a new version
of the MOMFBD image restoration code, with multiple enhancements and
new features. It uses projective transforms for the registration of
multiple detectors. We describe how image restoration is used with
CRISP and CHROMIS data. The science-ready output is delivered in FITS
files, with metadata compliant with the SOLARNET recommendations. Data
cube coordinates are specified within the World Coordinate System
(WCS). Cavity errors are specified as distortions of the WCS wavelength
coordinate with an extension of existing WCS notation. We establish
notation for specifying the reference system for Stokes vectors with
reference to WCS coordinate directions. The CRIsp SPectral EXplorer
(CRISPEX) data-cube browser has been extended to accept SSTRED output
and to take advantage of the SOLARNET metadata. <BR /> Results: SSTRED
is a mature data-processing pipeline for imaging instruments, developed
and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP
spectropolarimeter. SSTRED delivers well-characterized, science-ready,
archival-quality FITS files with well-defined metadata. The SSTRED
code, as well as REDUX and CRISPEX, is freely available through git
repositories.
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Title: Non-LTE inversions of a confined X2.2 flare. I. The vector
magnetic field in the photosphere and chromosphere
Authors: Vissers, G. J. M.; Danilovic, S.; de la Cruz Rodríguez,
J.; Leenaarts, J.; Morosin, R.; Díaz Baso, C. J.; Reid, A.; Pomoell,
J.; Price, D. J.; Inoue, S.
2021A&A...645A...1V Altcode: 2020arXiv200901537V
Context. Obtaining an accurate measurement of magnetic field vector
in the solar atmosphere is essential for studying changes in field
topology during flares and reliably modelling space weather. <BR />
Aims: We tackle this problem by applying various inversion methods to a
confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017
and comparing the photospheric and chromospheric magnetic field vector
with the results of two numerical models of this event. <BR /> Methods:
We obtained the photospheric magnetic field from Milne-Eddington
and (non-)local thermal equilibrium (non-LTE) inversions of Hinode
SOT/SP Fe I 6301.5 Å and 6302.5 Å. The chromospheric field was
obtained from a spatially regularised weak-field approximation (WFA)
and non-LTE inversions of Ca II 8542 Å observed with CRISP at the
Swedish 1 m Solar Telescope. We investigated the field strengths
and photosphere-to-chromosphere shear in the field vector. <BR />
Results: The LTE- and non-LTE-inferred photospheric magnetic field
components are strongly correlated across several optical depths in
the atmosphere, with a tendency towards a stronger field and higher
temperatures in the non-LTE inversions. For the chromospheric field,
the non-LTE inversions correlate well with the spatially regularised
WFA, especially in terms of the line-of-sight field strength and field
vector orientation. The photosphere exhibits coherent strong-field
patches of over 4.5 kG, co-located with similar concentrations
exceeding 3 kG in the chromosphere. The obtained field strengths
are up to two to three times higher than in the numerical models,
while the photosphere-to-chromosphere shear close to the polarity
inversion line is more concentrated and structured. <BR /> Conclusions:
In the photosphere, the assumption of LTE for Fe I line formation
does not yield significantly different magnetic field results in
comparison to the non-LTE case, while Milne-Eddington inversions
fail to reproduce the magnetic field vector orientation where Fe
I is in emission. In the chromosphere, the non-LTE-inferred field
is excellently approximated by the spatially regularised WFA. Our
inversions confirm the locations of flux rope footpoints that have
been predicted by numerical models. However, pre-processing and lower
spatial resolution lead to weaker and smoother field in the models than
what our data indicate. This highlights the need for higher spatial
resolution in the models to better constrain pre-eruptive flux ropes.
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Title: Probing chromospheric heating with millimeter interferometry
Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S.
2020AGUFMSH0010001D Altcode:
Observations at visible and ultraviolet wavelengths have shown that
solar active regions host different kinds of small-scale, transient,
bright structures that are believed to be heating events resulting
from the release of magnetic energy in the low atmosphere of the Sun,
especially at the early stages of flux emergence. It is of great
scientific interest to be able to accurately infer temperatures and
formation heights of the most localized events, which are still
matter of debate, in the hope of learning about the evolution of
active regions where occasionally more energetic phenomena lead to
much larger outbursts that propagate across the Solar System. The
millimeter (mm) continuum is a new complementary diagnostic for
chromospheric heating that is now available thanks to the Atacama
Large Millimeter/submillimeter Array (ALMA). <P />We report on the
first ALMA 3 mm observations of small-scale heating events in a
solar active region. In contrast with the low-amplitude brightness
temperature variations in the quiet-Sun, the interferometric maps show
that the active region consists of long, warm, fibril-like structures
that connect magnetic concentrations of opposite polarity and often
flare up along with compact, flickering mm-bursts -- reminiscent of
ultraviolet bursts -- with brightness temperatures of up to 14000 K at
1.2" scales. These events also show simultaneous EUV emission observed
by the Solar Dynamics Observatory (SDO). We find a weak correlation
between the photospheric bright patches and the 3 mm continuum
brightness and, in particular, we do not detect any mm counterpart of
Ellerman bombs which confirms that they are photospheric phenomena. <P
/>Our observations and modelling highlight the diagnostic capabilities
of ALMA for local heating in solar active regions and emphasize the
need for coordinated observations with IRIS and DKIST in the future.
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Title: ALMA observations of transient heating in a solar active region
Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White,
S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.
2020A&A...643A..41D Altcode: 2020arXiv200614564D
<BR /> Aims: We aim to investigate the temperature enhancements and
formation heights of solar active-region brightenings such as Ellerman
bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region
fibrils (FAFs) using interferometric observations in the millimeter
(mm) continuum provided by the Atacama Large Millimeter/submillimeter
Array (ALMA). <BR /> Methods: We examined 3 mm signatures of heating
events identified in Solar Dynamics Observatory observations of an
active region and compared the results with synthetic spectra from a 3D
radiative magnetohydrodynamic simulation. We estimated the contribution
from the corona to the mm brightness using differential emission measure
analysis. <BR /> Results: We report the null detection of EBs in the 3
mm continuum at ∼1.2″ spatial resolution, which is evidence that
they are sub-canopy events that do not significantly contribute to
heating the upper chromosphere. In contrast, we find the active region
to be populated with multiple compact, bright, flickering mm-bursts -
reminiscent of UVBs. The high brightness temperatures of up to ∼14 200
K in some events have a contribution (up to ∼7%) from the corona. We
also detect FAF-like events in the 3 mm continuum. These events show
rapid motions of > 10 kK plasma launched with high plane-of-sky
velocities (37 - 340 km s<SUP>-1</SUP>) from bright kernels. The mm FAFs
are the brightest class of warm canopy fibrils that connect magnetic
regions of opposite polarities. The simulation confirms that ALMA
should be able to detect the mm counterparts of UVBs and small flares
and thus provide a complementary diagnostic for localized heating in the
solar chromosphere. <P />Movie associated to Fig. 5 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038755/olm">https://www.aanda.org</A>
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Title: Stratification of canopy magnetic fields in a plage
region. Constraints from a spatially-regularized weak-field
approximation method
Authors: Morosin, Roberta; de la Cruz Rodríguez, Jaime; Vissers,
Gregal J. M.; Yadav, Rahul
2020A&A...642A.210M Altcode: 2020arXiv200614487M
Context. The role of magnetic fields in the chromospheric heating
problem remains greatly unconstrained. Most theoretical predictions from
numerical models rely on a magnetic configuration, field strength, and
connectivity; the details of which have not been well established with
observational studies for many chromospheric scenarios. High-resolution
studies of chromospheric magnetic fields in plage are very scarce
or non existent in general. <BR /> Aims: Our aim is to study the
stratification of the magnetic field vector in plage regions. Previous
studies predict the presence of a magnetic canopy in the chromosphere
that has not yet been studied with full-Stokes observations. We
use high-spatial resolution full-Stokes observations acquired with
the CRisp Imaging Spectro-Polarimeter (CRISP) at the Swedish 1-m
Solar Telescope in the Mg I 5173 Å, Na I 5896 Å and Ca II 8542
Å lines. <BR /> Methods: We have developed a spatially-regularized
weak-field approximation (WFA) method, based on the idea of spatial
regularization. This method allows for a fast computation of magnetic
field maps for an extended field of view. The fidelity of this new
technique has been assessed using a snapshot from a realistic 3D
magnetohydrodynamics simulation. <BR /> Results: We have derived the
depth-stratification of the line-of-sight component of the magnetic
field from the photosphere to the chromosphere in a plage region. The
magnetic fields are concentrated in the intergranular lanes in the
photosphere and expand horizontally toward the chromosphere, filling
all the space and forming a canopy. Our results suggest that the
lower boundary of this canopy must be located around 400 - 600 km
from the photosphere. The mean canopy total magnetic field strength
in the lower chromosphere (z ≈ 760 km) is 658 G. At z = 1160 km,
we estimate ⟨B<SUB>∥</SUB>⟩ ≈ 417 G. <BR /> Conclusions:
In this study we propose a modification to the WFA that improves
its applicability to data with a worse signal-to-noise ratio. We
have used this technique to study the magnetic properties of the hot
chromospheric canopy that is observed in plage regions. The methods
described in this paper provide a quick and reliable way of studying
multi layer magnetic field observations without the many difficulties
inherent to other inversion methods.
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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: 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 & k, Si IV, and Ca
II 8542 Å and Ca II H & K lines, we aim to characterise their
temperature and velocity stratification, as well as their magnetic
field configuration. We find average temperature enhancements of
a few thousand kelvin, close to the classical temperature minimum
and similar to previous studies, but localised peak temperatures
of up to 10 000-15 000 K from Ca II inversions. Including Mg II
appears to generally dampen these temperature enhancements to below
8000 K, while Si IV requires temperatures in excess of 10 000 K at
low heights, but may also be reproduced with secondary temperature
enhancements of 35 000-60 000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The
line-of-sight velocity maps show clear bi-directional jet signatures
for some events and strong correlation with substructure in the
intensity images in general. Absolute line-of-sight velocities range
between 5 and 20 km s<SUP>-1</SUP> on average, with slightly larger
velocities towards, rather than away from, the observer. The inverted
magnetic field parameters show an enhancement of the horizontal
field co-located with the brightenings at heights similar to that of
the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with the UV burst signature
(e.g. intensities, profile asymmetries, morphology, and bi-directional
jet signatures), with temperature stratifications peaking close
to the classical temperature minimum. Correctly modelling the Si
IV emission in agreement with all other diagnostics is however an
outstanding issue and remains paramount in explaining its apparent
coincidence with Hα emission. Fine-tuning the approach (accounting for
resolution differences, fitting localised temperature enhancements,
and/or performing spatially coupled inversions) is likely necessary
in order to obtain better agreement between all considered diagnostics.
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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.
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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.
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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.
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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<SUP>-1</SUP> in multiple loop-top sources. We calculated the
density of the extreme-ultraviolet (EUV) plasma from the differential
emission measure of the multithermal source employing regularized
inversion. Assuming a pressure balance, we estimate the density of
the chromospheric component of rain to be 9.21 × 10<SUP>11</SUP>
± 1.76 × 10<SUP>11</SUP> cm<SUP>-3</SUP>, which is comparable with
quiescent coronal rain densities. With up to eight parallel strands
in the EUV loop cross section, we calculate the mass loss rate from
the postflare arcade to be as much as 1.98 × 10<SUP>12</SUP> ±
4.95 × 10<SUP>11</SUP> g s<SUP>-1</SUP>. Finally, we reveal a close
proximity between the model predictions of {10}<SUP>5.8</SUP> K and the
observed properties between {10}<SUP>5.9</SUP> and {10}<SUP>6.2</SUP>
K, which defines the temperature onset of catastrophic cooling. The
close correspondence between the observations and numerical models
suggests that indeed acoustic waves (with a sound travel time of 68 s)
could play an important role in redistributing energy and sustaining
the enthalpy-based radiative cooling.
---------------------------------------------------------
Title: 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. <P />The movies are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201628889/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: 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 & 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: 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: 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 10<SUP>5</SUP>
to 10<SUP>4</SUP> K occurs. The 0\buildrel{\prime\prime}\over{.}
2-0\buildrel{\prime\prime}\over{.} 8 width of the distribution
of coronal rain is found to be independent of temperature. The
sharp increase in the number of clumps at the coolest temperatures,
especially at higher resolution, suggests that the bulk distribution
of the rain remains undetected. Rain clumps appear organized in
strands in both chromospheric and TR temperatures. We further find
structure reminiscent of the magnetohydrodynamic (MHD) thermal mode
(also known as entropy mode), thereby suggesting an important role of
thermal instability in shaping the basic loop substructure. Rain core
densities are estimated to vary between 2 × 10<SUP>10</SUP> and 2.5×
{{10}<SUP>11</SUP>} cm<SUP>-3</SUP>, leading to significant downward
mass fluxes per loop of 1-5 × 10<SUP>9</SUP> g s<SUP>-1</SUP>, thus
suggesting a major role in the chromosphere-corona mass cycle.
---------------------------------------------------------
Title: Ellerman bombs: Advances driven by high-resolution observations
Authors: Vissers, Gregal
2014cosp...40E3533V Altcode:
Ellerman bombs, transient brightenings that have traditionally been
observed in the wings of the Balmer Halpha line, are a ubiquitous
phenomenon in the lower atmosphere of active regions with considerable
flux emergence. These explosive events display sub-arcsecond fine
structure, fast dynamical evolution and their energies tend to
fall in the nanoflare ballpark. Over the past decade and a half,
several high-resolution ground-based and space-based telescopes
have contributed greatly to further characterising Ellerman bombs,
offering a view in spectral diagnostics ranging from the UV to the
infrared. I will highlight some of the recent advances that have been
made - both observationally and from a theoretical point of view -
in determining their properties (quantitative morphology, energies,
flows and proper motion, driving mechanism, etc.), and discuss the
potential of observations from relatively new space-based telescopes,
such as SDO and IRIS, to add to our understanding of this phenomenon.
---------------------------------------------------------
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: 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: 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: 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: 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<SUP>-1</SUP> are found,
with an average value of 36.5 ± 5.9 km s<SUP>-1</SUP> and slightly
higher typical velocities for features moving toward the sunspot than
away. These velocities are much higher than those determined from the
shift of the line core, which shows patches around the sunspot with
velocity enhancements of up to 10-15 km s<SUP>-1</SUP> (both red- and
blueshifted). Accelerations are determined for a subsample of features
that show clear accelerating or decelerating behavior, yielding an
average of 270 ± 63 m s<SUP>-2</SUP> and 149 ± 63 m s<SUP>-2</SUP>
for the accelerating and decelerating features, respectively. Typical
flocculent features measure 627 ± 44 km in length and 304 ± 30 km
in width. On average, 68 features are detected per minute, with an
average lifetime of 67.7 ± 8.8 s. The dynamics and phenomenology
of the flocculent flows suggest they may be driven by a siphon flow,
where the flocculence could arise from a density perturbation close
to one of the footpoints or along the loop structure.
---------------------------------------------------------
Title: 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<SUP>-1</SUP>. The bombs flare repetitively with much
rapid variation at timescales of seconds only, in the form of upward
jet-shaped brightness features. These reach heights of 600-1200 km and
tend to show blueshifts; some show bi-directional Doppler signature
and some seem accompanied with an Hα surge. They are not seen in the
core of Hα due to shielding by overlying chromospheric fibrils. The
network where they originate has normal properties. The morphology of
these jets strongly supports deep-seated photospheric reconnection of
emergent or moat-driven magnetic flux with pre-existing strong vertical
network fields as the mechanism underlying the Ellerman bomb phenomenon.
---------------------------------------------------------
Title: Dynamics of fine structure in the atmosphere of solar active
regions
Authors: Vissers, Gregal Joan María
2011PhDT.......106V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Rain as a Marker for Coronal Heating Mechanisms
Authors: Antolin, P.; Shibata, K.; Vissers, G.
2010ApJ...716..154A Altcode: 2009arXiv0910.2383A
Reported observations in Hα, Ca II H, and K or other chromospheric
lines of coronal rain trace back to the days of the Skylab
mission. Corresponding to cool and dense plasma, coronal rain is often
observed falling down along coronal loops in active regions. A physical
explanation for this spectacular phenomenon has been put forward
thanks to numerical simulations of loops with footpoint-concentrated
heating, a heating scenario in which cool condensations naturally
form in the corona. This effect has been termed "catastrophic cooling"
and is the predominant explanation for coronal rain. In this work, we
further investigate the link between this phenomenon and the heating
mechanisms acting in the corona. We start by analyzing observations of
coronal rain at the limb in the Ca II H line performed by the Hinode
satellite, and derive interesting statistical properties concerning
the dynamics. We then compare the observations with 1.5-dimensional
MHD simulations of loops being heated by small-scale discrete events
concentrated toward the footpoints (that could come, for instance,
from magnetic reconnection events), and by Alfvén waves generated at
the photospheric level. Both our observation and simulation results
suggest that coronal rain is a far more common phenomenon than
previously thought. Also, we show that the structure and dynamics of
condensations are far more sensitive to the internal pressure changes
in loops than to gravity. Furthermore, it is found that if a loop is
predominantly heated from Alfvén waves, coronal rain is inhibited due
to the characteristic uniform heating they produce. Hence, coronal
rain may not only point to the spatial distribution of the heating
in coronal loops but also to the agent of the heating itself. We thus
propose coronal rain as a marker for coronal heating mechanisms.
---------------------------------------------------------
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<SUP>-1</SUP>)
than the classical spicules. Since they have been implicated in
providing hot plasma to coronal loops, their formation, evolution,
and properties are important ingredients for a better understanding
of the mass and energy balance of the low solar atmosphere. Here, we
report on the discovery of the disk counterparts of type II spicules
using spectral imaging data in the Ca II 854.2 nm and Hα lines with
the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope in
La Palma. We find rapid blueward excursions in the line profiles of
both chromospheric lines that correspond to thin, jet-like features
that show apparent velocities of order 50 km s<SUP>-1</SUP>. These
blueward excursions seem to form a separate absorbing component with
Doppler shifts of order 20 and 50 km s<SUP>-1</SUP> for the Ca II 854.2
nm and Hα line, respectively. We show that the appearance, lifetimes,
longitudinal and transverse velocities, and occurrence rate of these
rapid blue excursions on the disk are very similar to those of the type
II spicules at the limb. A detailed study of the spectral line profiles
in these events suggests that plasma is accelerated along the jet,
and plasma is being heated throughout the short lifetime of the event.
