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Author name code: chitta
ADS astronomy entries on 2022-09-14
author:"Chitta, Lakshmi Pradeep"
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Title: What drives decayless kink oscillations in active region
coronal loops on the Sun?
Authors: Mandal, Sudip; Chitta, Lakshmi P.; Antolin, Patrick; Peter,
Hardi; Solanki, Sami K.; Auchère, Frédéric; Berghmans, David;
Zhukov, Andrei N.; Teriaca, Luca; Cuadrado, Regina A.; Schühle,
Udo; Parenti, Susanna; Buchlin, Éric; Harra, Louise; Verbeeck, Cis;
Kraaikamp, Emil; Long, David M.; Rodriguez, Luciano; Pelouze, Gabriel;
Schwanitz, Conrad; Barczynski, Krzysztof; Smith, Phil J.
2022arXiv220904251M Altcode:
We study here the phenomena of decayless kink oscillations in a system
of active region (AR) coronal loops. Using high resolution observations
from two different instruments, namely the Extreme Ultraviolet Imager
(EUI) on board Solar Orbiter and the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory, we follow these AR
loops for an hour each on three consecutive days. Our results show
significantly more resolved decayless waves in the higher-resolution
EUI data compared with the AIA data. Furthermore, the same system of
loops exhibits many of these decayless oscillations on Day-2, while on
Day-3, we detect very few oscillations and on Day-1, we find none at
all. Analysis of photospheric magnetic field data reveals that at most
times, these loops were rooted in sunspots, where supergranular flows
are generally absent. This suggests that supergranular flows, which
are often invoked as drivers of decayless waves, are not necessarily
driving such oscillations in our observations. Similarly, our findings
also cast doubt on other possible drivers of these waves, such as a
transient driver or mode conversion of longitudinal waves near the loop
footpoints. In conclusion, through our analysis we find that none of
the commonly suspected sources proposed to drive decayless oscillations
in active region loops seems to be operating in this event and hence,
the search for that elusive wave driver needs to continue.
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Title: Defining the Middle Corona
Authors: West, Matthew J.; Seaton, Daniel B.; Wexler, David B.;
Raymond, John C.; Del Zanna, Giulio; Rivera, Yeimy J.; Kobelski,
Adam R.; DeForest, Craig; Golub, Leon; Caspi, Amir; Gilly, Chris R.;
Kooi, Jason E.; Alterman, Benjamin L.; Alzate, Nathalia; Banerjee,
Dipankar; Berghmans, David; Chen, Bin; Chitta, Lakshmi Pradeep; Downs,
Cooper; Giordano, Silvio; Higginson, Aleida; Howard, Russel A.; Mason,
Emily; Mason, James P.; Meyer, Karen A.; Nykyri, Katariina; Rachmeler,
Laurel; Reardon, Kevin P.; Reeves, Katharine K.; Savage, Sabrina;
Thompson, Barbara J.; Van Kooten, Samuel J.; Viall, Nicholeen M.;
Vourlidas, Angelos
2022arXiv220804485W Altcode:
The middle corona, the region roughly spanning heliocentric altitudes
from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential
physical transitions and processes that govern the behavior of
coronal outflow into the heliosphere. Eruptions that could disrupt
the near-Earth environment propagate through it. Importantly, it
modulates inflow from above that can drive dynamic changes at lower
heights in the inner corona. Consequently, this region is essential
for comprehensively connecting the corona to the heliosphere and for
developing corresponding global models. Nonetheless, because it is
challenging to observe, the middle corona has been poorly studied by
major solar remote sensing missions and instruments, extending back to
the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent
advances in instrumentation, observational processing techniques,
and a realization of the importance of the region, interest in the
middle corona has increased. Although the region cannot be intrinsically
separated from other regions of the solar atmosphere, there has emerged
a need to define the region in terms of its location and extension
in the solar atmosphere, its composition, the physical transitions
it covers, and the underlying physics believed to be encapsulated by
the region. This paper aims to define the middle corona and give an
overview of the processes that occur there.
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Title: A highly dynamic small-scale jet in a polar coronal hole
Authors: Mandal, Sudip; Chitta, Lakshmi Pradeep; Peter, Hardi;
Solanki, Sami K.; Cuadrado, Regina Aznar; Teriaca, Luca; Schühle,
Udo; Berghmans, David; Auchère, Frédéric
2022A&A...664A..28M Altcode: 2022arXiv220602236M
We present an observational study of the plasma dynamics at the base
of a solar coronal jet, using high resolution extreme ultraviolet
imaging data taken by the Extreme Ultraviolet Imager on board Solar
Orbiter, and by the Atmospheric Imaging Assembly on board Solar
Dynamics Observatory. We observed multiple plasma ejection events
over a period of ∼1 h from a dome-like base that is ca. 4 Mm wide
and is embedded in a polar coronal hole. Within the dome below the
jet spire, multiple plasma blobs with sizes around 1−2 Mm propagate
upwards to the dome apex with speeds of the order of the sound speed
(ca. 120 km s<SUP>−1</SUP>). Upon reaching the apex, some of these
blobs initiate flows with similar speeds towards the other footpoint
of the dome. At the same time, high speed super-sonic outflows
(∼230 km s<SUP>−1</SUP>) are detected along the jet spire. These
outflows as well as the intensity near the dome apex appear to be
repetitive. Furthermore, during its evolution, the jet undergoes
many complex morphological changes, including transitions between
the standard and blowout type eruption. These new observational
results highlight the underlying complexity of the reconnection
process that powers these jets and they also provide insights into
the plasma response when subjected to rapid energy injection. <P
/>Movies associated to Figs. 1, 2, and 4 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243765/olm">https://www.aanda.org</A>
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Title: Tracing the Drivers of Slow Solar Wind in the Middle Corona
Authors: Chitta, Lakshmi Pradeep; DeForest, Craig; Downs, Cooper;
Seaton, Daniel; Higginson, Aleida
2022cosp...44.1328C Altcode:
Compared to the so-called fast solar wind that originates from polar
coronal holes during the solar minimum, low-latitude wind streams
generally have lower speeds ($\le$ 500 km/s). These slow solar wind
streams closer to the ecliptic plane are characterised by their high
spatial structuring, temporal variability, and coronal compositions. The
magnetic driver responsible for the origin of this slow solar wind
and its characteristics, however, is a subject of active debate. Using
coronal observations from the 2018 off-pointing campaign of the GOES
Solar Ultraviolet Imager (SUVI) as well as images from SOHO/LASCO, we
found signatures of solar wind streams driven by magnetic reconnection
in the highly structured middle corona. In particular, elongated
coronal loops in the middle corona over a coronal-hole-active-region
complex are observed to reconnect and retract while in the process,
some plasma is propelled away from the Sun as streams or blobs. Using
STEREO observations we found that similar streams forming over the
same complex escape into the heliosphere. In this talk, we will
present these new results and discuss them in the context of models
of slow solar wind sources and drivers. our findings emphasise the
key necessity of having routine extreme ultraviolet observations of
the middle coronal processes from ecliptic and polar vantage points,
along with extended coronal diagnostics to develop better understanding
of the heliospheric impact of Sun's activity.
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Title: Small-scale coronal brightenings as seen by Solar Orbiter
Authors: Peter, Hardi; Berghmans, David; Chitta, Lakshmi Pradeep
2022cosp...44.1323P Altcode:
The corona of the Sun shows variability over a wide range of scales,
in space, time and energy. The power-law-like distributions small-scale
coronal brightenings events have been used to propose self-similarity
of the involved processes. Already during the cruise phase, Solar
Orbiter was close enough to the Sun so that images provided by the
Extreme Ultraviolet Imager (EUI) are among the highest resolution
coronal data acquired so far. The small brightenings found in the
quiet Sun, now often termed campfires, could be considered as the small
end of the distribution of coronal transients. Mostly, these coronal
brightenings occur very low in the atmosphere, essentially just above
the chromosphere. Still they show a variety of morphology, ranging
from dot-like to loop-like with propagating disturbances, small jets,
or miniature flux-rope eruptions. This variety of the phenomenology on
the smallest resolvable scales points towards a conclusion that there is
not one single process that can drive small-scale brightenings. This is
supported by studies relating the EUV brightenings to the underlying
magnetic field: a part of the cases shows a relation to changes of
the surface magnetic flux, while in other cases it is very hard to
find any connection to the magnetic field. In the quiet Sun these
small brightenings are mostly found at the edges of bright elements
of the chromospheric network, which they have in common with another
class of brightenings seen at lower temperatures in the transition
region, namely explosive events. These transients have been proposed
as being due to reconnection, have a similar size and lifetime as
the small brightenings, but mostly seem to lack a component at high
temperatures. It might well be that these explosive events are related
to one particular type of the campfires, e.g. the jet-types, but that
remains to be studied. Modelling work on small brightenings is not
yet very abundant. One 3D MHD model of the quiet Sun shows coronal
brightenings that share properties with the observations. Here the
brightening is caused (mostly) by component reconnection, but also
one case of a twisted flux rope is found in the simulation data,
other (future) models most likely will reveal that also different
processes could produce similar brightenings. Because of timing and
the mission profile, so far remote sensing observations have been
taken mostly in regions of quiet Sun. With the perihelion in March
2022 Solar Orbiter will not only be closer than 0.3 AU from the Sun,
providing coronal observations at even higher resolution than before,
but also active region observations are planned. These might extend
the zoo of the small-scale coronal transients to small features in
active regions that might show properties similar or different from
the quiet Sun coronal brightenings. Either way, this will provide a
challenge for our understanding of the small-scale corona.
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Title: Parallel Plasma Loops and the Energization of the Solar Corona
Authors: Peter, Hardi; Chitta, Lakshmi Pradeep; Chen, Feng; Pontin,
David I.; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.;
Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain,
Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.;
Testa, Paola; Tiwari, Sanjiv K.; Walsh, Robert W.; Warren, Harry P.
2022ApJ...933..153P Altcode: 2022arXiv220515919P
The outer atmosphere of the Sun is composed of plasma heated to
temperatures well in excess of the visible surface. We investigate
short cool and warm (<1 MK) loops seen in the core of an active
region to address the role of field-line braiding in energizing these
structures. We report observations from the High-resolution Coronal
imager (Hi-C) that have been acquired in a coordinated campaign with
the Interface Region Imaging Spectrograph (IRIS). In the core of the
active region, the 172 Å band of Hi-C and the 1400 Å channel of IRIS
show plasma loops at different temperatures that run in parallel. There
is a small but detectable spatial offset of less than 1″ between
the loops seen in the two bands. Most importantly, we do not see
observational signatures that these loops might be twisted around each
other. Considering the scenario of magnetic braiding, our observations
of parallel loops imply that the stresses put into the magnetic field
have to relax while the braiding is applied: the magnetic field never
reaches a highly braided state on these length scales comparable to
the separation of the loops. This supports recent numerical 3D models
of loop braiding in which the effective dissipation is sufficiently
large that it keeps the magnetic field from getting highly twisted
within a loop.
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Title: Automatic detection of small-scale EUV brightenings observed
by the Solar Orbiter/EUI
Authors: Alipour, N.; Safari, H.; Verbeeck, C.; Berghmans, D.;
Auchère, F.; Chitta, L. P.; Antolin, P.; Barczynski, K.; Buchlin,
É.; Aznar Cuadrado, R.; Dolla, L.; Georgoulis, M. K.; Gissot, S.;
Harra, L.; Katsiyannis, A. C.; Long, D. M.; Mandal, S.; Parenti,
S.; Podladchikova, O.; Petrova, E.; Soubrié, É.; Schühle, U.;
Schwanitz, C.; Teriaca, L.; West, M. J.; Zhukov, A. N.
2022A&A...663A.128A Altcode: 2022arXiv220404027A
Context. Accurate detections of frequent small-scale extreme ultraviolet
(EUV) brightenings are essential to the investigation of the physical
processes heating the corona. <BR /> Aims: We detected small-scale
brightenings, termed campfires, using their morphological and
intensity structures as observed in coronal EUV imaging observations
for statistical analysis. <BR /> Methods: We applied a method based
on Zernike moments and a support vector machine (SVM) classifier
to automatically identify and track campfires observed by Solar
Orbiter/Extreme Ultraviolet Imager (EUI) and Solar Dynamics Observatory
(SDO)/Atmospheric Imaging Assembly (AIA). <BR /> Results: This method
detected 8678 campfires (with length scales between 400 km and 4000 km)
from a sequence of 50 High Resolution EUV telescope (HRI<SUB>EUV</SUB>)
174 Å images. From 21 near co-temporal AIA images covering the same
field of view as EUI, we found 1131 campfires, 58% of which were
also detected in HRI<SUB>EUV</SUB> images. In contrast, about 16%
of campfires recognized in HRI<SUB>EUV</SUB> were detected by AIA. We
obtain a campfire birthrate of 2 × 10<SUP>−16</SUP> m<SUP>−2</SUP>
s<SUP>−1</SUP>. About 40% of campfires show a duration longer than 5
s, having been observed in at least two HRI<SUB>EUV</SUB> images. We
find that 27% of campfires were found in coronal bright points and
the remaining 73% have occurred out of coronal bright points. We
detected 23 EUI campfires with a duration greater than 245 s. We found
that about 80% of campfires are formed at supergranular boundaries,
and the features with the highest total intensities are generated at
network junctions and intense H I Lyman-α emission regions observed
by EUI/HRI<SUB>Lya</SUB>. The probability distribution functions for
the total intensity, peak intensity, and projected area of campfires
follow a power law behavior with absolute indices between 2 and 3. This
self-similar behavior is a possible signature of self-organization,
or even self-organized criticality, in the campfire formation
process. <P />Supplementary material (S1-S3) is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243257/olm">https://www.aanda.org</A>
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Title: Observation of Magnetic Switchback in the Solar Corona
Authors: Telloni, Daniele; Zank, Gary P.; Stangalini, Marco;
Downs, Cooper; Liang, Haoming; Nakanotani, Masaru; Andretta,
Vincenzo; Antonucci, Ester; Sorriso-Valvo, Luca; Adhikari, Laxman;
Zhao, Lingling; Marino, Raffaele; Susino, Roberto; Grimani, Catia;
Fabi, Michele; D'Amicis, Raffaella; Perrone, Denise; Bruno, Roberto;
Carbone, Francesco; Mancuso, Salvatore; Romoli, Marco; Da Deppo, Vania;
Fineschi, Silvano; Heinzel, Petr; Moses, John D.; Naletto, Giampiero;
Nicolini, Gianalfredo; Spadaro, Daniele; Teriaca, Luca; Frassati,
Federica; Jerse, Giovanna; Landini, Federico; Pancrazzi, Maurizio;
Russano, Giuliana; Sasso, Clementina; Berghmans, David; Auchère,
Frédéric; Aznar Cuadrado, Regina; Chitta, Lakshmi P.; Harra, Louise;
Kraaikamp, Emil; Long, David M.; Mandal, Sudip; Parenti, Susanna;
Pelouze, Gabriel; Peter, Hardi; Rodriguez, Luciano; Schühle, Udo;
Schwanitz, Conrad; Smith, Phil J.; Verbeeck, Cis; Zhukov, Andrei N.
2022arXiv220603090T Altcode:
Switchbacks are sudden, large radial deflections of the solar wind
magnetic field, widely revealed in interplanetary space by the Parker
Solar Probe. The switchbacks' formation mechanism and sources are still
unresolved, although candidate mechanisms include Alfvénic turbulence,
shear-driven Kelvin-Helmholtz instabilities, interchange reconnection,
and geometrical effects related to the Parker spiral. This Letter
presents observations from the Metis coronagraph onboard Solar Orbiter
of a single large propagating S-shaped vortex, interpreted as first
evidence of a switchback in the solar corona. It originated above
an active region with the related loop system bounded by open-field
regions to the East and West. Observations, modeling, and theory provide
strong arguments in favor of the interchange reconnection origin of
switchbacks. Metis measurements suggest that the initiation of the
switchback may also be an indicator of the origin of slow solar wind.
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Title: The magnetic drivers of campfires seen by the Polarimetric
and Helioseismic Imager (PHI) on Solar Orbiter
Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.;
Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert,
K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco
Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez
Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.;
Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis
Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti,
D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina,
A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.;
Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.;
Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai,
E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.;
Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans,
D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.
2022A&A...660A.143K Altcode: 2022arXiv220213859K
Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter
(SO) spacecraft observed small extreme ultraviolet (EUV) bursts,
termed campfires, that have been proposed to be brightenings near the
apexes of low-lying loops in the quiet-Sun atmosphere. The underlying
magnetic processes driving these campfires are not understood. <BR
/> Aims: During the cruise phase of SO and at a distance of 0.523
AU from the Sun, the Polarimetric and Helioseismic Imager on Solar
Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI,
offering the possibility to investigate the surface magnetic field
dynamics underlying campfires at a spatial resolution of about 380
km. <BR /> Methods: We used co-spatial and co-temporal data of the
quiet-Sun network at disc centre acquired with the High Resolution
Imager of SO/EUI at 17.4 nm (HRI<SUB>EUV</SUB>, cadence 2 s) and the
High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5
min). Campfires that are within the SO/PHI−SO/EUI common field
of view were isolated and categorised according to the underlying
magnetic activity. <BR /> Results: In 71% of the 38 isolated events,
campfires are confined between bipolar magnetic features, which seem to
exhibit signatures of magnetic flux cancellation. The flux cancellation
occurs either between the two main footpoints, or between one of the
footpoints of the loop housing the campfire and a nearby opposite
polarity patch. In one particularly clear-cut case, we detected the
emergence of a small-scale magnetic loop in the internetwork followed
soon afterwards by a campfire brightening adjacent to the location
of the linear polarisation signal in the photosphere, that is to
say near where the apex of the emerging loop lays. The rest of the
events were observed over small scattered magnetic features, which
could not be identified as magnetic footpoints of the campfire hosting
loops. <BR /> Conclusions: The majority of campfires could be driven
by magnetic reconnection triggered at the footpoints, similar to the
physical processes occurring in the burst-like EUV events discussed
in the literature. About a quarter of all analysed campfires, however,
are not associated to such magnetic activity in the photosphere, which
implies that other heating mechanisms are energising these small-scale
EUV brightenings.
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Title: Spectroscopic observation of a transition region network jet
Authors: Gorman, J.; Chitta, L. P.; Peter, H.
2022A&A...660A.116G Altcode: 2022arXiv220211375G
<BR /> Aims: Ubiquitous transition region (TR) network jets are
considered to be substantial sources of mass and energy to the corona
and solar wind. We conduct a case study of a network jet to better
understand the nature of mass flows along its length and the energetics
involved in its launch. <BR /> Methods: We present an observation of a
jet with the Interface Region Imaging Spectrograph (IRIS), while also
using data from the Solar Dynamics Observatory (SDO) to provide further
context. The jet was located within a coronal hole close to the disk
center. <BR /> Results: We find that a blueshifted secondary component
of TR emission is associated with the jet and is persistent along its
spire. This component exhibits upward speeds of approximately 20-70 km
s<SUP>−1</SUP> and shows enhanced line broadening. However, plasma
associated with the jet in the upper chromosphere shows downflows of
5-10 km s<SUP>−1</SUP>. Finally, the jet emanates from a seemingly
unipolar magnetic footpoint. <BR /> Conclusions: While a definitive
magnetic driver is not discernible for this event, we infer that
the energy driving the network jet is deposited at the top of the
chromosphere, indicating that TR network jets are driven from the
mid-atmospheric layers of the Sun. The energy flux associated with the
line broadening indicates that the jet could be powered all the way into
the solar wind. <P />Movie associated with Fig. 3 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142995/olm">https://www.aanda.org</A>
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Title: Coronal condensation as the source of transition-region
supersonic downflows above a sunspot
Authors: Chen, Hechao; Tian, Hui; Li, Leping; Peter, Hardi; Chitta,
Lakshmi Pradeep; Hou, Zhenyong
2022A&A...659A.107C Altcode: 2021arXiv211201354C
Context. Plasma loops or plumes rooted in sunspot umbrae often harbor
downflows with speeds of 100 km s<SUP>−1</SUP>. These downflows
are supersonic at transition region temperatures of ∼0.1 MK. The
source of these flows is not well understood. <BR /> Aims: We aim
to investigate the source of sunspot supersonic downflows (SSDs)
in active region 12740 using simultaneous spectroscopic and imaging
observations. <BR /> Methods: We identified SSD events from multiple
raster scans of a sunspot by the Interface Region Imaging Spectrograph,
and we calculated the electron densities, mass fluxes, and velocities
of these SSDs. The extreme-ultraviolet (EUV) images provided by the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and
the EUV Imager onboard the Solar Terrestrial Relations Observatory were
employed to investigate the origin of these SSDs and their associated
coronal rain. <BR /> Results: Almost all the identified SSDs appear
at the footpoints of sunspot plumes and are temporally associated
with the appearance of chromospheric bright dots inside the sunspot
umbra. Dual-perspective EUV imaging observations reveal a large-scale
closed magnetic loop system spanning the sunspot region and a remote
region. We observed that the SSDs are caused by repeated coronal
rain that forms and flows along these closed magnetic loops toward the
sunspot. One episode of coronal rain clearly indicates that reconnection
near a coronal X-shaped structure first leads to the formation of
a magnetic dip. Subsequently, hot coronal plasma catastrophically
cools from ∼2 MK in the dip region via thermal instability. This
results in the formation of a transient prominence in the dip, from
which the cool gas mostly slides into the sunspot along inclined
magnetic fields under the gravity. This drainage process manifests
as a continuous rain flow, which lasts for ∼2 h and concurrently
results in a nearly steady SSD event. The total mass of condensation
(1.3 × 10<SUP>14</SUP> g) and condensation rate (1.5 × 10<SUP>10</SUP>
g s<SUP>−1</SUP>) in the dip region were found to be sufficient to
sustain this long-lived SSD event, which has a mass transport rate of
0.7 − 1.2 × 10<SUP>10</SUP> g s<SUP>−1</SUP>. <BR /> Conclusions:
Our results demonstrate that coronal condensation in magnetic dips
can result in the quasi-steady sunspot supersonic downflows. <P
/>Movies associated to Figs. 1, 3, 6, 7, and 11 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142093/olm">https://www.aanda.org</A>
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Title: A solar coronal loop in a box: Energy generation and heating
Authors: Breu, C.; Peter, H.; Cameron, R.; Solanki, S. K.; Przybylski,
D.; Rempel, M.; Chitta, L. P.
2022A&A...658A..45B Altcode: 2021arXiv211211549B
Context. Coronal loops are the basic building block of the upper solar
atmosphere as seen in the extreme UV and X-rays. Comprehending how
these are energized, structured, and evolve is key to understanding
stellar coronae. <BR /> Aims: Here we investigate how the energy
to heat the loop is generated by photospheric magneto-convection,
transported into the upper atmosphere, and how the internal
structure of a coronal magnetic loop forms. <BR /> Methods: In a 3D
magnetohydrodynamics model, we study an isolated coronal loop rooted
with both footpoints in a shallow layer within the convection zone
using the MURaM code. To resolve its internal structure, we limited
the computational domain to a rectangular box containing a single
coronal loop as a straightened magnetic flux tube. Field-aligned heat
conduction, gray radiative transfer in the photosphere and chromosphere,
and optically thin radiative losses in the corona were taken into
account. The footpoints were allowed to interact self-consistently
with the granulation surrounding them. <BR /> Results: The loop is
heated by a Poynting flux that is self-consistently generated through
small-scale motions within individual magnetic concentrations in
the photosphere. Turbulence develops in the upper layers of the
atmosphere as a response to the footpoint motions. We see little
sign of heating by large-scale braiding of magnetic flux tubes
from different photospheric concentrations at a given footpoint. The
synthesized emission, as it would be observed by the Atmospheric Imaging
Assembly or the X-Ray Telescope, reveals transient bright strands that
form in response to the heating events. Overall, our model roughly
reproduces the properties and evolution of the plasma as observed
within (the substructures of) coronal loops. <BR /> Conclusions:
With this model we can build a coherent picture of how the energy
flux to heat the upper atmosphere is generated near the solar surface
and how this process drives and governs the heating and dynamics of
a coronal loop. <P />Movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141451/olm">https://www.aanda.org</A>
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Title: Tracing the Drivers of Slow Solar Wind in the Middle Corona
Authors: Chitta, Lakshmi Pradeep; Seaton, Daniel; Downs, Cooper;
DeForest, Craig; Higginson, Aleida
2021AGUFMSH24C..01C Altcode:
Compared to the so-called fast solar wind that originates from polar
coronal holes during the solar minimum, low-latitude wind streams
generally have lower speeds ( 500 km/s). These slow solar wind streams
closer to the ecliptic plane are characterized by their high spatial
structuring, temporal variability, and coronal compositions. The
magnetic driver responsible for the origin of this slow solar wind and
its characteristics, however, is not well understood. Using coronal
observations from the 2018 off-pointing campaign of the GOES Solar
Ultraviolet Imager (SUVI) as well as images from SOHO/LASCO, we found
signatures of solar wind streams driven by magnetic reconnection
in the highly structured middle corona. In particular, elongated
coronal loops in the middle corona over a coronal-hole-active-region
complex are observed to reconnect and retract while in the process,
some plasma is propelled away from the Sun as streams or blobs. Using
STEREO observations we found that similar streams forming over the same
complex escape into the heliosphere. Our observations of reconnection
effects, however, are limited by the time resolution of the SUVI data
in that they clearly represent the tail of a distribution of event
durations, with a strong likelihood that many more events remain
unresolved in the current data. In this talk, we will present these
new results and discuss them in the context of models of slow solar
wind sources and drivers. our findings emphasize the key necessity of
having routine extreme ultraviolet observations of the middle coronal
processes from ecliptic and polar vantage points, along with extended
coronal diagnostics to develop better understanding of the heliospheric
impact of Sun's activity.
---------------------------------------------------------
Title: Campfires observed by EUI: What have we learned so far?
Authors: Berghmans, David; Auchere, F.; Zhukov, Andrei; Mierla,
Marilena; Chen, Yajie; Peter, Hardi; Panesar, Navdeep; Chitta, Lakshmi
Pradeep; Antolin, Patrick; Aznar Cuadrado, Regina; Tian, Hui; Hou,
Zhenyong; Podladchikova, Olena
2021AGUFMSH21A..02B Altcode:
Since its very first light images of the corona, the EUI/HRIEUV
telescope onboard Solar Orbiter has observed small localised
brightenings in the Quiet Sun. These small localised brightenings,
have become known as campfires, and are observed with length scales
between 400 km and 4000 km and durations between 10 sec and 200
sec. The smallest and weakest of these HRIEUV brightenings have
not been previously observed. Simultaneous observations from the
EUI High-resolution Lyman- telescope (HRILYA) do not show localised
brightening events, but the locations of the HRIEUV events clearly
correspond to the chromospheric network. Comparisons with simultaneous
AIA images shows that most events can also be identified in the
17.1 nm, 19.3 nm, 21.1 nm, and 30.4 nm pass-bands of AIA, although
they appear weaker and blurred. Some of the larger campfires have
the appearance of small interacting loops with the brightening
expanding from the contact point of the loops. Our differential
emission measure (DEM) analysis indicated coronal temperatures. We
determined the height for a few of these campfires to be between 1
and 5 Mm above the photosphere. We interpret these events as a new
extension to the flare-microflare-nanoflare family. Given their low
height, the EUI campfires could stand as a new element of the fine
structure of the transition region-low corona, that is, as apexes
of small-scale loops that undergo internal heating all the way up to
coronal temperatures. 3D MHD simulations with the MURaM code revealed
brightenings that are in many ways similar to the campfires by EUI. The
brightenings in the simulations suggest that campfires are triggered by
component reconnection inside flux bundles rather than flux emergence
or cancellation. Nevertheless, some of the observed campfires can
be clearly linked to flux cancellation events and, intriguingly,
are preceded by an erupting cool plasma structure. Analysis of the
dynamics of campfires revealed that some have the appearance of coronal
microjets, the smallest coronal jets observed in the quiet Sun. The
HRIEUV images also reveal transient jets on a somewhat bigger scale
with repeated outflows on the order of 100 km s1. In this paper we
will provide an overview of the campfire related phenomena that EUI
has observed and discuss the possible relevance for coronal heating.
---------------------------------------------------------
Title: Propagating brightenings in small loop-like structures in
the quiet-Sun corona: Observations from Solar Orbiter/EUI
Authors: Mandal, Sudip; Peter, Hardi; Chitta, Lakshmi Pradeep;
Solanki, Sami K.; Aznar Cuadrado, Regina; Teriaca, Luca; Schühle,
Udo; Berghmans, David; Auchère, Frédéric
2021A&A...656L..16M Altcode: 2021arXiv211108106M
Brightenings observed in solar extreme-ultraviolet images are generally
interpreted as signatures of micro- or nanoflares occurring in the
transition region or at coronal temperatures. Recent observations
with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have
revealed the smallest of such brightenings (called campfires) in the
quiet-Sun corona. Analyzing EUI 174 Å data obtained at a resolution
of about 400 km on the Sun with a cadence of 5 s on 30 May 2020,
we report here a number of cases in which these campfires exhibit
propagating signatures along their apparently small (3-5 Mm) loop-like
structures. The measured propagation speeds are generally between 25
km s<SUP>−1</SUP> and 60 km s<SUP>−1</SUP>. If the loop plasma is
assumed to be at a million Kelvin, these apparent motions would be
slower than the local sound speed. Furthermore, these brightenings
exhibit nontrivial propagation characteristics such as bifurcation,
merging, reflection, and repeated plasma ejections. We suggest that
these features are manifestations of the internal dynamics of these
small-scale magnetic structures and could provide important insights
into the dynamic response (∼40 s) of the loop plasma to the heating
events and also into the locations of the heating events themselves. <P
/>Movies associated to Figs 2-5, A.1, and B.1 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142041/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Capturing transient plasma flows and jets in the solar corona
Authors: Chitta, L. P.; Solanki, S. K.; Peter, H.; Aznar Cuadrado,
R.; Teriaca, L.; Schühle, U.; Auchère, F.; Berghmans, D.; Kraaikamp,
E.; Gissot, S.; Verbeeck, C.
2021A&A...656L..13C Altcode: 2021arXiv210915106C
Intensity bursts in ultraviolet (UV) to X-ray wavelengths and plasma
jets are typical signatures of magnetic reconnection and the associated
impulsive heating of the solar atmospheric plasma. To gain new insights
into the process, high-cadence observations are required to capture
the rapid response of plasma to magnetic reconnection as well as the
highly dynamic evolution of jets. Here, we report the first 2 s cadence
extreme-UV observations recorded by the 174 Å High Resolution Imager of
the Extreme Ultraviolet Imager on board the Solar Orbiter mission. These
observations, covering a quiet-Sun coronal region, reveal the onset
signatures of magnetic reconnection as localized heating events. These
localized sources then exhibit repeated plasma eruptions or jet
activity. Our observations show that this spatial morphological change
from localized sources to jet activity could occur rapidly on timescales
of about 20 s. The jets themselves are intermittent and are produced
from the source region on timescales of about 20 s. In the initial
phases of these events, plasma jets are observed to exhibit speeds,
as inferred from propagating intensity disturbances, in the range of
100 km s<SUP>−1</SUP> to 150 km s<SUP>−1</SUP>. These jets then
propagate to lengths of about 5 Mm. We discuss examples of bidirectional
and unidirectional jet activity observed to have been initiated from
the initially localized bursts in the corona. The transient nature
of coronal bursts and the associated plasma flows or jets along with
their dynamics could provide a benchmark for magnetic reconnection
models of coronal bursts and jets. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141683/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: First observations from the SPICE EUV spectrometer on Solar
Orbiter
Authors: Fludra, A.; Caldwell, M.; Giunta, A.; Grundy, T.; Guest,
S.; Leeks, S.; Sidher, S.; Auchère, F.; Carlsson, M.; Hassler, D.;
Peter, H.; Aznar Cuadrado, R.; Buchlin, É.; Caminade, S.; DeForest,
C.; Fredvik, T.; Haberreiter, M.; Harra, L.; Janvier, M.; Kucera, T.;
Müller, D.; Parenti, S.; Schmutz, W.; Schühle, U.; Solanki, S. K.;
Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R.;
Chitta, L. P.
2021A&A...656A..38F Altcode: 2021arXiv211011252F
<BR /> Aims: We present first science observations taken during the
commissioning activities of the Spectral Imaging of the Coronal
Environment (SPICE) instrument on the ESA/NASA Solar Orbiter
mission. SPICE is a high-resolution imaging spectrometer operating at
extreme ultraviolet (EUV) wavelengths. In this paper we illustrate
the possible types of observations to give prospective users a
better understanding of the science capabilities of SPICE. <BR />
Methods: We have reviewed the data obtained by SPICE between April
and June 2020 and selected representative results obtained with
different slits and a range of exposure times between 5 s and 180
s. Standard instrumental corrections have been applied to the raw
data. <BR /> Results: The paper discusses the first observations
of the Sun on different targets and presents an example of the full
spectra from the quiet Sun, identifying over 40 spectral lines from
neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur,
magnesium, and iron. These lines cover the temperature range between
20 000 K and 1 million K (10 MK in flares), providing slices of the
Sun's atmosphere in narrow temperature intervals. We provide a list
of count rates for the 23 brightest spectral lines. We show examples
of raster images of the quiet Sun in several strong transition region
lines, where we have found unusually bright, compact structures in the
quiet Sun network, with extreme intensities up to 25 times greater
than the average intensity across the image. The lifetimes of these
structures can exceed 2.5 hours. We identify them as a transition
region signature of coronal bright points and compare their areas and
intensity enhancements. We also show the first above-limb measurements
with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and
far off limb measurements in the equatorial plane in Mg IX, Ne VIII,
and O VI lines. We discuss the potential to use abundance diagnostics
methods to study the variability of the elemental composition that can
be compared with in situ measurements to help confirm the magnetic
connection between the spacecraft location and the Sun's surface,
and locate the sources of the solar wind. <BR /> Conclusions: The
SPICE instrument successfully performs measurements of EUV spectra
and raster images that will make vital contributions to the scientific
success of the Solar Orbiter mission.
---------------------------------------------------------
Title: An Analysis of Spikes in Atmospheric Imaging Assembly
(AIA) Data
Authors: Young, Peter R.; Viall, Nicholeen M.; Kirk, Michael S.;
Mason, Emily I.; Chitta, Lakshmi Pradeep
2021SoPh..296..181Y Altcode: 2021arXiv210802624Y
The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) returns high-resolution images of the solar atmosphere
in seven extreme ultraviolet (EUV) wavelength channels. The images
are processed on the ground to remove intensity spikes arising from
energetic particles hitting the instrument, and the despiked images
are provided to the community. In this article, a three-hour series of
images from the 171 Å channel obtained on 28 February 2017 was studied
to investigate how often the despiking algorithm gave false positives
caused by compact brightenings in the solar atmosphere. The latter
were identified through spikes appearing in the same detector pixel
for three consecutive frames. 1096 examples were found from the 900
image frames. These "three-spikes" were assigned to 126 dynamic solar
features, and it is estimated that the three-spike method identifies
19% of the total number of features affected by despiking. For any
ten-minute sequence of AIA 171 Å images there are around 37 solar
features that have their intensity modified by despiking. The features
are found in active regions, quiet Sun, and coronal holes and, in
relation to solar surface area, there is a greater proportion within
coronal holes. In 96% of the cases, the despiked structure is a compact
brightening with a size of two arcsec or less, and the remaining 4%
have narrow, elongated structures. By applying an EUV burst detection
algorithm, we found that 96% of the events could be classified as EUV
bursts. None of the spike events are rendered invisible by the AIA
processing pipeline, but the total intensity over an event's lifetime
can be reduced by up to 67%. Users are recommended to always restore
the original intensities in AIA data when studying short-lived or
rapidly evolving features that exhibit fine-scale structure.
---------------------------------------------------------
Title: Revisiting the formation mechanism for coronal rain from
previous studies
Authors: Li, Le-Ping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
Hong-Qiang
2021RAA....21..255L Altcode: 2021arXiv210701339L
Solar coronal rain is classified generally into two categories:
flare-driven and quiescent coronal rain. Th latter is observed to form
along both closed and open magnetic field structures. Recently, we
proposed that some of the quiescent coronal rain events, detected in the
transition region and chromospheric diagnostics, along loop-like paths
could be explained by the formation mechanism for quiescent coronal
rain facilitated by interchange magnetic reconnection between open and
closed field lines. In this study, we revisited 38 coronal rain reports
from the literature. From theseearlier works, we picked 15 quiescent
coronal rain events out of the solar limb, mostly suggested to occur in
active region closed loops due to thermal nonequilibrium, to scrutinize
their formation mechanism. Employing the extreme ultraviolet images
and line-of-sight magnetograms, the evolution of the quiescent coronal
rain events and their magnetic fields and context coronal structures
is examined. We find that six, comprising 40%, of the 15 quiescent
coronal rain events could be totally or partially interpreted by the
formation mechanism for quiescent coronal rain along open structures
facilitated by interchange reconnection. The results suggest that the
quiescent coronal rain facilitated by interchange reconnection between
open and closed field lines deserves more attention.
---------------------------------------------------------
Title: Formation of a Solar Filament by Magnetic Reconnection and
Coronal Condensation
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
Hongqiang
2021ApJ...919L..21L Altcode: 2021arXiv210905669L
In solar filament formation mechanisms, magnetic reconnection
between two sets of sheared arcades forms helical structures of the
filament with numerous magnetic dips, and cooling and condensation
of plasma trapped inside the helical structures supply mass to
the filament. Although each of these processes, namely, magnetic
reconnection and coronal condensation have been separately reported,
observations that show the whole process of filament formation are
rare. In this Letter, we present the formation of a sigmoid via
reconnection between two sets of coronal loops, and the subsequent
formation of a filament through cooling and condensation of plasma
inside the newly formed sigmoid. On 2014 August 27, a set of loops
in the active region 12151 reconnected with another set of loops
that are located to the east. A longer twisted sigmoidal structure
and a set of shorter lower-lying loops then formed. The observations
coincide well with the tether-cutting model. The newly formed sigmoid
remains stable and does not erupt as a coronal mass ejection. From the
eastern endpoint, signatures of injection of material into the sigmoid
(as brightenings) are detected, which closely outline the features of
increasing emission measure at these locations. This may indicate the
chromospheric evaporation caused by reconnection, supplying heated
plasma into the sigmoid. In the sigmoid, thermal instability occurs,
and rapid cooling and condensation of plasma take place, forming a
filament. The condensations then flow bidirectionally to the filament
endpoints. Our results provide a clear observational evidence of the
filament formation via magnetic reconnection and coronal condensation.
---------------------------------------------------------
Title: Magnetic imaging of the outer solar atmosphere (MImOSA)
Authors: Peter, H.; Ballester, E. Alsina; Andretta, V.; Auchère, F.;
Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.;
Chitta, L. P.; Dalmasse, K.; Alemán, T. del Pino; Feller, A.; Froment,
C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski,
D.; Solanki, S. K.; Štěpán, J.; Teriaca, L.; Bueno, J. Trujillo
2021ExA...tmp...95P Altcode:
The magnetic activity of the Sun directly impacts the Earth and human
life. Likewise, other stars will have an impact on the habitability of
planets orbiting these host stars. Although the magnetic field at the
surface of the Sun is reasonably well characterised by observations,
the information on the magnetic field in the higher atmospheric layers
is mainly indirect. This lack of information hampers our progress in
understanding solar magnetic activity. Overcoming this limitation would
allow us to address four paramount long-standing questions: (1) How
does the magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic field
structure, drive and interact with the plasma in the chromosphere and
upper atmosphere? (3) How does the magnetic field destabilise the outer
solar atmosphere and thus affect the interplanetary environment? (4)
How do magnetic processes accelerate particles to high energies? New
ground-breaking observations are needed to address these science
questions. We suggest a suite of three instruments that far exceed
current capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-IR
telescope of the 1-3 m class aimed mainly to measure the magnetic
field in the chromosphere by combining high spatial resolution
and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is
designed to measure the large-scale magnetic field in the corona with
an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter
based on a 30 cm telescope that combines high throughput in the
extreme UV with polarimetry to connect the magnetic measurements
of the other two instruments. Placed in a near-Earth orbit, the data
downlink would be maximised, while a location at L4 or L5 would provide
stereoscopic observations of the Sun in combination with Earth-based
observatories. This mission to measure the magnetic field will finally
unlock the driver of the dynamics in the outer solar atmosphere and
thereby will greatly advance our understanding of the Sun and the
heliosphere.
---------------------------------------------------------
Title: Characterizing a "Solar FRB"
Authors: Hudson, H.; Briggs, M.; Chitta, L.; Fletcher, L.; Gary, D.;
Monstein, C.; Nimmo, K.; Saint-Hilaire, P.; White, S.
2021AAS...23812716H Altcode:
A remarkable solar microwave (1.4 GHz) burst,
SOL2019-05-06T17:47:35.385, has been reported by the STARE2 fast cosmic
transient survey (Bochenek et al., 2020). Its behavior resembles
that of the Fast Radio Burst (FRB) extragalactic events in having a
relatively broad spectral bandwidth and brief (19-msec) duration. It
also had no measureable dispersion. The associated flare, GOES class
C1, had a relatively hard X-ray spectrum as observed by Fermi/GBM,
but no temporal association at the msec time scale suggested by the
microwaves. Although msec variability in the microwave domain has
been known to solar radio astronomy since the 1970s, the brightness
and isolation of this event (both spatial and temporal) suggests
novelty. Accordingly we survey the available correlative data from
many sources and discuss possible interpretations in terms of type
III-like behavior, electron cyclotron masering, and gyrosynchrotron
emission. We note that the radio data (e-Callisto and EOVSA) revealed
abundant type III activity in the vicinity, and the related flares
as observed by GOES had exceptionally short time scales, suggesting
burst origins in the lower solar atmosphere.
---------------------------------------------------------
Title: Dynamic Evolution Of A Solar Flare Current Sheet
Authors: Chitta, L.; Priest, E. R.; Cheng, X.
2021AAS...23830301C Altcode:
Current sheets play a key role in solar flares as they are the locations
where magnetic energy is liberated through reconnection and is converted
to other forms. Yet, their formation and evolution during the impulsive
phase of a flare remain elusive. In this talk, we will report new
observations of a current-sheet formation and subsequent evolution
in the early stages of a solar flare. In particular, we will present
multi-phase evolution of a dynamic current sheet from its formation
to quasi-stable evolution and disruption. Implications for the onset
and evolution of reconnection will be discussed.
---------------------------------------------------------
Title: Coronal loops in a box: 3D models of their internal structure,
dynamics and heating
Authors: Breu, C. A.; Peter, H.; Cameron, R.; Solanki, S.; Przybylski,
D.; Chitta, L.
2021AAS...23810606B Altcode:
The corona of the Sun, and probably also of other stars, is built
up by loops defined through the magnetic field. They vividly appear
in solar observations in the extreme UV and X-rays. High-resolution
observations show individual strands with diameters down to a few 100
km, and so far it remains open what defines these strands, in particular
their width, and where the energy to heat them is generated. <P />The
aim of our study is to understand how the magnetic field couples the
different layers of the solar atmosphere, how the energy generated
by magnetoconvection is transported into the upper atmosphere and
dissipated, and how this process determines the scales of observed
bright strands in the loop. <P />To this end, we conduct 3D resistive
MHD simulations with the MURaM code. We include the effects of heat
conduction, radiative transfer and optically thin radiative losses.We
study an isolated coronal loop that is rooted with both footpoints
in a shallow convection zone layer. To properly resolve the internal
structure of the loop while limiting the size of the computational box,
the coronal loop is modelled as a straightened magnetic flux tube. By
including part of the convection zone, we drive the evolution of
the corona self-consistently by magnetoconvection. <P />We find that
the energy injected into the loop is generated by internal coherent
motions within strong magnetic elements. The resulting Poynting
flux is channelled into the loop in vortex tubes forming a magnetic
connection between the photosphere and corona, where it is dissipated
and heats the upper atmosphere. <P />The coronal emission as it would
be observed in solar extreme UV or X-ray observations, e.g. with AIA
or XRT, shows transient bright strands.The widths of these strands are
consistent with observations. From our model we find that the width
of the strands is governed by the size of the individual photospheric
magnetic field concentrations where the field line through these strands
are rooted. Essentially, each coronal strand is rooted in a single
magnetic patch in the photosphere, and the energy to heat the strand is
generated by internal motions within this magnetic concentration. <P
/>With this model we can build a coherent picture of how energy and
matter are transported into the upper solar atmosphere and how these
processes structure the interior of coronal loops.
---------------------------------------------------------
Title: From Formation to Disruption: Observing the Multiphase
Evolution of a Solar Flare Current Sheet
Authors: Chitta, L. P.; Priest, E. R.; Cheng, X.
2021ApJ...911..133C Altcode: 2021arXiv210302352C
A current sheet, where magnetic energy is liberated through
reconnection and converted to other forms, is thought to play
the central role in solar flares, the most intense explosions in
the heliosphere. However, the evolution of a current sheet and its
subsequent role in flare-related phenomena such as particle acceleration
is poorly understood. Here we report observations obtained with
NASA's Solar Dynamics Observatory that reveal a multiphase evolution
of a current sheet in the early stages of a solar flare, from its
formation to quasi-stable evolution and disruption. Our observations
have implications for the understanding of the onset and evolution of
reconnection in the early stages of eruptive solar flares.
---------------------------------------------------------
Title: On-disk Solar Coronal Condensations Facilitated by Magnetic
Reconnection between Open and Closed Magnetic Structures
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
Hongqiang
2021ApJ...910...82L Altcode: 2021arXiv210204605L
Coronal condensation and rain are a crucial part of the mass cycle
between the corona and chromosphere. In some cases, condensation and
subsequent rain originate in the magnetic dips formed during magnetic
reconnection. This provides a new and alternative formation mechanism
for coronal rain. Until now, only off-limb, rather than on-disk,
condensation events during reconnection have been reported. In
this paper, employing extreme-ultraviolet (EUV) images of the
Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics
Observatory (SDO), we investigate the condensations facilitated by
reconnection from 2011 July 14-15, when STEREO was in quadrature with
respect to the Sun-Earth line. Above the limb, in STEREO/EUV Imager
(EUVI) 171 Å images, higher-lying open structures move downward,
reconnect with the lower-lying closed loops, and form dips. Two
sets of newly reconnected structures then form. In the dips, bright
condensations occur in the EUVI 304 Å images repeatedly, which
then flow downward to the surface. In the on-disk observations by
SDO/Atmospheric Imaging Assembly (AIA) in the 171 Å channel, these
magnetic structures are difficult to identify. Dark condensations
appear in the AIA 304 Å images, and then move to the surface as
on-disk coronal rain. The cooling and condensation of coronal plasma
is revealed by the EUV light curves. If only the on-disk observations
were be available, the relation between the condensations and
reconnection, shown clearly by the off-limb observations, could not
be identified. Thus, we suggest that some on-disk condensation events
seen in transition region and chromospheric lines may be facilitated
by reconnection.
---------------------------------------------------------
Title: Extreme-ultraviolet bursts and nanoflares in the quiet-Sun
transition region and corona
Authors: Chitta, L. P.; Peter, H.; Young, P. R.
2021A&A...647A.159C Altcode: 2021arXiv210200730C
The quiet solar corona consists of myriads of loop-like features, with
magnetic fields originating from network and internetwork regions on
the solar surface. The continuous interaction between these different
magnetic patches leads to transient brightenings or bursts that might
contribute to the heating of the solar atmosphere. The literature
on a variety of such burst phenomena in the solar atmosphere is
rich. However, it remains unclear whether such transients, which are
mostly observed in the extreme ultraviolet (EUV), play a significant
role in atmospheric heating. We revisit the open question of these
bursts as a prelude to the new high-resolution EUV imagery expected
from the recently launched Solar Orbiter. We use EUV image sequences
recorded by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO) to investigate statistical properties of
the bursts. We detect the bursts in the 171 Å filter images of AIA in
an automated way through a pixel-wise analysis by imposing different
intensity thresholds. By exploiting the high cadence (12 s) of the
AIA observations, we find that the distribution of lifetimes of these
events peaks at about 120 s. However, a significant number of events
also have lifetimes shorter than 60 s. The sizes of the detected bursts
are limited by the spatial resolution, which indicates that a larger
number of events might be hidden in the AIA data. We estimate that about
100 new bursts appear per second on the whole Sun. The detected bursts
have nanoflare-like energies of 10<SUP>24</SUP> erg per event. Based
on this, we estimate that at least 100 times more events of a similar
nature would be required to account for the energy that is required
to heat the corona. When AIA observations are considered alone, the
EUV bursts discussed here therefore play no significant role in the
coronal heating of the quiet Sun. If the coronal heating of the quiet
Sun is mainly bursty, then the high-resolution EUV observations from
Solar Orbiter may be able to reduce the deficit in the number of EUV
bursts seen with SDO/AIA at least partly by detecting more such events.
---------------------------------------------------------
Title: Magnetic Reconnection between Loops Accelerated By a Nearby
Filament Eruption
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
Hongqiang; Ji, Kaifan; Xiang, YongYuan
2021ApJ...908..213L Altcode: 2020arXiv201208710L
Magnetic reconnection modulated by nonlocal disturbances in the
solar atmosphere has been investigated theoretically, but rarely
observed. In this study, employing Hα and extreme ultraviolet (EUV)
images and line-of-sight magnetograms, we report the acceleration of
reconnection by an adjacent filament eruption. In Hα images, four
groups of chromospheric fibrils are observed to form a saddle-like
structure. Among them, two groups of fibrils converge and reconnect. Two
newly reconnected fibrils then form and retract away from the
reconnection region. In EUV images, similar structures and evolution
of coronal loops are identified. The current sheet forms repeatedly
at the interface of reconnecting loops, with a width and length of
1-2 and 5.3-7.2 Mm and a reconnection rate of 0.18-0.3. It appears
in the EUV low-temperature channels, with an average differential
emission measure (DEM) weighed temperature and EM of 2 MK and 2.5
× 10<SUP>27</SUP> cm<SUP>-5</SUP>. Plasmoids appear in the current
sheet and propagate along it, and then further along the reconnection
loops. The filament, located to the southeast of the reconnection
region, erupts and pushes away the loops covering the reconnection
region. Thereafter, the current sheet has a width and length of 2
and 3.5 Mm and a reconnection rate of 0.57. It becomes much brighter
and appears in the EUV high-temperature channels, with an average
DEM-weighed temperature and EM of 5.5 MK and 1.7 × 10<SUP>28</SUP>
cm<SUP>-5</SUP>. In the current sheet, more hotter plasmoids form. More
thermal and kinetic energy is hence converted. These results suggest
that the reconnection is significantly accelerated by the propagating
disturbance caused by the nearby filament eruption.
---------------------------------------------------------
Title: Magnetic Imaging of the Outer Solar Atmosphere (MImOSA):
Unlocking the driver of the dynamics in the upper solar atmosphere
Authors: Peter, H.; Alsina Ballester, E.; Andretta, V.; Auchere, F.;
Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.;
Chitta, L. P.; Dalmasse, K.; del Pino Aleman, T.; Feller, A.; Froment,
C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski,
D.; Solanki, S. K.; Stepan, J.; Teriaca, L.; Trujillo Bueno, J.
2021arXiv210101566P Altcode:
The magnetic activity of the Sun directly impacts the Earth and human
life. Likewise, other stars will have an impact on the habitability
of planets orbiting these host stars. The lack of information on the
magnetic field in the higher atmospheric layers hampers our progress in
understanding solar magnetic activity. Overcoming this limitation would
allow us to address four paramount long-standing questions: (1) How
does the magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic field
structure, drive and interact with the plasma in the chromosphere and
upper atmosphere? (3) How does the magnetic field destabilise the outer
solar atmosphere and thus affect the interplanetary environment? (4)
How do magnetic processes accelerate particles to high energies? New
ground-breaking observations are needed to address these science
questions. We suggest a suite of three instruments that far exceed
current capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-IR
telescope of the 1-3 m class aimed mainly to measure the magnetic
field in the chromosphere by combining high spatial resolution and high
sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to
measure the large-scale magnetic field in the corona with an aperture
of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30
cm telescope that combines high throughput in the extreme UV with
polarimetry to connect the magnetic measurements of the other two
instruments. This mission to measure the magnetic field will unlock
the driver of the dynamics in the outer solar atmosphere and thereby
greatly advance our understanding of the Sun and the heliosphere.
---------------------------------------------------------
Title: Spectroscopic Observations of the Eruption of an Filament
and Associated Magnetic Reconnection
Authors: Hu, H.; Liu, Y. D.; Peter, H.; Chitta, L. P.; Wang, R.
2020AGUFMSH0010013H Altcode:
We analyze the spectroscopic data from Interface Region Imaging
Spectrograph (IRIS) and images from Solar Dynamics Observatory (SDO)
to investigate the eruption and associated magnetic reconnection of
a filament structure in an active region (AR). Doppler maps derived
from Si IV 1394 Å based on single Gaussian fitting indicate that the
filament structure ascended rapidly after the birth of the situated
AR. The ascent apparently ceased ~2 days after the rapid ascent. The
filament structure gradually ascended again ~1 day before the
eruption. Blue and red shifts of ~30 km/s are observed in a narrow flare
region between two filament threads ~2 hours before the eruption, which
are probably the signatures of the outflows of magnetic reconnection
between the two filament threads. Eventually the upper filament thread
erupted and the lower filament thread remained. Downward motion with a
red shift of ~30 km/s and density enhancement are also observed in the
regions of flare ribbons during the eruption. This work pictures the
evolution of a filament structure before an eruption and the associated
magnetic reconnection between two filament threads.
---------------------------------------------------------
Title: Relation of Coronal Rain Originating from Coronal Condensations
to Interchange Magnetic Reconnection
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
Hongqiang
2020ApJ...905...26L Altcode: 2020arXiv201100709L
Using extreme-ultraviolet images, we recently proposed a new and
alternative formation mechanism for coronal rain along magnetically
open field lines due to interchange magnetic reconnection. In this
paper we report coronal rain at chromospheric and transition region
temperatures originating from the coronal condensations facilitated
by reconnection between open and closed coronal loops. For this,
we employ the Interface Region Imaging Spectrograph (IRIS) and
the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics
Observatory. Around 2013 October 19, a coronal rain along curved
paths was recorded by IRIS over the southeastern solar limb. Related
to this, we found reconnection between a system of higher-lying open
features and lower-lying closed loops that occurs repeatedly in AIA
images. In this process, the higher-lying features form magnetic
dips. In response, two sets of newly reconnected loops appear and
retract away from the reconnection region. In the dips, seven events
of cooling and condensation of coronal plasma repeatedly occur due
to thermal instability over several days, from October 18 to 20. The
condensations flow downward to the surface as coronal rain, with a
mean interval between condensations of ∼6.6 hr. In the cases where
IRIS data were available we found the condensations to cool all the
way down to chromospheric temperatures. Based on our observations we
suggest that some of the coronal rain events observed at chromospheric
temperatures could be explained by the new and alternative scenario for
the formation of coronal rain, where the condensation is facilitated
by interchange reconnection.
---------------------------------------------------------
Title: Impulsive coronal heating during the interaction of surface
magnetic fields in the lower solar atmosphere
Authors: Chitta, L. P.; Peter, H.; Priest, E. R.; Solanki, S. K.
2020A&A...644A.130C Altcode: 2020arXiv201012560C
Coronal plasma in the cores of solar active regions is impulsively
heated to more than 5 MK. The nature and location of the magnetic
energy source responsible for such impulsive heating is poorly
understood. Using observations of seven active regions from the Solar
Dynamics Observatory, we found that a majority of coronal loops hosting
hot plasma have at least one footpoint rooted in regions of interacting
mixed magnetic polarity at the solar surface. In cases when co-temporal
observations from the Interface Region Imaging Spectrograph space
mission are available, we found spectroscopic evidence for magnetic
reconnection at the base of the hot coronal loops. Our analysis suggests
that interactions of magnetic patches of opposite polarity at the
solar surface and the associated energy release during reconnection
are key to impulsive coronal heating. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039099/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
2020A&A...642A...3Z Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
---------------------------------------------------------
Title: Non-thermal line broadening due to braiding-induced turbulence
in solar coronal loops
Authors: Pontin, D. I.; Peter, H.; Chitta, L. P.
2020A&A...639A..21P Altcode: 2020arXiv200811915P
<BR /> Aims: Emission line profiles from solar coronal loops exhibit
properties that are unexplained by current models. We investigate
the non-thermal broadening associated with plasma heating in coronal
loops that is induced by magnetic field line braiding. <BR /> Methods:
We describe the coronal loop by a 3D magnetohydrodynamic model of
the turbulent decay of an initially-braided magnetic field. From
this, we synthesised the Fe XII line at 193 Å that forms around
1.5 MK. <BR /> Results: The key features of current observations of
extreme ultraviolet (UV) lines from the corona are reproduced in the
synthesised spectra: (i) Typical non-thermal widths range from 15 to
20 km s<SUP>-1</SUP>. (ii) The widths are approximately independent
of the size of the field of view. (iii) There is a correlation between
the line intensity and non-thermal broadening. (iv) Spectra are found
to be non-Gaussian, with enhanced power in the wings of the order
of 10-20%. <BR /> Conclusions: Our model provides an explanation
that self-consistently connects the heating process to the observed
non-thermal line broadening. The non-Gaussian nature of the spectra
is a consequence of the non-Gaussian nature of the underlying velocity
fluctuations, which is interpreted as a signature of intermittency in
the turbulence.
---------------------------------------------------------
Title: Solar Photosphere
Authors: Chitta, L. P.; Smitha, H. N.; Solanki, S. K.
2020orep.bookE...1C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Onset of Turbulent Fast Magnetic Reconnection Observed in
the Solar Atmosphere
Authors: Chitta, L. P.; Lazarian, A.
2020ApJ...890L...2C Altcode: 2020arXiv200108595C
Fast magnetic reconnection powers explosive events throughout
the universe, from gamma-ray bursts to solar flares. Despite its
importance, the onset of astrophysical fast reconnection is the subject
of intense debate and remains an open question in plasma physics. Here
we report high-cadence observations of two reconnection-driven solar
microflares obtained by the Interface Region Imaging Spectrograph
that show persistent turbulent flows preceding flaring. The speeds
of these flows are comparable to the local sound speed initially,
suggesting the onset of fast reconnection in a highly turbulent
plasma environment. Our results are in close quantitative agreement
with the theory of turbulence-driven reconnection as well as with
numerical simulations in which fast magnetic reconnection is induced
by turbulence.
---------------------------------------------------------
Title: Repeated Coronal Condensations Caused by Magnetic Reconnection
between Solar Coronal Loops
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Zhang,
Jun; Su, Jiangtao; Song, Hongqiang; Hou, Yijun; Xia, Chun
2019ApJ...884...34L Altcode:
We recently presented coronal condensations, caused by magnetic
reconnection (MR) between coronal loops from extreme ultraviolet
observations, over the course of one day, on 2012 January 19. In
this paper, by investigating the loops over an extended period of
time from January 16 to 20, we present a case for repeated coronal
condensations caused by repeated MR between them. In these five days,
MR between higher-lying open loops and lower-lying closed loops occurs
repeatedly, forming magnetic dips in the higher-lying open loops. During
the MR process, cooling and condensation of coronal plasma occur
repeatedly. Early on January 16, cooling, but not condensation, of
coronal plasma happens. Later, condensation appears at the edge of the
dips and falls down along the loops as coronal rains. On January 17,
a similar condensation happens at the edge of the higher-lying dips
and falls down along the loops. However, another condensation appears
in the lower-lying dips and rains down across them. From January 18
to 19, multiple condensations mostly occur at the edge of the dips
and fall down both along the loops and across the dips. On January
20, five condensations sequentially appear and rain down across the
dips. Overall, 15 condensation events occur in five days, lasting from
0.5 to 15.6 hr. We suggest that the formation of coronal condensations
by MR between loops is common in the solar corona. The repeated MR
between loops thus plays an essential role in the mass cycle of coronal
plasma by initiating repeated catastrophic cooling and condensation.
---------------------------------------------------------
Title: Plasmoid-mediated reconnection in solar UV bursts
Authors: Peter, H.; Huang, Y. -M.; Chitta, L. P.; Young, P. R.
2019A&A...628A...8P Altcode: 2019arXiv190704335P
Context. Ultraviolet bursts are transients in the solar atmosphere with
an increased impulsive emission in the extreme UV lasting for one to
several tens of minutes. They often show spectral profiles indicative
of a bi-directional outflow in response to magnetic reconnection. <BR
/> Aims: To understand UV bursts, we study how motions of magnetic
elements at the surface can drive the self-consistent formation
of a current sheet resulting in plasmoid-mediated reconnection. In
particular, we want to study the role of the height of the reconnection
in the atmosphere. <BR /> Methods: We conducted numerical experiments
solving the 2D magnetohydrodynamic equations from the solar surface
to the upper atmosphere. Motivated by observations, we drove a small
magnetic patch embedded in a larger system of magnetic field of opposite
polarity. This type of configuration creates an X-type neutral point
in the initial potential field. The models are characterized by the
(average) plasma-β at the height of this X point. <BR /> Results:
The driving at the surface stretches the X-point into a thin current
sheet, where plasmoids appear, accelerating the reconnection, and a
bi-directional jet forms. This is consistent with what is expected
for UV bursts or explosive events, and we provide a self-consistent
model of the formation of the reconnection region in such events. The
gravitational stratification gives a natural explanation for why
explosive events are restricted to a temperature range around a few
0.1 MK, and the presence of plasmoids in the reconnection process
provides an understanding of the observed variability during the
transient events on a timescale of minutes. <BR /> Conclusions: Our
numerical experiments provide a comprehensive understanding of UV bursts
and explosive events, in particular of how the atmospheric response
changes if the reconnection happens at different plasma-β, that is,
at different heights in the atmosphere. This analysis also gives
new insight into how UV bursts might be related to the photospheric
Ellerman bombs. <P />Movie attached to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935820/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Hot prominence spicules launched from turbulent cool solar
prominences
Authors: Chitta, L. P.; Peter, H.; Li, L.
2019A&A...627L...5C Altcode: 2019arXiv190609125C
A solar filament is a dense cool condensation that is supported and
thermally insulated by magnetic fields in the rarefied hot corona. Its
evolution and stability, leading to either an eruption or disappearance,
depend on its coupling with the surrounding hot corona through a
thin transition region, where the temperature steeply rises. However,
the heating and dynamics of this transition region remain elusive. We
report extreme-ultraviolet observations of quiescent filaments from the
Solar Dynamics Observatory that reveal prominence spicules propagating
through the transition region of the filament-corona system. These thin
needle-like jet features are generated and heated to at least 0.7 MK
by turbulent motions of the material in the filament. We suggest that
the prominence spicules continuously channel the heated mass into the
corona and aid in the filament evaporation and decay. Our results shed
light on the turbulence-driven heating in magnetized condensations
that are commonly observed on the Sun and in the interstellar
medium. <P />The movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936027/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. <BR /> Aims: We aim to explore the transient nature of
the magnetic field, its mixed-polarity characteristics, and the
associated energetics in the active region plage using high spatial
resolution observations and numerical simulations. <BR /> Methods:
We used photospheric Fe I 6173 Å spectropolarimetric observations of
a decaying active region obtained from the Swedish 1-m Solar Telescope
(SST). These data were inverted to retrieve the photospheric magnetic
field underlying the plage as identified in the extreme-ultraviolet
emission maps obtained from the Atmospheric Imaging Assembly (AIA)
on board the Solar Dynamics Observatory (SDO). To obtain better
insight into the evolution of extended unidirectional magnetic field
patches on the Sun, we performed 3D radiation magnetohydrodynamic
simulations of magnetoconvection using the MURaM code. <BR />
Results: The observations show transient magnetic flux emergence
and cancellation events within the extended predominantly unipolar
patch on timescales of a few 100 s and on spatial scales comparable
to granules. These transient events occur at the footpoints of active
region plage loops. In one case the coronal response at the footpoints
of these loops is clearly associated with the underlying transient. The
numerical simulations also reveal similar magnetic flux emergence and
cancellation events that extend to even smaller spatial and temporal
scales. Individual simulated transient events transfer an energy
flux in excess of 1 MW m<SUP>-2</SUP> through the photosphere. <BR
/> Conclusions: We suggest that the magnetic transients could play
an important role in the energetics of active region plage. Both in
observations and simulations, the opposite-polarity magnetic field
brought up by transient flux emergence cancels with the surrounding
plage field. Magnetic reconnection associated with such transient events
likely conduits magnetic energy to power the overlying chromosphere
and coronal loops.
---------------------------------------------------------
Title: A Cancellation Nanoflare Model for Solar Chromospheric and
Coronal Heating. II. 2D Theory and Simulations
Authors: Syntelis, P.; Priest, E. R.; Chitta, L. P.
2019ApJ...872...32S Altcode: 2019arXiv190102798S
Recent observations at high spatial resolution have shown that magnetic
flux cancellation occurs on the solar surface much more frequently than
previously thought, and so this led Priest et al. (2018) to propose
magnetic reconnection driven by photospheric flux cancellation as
a mechanism for chromospheric and coronal heating. In particular,
they estimated analytically the amount of energy released as heat
and the height of the energy release during flux cancellation. In
the present work, we take the next step in the theory by setting up a
two-dimensional resistive MHD simulation of two canceling polarities
in the presence of a horizontal external field and a stratified
atmosphere in order to check and improve upon the analytical
estimates. Computational evaluation of the energy release during
reconnection is found to be in good qualitative agreement with the
analytical estimates. In addition, we go further and undertake an
initial study of the atmospheric response to reconnection. We find
that, during the cancellation, either hot ejections or cool ones or a
combination of both hot and cool ejections can be formed, depending
on the height of the reconnection location. The hot structures can
have the density and temperature of coronal loops, while the cooler
structures are suggestive of surges and large spicules.
---------------------------------------------------------
Title: Quasi-periodic Fast Propagating Magnetoacoustic Waves during
the Magnetic Reconnection Between Solar Coronal Loops
Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Chitta, Lakshmi Pradeep;
Su, Jiangtao; Song, Hongqiang; Xia, Chun; Hou, Yijun
2018ApJ...868L..33L Altcode: 2018arXiv181108553L
Employing Solar Dynamics Observatory/Atmospheric Imaging Assembly
(AIA) multi-wavelength images, we have presented coronal condensations
caused by magnetic reconnection between a system of open and closed
solar coronal loops. In this Letter, we report the quasi-periodic fast
magnetoacoustic waves propagating away from the reconnection region
upward across the higher-lying open loops during the reconnection
process. On 2012 January 19, reconnection between the higher-lying
open loops and lower-lying closed loops took place, and two sets of
newly reconnected loops formed. Thereafter, cooling and condensations
of coronal plasma occurred in the magnetic dip region of higher-lying
open loops. During the reconnection process, disturbances originating
from the reconnection region propagate upward across the magnetic
dip region of higher-lying loops with the mean speed and mean speed
amplitude of 200 and 30 km s<SUP>-1</SUP>, respectively. The mean speed
of the propagating disturbances decreases from ∼230 km s<SUP>-1</SUP>
to ∼150 km s<SUP>-1</SUP> during the coronal condensation process,
and then increases to ∼220 km s<SUP>-1</SUP>. This temporal evolution
of the mean speed anti-correlates with the light curves of the AIA 131
and 304 Å channels that show the cooling and condensation process
of coronal plasma. Furthermore, the propagating disturbances appear
quasi-periodically with a peak period of 4 minutes. Our results suggest
that the disturbances represent the quasi-periodic fast propagating
magnetoacoustic (QFPM) waves originating from the magnetic reconnection
between coronal loops.
---------------------------------------------------------
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: Observations of solar chromospheric heating at sub-arcsec
spatial resolution
Authors: Smitha, H. N.; Chitta, L. P.; Wiegelmann, T.; Solanki, S. K.
2018A&A...617A.128S Altcode: 2018arXiv180701078S
A wide variety of phenomena such as gentle but persistent
brightening, dynamic slender features (∼100 km), and compact
(∼1″) ultraviolet (UV) bursts are associated with the heating of
the solar chromosphere. High spatio-temporal resolution is required to
capture the finer details of the likely magnetic reconnection-driven,
rapidly evolving bursts. Such observations are also needed to reveal
their similarities to large-scale flares, which are also thought to be
reconnection driven, and more generally their role in chromospheric
heating. Here we report observations of chromospheric heating in
the form of a UV burst obtained with the balloon-borne observatory
SUNRISE. The observed burst displayed a spatial morphology similar
to that of a large-scale solar flare with a circular ribbon. While
the co-temporal UV observations at 1.5″ spatial resolution and
24 s cadence from the Solar Dynamics Observatory showed a compact
brightening, the SUNRISE observations at diffraction-limited spatial
resolution of 0.1″ at 7 s cadence revealed a dynamic substructure
of the burst that it is composed of an extended ribbon-like feature
and a rapidly evolving arcade of thin (∼0.1″) magnetic loop-like
features, similar to post-flare loops. Such a dynamic substructure
reveals the small-scale nature of chromospheric heating in these
bursts. Furthermore, based on magnetic field extrapolations, this
heating event is associated with a complex fan-spine magnetic
topology. Our observations strongly hint at a unified picture of
magnetic heating in the solar atmosphere from some large-scale
flares to small-scale bursts, all associated with such a magnetic
topology. <P />The movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833276/olm">https://www.aanda.org/</A>
---------------------------------------------------------
Title: Emission of solar chromospheric and transition region features
related to the underlying magnetic field
Authors: Barczynski, K.; Peter, H.; Chitta, L. P.; Solanki, S. K.
2018A&A...619A...5B Altcode: 2018arXiv180702372B
Context. The emission of the upper atmosphere of the Sun is closely
related to magnetic field concentrations at the solar surface. <BR />
Aims: It is well established that this relation between chromospheric
emission and magnetic field is nonlinear. Here we investigate
systematically how this relation, characterised by the exponent
of a power-law fit, changes through the atmosphere, from the upper
photosphere through the temperature minimum region and chromosphere
to the transition region. <BR /> Methods: We used spectral maps from
the Interface Region Imaging Spectrograph (IRIS) covering Mg II and
its wings, C II, and Si IV together with magnetograms and UV continuum
images from the Solar Dynamics Observatory. After a careful alignment
of the data we performed a power-law fit for the relation between each
pair of observables and determine the power-law index (or exponent) for
these. This was done for different spatial resolutions and different
features on the Sun. <BR /> Results: While the correlation between
emission and magnetic field drops monotonically with temperature,
the power-law index shows a hockey-stick-type variation: from the
upper photosphere to the temperature-minimum it drops sharply and then
increases through the chromosphere into the transition region. This
is even seen through the features of the Mg II line, this is,
from k1 to k2 and k3. It is irrespective of spatial resolution or
whether we investigate active regions, plage areas, quiet Sun, or
coronal holes. <BR /> Conclusions: In accordance with the general
picture of flux-flux relations from the chromosphere to the corona,
above the temperature minimum the sensitivity of the emission to the
plasma heating increases with temperature. Below the temperature
minimum a different mechanism has to govern the opposite trend of
the power-law index with temperature. We suggest four possibilities,
in other words, a geometric effect of expanding flux tubes filling the
available chromospheric volume, the height of formation of the emitted
radiation, the dependence on wavelength of the intensity-temperature
relationship, and the dependence of the heating of flux tubes on the
magnetic flux density.
---------------------------------------------------------
Title: Coronal Condensations Caused by Magnetic Reconnection between
Solar Coronal Loops
Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Chitta, Lakshmi Pradeep;
Su, Jiangtao; Xia, Chun; Song, Hongqiang; Hou, Yijun
2018ApJ...864L...4L Altcode: 2018arXiv180809626L
Employing Solar Dynamics Observatory/Atmospheric Imaging Assembly
(AIA) multi-wavelength images, we report the coronal condensation
during the magnetic reconnection (MR) between a system of open and
closed coronal loops. Higher-lying magnetically open structures,
observed in AIA 171 Å images above the solar limb, move downward and
interact with the lower-lying closed loops, resulting in the formation
of dips in the former. An X-type structure forms at the interface. The
interacting loops reconnect and disappear. Two sets of newly reconnected
loops then form and recede from the MR region. During the MR process,
bright emission appears sequentially in the AIA 131 and 304 Å channels
repeatedly in the dips of higher-lying open structures. This indicates
the cooling and condensation process of hotter plasma from ∼0.9
MK down to ∼0.6 MK, and then to ∼0.05 MK, also supported by the
light curves of the AIA 171, 131, and 304 Å channels. The part of
higher-lying open structures supporting the condensation participate
in the successive MR. Without support from underlying loops, the
condensation then rains back to the solar surface along the newly
reconnected loops. Our results suggest that the MR between coronal loops
leads to the condensation of hotter coronal plasma and its downflows. MR
thus plays an active role in the mass cycle of coronal plasma because it
can initiate the catastrophic cooling and condensation. This underlines
that the magnetic and thermal evolution has to be treated together
and cannot be separated, even in the case of catastrophic cooling.
---------------------------------------------------------
Title: A Cancellation Nanoflare Model for Solar Chromospheric and
Coronal Heating
Authors: Priest, E. R.; Chitta, L. P.; Syntelis, P.
2018ApJ...862L..24P Altcode: 2018arXiv180708161P
Nanoflare models for heating the solar corona usually assume magnetic
braiding and reconnection as the source of the energy. However,
recent observations at record spatial resolution from the SUNRISE
balloon mission suggest that photospheric magnetic flux cancellation
is much more common than previously realized. We therefore examine
the possibility of three-dimensional reconnection driven by flux
cancellation as a cause of chromospheric and coronal heating. In
particular, we estimate how the heights and amount of energy release
produced by flux cancellation depend on flux size, flux cancellation
speed, and overlying field strength.
---------------------------------------------------------
Title: Nature of the energy source powering solar coronal loops
driven by nanoflares
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.
2018A&A...615L...9C Altcode: 2018arXiv180611045C
Context. Magnetic energy is required to heat the corona, the outer
atmosphere of the Sun, to millions of degrees. <BR /> Aims: We study the
nature of the magnetic energy source that is probably responsible for
the brightening of coronal loops driven by nanoflares in the cores of
solar active regions. <BR /> Methods: We consider observations of two
active regions (ARs), 11890 and 12234, in which nanoflares have been
detected. To this end, we use ultraviolet (UV) and extreme ultraviolet
(EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the
Solar Dynamics Observatory (SDO) for coronal loop diagnostics. These
images are combined with the co-temporal line-of-sight magnetic
field maps from the Helioseismic and Magnetic Imager (HMI) onboard
SDO to investigate the connection between coronal loops and their
magnetic roots in the photosphere. <BR /> Results: The core of
these ARs exhibit loop brightening in multiple EUV channels of AIA,
particularly in its 9.4 nm filter. The HMI magnetic field maps reveal
the presence of a complex mixed polarity magnetic field distribution
at the base of these loops. We detect the cancellation of photospheric
magnetic flux at these locations at a rate of about 10<SUP>15</SUP>
Mx s<SUP>-1</SUP>. The associated compact coronal brightenings directly
above the cancelling magnetic features are indicative of plasma heating
due to chromospheric magnetic reconnection. <BR /> Conclusions:
We suggest that the complex magnetic topology and the evolution of
magnetic field, such as flux cancellation in the photosphere and
the resulting chromospheric reconnection, can play an important role
in energizing active region coronal loops driven by nanoflares. Our
estimate of magnetic energy release during flux cancellation in the
quiet Sun suggests that chromospheric reconnection can also power the
quiet corona. <P />The movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833404/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Compact solar UV burst triggered in a magnetic field with a
fan-spine topology
Authors: Chitta, L. P.; Peter, H.; Young, P. R.; Huang, Y. -M.
2017A&A...605A..49C Altcode: 2017arXiv170608059C
Context. Solar ultraviolet (UV) bursts are small-scale features
that exhibit intermittent brightenings that are thought to be due to
magnetic reconnection. They are observed abundantly in the chromosphere
and transition region, in particular in active regions. <BR /> Aims:
We investigate in detail a UV burst related to a magnetic feature that
is advected by the moat flow from a sunspot towards a pore. The moving
feature is parasitic in that its magnetic polarity is opposite to that
of the spot and the pore. This comparably simple photospheric magnetic
field distribution allows for an unambiguous interpretation of the
magnetic geometry leading to the onset of the observed UV burst. <BR />
Methods: We used UV spectroscopic and slit-jaw observations from the
Interface Region Imaging Spectrograph (IRIS) to identify and study
chromospheric and transition region spectral signatures of said UV
burst. To investigate the magnetic topology surrounding the UV burst,
we used a two-hour-long time sequence of simultaneous line-of-sight
magnetograms from the Helioseismic and Magnetic Imager (HMI) and
performed data-driven 3D magnetic field extrapolations by means of
a magnetofrictional relaxation technique. We can connect UV burst
signatures to the overlying extreme UV (EUV) coronal loops observed
by the Atmospheric Imaging Assembly (AIA). <BR /> Results: The UV
burst shows a variety of extremely broad line profiles indicating
plasma flows in excess of ±200 km s<SUP>-1</SUP> at times. The whole
structure is divided into two spatially distinct zones of predominantly
up- and downflows. The magnetic field extrapolations show a persistent
fan-spine magnetic topology at the UV burst. The associated 3D magnetic
null point exists at a height of about 500 km above the photosphere
and evolves co-spatially with the observed UV burst. The EUV emission
at the footpoints of coronal loops is correlated with the evolution of
the underlying UV burst. <BR /> Conclusions: The magnetic field around
the null point is sheared by photospheric motions, triggering magnetic
reconnection that ultimately powers the observed UV burst and energises
the overlying coronal loops. The location of the null point suggests
that the burst is triggered low in the solar chromosphere. <P
/>Movies associated to Figs. 2 and 4 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201730830/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity
Surface Magnetic Fields
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.;
Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van
Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco
Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....4C Altcode: 2016arXiv161007484C
How and where are coronal loops rooted in the solar lower
atmosphere? The details of the magnetic environment and its evolution
at the footpoints of coronal loops are crucial to understanding the
processes of mass and energy supply to the solar corona. To address
the above question, we use high-resolution line-of-sight magnetic
field data from the Imaging Magnetograph eXperiment instrument on the
Sunrise balloon-borne observatory and coronal observations from the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
of an emerging active region. We find that the coronal loops are
often rooted at the locations with minor small-scale but persistent
opposite-polarity magnetic elements very close to the larger dominant
polarity. These opposite-polarity small-scale elements continually
interact with the dominant polarity underlying the coronal loop through
flux cancellation. At these locations we detect small inverse Y-shaped
jets in chromospheric Ca II H images obtained from the Sunrise Filter
Imager during the flux cancellation. Our results indicate that magnetic
flux cancellation and reconnection at the base of coronal loops due
to mixed polarity fields might be a crucial feature for the supply of
mass and energy into the corona.
---------------------------------------------------------
Title: Coronal loop footpoints threaded with small-scale mixed
polarity surface magnetic fields
Authors: Chitta, L. P.
2017psio.confE..62C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A closer look at a coronal loop rooted in a sunspot umbra
Authors: Chitta, L. P.; Peter, H.; Young, P. R.
2016A&A...587A..20C Altcode: 2015arXiv151203831C
Context. Extreme UV (EUV) and X-ray loops in the solar corona connect
regions of enhanced magnetic activity, but they are not usually rooted
in the dark umbrae of sunspots because the strong magnetic field
found there suppresses convection. This means that the Poynting flux
of magnetic energy into the upper atmosphere is not significant within
the umbra as long as there are no light bridges or umbral dots. <BR />
Aims: Here we report a rare observation of a coronal loop rooted in the
dark umbra of a sunspot without any traces of light bridges or umbral
dots. This allows us to investigate the loop without much confusion
from background or line-of-sight integration effects. <BR /> Methods:
We used the slit-jaw images and spectroscopic data from the Interface
Region Imaging Spectrograph (IRIS) and concentrate on the line profiles
of O iv and Si iv that show persistent strong redshifted components in
the loop rooted in the umbra. Using the ratios of O iv, we can estimate
the density and thus investigate the mass flux. The coronal context
and temperature diagnostics of these observations is provided through
the EUV channels of the Atmospheric Imaging Assembly (AIA). <BR />
Results: The coronal loop, embedded within cooler downflows, hosts
supersonic downflows. The speed of more than 100 km s<SUP>-1</SUP> is
on the same order of magnitude in the transition region lines of O iv
and Si iv, and is even seen at comparable speed in the chromospheric
Mg II lines. At a projected distance of within 1” of the footpoint,
we see a shock transition to smaller downflow speeds of about 15
km s<SUP>-1</SUP> being consistent with mass conservation across a
stationary isothermal shock. <BR /> Conclusions: We see no direct
evidence for energy input into the loop because the loop is rooted
in the dark uniform part of the umbra with no light bridges or umbral
dots near by. Thus one might conclude that we are seeing a siphon flow
driven from the footpoint at the other end of the loop. However, for a
final result data of similar quality at the other footpoint are needed,
but this is too far away to be covered by the IRIS field of view.
---------------------------------------------------------
Title: Limitations of force-free magnetic field extrapolations:
Revisiting basic assumptions
Authors: Peter, H.; Warnecke, J.; Chitta, L. P.; Cameron, R. H.
2015A&A...584A..68P Altcode: 2015arXiv151004642P
Context. Force-free extrapolations are widely used to study the magnetic
field in the solar corona based on surface measurements. <BR /> Aims:
The extrapolations assume that the ratio of internal energy of the
plasma to magnetic energy, the plasma β, is negligible. Despite the
widespread use of this assumption observations, models, and theoretical
considerations show that β is of the order of a few percent to more
than 10%, and thus not small. We investigate what consequences this
has for the reliability of extrapolation results. <BR /> Methods: We
use basic concepts starting with force and energy balance to infer
relations between plasma β and free magnetic energy to study the
direction of currents in the corona with respect to the magnetic
field, and to estimate the errors in the free magnetic energy by
neglecting effects of the plasma (β ≪ 1). A comparison with a 3D
magneto-hydrodynamics (MHD) model supports our basic considerations. <BR
/> Results: If plasma β is of the order of the relative free energy
(the ratio of the free magnetic energy to the total magnetic energy)
then the pressure gradient can balance the Lorentz force. This is the
case in solar corona, and therefore the currents are not properly
described. In particular, the error in terms of magnetic energy by
neglecting the plasma is of the order of the free magnetic energy, so
that the latter cannot be reliably determined by an extrapolation. <BR
/> Conclusions: While a force-free extrapolation might capture the
magnetic structure and connectivity of the coronal magnetic field,
the derived currents and free magnetic energy are not reliable. Thus
quantitative results of extrapolations on the location and amount of
heating in the corona (through current dissipation) and on the energy
storage of the magnetic field (e.g. for eruptive events) are limited.
---------------------------------------------------------
Title: VLBI Imaging of the Double Peaked Emission Line Seyfert
KISSR 1494
Authors: Kharb, P.; Das, M.; Paragi, Z.; Subramanian, S.; Chitta, L. P.
2015ApJ...799..161K Altcode: 2014arXiv1412.0400K
We present here the results from dual-frequency phase-referenced Very
Long Baseline Interferometry observations of the Seyfert galaxy KISSR
1494, which exhibits double peaked emission lines in its Sloan Digital
Sky Survey spectrum. We detect a single radio component at 1.6 GHz,
but not at 5 GHz, implying a spectral index steeper than -1.5 ± 0.5
(S <SUB>ν</SUB>vpropν<SUP>α</SUP>). The high brightness temperature
of the radio component (~1.4 × 10<SUP>7</SUP> K) and the steep radio
spectrum support a non-thermal synchrotron origin. A crude estimate of
the black hole mass derived from the M <SUB>BH</SUB>-σ<SUB>sstarf</SUB>
relation is ~1.4 ± 1.0 × 10<SUP>8</SUP> M <SUB>⊙</SUB> it is
accreting at an Eddington rate of ~0.02. The radio data are consistent
with either the radio emission coming from the parsec-scale base of
a synchrotron wind originating in the magnetized corona above the
accretion disk, or from the inner ionized edge of the accretion disk
or torus. In the former case, the narrow line region (NLR) clouds may
form a part of the broad outflow, while in the latter, the NLR clouds
may form a part of an extended disk beyond the torus. The radio and NLR
emission may also be decoupled so that the radio emission originates
in an outflow while the NLR is in a disk and vice versa. While
with the present data it is not possible to clearly distinguish
between these scenarios, there appears to be greater circumstantial
evidence supporting the coronal wind picture in KISSR 1494. From
the kiloparsec-scale radio emission, the time-averaged kinetic power
of this outflow is estimated to be Q ≈ 1.5 × 10<SUP>42</SUP> erg
s<SUP>-1</SUP>, which is typical of radio outflows in low-luminosity
active galactic nuclei. This supports the idea that radio "jets"
and outflowing coronal winds are indistinguishable in Seyfert galaxies.
---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of the Solar Magnetic
Carpet and Comparison with SDO/HMI and Sunrise/IMaX Observations
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
DeLuca, E. E.; Solanki, S. K.
2014ApJ...793..112C Altcode: 2014arXiv1408.0497C
In the quiet solar photosphere, the mixed polarity fields form a
magnetic carpet that continuously evolves due to dynamical interaction
between the convective motions and magnetic field. This interplay is a
viable source to heat the solar atmosphere. In this work, we used the
line-of-sight (LOS) magnetograms obtained from the Helioseismic and
Magnetic Imager on the Solar Dynamics Observatory, and the Imaging
Magnetograph eXperiment instrument on the Sunrise balloon-borne
observatory, as time-dependent lower boundary conditions, to study the
evolution of the coronal magnetic field. We use a magneto-frictional
relaxation method, including hyperdiffusion, to produce a time series
of three-dimensional nonlinear force-free fields from a sequence
of photospheric LOS magnetograms. Vertical flows are added up to a
height of 0.7 Mm in the modeling to simulate the non-force-freeness
at the photosphere-chromosphere layers. Among the derived quantities,
we study the spatial and temporal variations of the energy dissipation
rate and energy flux. Our results show that the energy deposited in
the solar atmosphere is concentrated within 2 Mm of the photosphere and
there is not sufficient energy flux at the base of the corona to cover
radiative and conductive losses. Possible reasons and implications are
discussed. Better observational constraints of the magnetic field in
the chromosphere are crucial to understand the role of the magnetic
carpet in coronal heating.
---------------------------------------------------------
Title: Segmentation of coronal features to understand the solar EUV
and UV irradiance variability
Authors: Kumara, S. T.; Kariyappa, R.; Zender, J. J.; Giono, G.;
Delouille, V.; Chitta, L. P.; Damé, L.; Hochedez, J. -F.; Verbeeck,
C.; Mampaey, B.; Doddamani, V. H.
2014A&A...561A...9K Altcode:
Context. The study of solar irradiance variability is of great
importance in heliophysics, the Earth's climate, and space weather
applications. These studies require careful identifying, tracking
and monitoring of active regions (ARs), coronal holes (CHs), and the
quiet Sun (QS). <BR /> Aims: We studied the variability of solar
irradiance for a period of two years (January 2011-December 2012)
using the Large Yield Radiometer (LYRA), the Sun Watcher using APS and
image Processing (SWAP) on board PROBA2, and the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO). <BR />
Methods: We used the spatial possibilistic clustering algorithm (SPoCA)
to identify and segment coronal features from the EUV observations of
AIA. The AIA segmentation maps were then applied on SWAP images, and
parameters such as the intensity, fractional area, and contribution
of ARs/CHs/QS features were computed and compared with the full-disk
integrated intensity and LYRA irradiance measurements. <BR /> Results:
We report the results obtained from SDO/AIA and PROBA2/SWAP images
taken from January 2011 to December 2012 and compare the resulting
integrated full-disk intensity with PROBA2/LYRA irradiance. We
determine the contributions of the segmented features to EUV and UV
irradiance variations. The variations of the parameters resulting
from the segmentation, namely the area, integrated intensity, and
relative contribution to the solar irradiance, are compared with LYRA
irradiance. We find that the active regions have a great impact on the
irradiance fluctuations. In the EUV passbands considered in this study,
the QS is the greatest contributor to the solar irradiance, with up
to 63% of total intensity values. Active regions, on the other hand,
contribute to about 10%, and off-limb structures to about 24%. We
also find that the area of the features is highly variable suggesting
that their area has to be taken into account in irradiance models,
in addition to their intensity variations. <BR /> Conclusions:
We successfully show that the feature extraction allows us to use
EUV telescopes to measure irradiance fluctuations and to quantify
the contribution of each part to the EUV spectral solar irradiance
observed with a calibrated radiometer. This study also shows that
SPoCA is viable, and that the segmentation of images can be a useful
tool. We also provide the measurement correlation between SWAP and
AIA during this analysis.
---------------------------------------------------------
Title: Observations and Modeling of the Emerging Extreme-ultraviolet
Loops in the Quiet Sun as Seen with the Solar Dynamics Observatory
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
DeLuca, E. E.; Hasan, S. S.; Hanslmeier, A.
2013ApJ...768...32C Altcode: 2013arXiv1303.3426C
We used data from the Helioseismic and Magnetic Imager (HMI) and the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO) to study coronal loops at small scales, emerging in the quiet
Sun. With HMI line-of-sight magnetograms, we derive the integrated
and unsigned photospheric magnetic flux at the loop footpoints in the
photosphere. These loops are bright in the EUV channels of AIA. Using
the six AIA EUV filters, we construct the differential emission measure
(DEM) in the temperature range 5.7-6.5 in log T (K) for several hours
of observations. The observed DEMs have a peak distribution around
log T ≈ 6.3, falling rapidly at higher temperatures. For log T <
6.3, DEMs are comparable to their peak values within an order of
magnitude. The emission-weighted temperature is calculated, and its
time variations are compared with those of magnetic flux. We present
two possibilities for explaining the observed DEMs and temperatures
variations. (1) Assuming that the observed loops are composed of
a hundred thin strands with certain radius and length, we tested
three time-dependent heating models and compared the resulting DEMs
and temperatures with the observed quantities. This modeling used
enthalpy-based thermal evolution of loops (EBTEL), a zero-dimensional
(0D) hydrodynamic code. The comparisons suggest that a medium-frequency
heating model with a population of different heating amplitudes can
roughly reproduce the observations. (2) We also consider a loop model
with steady heating and non-uniform cross-section of the loop along
its length, and find that this model can also reproduce the observed
DEMs, provided the loop expansion factor γ ~ 5-10. More observational
constraints are required to better understand the nature of coronal
heating in the short emerging loops on the quiet Sun.
---------------------------------------------------------
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 km<SUP>2</SUP> s<SUP>-2</SUP>
to the variance of the observed velocities. The raw and corrected
velocity measurements in both directions, i.e., (v<SUB>x</SUB> ,
v<SUB>y</SUB> ), have Gaussian distributions with standard deviations
of (1.32, 1.22) and (1.00, 0.86) km s<SUP>-1</SUP>, respectively. The
BP motions have correlation times of about 22-30 s. We construct the
power spectrum of the horizontal motions as a function of frequency,
a quantity that is useful and relevant to the studies of generation
of Alfvén waves. Photospheric turbulent diffusion at timescales less
than 200 s is found to satisfy a power law with an index of 1.59.
---------------------------------------------------------
Title: 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: Preliminary Results on Irradiance Measurements from Lyra
and Swap
Authors: Kumara, S. T.; Kariyappa, R.; Dominique, M.; Berghmans, D.;
Damé, L.; Hochedez, J. F.; Doddamani, V. H.; Chitta, Lakshmi Pradeep
2012AdAst2012E...5K Altcode: 2012AdAst2012E..10K
No abstract at ADS
---------------------------------------------------------
Title: Observations of the Interaction of Acoustic Waves and
Small-scale Magnetic Fields in a Quiet Sun
Authors: Chitta, Lakshmi Pradeep; Jain, Rekha; Kariyappa, R.;
Jefferies, Stuart M.
2012ApJ...744...98C Altcode: 2012ApJ...744...98P
The effect of the magnetic field on photospheric intensity and
velocity oscillations at the sites of small-scale magnetic fields
(SMFs) in a quiet Sun near the solar disk center is studied. We use
observations made by the G-band filter in the Solar Optical Telescope
on board Hinode for intensity oscillations; Doppler velocity, magnetic
field, and continuum intensity are derived from an Ni I photospheric
absorption line at 6767.8 Å using the Michelson Doppler Imager on
board the Solar and Heliospheric Observatory. Our analysis shows that
both the high-resolution intensity observed in the G band and velocity
oscillations are influenced by the presence of a magnetic field. While
intensity oscillations are suppressed at all frequencies in strong
magnetic field regions compared to weak magnetic field regions,
velocity oscillations show an enhancement of power in the frequency
band 5.5-7 mHz. We find that there is a drop of 20%-30% in the p-mode
power of velocity oscillations within the SMFs when compared to the
regions surrounding them. Our findings indicate that the nature of the
interaction of acoustic waves with the quiet Sun SMFs is similar to
that of large-scale magnetic fields in active regions. We also report
the first results of the center-to-limb variation of such effects
using the observations of the quiet Sun from the Helioseismic and
Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). The
independent verification of these interactions using SDO/HMI suggests
that the velocity power drop of 20%-30% in p-modes is fairly constant
across the solar disk.
