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Author name code: antolin
ADS astronomy entries on 2022-09-14
author:"Antolin, Patrick"
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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: Observations of Instability-driven Nanojets in Coronal Loops
Authors: Sukarmadji, A. Ramada C.; Antolin, Patrick; McLaughlin,
James A.
2022ApJ...934..190S Altcode: 2022arXiv220210960S
The recent discovery of nanojets by Antolin et al. represents
magnetic reconnection in a braided field, thus clearly identifying
reconnection-driven nanoflares. Due to their small scale (500 km
in width, 1500 km in length) and short timescales (<15 s), it is
unclear how pervasive nanojets are in the solar corona. In this paper,
we present Interface Region Imaging Spectrograph and Solar Dynamics
Observatory observations of nanojets found in multiple coronal
structures, namely, in a coronal loop powered by a blowout jet,
and in two other coronal loops with coronal rain. In agreement with
previous findings, we observe that nanojets are accompanied by small
nanoflare-like intensity bursts in the (E)UV, have velocities of 150-250
km s<SUP>-1</SUP> and occur transversely to the field line of origin,
which is sometimes observed to split. However, we find a variety of
nanojet directions in the plane transverse to the loop axis. These
nanojets are found to have kinetic and thermal energies within the
nanoflare range, and often occur in clusters. In the blowout jet case
study, the Kelvin-Helmholtz instability (KHI) is directly identified
as the reconnection driver. For the other two loops, we find that
both, KHI and Rayleigh-Taylor instability (RTI) are likely to be the
drivers. However, we find that KHI and RTI are each more likely in one
of the other two cases. These observations of nanojets in a variety
of structures and environments support nanojets being a general result
of reconnection that are driven here by dynamic instabilities.
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Title: Coronal oscillations in the self-consistent 3D MHD simulations
of the solar atmosphere
Authors: Kohutova, Petra; Antolin, Patrick; Carlsson, Mats; Popovas,
Andrius
2022cosp...44.2494K Altcode:
Solar coronal loops are commonly subject to oscillations. Coronal
oscillations are typically studied using highly idealised models of
magnetic flux-tubes. In order to improve our understanding of coronal
oscillations, it is necessary to consider the effect of realistic
magnetic field topology and evolution. To do this, we study excitation,
evolution and damping of coronal oscillations in three-dimensional
self-consistent simulations of solar atmosphere spanning from convection
zone to solar corona using the radiation-MHD code Bifrost. We use
forward-modelled EUV emission and three-dimensional tracing of magnetic
field to analyse oscillatory behaviour of individual magnetic loops. We
show that coronal loop oscillations are abundant in such models and
the oscillation modes and characteristics match those detected in solar
observations. Finally, we discuss the dynamics and variability of the
oscillating loops and the implications for coronal seismology.
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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: Prevalence of Thermal Nonequilibrium over an Active Region
Authors: Şahin, Seray; Antolin, Patrick
2022ApJ...931L..27S Altcode: 2022arXiv220510794S
Recent observations have shown that besides the characteristic
multimillion degree component, the corona also contains a large
amount of cool material called coronal rain, whose clumps are 10-100
times cooler and denser than the surroundings and are often organized
in larger events, termed showers. Thermal instability (TI) within a
coronal loop in a state of thermal nonequilibrium (TNE) is the leading
mechanism behind the formation of coronal rain but no investigation on
showers exists to date. In this study, we conduct a morphological and
thermodynamic multiwavelength study of coronal rain showers observed
in an active region (AR) off-limb with IRIS and the Solar Dynamics
Observatory, spanning chromospheric to transition region and coronal
temperatures. Rain showers were found to be widespread across the
AR over the 5.45 hr observing time, with an average length, width,
and duration of 27.37 ± 11.95 Mm, 2.14 ± 0.74 Mm, and 35.22 ±
20.35 minutes, respectively. We find a good correspondence between
showers and the cooling coronal structures consistent with the TNE-TI
scenario, thereby properly identifying coronal loops in the "coronal
veil", including the strong expansion at low heights and an almost zero
expansion in the corona. This agrees with previous work suggesting that
the observed zero expansion in the EUV is due to specific cross-field
temperature distribution. We estimate the total number of showers to be
155 ± 40, leading to a TNE volume of 4.56 ± [3.71] × 10<SUP>28</SUP>
cm<SUP>3</SUP>, i.e., on the same order of the AR volume. This suggests
a prevalence of TNE over the AR indicating strongly stratified and
high-frequency heating on average.
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Title: First high resolution interferometric observation of a solar
prominence with ALMA
Authors: Labrosse, Nicolas; Rodger, Andrew S.; Radziszewski, Krzysztof;
Rudawy, Paweł; Antolin, Patrick; Fletcher, Lyndsay; Levens, Peter J.;
Peat, Aaron W.; Schmieder, Brigitte; Simões, Paulo J. A.
2022MNRAS.513L..30L Altcode: 2022arXiv220212434L; 2022MNRAS.tmpL..22L
We present the first observation of a solar prominence at 84 - 116 GHz
using the high resolution interferometric imaging of ALMA. Simultaneous
observations in Hα from Białkaw Observatory and with SDO/AIA reveal
similar prominence morphology to the ALMA observation. The contribution
functions of 3 mm and Hα emission are shown to have significant
overlap across a range of gas pressures. We estimate the maximum
millimetre-continuum optical thickness to be τ<SUB>3mm</SUB> ≍ 2,
and the brightness temperature from the observed Hα intensity. The
brightness temperature measured by ALMA is ~6000 - 7000 K in the
prominence spine, which correlates well with the estimated brightness
temperature for a kinetic temperature of 8000 K.
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Title: Oscilations in the Tails of Comets may Reveal the Rotational
Period of the Nucleus
Authors: Ferrin; I.; Antolin; L.
2022ATel15357....1F Altcode:
We have compiled observations of several comets that exhibit oscillation
in their tails, and we have been able to extract the period of
oscillation using the Periodogram Tool of the exoplanet archive of NASA.
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Title: Construction of coronal hole and active region
magnetohydrostatic solutions in two dimensions: Force and energy
balance
Authors: Terradas, J.; Soler, R.; Oliver, R.; Antolin, P.; Arregui,
I.; Luna, M.; Piantschitsch, I.; Soubrié, E.; Ballester, J. L.
2022A&A...660A.136T Altcode: 2022arXiv220206800T; 2022arXiv220206800J
Coronal holes and active regions are typical magnetic structures
found in the solar atmosphere. We propose several magnetohydrostatic
equilibrium solutions that are representative of these structures in
two dimensions. Our models include the effect of a finite plasma-β and
gravity, but the distinctive feature is that we incorporate a thermal
structure with properties similar to those reported by observations. We
developed a semi-analytical method to compute the equilibrium
configuration. Using this method, we obtain cold and under-dense
plasma structures in open magnetic fields representing coronal holes,
while in closed magnetic configurations, we achieve the characteristic
hot and over-dense plasma arrangements of active regions. Although
coronal holes and active regions seem to be antagonistic structures,
we find that they can be described using a common thermal structure
that depends on the flux function. In addition to the force balance,
the energy balance is included in the constructed models using an a
posteriori approach. From the two-dimensional computation of thermal
conduction and radiative losses in our models, we infer the required
heating function to achieve energy equilibrium. We find that the
temperature dependence on height is an important parameter that may
prevent the system from accomplishing thermal balance at certain spatial
locations. The implications of these results are discussed in detail.
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Title: Novel Data Analysis Techniques in Coronal Seismology
Authors: Anfinogentov, Sergey A.; Antolin, Patrick; Inglis, Andrew
R.; Kolotkov, Dmitrii; Kupriyanova, Elena G.; McLaughlin, James A.;
Nisticò, Giuseppe; Pascoe, David J.; Krishna Prasad, S.; Yuan, Ding
2022SSRv..218....9A Altcode: 2021arXiv211213577A
We review novel data analysis techniques developed or adapted for
the field of coronal seismology. We focus on methods from the last
ten years that were developed for extreme ultraviolet (EUV) imaging
observations of the solar corona, as well as for light curves from
radio and X-ray. The review covers methods for the analysis of
transverse and longitudinal waves; spectral analysis of oscillatory
signals in time series; automated detection and processing of large
data sets; empirical mode decomposition; motion magnification;
and reliable detection, including the most common pitfalls causing
artefacts and false detections. We also consider techniques for the
detailed investigation of MHD waves and seismological inference of
physical parameters of the coronal plasma, including restoration of
the three-dimensional geometry of oscillating coronal loops, forward
modelling and Bayesian parameter inference.
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Title: Multi-Scale Variability of Coronal Loops Set by Thermal
Non-Equilibrium and Instability as a Probe for Coronal Heating
Authors: Antolin, Patrick; Froment, Clara
2022FrASS...920116A Altcode:
Solar coronal loops are the building blocks of the solar corona. These
dynamic structures are shaped by the magnetic field that expands
into the solar atmosphere. They can be observed in X-ray and extreme
ultraviolet (EUV), revealing the high plasma temperature of the
corona. However, the dissipation of magnetic energy to heat the
plasma to millions of degrees and, more generally, the mechanisms
setting the mass and energy circulation in the solar atmosphere are
still a matter of debate. Furthermore, multi-dimensional modelling
indicates that the very concept of a coronal loop as an individual
entity and its identification in EUV images is ill-defined due to
the expected stochasticity of the solar atmosphere with continuous
magnetic connectivity changes combined with the optically thin
nature of the solar corona. In this context, the recent discovery
of ubiquitous long-period EUV pulsations, the observed coronal rain
properties and their common link in between represent not only major
observational constraints for coronal heating theories but also major
theoretical puzzles. The mechanisms of thermal non-equilibrium (TNE)
and thermal instability (TI) appear in concert to explain these
multi-scale phenomena as evaporation-condensation cycles. Recent
numerical efforts clearly illustrate the specific but large parameter
space involved in the heating and cooling aspects, and the geometry of
the loop affecting the onset and properties of such cycles. In this
review we will present and discuss this new approach into inferring
coronal heating properties and understanding the mass and energy cycle
based on the multi-scale intensity variability and cooling properties
set by the TNE-TI scenario. We further discuss the major numerical
challenges posed by the existence of TNE cycles and coronal rain,
and similar phenomena at much larger scales in the Universe.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). I. Coronal Heating
Authors: De Pontieu, Bart; Testa, Paola; Martínez-Sykora, Juan;
Antolin, Patrick; Karampelas, Konstantinos; Hansteen, Viggo; Rempel,
Matthias; Cheung, Mark C. M.; Reale, Fabio; Danilovic, Sanja; Pagano,
Paolo; Polito, Vanessa; De Moortel, Ineke; Nóbrega-Siverio, Daniel;
Van Doorsselaere, Tom; Petralia, Antonino; Asgari-Targhi, Mahboubeh;
Boerner, Paul; Carlsson, Mats; Chintzoglou, Georgios; Daw, Adrian;
DeLuca, Edward; Golub, Leon; Matsumoto, Takuma; Ugarte-Urra, Ignacio;
McIntosh, Scott W.; the MUSE Team
2022ApJ...926...52D Altcode: 2021arXiv210615584D
The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of
a multislit extreme ultraviolet (EUV) spectrograph (in three spectral
bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in
two passbands around 195 Å and 304 Å). MUSE will provide unprecedented
spectral and imaging diagnostics of the solar corona at high spatial
(≤0.″5) and temporal resolution (down to ~0.5 s for sit-and-stare
observations), thanks to its innovative multislit design. By obtaining
spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX-Fe
XXI 108 Å) covering a wide range of transition regions and coronal
temperatures along 37 slits simultaneously, MUSE will, for the first
time, "freeze" (at a cadence as short as 10 s) with a spectroscopic
raster the evolution of the dynamic coronal plasma over a wide range of
scales: from the spatial scales on which energy is released (≤0.″5)
to the large-scale (~170″ × 170″) atmospheric response. We use
numerical modeling to showcase how MUSE will constrain the properties of
the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and
the large field of view on which state-of-the-art models of the physical
processes that drive coronal heating, flares, and coronal mass ejections
(CMEs) make distinguishing and testable predictions. We describe the
synergy between MUSE, the single-slit, high-resolution Solar-C EUVST
spectrograph, and ground-based observatories (DKIST and others), and
the critical role MUSE plays because of the multiscale nature of the
physical processes involved. In this first paper, we focus on coronal
heating mechanisms. An accompanying paper focuses on flares and CMEs.
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Title: Implications of spicule activity on coronal loop heating and
catastrophic cooling
Authors: Nived, V. N.; Scullion, E.; Doyle, J. G.; Susino, R.; Antolin,
P.; Spadaro, D.; Sasso, C.; Sahin, S.; Mathioudakis, M.
2022MNRAS.509.5523N Altcode: 2021arXiv211107967N; 2021MNRAS.tmp.3004N
We report on the properties of coronal loop foot-point heating
with observations at the highest resolution, from the CRisp Imaging
Spectro-Polarimeter located at the Swedish 1-m Solar Telescope and
co-aligned NASA Solar Dynamics Observatory observations, of Type II
spicules in the chromosphere and their signatures in the extreme
ultraviolet (EUV) corona. Here, we address one important issue,
as to why there is not always a one-to-one correspondence, between
Type II spicules and hot coronal plasma signatures, i.e. beyond
TR temperatures. We do not detect any difference in their spectral
properties in a quiet Sun region compared to a region dominated by
coronal loops. On the other hand, the number density close to the
foot-points in the active region is found to be an order of magnitude
higher than in the quiet Sun case. A differential emission measure
analysis reveals a peak at ~5 × 10<SUP>5</SUP> K of the order of
10<SUP>22</SUP> cm<SUP>-5</SUP> K<SUP>-1</SUP>. Using this result as
a constraint, we conduct numerical simulations and show that with an
energy input of 1.25 × 10<SUP>24</SUP> erg (corresponding to ~10 RBEs
contributing to the burst) we manage to reproduce the observation very
closely. However, simulation runs with lower thermal energy input do not
reproduce the synthetic AIA 171 Å signatures, indicating that there
is a critical number of spicules required in order to account for the
AIA 171 Å signatures in the simulation. Furthermore, the higher energy
(1.25 × 10<SUP>24</SUP> erg) simulations reproduce catastrophic cooling
with a cycle duration of ~5 h, matching a periodicity we observe in
the EUV observations.
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Title: Thermal Instability-Induced Fundamental Magnetic Field Strands
in the Solar Corona
Authors: Antolin, Patrick; Martínez-Sykora, Juan; Şahin, Seray
2022ApJ...926L..29A Altcode:
Thermal instability is a fundamental process of astrophysical
plasmas. It is expected to occur whenever the cooling is dominated
by radiation and cannot be compensated for by heating. In this work,
we conduct 2.5D radiation MHD simulations with the Bifrost code
of an enhanced activity network in the solar atmosphere. Coronal
loops are produced self-consistently, mainly through Joule heating,
which is sufficiently stratified and symmetric to produce thermal
nonequilibrium. During the cooling and driven by thermal instability,
coronal rain is produced along the loops. Due to flux freezing,
the catastrophic cooling process leading to a rain clump produces a
local enhancement of the magnetic field, thereby generating a distinct
magnetic strand within the loop up to a few Gauss stronger than the
surrounding coronal field. These strands, which can be considered
fundamental, are a few hundred kilometers in width, span most of
the loop leg, and emit strongly in the UV and extreme UV (EUV),
thereby establishing a link between the commonly seen rain strands
in the visible spectrum with the observed EUV coronal strands at
high resolution. The compression downstream leads to an increase in
temperature that generates a plume-like structure, a strongly emitting
spicule-like feature, and short-lived brightening in the UV during
the rain impact, providing an explanation for similar phenomena seen
with IRIS. Thermal instability and nonequilibrium can therefore be
associated with localized and intermittent UV brightening in the
transition region and chromosphere, as well as contribute to the
characteristic filamentary morphology of the solar corona in the EUV.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
Leon; The
2022ApJ...926...53C Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
2021AGUFMSH51A..08C Altcode:
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (sub-arcseconds) and temporal scales (less than a few tens
of seconds) of the coronal dynamics of solar flares and eruptive
phenomena. The highest resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by IRIS for the low solar atmosphere,
we need high-resolution spectroscopic measurements with simultaneous
imaging to understand the dominant processes. In this paper: (1)
we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne
observatory to fill this observational gap by providing high-cadence
(<20 s), sub-arcsecond resolution spectroscopic rasters over an
active region size of the solar transition region and corona; (2)
using advanced numerical models, we demonstrate the unique diagnostic
capabilities of MUSE for exploring solar coronal dynamics, and for
constraining and discriminating models of solar flares and eruptions;
(3) we discuss the key contributions MUSE would make in addressing the
science objectives of the Next Generation Solar Physics Mission (NGSPM),
and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the
Daniel K Inouye Solar Telescope (and other ground-based observatories)
can operate as a distributed implementation of the NGSPM. This is a
companion paper to De Pontieu et al. (2021, also submitted to SH-17),
which focuses on investigating coronal heating with MUSE.
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Title: Stereoscopy of extreme UV quiet Sun brightenings observed by
Solar Orbiter/EUI
Authors: Zhukov, A. N.; Mierla, M.; Auchère, F.; Gissot, S.;
Rodriguez, L.; Soubrié, E.; Thompson, W. T.; Inhester, B.; Nicula, B.;
Antolin, P.; Parenti, S.; Buchlin, É.; Barczynski, K.; Verbeeck, C.;
Kraaikamp, E.; Smith, P. J.; Stegen, K.; Dolla, L.; Harra, L.; Long,
D. M.; Schühle, U.; Podladchikova, O.; Aznar Cuadrado, R.; Teriaca,
L.; Haberreiter, M.; Katsiyannis, A. C.; Rochus, P.; Halain, J. -P.;
Jacques, L.; Berghmans, D.
2021A&A...656A..35Z Altcode: 2021arXiv210902169Z
Context. The three-dimensional fine structure of the solar atmosphere
is still not fully understood as most of the available observations
are taken from a single vantage point. <BR /> Aims: The goal of the
paper is to study the three-dimensional distribution of the small-scale
brightening events ("campfires") discovered in the extreme-UV quiet Sun
by the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter. <BR />
Methods: We used a first commissioning data set acquired by the EUI's
High Resolution EUV telescope on 30 May 2020 in the 174 Å passband and
we combined it with simultaneous data taken by the Atmospheric Imaging
Assembly (AIA) aboard the Solar Dynamics Observatory in a similar 171
Å passband. The two-pixel spatial resolution of the two telescopes
is 400 km and 880 km, respectively, which is sufficient to identify
the campfires in both data sets. The two spacecraft had an angular
separation of around 31.5° (essentially in heliographic longitude),
which allowed for the three-dimensional reconstruction of the campfire
position. These observations represent the first time that stereoscopy
was achieved for brightenings at such a small scale. Manual and
automatic triangulation methods were used to characterize the campfire
data. <BR /> Results: The height of the campfires is located between
1000 km and 5000 km above the photosphere and we find a good agreement
between the manual and automatic methods. The internal structure of
campfires is mostly unresolved by AIA; however, for a particularly
large campfire, we were able to triangulate a few pixels, which are
all in a narrow range between 2500 and 4500 km. <BR /> Conclusions: We
conclude that the low height of EUI campfires suggests that they belong
to the previously unresolved fine structure of the transition region and
low corona of the quiet Sun. They are probably apexes of small-scale
dynamic loops heated internally to coronal temperatures. This work
demonstrates that high-resolution stereoscopy of structures in the
solar atmosphere has become feasible.
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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: Stereoscopy of extreme UV quiet Sun brightenings observed by
Solar Orbiter/EUI
Authors: Zhukov, Andrei; Mierla, Marilena; Auchere, F.; Gissot,
Samuel; Rodriguez, Luciano; Soubrie, Elie; Thompson, William; Inhester,
Bernd; Nicula, Bogdan; Antolin, Patrick; Parenti, Susanna; Buchlin,
Eric; Barczynski, Krzysztof; Verbeeck, Cis; Kraaikamp, Emil; Smith,
Philip; Stegen, Koen; Dolla, Laurent; Harra, Louise; Long, David;
Schuhle, Udo; Podladchikova, Olena; Aznar Cuadrado, Regina; Teriaca,
Luca; Haberreiter, Margit; Katsiyannis, Athanassios; Rochus, Pierre;
Halain, Jean-Philippe; Jacques, Lionel; Berghmans, David
2021AGUFMSH21A..03Z Altcode:
We study the three-dimensional distribution of small-scale brightening
events (campfires) discovered in the extreme-ultraviolet (EUV) quiet Sun
by the EUI telescope onboard the Solar Orbiter mission. We use one of
the first commissioning data sets acquired by the HRI_EUV telescope of
EUI on 2020 May 30 in the 174 A passband, combined with the simultaneous
SDO/AIA dataset taken in the very similar 171 A passband. The spatial
resolution of the two telescopes is sufficient to identify the campfires
in both datasets. The angular separation between the two spacecraft of
around 31.5 degrees allowed for the three-dimensional reconstruction
of the position of campfires. This is the first time that stereoscopy
was achieved for structures at such a small scale. Manual and automatic
triangulation methods were used. The height of campfires is between 1000
km and 5000 km above the photosphere, and there is a good agreement
between the results of manual and automatic methods. The internal
structure of campfires is mostly not resolved by AIA, but for a large
campfire we could triangulate a few pixels, which are all in a narrow
height range between 2500 and 4500 km. The low height of campfires
suggests that they belong to the previously unresolved fine structure
of the transition region and low corona of the quiet Sun. They are
probably apexes of small-scale dynamic loops internally heated to
coronal temperatures. This work demonstrates that high-resolution
stereoscopy of structures in the solar atmosphere has become possible.
---------------------------------------------------------
Title: Extreme-UV quiet Sun brightenings observed by the Solar
Orbiter/EUI
Authors: Berghmans, D.; Auchère, F.; Long, D. M.; Soubrié, E.;
Mierla, M.; Zhukov, A. N.; Schühle, U.; Antolin, P.; Harra, L.;
Parenti, S.; Podladchikova, O.; Aznar Cuadrado, R.; Buchlin, É.;
Dolla, L.; Verbeeck, C.; Gissot, S.; Teriaca, L.; Haberreiter, M.;
Katsiyannis, A. C.; Rodriguez, L.; Kraaikamp, E.; Smith, P. J.;
Stegen, K.; Rochus, P.; Halain, J. P.; Jacques, L.; Thompson, W. T.;
Inhester, B.
2021A&A...656L...4B Altcode: 2021arXiv210403382B
Context. The heating of the solar corona by small heating events
requires an increasing number of such events at progressively smaller
scales, with the bulk of the heating occurring at scales that are
currently unresolved. <BR /> Aims: The goal of this work is to study the
smallest brightening events observed in the extreme-UV quiet Sun. <BR />
Methods: We used commissioning data taken by the Extreme Ultraviolet
Imager (EUI) on board the recently launched Solar Orbiter mission. On
30 May 2020, the EUI was situated at 0.556 AU from the Sun. Its
High Resolution EUV telescope (HRI<SUB>EUV</SUB>, 17.4 nm passband)
reached an exceptionally high two-pixel spatial resolution of 400
km. The size and duration of small-scale structures was determined
by the HRI<SUB>EUV</SUB> data, while their height was estimated
from triangulation with simultaneous images from the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory
mission. This is the first stereoscopy of small-scale brightenings
at high resolution. <BR /> Results: We observed small localised
brightenings, also known as `campfires', in a quiet Sun region with
length scales between 400 km and 4000 km and durations between 10 s and
200 s. The smallest and weakest of these HRI<SUB>EUV</SUB> brightenings
have not been previously observed. Simultaneous observations from the
EUI High-resolution Lyman-α telescope (HRI<SUB>Lya</SUB>) do not show
localised brightening events, but the locations of the HRI<SUB>EUV</SUB>
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. Our differential emission
measure analysis indicated coronal temperatures peaking at log T ≈
6.1 − 6.15. We determined the height for a few of these campfires to
be between 1000 and 5000 km above the photosphere. <BR /> Conclusions:
We find that `campfires' are mostly coronal in nature and rooted in the
magnetic flux concentrations of the chromospheric network. 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.
---------------------------------------------------------
Title: Modelling of asymmetric nanojets in coronal loops
Authors: Pagano, P.; Antolin, P.; Petralia, A.
2021A&A...656A.141P Altcode: 2021arXiv210904854P
Context. Observations of reconnection jets in the solar corona are
emerging as a possible diagnostic for studying highly elusive coronal
heating. Such jets, and in particular those termed nanojets, can be
observed in coronal loops and have been linked to nanoflares. However,
while models successfully describe the bilateral post-reconnection
magnetic slingshot effect that leads to the jets, observations
reveal that nanojets are unidirectional or highly asymmetric, with
only the jet travelling inward with respect to the coronal loop's
curvature being clearly observed. <BR /> Aims: The aim of this work
is to address the role of the curvature of the coronal loop in the
generation and evolution of asymmetric reconnection jets. <BR />
Methods: We first use a simplified analytical model in which we
estimate the post-reconnection tension forces based on the local
intersection angle between the pre-reconnection magnetic field
lines and their post-reconnection retracting length towards new
equilibria. Second, we use a simplified numerical magnetohydrodynamic
(MHD) model to study how two opposite propagating jets evolve in curved
magnetic field lines. <BR /> Results: Through our analytical model,
we demonstrate that in the post-reconnection reorganised magnetic
field, the inward directed magnetic tension is inherently stronger
(by up to three orders of magnitude) than the outward directed one
and that, with a large enough retracting length, a regime exists where
the outward directed tension disappears, leading to no outward jet at
large, observable scales. Our MHD numerical model provides support for
these results, proving also that in the subsequent time evolution the
inward jets are consistently more energetic. The degree of asymmetry
is also found to increase for small-angle reconnection and for more
localised reconnection regions. <BR /> Conclusions: This work shows
that the curvature of the coronal loops can play a major role in the
asymmetry of the reconnection jets and that inward directed jets are
more likely to occur and are more energetic than the corresponding
outward directed ones.
---------------------------------------------------------
Title: Forward Modeling of Simulated Transverse Oscillations in
Coronal Loops and the Influence of Background Emission
Authors: Shi, Mijie; Van Doorsselaere, Tom; Antolin, Patrick; Li, Bo
2021ApJ...922...60S Altcode: 2021arXiv210902338S
We simulate transverse oscillations in radiatively cooling coronal
loops and forward-model their spectroscopic and imaging signatures,
paying attention to the influence of background emission. The
transverse oscillations are driven at one footpoint by a periodic
velocity driver. A standing kink wave is subsequently formed and the
loop cross section is deformed due to the Kelvin-Helmholtz instability,
resulting in energy dissipation and heating at small scales. Besides the
transverse motions, a long-period longitudinal flow is also generated
due to the ponderomotive force induced slow wave. We then transform the
simulated straight loop to a semi-torus loop and forward-model their
spectrometer and imaging emissions, mimicking observations of Hinode/EIS
and SDO/AIA. We find that the oscillation amplitudes of the intensity
are different at different slit positions, but are roughly the same in
different spectral lines or channels. X-t diagrams of both the Doppler
velocity and the Doppler width show periodic signals. We also find that
the background emission dramatically decreases the Doppler velocity,
making the estimated kinetic energy two orders of magnitude smaller
than the real value. Our results show that background subtraction can
help recover the real oscillation velocity. These results are helpful
for further understanding transverse oscillations in coronal loops
and their observational signatures. However, they cast doubt on the
spectroscopically estimated energy content of transverse waves using
the Doppler velocity.
---------------------------------------------------------
Title: Magnetohydrodynamic Waves in Open Coronal Structures
Authors: Banerjee, D.; Krishna Prasad, S.; Pant, V.; McLaughlin, J. A.;
Antolin, P.; Magyar, N.; Ofman, L.; Tian, H.; Van Doorsselaere, T.;
De Moortel, I.; Wang, T. J.
2021SSRv..217...76B Altcode: 2020arXiv201208802B
Modern observatories have revealed the ubiquitous presence of
magnetohydrodynamic waves in the solar corona. The propagating waves
(in contrast to the standing waves) are usually originated in the lower
solar atmosphere which makes them particularly relevant to coronal
heating. Furthermore, open coronal structures are believed to be the
source regions of solar wind, therefore, the detection of MHD waves
in these structures is also pertinent to the acceleration of solar
wind. Besides, the advanced capabilities of the current generation
telescopes have allowed us to extract important coronal properties
through MHD seismology. The recent progress made in the detection,
origin, and damping of both propagating slow magnetoacoustic waves and
kink (Alfvénic) waves is presented in this review article especially
in the context of open coronal structures. Where appropriate, we give
an overview on associated theoretical modelling studies. A few of the
important seismological applications of these waves are discussed. The
possible role of Alfvénic waves in the acceleration of solar wind is
also touched upon.
---------------------------------------------------------
Title: Kink Oscillations of Coronal Loops
Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Antolin, P.; Jain, R.;
Kolotkov, D. Y.; Kupriyanova, E. G.; Li, D.; Magyar, N.; Nisticò, G.;
Pascoe, D. J.; Srivastava, A. K.; Terradas, J.; Vasheghani Farahani,
S.; Verth, G.; Yuan, D.; Zimovets, I. V.
2021SSRv..217...73N Altcode: 2021arXiv210911220N
Kink oscillations of coronal loops, i.e., standing kink waves, is
one of the most studied dynamic phenomena in the solar corona. The
oscillations are excited by impulsive energy releases, such as low
coronal eruptions. Typical periods of the oscillations are from a
few to several minutes, and are found to increase linearly with the
increase in the major radius of the oscillating loops. It clearly
demonstrates that kink oscillations are natural modes of the loops,
and can be described as standing fast magnetoacoustic waves with the
wavelength determined by the length of the loop. Kink oscillations are
observed in two different regimes. In the rapidly decaying regime,
the apparent displacement amplitude reaches several minor radii of
the loop. The damping time which is about several oscillation periods
decreases with the increase in the oscillation amplitude, suggesting a
nonlinear nature of the damping. In the decayless regime, the amplitudes
are smaller than a minor radius, and the driver is still debated. The
review summarises major findings obtained during the last decade,
and covers both observational and theoretical results. Observational
results include creation and analysis of comprehensive catalogues of
the oscillation events, and detection of kink oscillations with imaging
and spectral instruments in the EUV and microwave bands. Theoretical
results include various approaches to modelling in terms of the
magnetohydrodynamic wave theory. Properties of kink oscillations are
found to depend on parameters of the oscillating loop, such as the
magnetic twist, stratification, steady flows, temperature variations
and so on, which make kink oscillations a natural probe of these
parameters by the method of magnetohydrodynamic seismology.
---------------------------------------------------------
Title: Magnetic field inference in active region coronal loops using
coronal rain clumps
Authors: Kriginsky, M.; Oliver, R.; Antolin, P.; Kuridze, D.; Freij, N.
2021A&A...650A..71K Altcode: 2021arXiv210403089K
<BR /> Aims: We aim to infer information about the magnetic field in
the low solar corona from coronal rain clumps using high-resolution
spectropolarimetric observations in the Ca II 8542 Å line
obtained with the Swedish 1 m Solar Telescope. <BR /> Methods: The
weak-field approximation (WFA) provides a simple tool to obtain the
line-of-sight component of the magnetic field from spectropolarimetric
observations. We adapted a method developed in a previous paper in
order to assess the different conditions that must be satisfied in
order to properly use the WFA for the data at hand. We also made use
of velocity measurements in order to estimate the plane-of-the-sky
magnetic field component, so that the magnetic field vector could be
inferred. <BR /> Results: We have inferred the magnetic field vector
from a data set totalling 100 spectral scans in the Ca II 8542 Å line,
containing an off-limb view of the lower portion of catastrophically
cooled coronal loops in an active region. Our results, albeit limited by
the cadence and signal-to-noise ratio of the data, suggest that magnetic
field strengths of hundreds of Gauss, even reaching up to 1000 G, are
omnipresent at coronal heights below 9 Mm from the visible limb. Our
results are also compatible with the presence of larger magnetic
field values such as those reported by previous works. However, for
large magnetic fields, the Doppler width from coronal rain is not
that much larger than the Zeeman width, thwarting the application
of the WFA. Furthermore, we have determined the temperature, T,
and microturbulent velocity, ξ, of coronal rain clumps and off-limb
spicules present in the same data set, and we have found that the former
ones have narrower T and ξ distributions, their average temperature is
similar, and coronal rain has microturbulent velocities smaller than
those of spicules. <P />Movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202140611/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: A New View of the Solar Interface Region from the Interface
Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, Bart; Polito, Vanessa; Hansteen, Viggo; Testa,
Paola; Reeves, Katharine K.; Antolin, Patrick; Nóbrega-Siverio,
Daniel Elias; Kowalski, Adam F.; Martinez-Sykora, Juan; Carlsson,
Mats; McIntosh, Scott W.; Liu, Wei; Daw, Adrian; Kankelborg, Charles C.
2021SoPh..296...84D Altcode: 2021arXiv210316109D
The Interface Region Imaging Spectrograph (IRIS) has been obtaining
near- and far-ultraviolet images and spectra of the solar atmosphere
since July 2013. IRIS is the highest resolution observatory to provide
seamless coverage of spectra and images from the photosphere into the
low corona. The unique combination of near- and far-ultraviolet spectra
and images at sub-arcsecond resolution and high cadence allows the
tracing of mass and energy through the critical interface between the
surface and the corona or solar wind. IRIS has enabled research into the
fundamental physical processes thought to play a role in the low solar
atmosphere such as ion-neutral interactions, magnetic reconnection, the
generation, propagation, and dissipation of waves, the acceleration of
non-thermal particles, and various small-scale instabilities. IRIS has
provided insights into a wide range of phenomena including the discovery
of non-thermal particles in coronal nano-flares, the formation and
impact of spicules and other jets, resonant absorption and dissipation
of Alfvénic waves, energy release and jet-like dynamics associated
with braiding of magnetic-field lines, the role of turbulence and the
tearing-mode instability in reconnection, the contribution of waves,
turbulence, and non-thermal particles in the energy deposition during
flares and smaller-scale events such as UV bursts, and the role of flux
ropes and various other mechanisms in triggering and driving CMEs. IRIS
observations have also been used to elucidate the physical mechanisms
driving the solar irradiance that impacts Earth's upper atmosphere,
and the connections between solar and stellar physics. Advances in
numerical modeling, inversion codes, and machine-learning techniques
have played a key role. With the advent of exciting new instrumentation
both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST)
and the Atacama Large Millimeter/submillimeter Array (ALMA), and
space-based, e.g. the Parker Solar Probe and the Solar Orbiter, we aim
to review new insights based on IRIS observations or related modeling,
and highlight some of the outstanding challenges.
---------------------------------------------------------
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
2021SoPh..296...70R Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
---------------------------------------------------------
Title: The First 3D Coronal Loop Model Heated by MHD Waves against
Radiative Losses
Authors: Shi, Mijie; Van Doorsselaere, Tom; Guo, Mingzhe; Karampelas,
Konstantinos; Li, Bo; Antolin, Patrick
2021ApJ...908..233S Altcode: 2021arXiv210101019S
In the quest to solve the long-standing coronal heating problem, it
was suggested half a century ago that coronal loops could be heated by
waves. Despite the accumulating observational evidence of the possible
importance of coronal waves, still no 3D MHD simulations exist that
show significant heating by MHD waves. Here we report on the first 3D
coronal loop model that heats the plasma against radiative cooling. The
coronal loop is driven at the footpoint by transverse oscillations,
and subsequently the induced Kelvin-Helmholtz instability deforms the
loop cross section and generates small-scale structures. Wave energy
is transferred to smaller scales where it is dissipated, overcoming
the internal energy losses by radiation. These results open up a new
avenue to address the coronal heating problem.
---------------------------------------------------------
Title: On Cooling Condensation Near Magnetic Null Points and the
Formation of Solar Coronal Rain and Prominences
Authors: Liu, Wei; Titov, Viacheslav; Downs, Cooper; Antolin, Patrick;
Luna, Manuel; Sun, Xudong; Berger, Thomas; Yu, Sijie; Yoffe, Luke
2021cosp...43E.975L Altcode:
The Sun's outer atmosphere, the corona, is million-degrees hot and
tenuous. Such hot plasma, under certain conditions, can enigmatically
undergo a radiative cooling instability and condense into material of
100 times cooler in the form of coronal rain or prominences. Where,
when, and how such cooling condensation takes place remain poorly
understood. Answers to these questions are not only important in their
own right, but also bear implications for the fundamental question
of coronal heating and the chromosphere-corona mass cycle. Magnetic
fields in the magnetized corona undoubtedly play a crucial role (e.g.,
by trapping the plasma), but where and how? We report recent imaging
and spectroscopic observations from SDO/AIA/HMI and IRIS that can
shed light on this puzzle. Through a systematic survey, we found that
a large fraction of quiet-Sun condensations preferentially occur at
the dips of coronal loops or funnels. Such dips are located at/near
magnetic topological features, such as null points and quasi-separatrix
layers (QSLs), which are regions characterized by high values of the
squashing factor. We also identified evidence of magnetic reconnection
at such locations, which can produce favorable conditions, e.g., density
enhancement by compression and/or mass trapping in plasmoids, that can
trigger run-away radiative cooling. We present proof-of-concept MHD
simulations that demonstrate the role of reconnection in transporting
cooled mass from overlying, long loops to underlying, short loops where
it slides down as coronal rain. We will discuss the significance and
broader implications of these results beyond the Sun.
---------------------------------------------------------
Title: Reconnection nanojets in the solar corona
Authors: Antolin, Patrick; Pagano, Paolo; Testa, Paola; Petralia,
Antonino; Reale, Fabio
2021NatAs...5...54A Altcode: 2020NatAs.tmp..201A; 2020NatAs.tmp..186A
The solar corona is shaped and mysteriously heated to millions of
degrees by the Sun's magnetic field. It has long been hypothesized
that the heating results from a myriad of tiny magnetic energy
outbursts called nanoflares, driven by the fundamental process of
magnetic reconnection. Misaligned magnetic field lines can break and
reconnect, producing nanoflares in avalanche-like processes. However,
no direct and unique observations of such nanoflares exist to date,
and the lack of a smoking gun has cast doubt on the possibility of
solving the coronal heating problem. From coordinated multi-band
high-resolution observations, we report on the discovery of very fast
and bursty nanojets, the telltale signature of reconnection-based
nanoflares resulting in coronal heating. Using state-of-the-art
numerical simulations, we demonstrate that the nanojet is a consequence
of the slingshot effect from the magnetically tensed, curved magnetic
field lines reconnecting at small angles. Nanojets are therefore the
key signature of reconnection-based coronal heating in action.
---------------------------------------------------------
Title: Reconnection Nanojets in the Solar Corona
Authors: Antolin, Patrick; Reale, Fabio; Testa, Paola; Pagano, Paolo;
Petralia, Antonino
2021cosp...43E1798A Altcode:
The solar corona is shaped and mysteriously heated to millions of
degrees by the Sun's magnetic field. It has long been hypothesised that
the heating results from a myriad of tiny magnetic energy outbursts
called nanoflares driven by the fundamental process of magnetic
reconnection. Misaligned magnetic field lines can break and reconnect,
producing nanoflares in avalanche-like processes. However, no direct
and unique observations of such nanoflares exist to date, and the
lack of a smoking gun has cast doubt on the possibility of solving the
coronal heating problem. From coordinated multi-band high-resolution
observations here we report on the discovery of very fast and bursty
nanojets, the telltale signature of reconnection-based nanoflares
resulting in coronal heating. Isolated and clustered nanojets are
detected, and a myriad are observed in an avalanche-like progression,
leading to the formation of a coronal loop. Using state-of-the-art
numerical simulations we demonstrate that the nanojet is a consequence
of the slingshot effect from the magnetically tensed, curved magnetic
field lines reconnecting at small angles. Nanojets are therefore the
key signature to look for reconnection-based coronal heating in action.
---------------------------------------------------------
Title: Thermal instability-induced fundamental magnetic strands in
coronal loops
Authors: Antolin, Patrick; Martinez-Sykora, Juan
2021cosp...43E.968A Altcode:
Thermal instability is a fundamental process of astrophysical
plasmas. It is expected to occur whenever the cooling is dominated
by radiation and cannot be compensated by heating. This mechanism has
been invoked to explain structures at multiple scales in the Universe,
from the filamentary structure of the ISM to the phenomenon of coronal
rain in the solar corona. In this work we conduct 2.5-D Radiation MHD
simulations with the Bifrost code of an enhanced activity network in
the solar atmosphere. Coronal loops are produced self-consistently,
mainly through Ohmic heating, which is stratified and of a high enough
frequency as to produce thermal non-equilibrium. During the cooling
and driven by thermal instability, coronal rain is produced along the
loops. Due to flux freezing, the catastrophic cooling process leading
to a rain clump produces a local enhancement of the magnetic field,
thereby generating a distinct magnetic strand within the loop up to a
few Gauss stronger than the ambient corona. The compression downstream
leads to an increase in temperature that generates a strongly emitting
spicule-like feature in the UV during the rain impact. The stronger
magnetic field strength in the rarefied upstream region has a stronger
Ohmic heating, leading to a filamentary coronal strand with enhanced
EUV emission. Thermal instability and _x0005_non-equilibrium can
therefore be associated with localised and intermittent UV brightening
in the transition region and chromosphere, as well as contribute to
the characteristic filamentary morphology of the solar corona in the
EUV. An additional effect of a strand with enhanced magnetic field is to
serve as a waveguide, which combined with the Ohmic heating can act as a
seed to sustain the coronal loop and the thermal non-equilibrium cycle.
---------------------------------------------------------
Title: Publisher Correction: Reconnection nanojets in the solar corona
Authors: Antolin, Patrick; Pagano, Paolo; Testa, Paola; Petralia,
Antonino; Reale, Fabio
2021NatAs...5..103A Altcode: 2020NatAs.tmp..204A
An amendment to this paper has been published and can be accessed via
a link at the top of the paper.
---------------------------------------------------------
Title: Magnetohydrodynamic Fast Sausage Waves in the Solar Corona
Authors: Li, B.; Antolin, P.; Guo, M. -Z.; Kuznetsov, A. A.; Pascoe,
D. J.; Van Doorsselaere, T.; Vasheghani Farahani, S.
2020SSRv..216..136L Altcode: 2020arXiv201016023L
Characterized by cyclic axisymmetric perturbations to both the magnetic
and fluid parameters, magnetohydrodynamic fast sausage modes (FSMs)
have proven useful for solar coronal seismology given their strong
dispersion. This review starts by summarizing the dispersive properties
of the FSMs in the canonical configuration where the equilibrium
quantities are transversely structured in a step fashion. With this
preparation we then review the recent theoretical studies on coronal
FSMs, showing that the canonical dispersion features have been better
understood physically, and further exploited seismologically. In
addition, we show that departures from the canonical equilibrium
configuration have led to qualitatively different dispersion features,
thereby substantially broadening the range of observations that FSMs
can be invoked to account for. We also summarize the advances in
forward modeling studies, emphasizing the intricacies in interpreting
observed oscillatory signals in terms of FSMs. All these advances
notwithstanding, we offer a list of aspects that remain to be better
addressed, with the physical connection of coronal FSMs to the
quasi-periodic pulsations in solar flares particularly noteworthy.
---------------------------------------------------------
Title: Coronal Heating by MHD Waves
Authors: Van Doorsselaere, Tom; Srivastava, Abhishek K.; Antolin,
Patrick; Magyar, Norbert; Vasheghani Farahani, Soheil; Tian, Hui;
Kolotkov, Dmitrii; Ofman, Leon; Guo, Mingzhe; Arregui, Iñigo; De
Moortel, Ineke; Pascoe, David
2020SSRv..216..140V Altcode: 2020arXiv201201371V
The heating of the solar chromosphere and corona to the observed high
temperatures, imply the presence of ongoing heating that balances
the strong radiative and thermal conduction losses expected in the
solar atmosphere. It has been theorized for decades that the required
heating mechanisms of the chromospheric and coronal parts of the active
regions, quiet-Sun, and coronal holes are associated with the solar
magnetic fields. However, the exact physical process that transport
and dissipate the magnetic energy which ultimately leads to the solar
plasma heating are not yet fully understood. The current understanding
of coronal heating relies on two main mechanism: reconnection and MHD
waves that may have various degrees of importance in different coronal
regions. In this review we focus on recent advances in our understanding
of MHD wave heating mechanisms. First, we focus on giving an overview
of observational results, where we show that different wave modes have
been discovered in the corona in the last decade, many of which are
associated with a significant energy flux, either generated in situ
or pumped from the lower solar atmosphere. Afterwards, we summarise
the recent findings of numerical modelling of waves, motivated by the
observational results. Despite the advances, only 3D MHD models with
Alfvén wave heating in an unstructured corona can explain the observed
coronal temperatures compatible with the quiet Sun, while 3D MHD wave
heating models including cross-field density structuring are not yet
able to account for the heating of coronal loops in active regions to
their observed temperature.
---------------------------------------------------------
Title: Ubiquitous hundred-Gauss magnetic fields in solar spicules
Authors: Kriginsky, M.; Oliver, R.; Freij, N.; Kuridze, D.; Asensio
Ramos, A.; Antolin, P.
2020A&A...642A..61K Altcode: 2020arXiv200601809K
<BR /> Aims: We aim to study the magnetic field in solar spicules
using high-resolution spectropolarimetric observations in the Ca II
8542 Å line obtained with the Swedish 1-m Solar Telescope. <BR />
Methods: The equations that result from the application of the weak
field approximation (WFA) to the radiative transfer equations were
used to infer the line-of-sight (LOS) component of the magnetic
field (B<SUB>LOS</SUB>). Two restrictive conditions were imposed
on the Stokes I and V profiles at each pixel before they could be
used in a Bayesian inversion to compute its B<SUB>LOS</SUB>. <BR />
Results: The LOS magnetic field component was inferred in six data sets
totalling 448 spectral scans in the Ca II 8542 Å line and containing
both active region and quiet Sun areas, with values of hundreds of
Gauss being abundantly inferred. There seems to be no difference,
from a statistical point of view, between the magnetic field strength
of spicules in the quiet Sun or near an active region. On the other
hand, the B<SUB>LOS</SUB> distributions present smaller values on
the disc than off-limb, a fact that can be explained by the effect of
superposition on the chromosphere of on-disc structures. We show that
on-disc pixels in which the B<SUB>LOS</SUB> is determined are possibly
associated with spicular structures because these pixels are co-spatial
with the magnetic field concentrations at the network boundaries and
the sign of their B<SUB>LOS</SUB> agrees with that of the underlying
photosphere. We find that spicules in the vicinity of a sunspot have
a magnetic field polarity (i.e. north or south) equal to that of the
sunspot. This paper also contains an analysis of the effect of off-limb
overlapping structures on the observed Stokes I and V parameters and
the B<SUB>LOS</SUB> obtained from the WFA. It is found that this value
is equal to or smaller than the largest LOS magnetic field components
of the two structures. In addition, using random B<SUB>LOS</SUB>,
Doppler velocities, and line intensities of these two structures
leads in ≃50% of the cases to Stokes I and V parameters that are
unsuitable to be used with the WFA. <BR /> Conclusions: Our results
present a scarcity of LOS magnetic field components smaller than some
50 G, which must not be taken as evidence against the existence of
these magnetic field strengths in spicules. This fact possibly arises
as the consequence of signal superposition and noise in the data. We
also suggest that the failure of previous works to infer the strong
magnetic fields in spicules detected here is their coarser spatial
and/or temporal resolution.
---------------------------------------------------------
Title: Magnetic field inference in the chromosphere and lower corona
Authors: Kriginsky, M.; Oliver, R.; Freij, N.; Kuridze, D.; Asensio
Ramos, A.; Antolin, P.
2020sea..confE.201K Altcode:
The Weak Field Approximation (WFA) is used to infer the line-of-sight
magnetic field of the solar chromosphere and lower corona. Using near
limb spectropolarimetric observations in the Ca II 8542 Å line taken
with the CRISP instrument at the Swedish 1-metre telescope in La Palma,
the presence of an active region near/in the field of view allows
for the presence of chromospheric spicules and coronal rain blobs
to be detected. This work focuses mostly in the inference of magnetic
fields of off-limb spicules, but a successful attempt to obtain Stokes V
signal from the coronal rain blobs allowed for the inference of coronal
magnetic fields. A careful treatment of the data pixels is undertaken in
order to guarantee the correct application of the WFA, and the results
show the presence of ubiquitous hundred-Gauss magnetic fields in the
spicular material and in the coronal rain blobs. A Bayesian approach
is used to infer the results.
---------------------------------------------------------
Title: Self-consistent 3D radiative magnetohydrodynamic simulations
of coronal rain formation and evolution
Authors: Kohutova, P.; Antolin, P.; Popovas, A.; Szydlarski, M.;
Hansteen, V. H.
2020A&A...639A..20K Altcode: 2020arXiv200503317K
Context. Coronal rain consists of cool and dense plasma condensations
formed in coronal loops as a result of thermal instability. <BR />
Aims: Previous numerical simulations of thermal instability and
coronal rain formation have relied on the practice of artificially
adding a coronal heating term to the energy equation. To reproduce
large-scale characteristics of the corona, the use of more realistic
coronal heating prescription is necessary. <BR /> Methods: We analysed
coronal rain formation and evolution in a three-dimensional radiative
magnetohydrodynamic simulation spanning from convection zone to
corona which is self-consistently heated by magnetic field braiding
as a result of convective motions. <BR /> Results: We investigate
the spatial and temporal evolution of energy dissipation along
coronal loops which become thermally unstable. Ohmic dissipation in
the model leads to the heating events capable of inducing sufficient
chromospheric evaporation into the loop to trigger thermal instability
and condensation formation. The cooling of the thermally unstable
plasma occurs on timescales that are comparable to the duration
of the individual impulsive heating events. The impulsive heating
has sufficient duration to trigger thermal instability in the
loop but does not last long enough to lead to coronal rain limit
cycles. We show that condensations can either survive and fall into
the chromosphere or be destroyed by strong bursts of Joule heating
associated with a magnetic reconnection events. In addition, we find
that condensations can also form along open magnetic field lines. <BR />
Conclusions: We modelled, for the first time, coronal rain formation in
a self-consistent 3D radiative magnetohydrodynamic simulation, in which
the heating occurs mainly through the braiding and subsequent Ohmic
dissipation of the magnetic field. The heating is stratified enough
and lasts for long enough along specific field lines to produce the
necessary chromospheric evaporation that triggers thermal instability
in the corona. <P />Movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202037899/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Temporal and Spatial Scales in Coronal Rain Revealed by UV
Imaging and Spectroscopic Observations
Authors: Ishikawa, Ryohtaroh T.; Katsukawa, Yukio; Antolin, Patrick;
Toriumi, Shin
2020SoPh..295...53I Altcode: 2020arXiv200313214I
Coronal rain corresponds to cool and dense clumps in the corona
accreting towards the solar surface; it is often observed above solar
active regions. These clumps are generally thought to be produced by
a thermal instability in the corona and their lifetime is limited by
the time they take to reach the chromosphere. Although the rain usually
fragments into smaller clumps while falling down, their specific spatial
and temporal scales remain unclear. In addition, the observational
signatures of the impact of the rain with the chromosphere have not been
clarified yet. In this study, we investigate the time evolution of the
velocity and intensity of coronal rain above a sunspot by analyzing
coronal images obtained by the Atmospheric Imaging Assembly (AIA)
onboard the Solar Dynamics Observatory (SDO) as well as the slit-jaw
images (SJIs) and spectral data taken by the Interface Region Imaging
Spectrograph (IRIS) satellite. We identify dark and bright threads
moving towards the umbra in AIA images and in SJIs, respectively,
and co-spatial chromospheric intensity enhancements and redshifts in
three IRIS spectral lines, Mg II k 2796 Å, Si IV 1394 Å, and C II
1336 Å. The intensity enhancements and coronal rain redshifts occur
almost concurrently in all the three lines, which clearly demonstrates
the causal relationship with coronal rain. Furthermore, we detect bursty
intensity variation with a time scale shorter than 1 minute in Mg II k,
Si IV, and C II, indicating that a length scale of rain clumps is about
2.7 Mm if we multiply the typical time scale of the busty intensity
variation at 30 sec by the rain velocity at 90 kms−<SUP>1</SUP>. Such
rapid enhancements in the IRIS lines are excited within a time lag
of 5.6 sec limited by the temporal resolution. These temporal and
spatial scales may reflect the physical processes responsible for
the rain morphology, and are suggestive of instabilities such as the
Kelvin-Helmholtz instability.
---------------------------------------------------------
Title: Electron Beams Cannot Directly Produce Coronal Rain
Authors: Reep, Jeffrey W.; Antolin, Patrick; Bradshaw, Stephen J.
2020ApJ...890..100R Altcode: 2020arXiv200207669R
Coronal rain is ubiquitous in flare loops, forming shortly after the
onset of the solar flare. Rain is thought to be caused by a thermal
instability, a localized runaway cooling of material in the corona. The
models that demonstrate this require extremely long duration heating on
the order of the radiative cooling time, localized near the footpoints
of the loops. In flares, electron beams are thought to be the primary
energy transport mechanism, driving strong footpoint heating during
the impulsive phase that causes evaporation, filling and heating flare
loops. Electron beams, however, do not act for a long period of time,
and even supposing that they did, their heating would not remain
localized at the footpoints. With a series of numerical experiments,
we show directly that these two issues mean that electron beams are
incapable of causing the formation of rain in flare loops. This result
suggests that either there is another mechanism acting in flare loops
responsible for rain, or that the modeling of the cooling of flare loops
is somehow deficient. To adequately describe flares, the standard model
must address this issue to account for the presence of coronal rain.
---------------------------------------------------------
Title: Multi-scale observations of thermal non-equilibrium cycles
in coronal loops
Authors: Froment, C.; Antolin, P.; Henriques, V. M. J.; Kohutova,
P.; Rouppe van der Voort, L. H. M.
2020A&A...633A..11F Altcode: 2019arXiv191109710F
Context. Thermal non-equilibrium (TNE) is a phenomenon that can
occur in solar coronal loops when the heating is quasi-constant and
highly-stratified. Under such heating conditions, coronal loops undergo
cycles of evaporation and condensation. The recent observations of
ubiquitous long-period intensity pulsations in coronal loops and their
relationship with coronal rain have demonstrated that understanding the
characteristics of TNE cycles is an essential step in constraining
the circulation of mass and energy in the corona. <BR /> Aims:
We report unique observations with the Solar Dynamics Observatory
(SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured
thermal properties across the extreme spatiotemporal scales covered by
TNE processes. <BR /> Methods: Within the same coronal loop bundle,
we captured 6 h period coronal intensity pulsations in SDO/AIA and
coronal rain observed off-limb in the chromospheric Hα and Ca
II K spectral lines with SST/CRISP and SST/CHROMIS. We combined
a multi-thermal analysis of the cycles with AIA and an extensive
spectral characterisation of the rain clumps with the SST. <BR />
Results: We find clear evidence of evaporation-condensation cycles in
the corona which are linked with periodic coronal rain showers. The
high-resolution spectroscopic instruments at the SST reveal the
fine-structured rain strands and allow us to probe the cooling
phase of one of the cycles down to chromospheric temperatures. <BR />
Conclusions: These observations reinforce the link between long-period
intensity pulsations and coronal rain. They also demonstrate the
capability of TNE to shape the dynamics of active regions on the large
scales as well as on the smallest scales currently resolvable. <P
/>Movies associated to Figs. 3-5, and 8 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936717/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Thermal instability and non-equilibrium in solar coronal loops:
from coronal rain to long-period intensity pulsations
Authors: Antolin, Patrick
2020PPCF...62a4016A Altcode:
The complex interaction of the magnetic field with matter is the key
to some of the most puzzling observed phenomena at multiple scales
across the Universe, from tokamak plasma confinement experiments in the
laboratory to the filamentary structure of the interstellar medium. A
major astrophysical puzzle is the phenomenon of coronal heating, upon
which the most external layer of the solar atmosphere, the corona,
is sustained at multi-million degree temperatures on average. However,
the corona also conceals a cooling problem. Indeed, recent observations
indicate that, even more mysteriously, like snowflakes in the oven,
the corona hosts large amounts of cool material termed coronal
rain, hundreds of times colder and denser, that constitute the seed
of the famous prominences. Numerical simulations have shown that
this cold material does not stem from the inefficiency of coronal
heating mechanisms, but results from the specific spatio-temporal
properties of these. As such, a large fraction of coronal loops,
the basic constituents of the solar corona, are suspected to be in
a state of thermal non-equilibrium (TNE), characterised by heating
(evaporation) and cooling (condensation) cycles whose telltale
observational signatures are long-period intensity pulsations in hot
lines and thermal instability-driven coronal rain in cool lines, both
now ubiquitously observed. In this paper, we review this yet largely
unexplored strong connection between the observed properties of hot
and cool material in TNE and instability and the underlying coronal
heating mechanisms. Focus is set on the long-observed coronal rain,
for which significant research already exists, contrary to the recently
discovered long-period intensity pulsations. We further identify the
outstanding open questions in what constitutes a new, rapidly growing
field of solar physics.
---------------------------------------------------------
Title: Editorial: Magnetohydrodynamic Waves in the Solar Atmosphere:
Heating and Seismology
Authors: Van Doorsselaere, Tom; Nakariakov, Valery M.; Li, Bo;
Antolin, Patrick
2020FrASS...6...79V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Multi-slit Approach to Coronal Spectroscopy with the
Multi-slit Solar Explorer (MUSE)
Authors: De Pontieu, Bart; Martínez-Sykora, Juan; Testa, Paola;
Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo; Cheung, Mark C. M.;
Antolin, Patrick
2020ApJ...888....3D Altcode: 2019arXiv190908818D
The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed
at understanding the physical mechanisms driving the heating of the
solar corona and the eruptions that are at the foundation of space
weather. MUSE contains two instruments, a multi-slit extreme ultraviolet
(EUV) spectrograph and a context imager. It will simultaneously
obtain EUV spectra (along 37 slits) and context images with the
highest resolution in space (0.″33-0.″4) and time (1-4 s) ever
achieved for the transition region (TR) and corona. The MUSE science
investigation will exploit major advances in numerical modeling, and
observe at the spatial and temporal scales on which competing models
make testable and distinguishable predictions, thereby leading to a
breakthrough in our understanding of coronal heating and the drivers
of space weather. By obtaining spectra in four bright EUV lines (Fe
IX 171 Å, Fe XV 284 Å, Fe XIX 108Å, Fe XXI 108 Å) covering a wide
range of TR and coronal temperatures along 37 slits simultaneously,
MUSE will be able to “freeze” the evolution of the dynamic
coronal plasma. We describe MUSE’s multi-slit approach and show
that the optimization of the design minimizes the impact of spectral
lines from neighboring slits, generally allowing line parameters to
be accurately determined. We also describe a Spectral Disambiguation
Code to resolve multi-slit ambiguity in locations where secondary lines
are bright. We use simulations of the corona and eruptions to perform
validation tests and show that the multi-slit disambiguation approach
allows accurate determination of MUSE observables in locations where
significant multi-slit contamination occurs.
---------------------------------------------------------
Title: Cooling Condensation at Coronal Null Points and
Quasi-Separatrix Layers Involving Magnetic Reconnection
Authors: Liu, W.; Sun, X.; Yu, S.; Luna Bennasar, M.; Antolin, P.;
Titov, V. S.; Downs, C.; Berger, T. E.
2019AGUFMSH11C3394L Altcode:
The solar corona, Sun's outer atmosphere, is million-degrees hot and
tenuous. This hot plasma, under certain conditions, can enigmatically
undergo a radiative cooling instability and condense into material of
100 times cooler in the form of prominences or coronal rain. Where,
when, and how such cooling condensation takes place remain poorly
understood. Answers to these questions are not only of scientific
importance in their own right, but also bear implications for the
fundamental question of coronal heating and the chromosphere-corona
mass cycle. Magnetic fields in the magnetized corona undoubtedly play
a crucial role (e.g., by trapping the plasma), but where and how? We
report recent imaging and spectroscopic observations from SDO/AIA/HMI
and IRIS that can shed light on these puzzles. Through a systematic
survey, we found that a large fraction of quiet-Sun condensations
preferentially occur at the dips of coronal loops or funnels. Such dips
are located at/near magnetic topological features, such as null points
and quasi-separatrix layers (QSLs), which are regions characterized by
high values of the squashing factor. We also identified evidence of
magnetic reconnection at such locations, which can produce favorable
conditions, e.g., density enhancement by compression and/or mass
trapping in plasmoids, that can trigger run-away radiative cooling. We
present proof-of-concept MHD simulations that demonstrate the role of
reconnection in transporting cooled mass from overlying, long loops to
underlying, short loops where it slide down as coronal rain. We will
discuss the significance and broader implications of these results
beyond solar physics.
---------------------------------------------------------
Title: Resonant absorption in expanding coronal magnetic flux tubes
with uniform density
Authors: Howson, T. A.; De Moortel, I.; Antolin, P.; Van Doorsselaere,
T.; Wright, A. N.
2019A&A...631A.105H Altcode: 2019arXiv190910781H
<BR /> Aims: We investigate the transfer of energy between a fundamental
standing kink mode and azimuthal Alfvén waves within an expanding
coronal magnetic flux tube. We consider the process of resonant
absorption in a loop with a non-uniform Alfvén frequency profile but
in the absence of a radial density gradient. <BR /> Methods: Using the
three dimensional magnetohydrodynamic (MHD) code, Lare3d, we modelled a
transversely oscillating magnetic flux tube that expands radially with
height. An initially straight loop structure with a magnetic field
enhancement was allowed to relax numerically towards a force-free
state before a standing kink mode was introduced. The subsequent
dynamics, rate of wave damping and formation of small length scales are
considered. <BR /> Results: We demonstrate that the transverse gradient
in Alfvén frequency required for the existence of resonant field lines
can be associated with the expansion of a high field-strength flux tube
from concentrated flux patches in the lower solar atmosphere. This
allows for the conversion of energy between wave modes even in the
absence of the transverse density profile typically assumed in wave
heating models. As with standing modes in straight flux tubes, small
scales are dominated by the vorticity at the loop apex and by currents
close to the loop foot points. The azimuthal Alfvén wave exhibits the
structure of the expanded flux tube and is therefore associated with
smaller length scales close to the foot points of the flux tube than
at the loop apex. <BR /> Conclusions: Resonant absorption can proceed
throughout the coronal volume, even in the absence of visible, dense,
loop structures. The flux tube and MHD waves considered are difficult
to observe and our model highlights how estimating hidden wave power
within the Sun's atmosphere can be problematic. We highlight that,
for standing modes, the global properties of field lines are important
for resonant absorption and coronal conditions at a single altitude
will not fully determine the nature of MHD resonances. In addition,
we provide a new model in partial response to the criticism that wave
heating models cannot self-consistently generate or sustain the density
profile upon which they typically rely.
---------------------------------------------------------
Title: Kelvin-Helmholtz Instability and Alfvénic Vortex Shedding
in Solar Eruptions
Authors: Syntelis, P.; Antolin, P.
2019ApJ...884L...4S Altcode: 2019arXiv190905716S
We report on a three-dimensional MHD numerical experiment of a
small-scale coronal mass ejection (CME)-like eruption propagating though
a nonmagnetized solar atmosphere. We find that the Kelvin-Helmholtz
instability (KHI) develops at various but specific locations at
the boundary layer between the erupting field and the background
atmosphere, depending on the relative angle between the velocity
and magnetic field. KHI develops at the front and at two of the
four sides of the eruption. KHI is suppressed at the other two
sides of the eruption. We also find the development of Alfvénic
vortex shedding flows at the wake of the developing CME due to the 3D
geometry of the field. Forward modeling reveals that the observational
detectability of the KHI in solar eruptions is confined to a narrow
≈10° range when observing off-limb, and therefore its occurrence
could be underestimated due to projection effects. The new findings
can have significant implications for observations, for heating, and
for particle acceleration by turbulence from flow-driven instabilities
associated with solar eruptions of all scales.
---------------------------------------------------------
Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
2019PASJ...71R...1H Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
---------------------------------------------------------
Title: Fundamental transverse vibrations of the active region
solar corona
Authors: Luna, M.; Oliver, R.; Antolin, P.; Arregui, I.
2019A&A...629A..20L Altcode: 2019arXiv190705212L
Context. Some high-resolution observations have revealed that the
active region solar corona is filled with a myriad of thin strands
even in apparently uniform regions with no resolved loops. This fine
structure can host collective oscillations involving a large portion
of the corona due to the coupling of the motions of the neighbouring
strands. <BR /> Aims: We study these vibrations and the possible
observational effects. <BR /> Methods: We theoretically investigated the
collective oscillations inherent to the fine structure of the corona. We
have called them fundamental vibrations because they cannot exist in
a uniform medium. We used the T-matrix technique to find the normal
modes of random arrangements of parallel strands. We considered an
increasing number of tubes to understand the vibrations of a huge number
of tubes of a large portion of the corona. We additionally generated
synthetic time-distance Doppler and line-broadening diagrams of the
vibrations of a coronal region to compare with observations. <BR />
Results: We have found that the fundamental vibrations are in the
form of clusters of tubes where not all the tubes participate in
the collective mode. The periods are distributed over a wide band of
values. The width of the band increases with the number of strands
but rapidly reaches an approximately constant value. We have found an
analytic approximate expression for the minimum and maximum periods
of the band. The frequency band associated with the fine structure
of the corona depends on the minimum separation between strands. We
have found that the coupling between the strands is on a large
extent and the motion of one strand is influenced by the motions of
distant tubes. The synthetic Dopplergrams and line-broadening maps
show signatures of collective vibrations, not present in the case
of purely random individual kink vibrations. <BR /> Conclusions: We
conclude that the fundamental vibrations of the corona can contribute
to the energy budget of the corona and they may have an observational
signature. <P />A movie associated to Fig. 10 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935850/olm">http://
https://www.aanda.org </A>
---------------------------------------------------------
Title: Multi-component Decomposition of Astronomical Spectra by
Compressed Sensing
Authors: Cheung, Mark C. M.; De Pontieu, Bart; Martínez-Sykora,
Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo;
Antolin, Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young,
Peter; MUSE Team
2019ApJ...882...13C Altcode: 2019arXiv190203890C
The signal measured by an astronomical spectrometer may be due to
radiation from a multi-component mixture of plasmas with a range of
physical properties (e.g., temperature, Doppler velocity). Confusion
between multiple components may be exacerbated if the spectrometer
sensor is illuminated by overlapping spectra dispersed from different
slits, with each slit being exposed to radiation from a different
portion of an extended astrophysical object. We use a compressed sensing
method to robustly retrieve the different components. This method can
be adopted for a variety of spectrometer configurations, including
single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer
mission), and slot spectrometers (which produce overlappograms).
---------------------------------------------------------
Title: Coronal Condensation at Preferential Topological Locations:
The Birth of Solar Prominences and Coronal Rain
Authors: Liu, Wei; Sun, Xudong; Yu, Sijie; Antolin, Patrick; Titov,
Viacheslav; Downs, Cooper; Berger, Thomas
2019AAS...23412502L Altcode:
The million-degree hot and tenuous solar coronal plasma, under
certain conditions, can enigmatically undergo a radiative cooling
instability and condense into material of 100 times cooler in the form
of prominences or coronal rain. Where, when, and how such cooling
condensation takes place remain poorly understood. Answers to these
questions are not only of scientific importance in their own right,
but also bear implications for the fundamental question of coronal
heating and the chromosphere-corona mass cycle. Magnetic fields in the
magnetized corona undoubtedly play a crucial role (e.g., by trapping the
plasma), but where and how? We report recent imaging and spectroscopic
observations from SDO/AIA/HMI and IRIS that can shed light on these
puzzles. Through a systematic survey, we found that a large fraction
of quiet-Sun condensations preferentially occur at the dips of coronal
loops or funnels. Such dips are located at/near magnetic topological
features, such as null points and quasi-separatrix layers (QSLs), which
are regions characterized by high values of the squashing factor. We
also identified evidence of magnetic reconnection at such locations,
which can produce favorable conditions, e.g., density enhancement
by compression and/or mass trapping in plasmoids, that can trigger
run-away radiative cooling. We will discuss the significance and
broader implications of these novel observations.
---------------------------------------------------------
Title: MHD simulations of the in situ generation of kink and sausage
waves in the solar corona by collision of dense plasma clumps
Authors: Pagano, P.; Van Damme, H. J.; Antolin, P.; De Moortel, I.
2019A&A...626A..53P Altcode: 2019arXiv190503749P
Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar
corona where the highly structured magnetic fields provide efficient
wave guides for their propagation. While MHD waves have been observed
originating from lower layers of the solar atmosphere, recent studies
have shown that some can be generated in situ by the collision of dense
counter-propagating flows. <BR /> Aims: In this theoretical study, we
analyse the mechanism that triggers the propagation of kink and sausage
modes in the solar corona following the collision of counter-propagating
flows, and how the properties of the flows affect the properties of
the generated waves. <BR /> Methods: To study in detail this mechanism
we ran a series of ideal 2D and 3D MHD simulations where we varied
the properties of the counter-propagating flows; by means of a simple
technique to estimate the amplitudes of the kink and sausage modes,
we investigated their role in the generation and propagation of the
MHD waves. <BR /> Results: We find that the amplitude of the waves is
largely dependent on the kinetic energy of the flows, and that the
onset of kink or sausage modes depends on the asymmetries between
the colliding blobs. Moreover, the initial wavelength of the MHD
waves is associated with the magnetic configuration resulting from
the collision of the flows. We also find that genuine 3D systems
respond with smaller wave amplitudes. <BR /> Conclusions: In this
study, we present a parameter space description of the mechanism that
leads to the generation of MHD waves from the collision of flows
in the corona. Future observations of these waves can be used to
understand the properties of the plasma and magnetic field of the solar
corona. <P />The movies associated to Figs. 2 and 21 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935539/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Multi-component Decomposition of Astronomical Spectra by
Compressed Sensing
Authors: Cheung, Mark; De Pontieu, Bart; Martinez-Sykora, Juan; Testa,
Paola; Winebarger, Amy R.; Daw, Adrian N.; Hansteen, Viggo; Antolin,
Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young, Peter R.
2019AAS...23411603C Altcode:
The signal measured by an astronomical spectrometer may be due to
radiation from a multi-component mixture of plasmas with a range of
physical properties (e.g. temperature, Doppler velocity). Confusion
between multiple components may be exacerbated if the spectrometer
sensor is illuminated by overlapping spectra dispersed from different
slits, with each slit being exposed to radiation from a different
portion of an extended astrophysical object. We use a compressed sensing
method to robustly retrieve the different components. This method can
be adopted for a variety of spectrometer configurations, including
single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer
mission; MUSE) and slot spectrometers (which produce overlappograms).
---------------------------------------------------------
Title: Amplitudes and energy fluxes of simulated decayless kink
oscillations.
Authors: Karampelas, Konstantinos; Van Doorsselaere, Tom; Pascoe,
David J.; Guo, Mingzhe; Antolin, Patrick
2019FrASS...6...38K Altcode: 2019arXiv190602001K
Recent observations with the Atmospheric Imaging Assembly (AIA)
instrument on the SDO spacecraft have revealed the existence of
decayless coronal kink oscillations. These transverse oscillations
are not connected to any external phenomena like flares or coronal
mass ejections, and show significantly lower amplitudes than the
externally excited decaying oscillations. Numerical studies have
managed to reproduce such decayless oscillations in the form of
footpoint driven standing waves in coronal loops, and to treat them
as a possible mechanism for wave heating of the solar corona. Our aim
is to investigate the correlation between the observed amplitudes of
the oscillations and input the energy flux from different drivers. We
perform 3D MHD simulations in single, straight, density-enhanced coronal
flux tubes for different drivers, in the presence of gravity. Synthetic
images at different spectral lines are constructed with the use of the
FoMo code. The development of the Kelvin-Helmholtz instability leads
to mixing of plasma between the flux tube and the hot corona. Once the
KHI is fully developed, the amplitudes of the decayless oscillations
show only a weak correlation with the driver strength. We find
that low amplitude decayless kink oscillations may correspond to
significant energy fluxes of the order of the radiative losses for
the Quiet Sun. A clear correlation between the input energy flux and
the observed amplitudes from our synthetic imaging data cannot be
established. Stronger drivers lead to higher vales of the line width
estimated energy fluxes. Finally, estimations of the energy fluxes by
spectroscopic data are affected by the LOS angle, favoring combined
analysis of imaging and spectroscopic data for single oscillating loops.
---------------------------------------------------------
Title: The effects of numerical resolution, heating timescales and
background heating on thermal non-equilibrium in coronal loops
Authors: Johnston, C. D.; Cargill, P. J.; Antolin, P.; Hood, A. W.;
De Moortel, I.; Bradshaw, S. J.
2019A&A...625A.149J Altcode: 2019arXiv190407287J
Thermal non-equilibrium (TNE) is believed to be a potentially important
process in understanding some properties of the magnetically closed
solar corona. Through one-dimensional hydrodynamic models, this paper
addresses the importance of the numerical spatial resolution, footpoint
heating timescales and background heating on TNE. Inadequate transition
region (TR) resolution can lead to significant discrepancies in TNE
cycle behaviour, with TNE being suppressed in under-resolved loops. A
convergence on the periodicity and plasma properties associated with
TNE required spatial resolutions of less than 2 km for a loop of length
180 Mm. These numerical problems can be resolved using an approximate
method that models the TR as a discontinuity using a jump condition, as
proposed by Johnston et al. (2017a, A&A, 597, A81; 2017b, A&A,
605, A8). The resolution requirements (and so computational cost)
are greatly reduced while retaining good agreement with fully resolved
results. Using this approximate method we (i) identify different regimes
for the response of coronal loops to time-dependent footpoint heating
including one where TNE does not arise and (ii) demonstrate that TNE
in a loop with footpoint heating is suppressed unless the background
heating is sufficiently small. The implications for the generality of
TNE are discussed.
---------------------------------------------------------
Title: Heating Effects from Driven Transverse and Alfvén Waves in
Coronal Loops
Authors: Guo, Mingzhe; Van Doorsselaere, Tom; Karampelas, Konstantinos;
Li, Bo; Antolin, Patrick; De Moortel, Ineke
2019ApJ...870...55G Altcode: 2018arXiv181107608G
Recent numerical studies revealed that transverse motions of coronal
loops can induce the Kelvin-Helmholtz instability (KHI). This process
could be important in coronal heating because it leads to dissipation
of energy at small spatial scale plasma interactions. Meanwhile,
small-amplitude decayless oscillations in coronal loops have
been discovered recently in observations of SDO/AIA. We model
such oscillations in coronal loops and study wave heating effects,
considering a kink and Alfvén driver separately and a mixed driver at
the bottom of flux tubes. Both the transverse and Alfvén oscillations
can lead to the KHI. Meanwhile, the Alfvén oscillations established
in loops will experience phase mixing. Both processes will generate
small spatial scale structures, which can help the dissipation of
wave energy. Indeed, we observe the increase of internal energy and
temperature in loop regions. The heating is more pronounced for the
simulation containing the mixed kink and Alfvén driver. This means that
the mixed wave modes can lead to a more efficient energy dissipation
in the turbulent state of the plasma and that the KHI eddies act as an
agent to dissipate energy in other wave modes. Furthermore, we also
obtained forward-modeling results using the FoMo code. We obtained
forward models that are very similar to the observations of decayless
oscillations. Due to the limited resolution of instruments, neither
Alfvén modes nor the fine structures are observable. Therefore,
this numerical study shows that Alfvén modes probably can coexist
with kink modes, leading to enhanced heating.
---------------------------------------------------------
Title: The Coronal Monsoon: Thermal Nonequilibrium Revealed by
Periodic Coronal Rain
Authors: Auchère, Frédéric; Froment, Clara; Soubrié, Elie; Antolin,
Patrick; Oliver, Ramon; Pelouze, Gabriel; Voyeux, Alfred
2018csc..confE.114A Altcode:
We report on the discovery of periodic coronal rain in an off-limb
sequence of SDO/AIA images. The showers are co-spatial and in phase
with periodic (6.6 hr) intensity pulsations of coronal loops of the
sort described by Auchère et al. (2014) and Froment et al. (2015,
2017. These new observations make possible a unified description of
both phenomena. Coronal rain and periodic intensity pulsations of loops
are two manifestations of the same physical process: evaporation /
condensation cycles resulting from a state of thermal nonequilibrium
(TNE). The fluctuations around coronal temperatures produce the
intensity pulsations of loops, and rain falls along their legs
if thermal runaway cools the periodic condensations down and below
transition-region (TR)temperatures. This scenario is in line with the
predictions of numerical models of quasi-steadily and footpoint heated
loops. This event of periodic coronal rain is compared with a similar
event showing only pulsations at coronal temperatures but no significant
cool rain fall. For both events we have stereoscopic observations from
the SDO and STEREO spacecraft which allows reconstruction of the 3D loop
geometries. Comparison with numerical simulations suggest that these two
events correspond to two regimes of TNE: one with "full condensations"
(coronal rain) and another in which "incomplete condensations" start
to develop but are pushed down one loop leg before they can reach
chromospheric temperatures. These new observations impose severe
constrains on the spatio-temporal distribution of coronal heating.
---------------------------------------------------------
Title: Broadening of the differential emission measure by
multi-shelled and turbulent loops
Authors: Van Doorsselaere, T.; Antolin, P.; Karampelas, K.
2018A&A...620A..65V Altcode: 2018arXiv181006300V
Context. Broad differential emission measure (DEM) distributions
in the corona are a sign of multi-thermal plasma along the
line-of-sight. Traditionally, this is interpreted as evidence of
multi-stranded loops. Recently, however, it has been shown that
multi-stranded loops are unlikely to exist in the solar corona,
because of their instability to transverse perturbations. <BR />
Aims: We aim to test if loop models subject to the transverse
wave-induced Kelvin-Helmholtz (TWIKH) instability result in broad
DEMs, potentially explaining the observations. <BR /> Methods: We
took simulation snapshots and compute the numerical DEM. Moreover,
we performed forward-modelling in the relevant AIA channels before
reconstructing the DEM. <BR /> Results: We find that turbulent loop
models broaden their initial DEM, because of the turbulent mixing. The
width of the DEM is determined by the initial temperature contrast with
the exterior. <BR /> Conclusions: We conclude that impulsively excited
loop models have a rather narrow DEM, but that continuously driven
models result in broad DEMs that are comparable to the observations.
---------------------------------------------------------
Title: Evolution of the Transverse Density Structure of Oscillating
Coronal Loops Inferred by Forward Modeling of EUV Intensity
Authors: Goddard, C. R.; Antolin, P.; Pascoe, D. J.
2018ApJ...863..167G Altcode:
Recent developments in the observation and modeling of kink oscillations
of coronal loops have led to heightened interest over the last few
years. The modification of the Transverse Density Profile (TDP)
of oscillating coronal loops by nonlinear effects, particularly the
Kelvin-Helmholtz Instability (KHI), is investigated. How this evolution
may be detected is established, in particular, when the KHI vortices
may not be observed directly. A model for the loop’s TDP is used
that includes a finite inhomogeneous layer and homogeneous core, with a
linear transition between them. The evolution of the loop’s transverse
intensity profile from numerical simulations of kink oscillations is
analyzed. Bayesian inference and forward modeling techniques are applied
to infer the evolution of the TDP from the intensity profiles, in a
manner that may be applied to observations. The strongest observational
evidence for the development of the KHI is found to be a widening of the
loop’s inhomogeneous layer, which may be inferred for sufficiently
well resolved loops, i.e., >15 data points across the loop. The
main signatures when observing the core of the loop (for this specific
loop model) during the oscillation are a widening inhomogeneous layer,
decreasing intensity, an unchanged radius, and visible fine transverse
structuring when the resolution is sufficient. The appearance of these
signatures are delayed for loops with wider inhomogeneous layers,
and quicker for loops oscillating at higher amplitudes. These cases
should also result in stronger observational signatures, with visible
transverse structuring appearing for wide loops observed at the
resolution of current instruments.
---------------------------------------------------------
Title: Evolution of the transverse density structure of oscillating
coronal loops inferred by forward modelling of EUV intensity
Authors: Rhys Goddard, Christopher; Antolin, Patrick; Pascoe,
David James
2018arXiv180803476R Altcode:
Recent developments in the observation and modelling of kink
oscillations of coronal loops have led to heightened interest over the
last few years. The modification of the Transverse Density Profile (TDP)
of oscillating coronal loops by non-linear effects, in particular the
Kelvin-Helmholtz Instability (KHI), is investigated. How this evolution
may be detected is established, in particular, when the KHI vortices
may not be observed directly. A model for the loop's TDP is used which
includes a finite inhomogeneous layer and homogeneous core, with a
linear transition between them. The evolution of the loop's transverse
intensity profile from numerical simulations of kink oscillations
is analysed. Bayesian inference and forward modelling techniques
are applied to infer the evolution of the TDP from the intensity
profiles, in a manner which may be applied to observations. The
strongest observational evidence for the development of the KHI is
found to be a widening of the loop's inhomogeneous layer, which may
be inferred for sufficiently well resolved loops, i.e $>$ 15 data
points across the loop. The main signatures when observing the core of
the loop (for this specific loop model) during the oscillation are:
a widening inhomogeneous layer, decreasing intensity, an unchanged
radius, and visible fine transverse structuring when the resolution is
sufficient. The appearance of these signatures are delayed for loops
with wider inhomogeneous layers, and quicker for loops oscillating
at higher amplitudes. These cases should also result in stronger
observational signatures, with visible transverse structuring appearing
for wide loops observed at SDO/AIA resolution.
---------------------------------------------------------
Title: Excitation and Evolution of Transverse Loop Oscillations by
Coronal Rain
Authors: Verwichte, Erwin; Kohutova, Petra; Antolin, Patrick; Rowlands,
George; Neukirch, Thomas
2018IAUS..335...36V Altcode:
We present evidence of the excitation of vertically polarised transverse
loop oscillations triggered by a catastrophic cooling of a coronal
loop with two thirds of the loop mass comprising of cool rain mass. The
nature and excitation of oscillations associated with coronal rain is
not well understood. We consider observations of coronal rain using data
from IRIS, SOT/Hinode and AIA/SDO in a bid to elucidate the excitation
mechanism and evolution of wave characteristics. We apply an analytical
model of wave-rain interaction, that predicts the inertial excitation
amplitude of transverse loop oscillations as a function of the rain
mass, to deduce the relative rain mass. It is consistent with the
evolution of the oscillation period showing the loop losing a third
of its mass due to falling coronal rain in a 10-15 minute time period.
---------------------------------------------------------
Title: What brakes coronal rain?
Authors: Oliver, Ramon; Khodachenko, Maxim; Terradas, Jaume; Soler,
Roberto; Antolin, Patrick; Zaqarashvili, Teimuraz; Boulharrak, Adel
2018cosp...42E2505O Altcode:
Coronal rain blobs usually fall toward the solar surface with a smaller
than free-fall acceleration. After conducting numerical simulations with
different setups, we conclude that once a dense blob forms and starts
to fall along a coronal loop, a pressure gradient is established along
the loop such that higher (smaller) pressure can be found below (above)
the blob. This pressure gradient produces an upward force that partially
counteracts gravity and leads to the observed non-free-fall dynamics.
---------------------------------------------------------
Title: Cool Material in the Hot Solar Corona and the
Chromosphere-Corona Mass Cycle
Authors: Liu, Wei; Vial, Jean-Claude; Antolin, Patrick; Sun, Xudong;
Berger, Thomas
2018cosp...42E2052L Altcode:
In the million-degree hot and tenuous solar corona, under favorable
conditions, some mass can undergo a radiative cooling instability and
condense into material of 100 times cooler in two distinct forms -
prominences and coronal rain. Being at similar temperatures, they
exhibit contrasting morphologies and behaviors: a quiescent prominence
usually consists of numerous long-lasting, filamentary downflow
threads, while coronal rain consists of transient mass blobs falling
at comparably higher speeds along well-defined, curved paths (e.g.,
guided by coronal loops). We report recent imaging and spectroscopic
observations from SDO/AIA and IRIS of a hybrid prominence-coronal
rain complex structure that suggest different magnetic environments
being responsible for such distinctions. We also present an ensemble
of observations of the so-called funnel prominences that reside near
the dips of magnetic funnels. Regardless of their morphological and
behavioral differences, a large fraction of prominence and coronal
rain material eventually falls back to the chromosphere and serves as
the return flow of the so-called chromosphere-corona mass cycle (the
other half of this cycle is the upward transport of heated mass from
the chromosphere to the corona). We estimate the downflow mass fluxes
in prominences and coronal rain, and compare them with the coronal
mass budget in this cycle and with the mass loss to the solar wind
and coronal mass ejections (CMEs). We will discuss the broad physical
implications of these observations for fundamental questions, such as
coronal heating and beyond.
---------------------------------------------------------
Title: The Coronal Monsoon: Thermal Nonequilibrium Revealed by
Periodic Coronal Rain
Authors: Auchere, Frederic; Soubrie, Elie; Antolin, Patrick; Froment,
Clara; Oliver, Ramon; Pelouze, Gabriel
2018cosp...42E.144A Altcode:
We report on the discovery of periodic coronal rain in an off-limb
sequence of SDO/AIA images. The showers are co-spatial and in phase
with periodic (6.6 hr) intensity pulsations of coronal loops of the
sort described by Auchère et al. (2014) and Froment et al. (2015,
2017}. These new observations make possible a unified description of
both phenomena. Coronal rain and periodic intensity pulsations of loops
are two manifestations of the same physical process: evaporation /
condensation cycles resulting from a state of thermal nonequilibrium
(TNE). The fluctuations around coronal temperatures produce the
intensity pulsations of loops, and rain falls along their legs
if thermal runaway cools the periodic condensations down and below
transition-region (TR) temperatures. This scenario is in line with the
predictions of numerical models of quasi-steadily and footpoint heated
loops.This event of periodic coronal rain is compared with a similar
event showing only pulsations at coronal temperatures but no significant
cool rain fall. For both events we have stereoscopic observations from
the SDO and STEREO spacecraft which allows reconstruction of the 3D loop
geometries. Comparison with numerical simulations suggest that these two
events correspond to two regimes of TNE: one with "full condensations"
(coronal rain) and another in which "incomplete condensations" start
to develop but are pushed down one loop leg before they can reach
chromospheric temperatures.These new observations impose severe
constrains on the spatio-temporal distribution of coronal heating.
---------------------------------------------------------
Title: Reconnection Microjets in the Solar Corona
Authors: Antolin, Patrick; Pagano, Paolo; De Moortel, Ineke
2018cosp...42E..96A Altcode:
Coronal rain is one of the highest resolution tracers of the coronal
magnetic field. In this work the dynamics of a prominence/coronal rain
complex are analysed based on spectroscopic and imaging observations
with IRIS, Hinode/SOT and SDO/AIA. The loop-like magnetic field arcade
hosting the rain is observed to slowly expand in height. Prior and
especially during this movement, several ( 100) small ( 1 arcsec)
and short (<20 sec) bursts of plasma perpendicular to the loop
arcade are captured in the Si IV and Mg II lines. The line profiles are
broad and asymmetric with long tails above 100 km/s. These microjets
are accompanied with strong intensity enhancements co-spatially and
along the loop in most of the AIA channels, indicating significant
energy release increasing the temperature to several MK. Some events
generate transverse MHD waves and the strongest events are accompanied
by ejection of plasmoid-like structures. We interpret these microjets as
magnetic reconnection outflows, produced by component reconnection in a
strong guide field. The originally cold conditions of the rain allows,
in this case, a unique high resolution glance into the reconnection
dynamics in low beta plasmas, and marks the X-target in the Sun for
next-generation telescopes.
---------------------------------------------------------
Title: In Situ Generation of Transverse Magnetohydrodynamic Waves
from Colliding Flows in the Solar Corona
Authors: Antolin, Patrick; Pagano, Paolo; De Moortel, Ineke;
Nakariakov, Valery M.
2018ApJ...861L..15A Altcode: 2018arXiv180700395A
Transverse magnetohydrodynamic (MHD) waves permeate the solar
atmosphere and are a candidate for coronal heating. However, the
origin of these waves is still unclear. In this Letter, we analyze
coordinated observations from Hinode/Solar Optical Telescope (SOT) and
Interface Region Imaging Spectrograph ( IRIS) of a prominence/coronal
rain loop-like structure at the limb of the Sun. Cool and dense
downflows and upflows are observed along the structure. A collision
between a downward and an upward flow with an estimated energy flux
of 10<SUP>7</SUP>-10<SUP>8</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>
is observed to generate oscillatory transverse perturbations of the
strands with an estimated ≈40 km s<SUP>-1</SUP> total amplitude, and
a short-lived brightening event with the plasma temperature increasing
to at least 10<SUP>5</SUP> K. We interpret this response as sausage
and kink transverse MHD waves based on 2D MHD simulations of plasma
flow collision. The lengths, density, and velocity differences between
the colliding clumps and the strength of the magnetic field are major
parameters defining the response to the collision. The presence of
asymmetry between the clumps (angle of impact surface and/or offset
of flowing axis) is crucial for generating a kink mode. Using the
observed values, we successfully reproduce the observed transverse
perturbations and brightening, and show adiabatic heating to coronal
temperatures. The numerical modeling indicates that the plasma β
in this loop-like structure is confined between 0.09 and 0.36. These
results suggest that such collisions from counter-streaming flows can
be a source of in situ transverse MHD waves, and that for cool and dense
prominence conditions such waves could have significant amplitudes.
---------------------------------------------------------
Title: Transverse Wave Induced Kelvin-Helmholtz Rolls in Spicules
Authors: Antolin, P.; Schmit, D.; Pereira, T. M. D.; De Pontieu, B.;
De Moortel, I.
2018ApJ...856...44A Altcode: 2018arXiv180300821A
In addition to their jet-like dynamic behavior, spicules usually exhibit
strong transverse speeds, multi-stranded structure, and heating from
chromospheric to transition region temperatures. In this work we first
analyze Hinode and IRIS observations of spicules and find different
behaviors in terms of their Doppler velocity evolution and collective
motion of their sub-structure. Some have a Doppler shift sign change
that is rather fixed along the spicule axis, and lack coherence in
the oscillatory motion of strand-like structure, matching rotation
models, or long-wavelength torsional Alfvén waves. Others exhibit a
Doppler shift sign change at maximum displacement and coherent motion
of their strands, suggesting a collective magnetohydrodynamic (MHD)
wave. By comparing with an idealized 3D MHD simulation combined with
radiative transfer modeling, we analyze the role of transverse MHD
waves and associated instabilities in spicule-like features. We find
that transverse wave induced Kelvin-Helmholtz (TWIKH) rolls lead to
coherence of strand-like structure in imaging and spectral maps, as seen
in some observations. The rapid transverse dynamics and the density
and temperature gradients at the spicule boundary lead to ring-shaped
Mg II k and Ca II H source functions in the transverse cross-section,
potentially allowing IRIS to capture the Kelvin-Helmholtz instability
dynamics. Twists and currents propagate along the spicule at Alfvénic
speeds, and the temperature variations within TWIKH rolls, produce the
sudden appearance/disappearance of strands seen in Doppler velocity
and in Ca II H intensity. However, only a mild intensity increase in
higher-temperature lines is obtained, suggesting there is an additional
heating mechanism at work in spicules.
---------------------------------------------------------
Title: The Coronal Monsoon: Thermal Nonequilibrium Revealed by
Periodic Coronal Rain
Authors: Auchère, Frédéric; Froment, Clara; Soubrié, Elie; Antolin,
Patrick; Oliver, Ramon; Pelouze, Gabriel
2018ApJ...853..176A Altcode: 2018arXiv180201852A
We report on the discovery of periodic coronal rain in an off-limb
sequence of Solar Dynamics Observatory/Atmospheric Imaging Assembly
images. The showers are co-spatial and in phase with periodic (6.6 hr)
intensity pulsations of coronal loops of the sort described by Auchère
et al. and Froment et al. These new observations make possible a unified
description of both phenomena. Coronal rain and periodic intensity
pulsations of loops are two manifestations of the same physical
process: evaporation/condensation cycles resulting from a state of
thermal nonequilibrium. The fluctuations around coronal temperatures
produce the intensity pulsations of loops, and rain falls along their
legs if thermal runaway cools the periodic condensations down and
below transition-region temperatures. This scenario is in line with
the predictions of numerical models of quasi-steadily and footpoint
heated loops. The presence of coronal rain—albeit non-periodic—in
several other structures within the studied field of view implies that
this type of heating is at play on a large scale.
---------------------------------------------------------
Title: Driven Transverse Waves Lead to Turbulent Coronal Loops
and Heating
Authors: Van Doorsselaere, T.; Karampelas, K.; Magyar, N.; Antolin,
P.; Goossens, M. L.
2017AGUFMSH41C..05V Altcode:
In this talk, I will show our recent results on 3D simulations of
coronal loops driven with transverse waves at the footpoints. We find
that the transverse waves convert to turbulence via the Kelvin-Helmholtz
instability (for standing waves) or uniturbulence (for propagating
waves). The latter is turbulence generated from the interaction of
the driven propagating waves with the counterpropagating waves which
are generated in-situ because of the plasma structure. Both of these
turbulence generation mechanisms lead to fully turbulent loops, which
allow for efficient energy dissipation and heating.
---------------------------------------------------------
Title: Energetics of the Kelvin-Helmholtz instability induced by
transverse waves in twisted coronal loops
Authors: Howson, T. A.; De Moortel, I.; Antolin, P.
2017A&A...607A..77H Altcode: 2017arXiv170804124H
<BR /> Aims: We quantify the effects of twisted magnetic fields on
the development of the magnetic Kelvin-Helmholtz instability (KHI) in
transversely oscillating coronal loops. <BR /> Methods: We modelled a
fundamental standing kink mode in a straight, density-enhanced magnetic
flux tube using the magnetohydrodynamics code, Lare3d. In order to
evaluate the impact of an azimuthal component of the magnetic field,
various degrees of twist were included within the flux tube's magnetic
field. <BR /> Results: The process of resonant absorption is only
weakly affected by the presence of a twisted magnetic field. However,
the subsequent evolution of the KHI is sensitive to the strength of the
azimuthal component of the field. Increased twist values inhibit the
deformation of the loop's density profile, which is associated with
the growth of the instability. Despite this, much smaller scales in
the magnetic field are generated when there is a non-zero azimuthal
component present. Hence, the instability is more energetic in cases
with (even weakly) twisted fields. Field aligned flows at the loop
apex are established in a twisted regime once the instability has
formed. Further, in the straight field case, there is no net vertical
component of vorticity when integrated across the loop. However, the
inclusion of azimuthal magnetic field generates a preferred direction
for the vorticity which oscillates during the kink mode. <BR />
Conclusions: The KHI may have implications for wave heating in the
solar atmosphere due to the creation of small length scales and the
generation of a turbulent regime. Whilst magnetic twist does suppress
the development of the vortices associated with the instability, the
formation of the KHI in a twisted regime will be accompanied by greater
Ohmic dissipation due to the larger currents that are produced, even if
only weak twist is present. The presence of magnetic twist will likely
make the instability more difficult to detect in the corona, but will
enhance its contribution to heating the solar atmosphere. Further,
the development of velocities along the loop may have observational
applications for inferring the presence of magnetic twist within
coronal structures.
---------------------------------------------------------
Title: Fine Structure and Dynamics of the Solar Atmosphere
Authors: Vargas Domínguez, S.; Kosovichev, A. G.; Antolin, P.;
Harra, L.
2017IAUS..327.....V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Fate of Cool Material in the Hot Corona: Solar Prominences
and Coronal Rain
Authors: Liu, Wei; Antolin, Patrick; Sun, Xudong; Vial, Jean-Claude;
Berger, Thomas
2017SPD....4810501L Altcode:
As an important chain of the chromosphere-corona mass cycle,
some of the million-degree hot coronal mass undergoes a radiative
cooling instability and condenses into material at chromospheric or
transition-region temperatures in two distinct forms - prominences
and coronal rain (some of which eventually falls back to the
chromosphere). A quiescent prominence usually consists of numerous
long-lasting, filamentary downflow threads, while coronal rain consists
of transient mass blobs falling at comparably higher speeds along
well-defined paths. It remains puzzling why such material of similar
temperatures exhibit contrasting morphologies and behaviors. We report
recent SDO/AIA and IRIS observations that suggest different magnetic
environments being responsible for such distinctions. Specifically,
in a hybrid prominence-coronal rain complex structure, we found that
the prominence material is formed and resides near magnetic null points
that favor the radiative cooling process and provide possibly a high
plasma-beta environment suitable for the existence of meandering
prominence threads. As the cool material descends, it turns into
coronal rain tied onto low-lying coronal loops in a likely low-beta
environment. Such structures resemble to certain extent the so-called
coronal spiders or cloud prominences, but the observations reported
here provide critical new insights. We will discuss the broad physical
implications of these observations for fundamental questions, such as
coronal heating and beyond (e.g., in astrophysical and/or laboratory
plasma environments).
---------------------------------------------------------
Title: Heating by transverse waves in simulated coronal loops
Authors: Karampelas, K.; Van Doorsselaere, T.; Antolin, P.
2017A&A...604A.130K Altcode: 2017arXiv170602640K
Context. Recent numerical studies of oscillating flux tubes have
established the significance of resonant absorption in the damping of
propagating transverse oscillations in coronal loops. The nonlinear
nature of the mechanism has been examined alongside the Kelvin-Helmholtz
instability, which is expected to manifest in the resonant layers
at the edges of the flux tubes. While these two processes have
been hypothesized to heat coronal loops through the dissipation
of wave energy into smaller scales, the occurring mixing with the
hotter surroundings can potentially hide this effect. <BR /> Aims:
We aim to study the effects of wave heating from driven and standing
kink waves in a coronal loop. <BR /> Methods: Using the MPI-AMRVAC
code, we perform ideal, three dimensional magnetohydrodynamic (MHD)
simulations of both (a) footpoint driven and (b) free standing
oscillations in a straight coronal flux tube, in the presence of
numerical resistivity. <BR /> Results: We have observed the development
of Kelvin-Helmholtz eddies at the loop boundary layer of all three
models considered here, as well as an increase of the volume averaged
temperature inside the loop. The main heating mechanism in our setups
was Ohmic dissipation, as indicated by the higher values for the
temperatures and current densities located near the footpoints. The
introduction of a temperature gradient between the inner tube and
the surrounding plasma, suggests that the mixing of the two regions,
in the case of hotter environment, greatly increases the temperature
of the tube at the site of the strongest turbulence, beyond the
contribution of the aforementioned wave heating mechanism. <P />Three
movies associated to Fig. 1 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201730598/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: The effects of resistivity and viscosity on the Kelvin-
Helmholtz instability in oscillating coronal loops
Authors: Howson, T. A.; De Moortel, I.; Antolin, P.
2017A&A...602A..74H Altcode: 2017arXiv170302423H
<BR /> Aims: We investigate the effects of resistivity and viscosity
on the onset and growth of the Kelvin-Helmholtz instability (KHI) in
an oscillating coronal loop. <BR /> Methods: We modelled a standing
kink wave in a density-enhanced loop with the three dimensional (3D),
resistive magnetohydrodynamics code, Lare3d. We conducted a parameter
study on the viscosity and resistivity coefficients to examine the
effects of dissipation on the KHI. <BR /> Results: Enhancing the
viscosity (ν) and resistivity (η) acts to suppress the KHI. Larger
values of η and ν delay the formation of the instability and, in some
cases, prevent the onset completely. This leads to the earlier onset
of heating for smaller values of the transport coefficients. We note
that viscosity has a greater effect on the development of the KHI than
resistivity. Furthermore, when using anomalous resistivity, the Ohmic
heating rate associated with the KHI may be greater than that associated
with the phase mixing that occurs in an instability-suppressed regime
(using uniform resistivity). <BR /> Conclusions: From our study, it is
clear that the heating rate crucially depends on the formation of small
length scales (influenced by the numerical resolution) as well as the
values of resistivity and viscosity. As larger values of the transport
coefficients suppress the KHI, the onset of heating is delayed but
the heating rate is larger. As increased numerical resolution allows
smaller length scales to develop, the heating rate will be higher even
for the same values of η and ν.
---------------------------------------------------------
Title: Observational signatures of transverse MHD waves and associated
dynamic instabilities
Authors: Antolin, Patrick; De Moortel, Ineke; Van Doorsselaere, Tom;
Yokoyama, Takaaki
2017arXiv170200775A Altcode:
MHD waves permeate the solar atmosphere and constitute potential
coronal heating agents. Yet, the waves detected so far may be but a
small subset of the true existing wave power. Detection is limited by
instrumental constraints, but also by wave processes that localise the
wave power in undetectable spatial scales. In this study we conduct 3D
MHD simulations and forward modelling of standing transverse MHD waves
in coronal loops with uniform and non-uniform temperature variation in
the perpendicular cross-section. The observed signatures are largely
dominated by the combination of the Kelvin-Helmholtz instability (KHI),
resonant absorption and phase mixing. In the presence of a cross-loop
temperature gradient we find that emission lines sensitive to the
loop core catch different signatures than those more sensitive to the
loop boundary and the surrounding corona, leading to an out-of-phase
intensity modulation produced by the KHI mixing. Common signatures to
all considered models include an intensity and loop width modulation
at half the kink period, fine strand-like structure, a characteristic
arrow-shaped structure in the Doppler maps, overall line broadening in
time but particularly at the loop edges. For our model, most of these
features can be captured with a spatial resolution of $0.33\arcsec$ and
spectral resolution of 25~km~s$^{-1}$, although severe over-estimation
of the line width is obtained. Resonant absorption leads to a
significant decrease of the observed kinetic energy from Doppler
motions over time, which is not recovered by a corresponding increase
in the line width from phase mixing and the KHI motions. We estimate
this hidden wave energy to be a factor of $5-10$ of the observed value.
---------------------------------------------------------
Title: Kinematics of coronal rain in a transversely oscillating loop:
Ponderomotive force and rain-excited oscillations
Authors: Verwichte, E.; Antolin, P.; Rowlands, G.; Kohutova, P.;
Neukirch, T.
2017A&A...598A..57V Altcode:
Context. Coronal rain is composed of cool dense blobs that form in solar
coronal loops and are a manifestation of catastrophic cooling linked to
thermal instability. Once formed, rain falls towards the solar surface
at sub-ballistic speeds, which is not well understood. Pressure forces
seem to be the prime candidate to explain this. In many observations
rain is accompanied by transverse oscillations and the interaction
between rain and these oscillations needs to be explored. <BR /> Aims:
Therefore, an alternative kinematic model for coronal rain kinematics in
transversely oscillating loops is developed to understand the physical
nature of the observed sub-ballistic falling motion of rain. This
model explicitly explores the role of the ponderomotive force arising
from the transverse oscillation on the rain motion and the capacity
of rain to excite wave motion. <BR /> Methods: An analytical model is
presented that describes a rain blob guided by the coronal magnetic
field supporting a one-dimensional shear Alfvén wave as a point mass on
an oscillating string. The model includes gravity and the ponderomotive
force from the oscillation acting on the mass and the inertia of the
mass acting on the oscillation. <BR /> Results: The kinematics of
rain in the limit of negligible rain mass are explored and falling and
trapped regimes are found, depending on wave amplitude. In the trapped
regime for the fundamental mode, the rain blob bounces back and forth
around the loop top at a long period that is inversely proportional
to the oscillation amplitude. The model is compared with several
observational rain studies, including one in-depth comparison with
an observation that shows rain with up-and-down bobbing motion. The
role of rain inertia in exciting transverse oscillations is explored
in inclined loops. <BR /> Conclusions: It is found that the model
requires displacement amplitudes of the transverse oscillation that
are typically an order of magnitude larger than observed to explain
the measured sub-ballistic motion of the rain. Therefore, it is
concluded that the ponderomotive force is not the primary reason for
understanding sub-ballistic motion, but it plays a role in cases of
large loop oscillations. The appearance of rain causes the excitation
of small-amplitude transverse oscillations that may explain observed
events and provide a seismological tool to measure rain mass.
---------------------------------------------------------
Title: Observational Signatures of Transverse Magnetohydrodynamic
Waves and Associated Dynamic Instabilities in Coronal Flux Tubes
Authors: Antolin, P.; De Moortel, I.; Van Doorsselaere, T.; Yokoyama,
T.
2017ApJ...836..219A Altcode:
Magnetohydrodynamic (MHD) waves permeate the solar atmosphere
and constitute potential coronal heating agents. Yet, the waves
detected so far may be but a small subset of the true existing wave
power. Detection is limited by instrumental constraints but also by
wave processes that localize the wave power in undetectable spatial
scales. In this study, we conduct 3D MHD simulations and forward
modeling of standing transverse MHD waves in coronal loops with
uniform and non-uniform temperature variation in the perpendicular
cross-section. The observed signatures are largely dominated by the
combination of the Kelvin-Helmholtz instability (KHI), resonant
absorption, and phase mixing. In the presence of a cross-loop
temperature gradient, we find that emission lines sensitive to the
loop core catch different signatures compared to those that are more
sensitive to the loop boundary and the surrounding corona, leading to
an out-of-phase intensity and Doppler velocity modulation produced by
KHI mixing. In all of the considered models, common signatures include
an intensity and loop width modulation at half the kink period, a fine
strand-like structure, a characteristic arrow-shaped structure in the
Doppler maps, and overall line broadening in time but particularly at
the loop edges. For our model, most of these features can be captured
with a spatial resolution of 0.″33 and a spectral resolution of 25
km s<SUP>-1</SUP>, although we do obtain severe over-estimation of
the line width. Resonant absorption leads to a significant decrease
of the observed kinetic energy from Doppler motions over time, which
is not recovered by a corresponding increase in the line width from
phase mixing and KHI motions. We estimate this hidden wave energy to
be a factor of 5-10 of the observed value.
---------------------------------------------------------
Title: Reconnection Microjets in the Pre-eruption Phase of a
Prominence/Coronal Rain Complex
Authors: Antolin, P.; Mehta, T.; Conlon, T.; De Moortel, I.
2016AGUFMSH43C2582A Altcode:
Coronal rain is known to be one of the highest resolution tracers
of the coronal magnetic field. In this work the dynamics of a
prominence/coronal rain complex are analysed based on imaging and
spectroscopic observations with IRIS. Prior to eruption, the loop-like
magnetic field arcade hosting the rain is observed to slowly expand
in height. This movement is accompanied by several small ( 1 arsec)
and short (<20 sec) bursts of plasma perpendicular to the field,
captured in the Si IV and Mg II lines. The line profiles are broad
and asymmetric with long tails above 100 km/s. These microjets are
accompanied with strong intensity enhancements along the loop in most of
the AIA channels, indicating significant energy release. We interpret
these microjets as reconnection outflows, produced by component
reconnection as the magnetic structure expands transversely. The
originally cold conditions of the rain allows in this case a unique
high resolution glance at the reconnection dynamics in low beta plasmas.
---------------------------------------------------------
Title: Observing the Formation of Flare-driven Coronal Rain
Authors: Scullion, E.; Rouppe van der Voort, L.; Antolin, P.;
Wedemeyer, S.; Vissers, G.; Kontar, E. P.; Gallagher, P. T.
2016ApJ...833..184S Altcode: 2016arXiv161009255S
Flare-driven coronal rain can manifest from rapidly cooled plasma
condensations near coronal loop tops in thermally unstable postflare
arcades. We detect five phases that characterize the postflare decay:
heating, evaporation, conductive cooling dominance for ∼120 s,
radiative/enthalpy cooling dominance for ∼4700 s, and finally
catastrophic cooling occurring within 35-124 s, leading to rain
strands with a periodicity of 55-70 s. We find an excellent agreement
between the observations and model predictions of the dominant
cooling timescales and the onset of catastrophic cooling. At the
rain-formation site, we detect comoving, multithermal rain clumps
that undergo catastrophic cooling from ∼1 MK to ∼22,000 K. During
catastrophic cooling, the plasma cools at a maximum rate of 22,700
K s<SUP>-1</SUP> in multiple loop-top sources. We calculated the
density of the extreme-ultraviolet (EUV) plasma from the differential
emission measure of the multithermal source employing regularized
inversion. Assuming a pressure balance, we estimate the density of
the chromospheric component of rain to be 9.21 × 10<SUP>11</SUP>
± 1.76 × 10<SUP>11</SUP> cm<SUP>-3</SUP>, which is comparable with
quiescent coronal rain densities. With up to eight parallel strands
in the EUV loop cross section, we calculate the mass loss rate from
the postflare arcade to be as much as 1.98 × 10<SUP>12</SUP> ±
4.95 × 10<SUP>11</SUP> g s<SUP>-1</SUP>. Finally, we reveal a close
proximity between the model predictions of {10}<SUP>5.8</SUP> K and the
observed properties between {10}<SUP>5.9</SUP> and {10}<SUP>6.2</SUP>
K, which defines the temperature onset of catastrophic cooling. The
close correspondence between the observations and numerical models
suggests that indeed acoustic waves (with a sound travel time of 68 s)
could play an important role in redistributing energy and sustaining
the enthalpy-based radiative cooling.
---------------------------------------------------------
Title: Probing the Physical Connection between Solar Prominences
and Coronal Rain
Authors: Liu, W.; Antolin, P.; Sun, X.; Vial, J. C.; Guo, L.; Gibson,
S. E.; Berger, T. E.; Okamoto, J.; De Pontieu, B.
2016AGUFMSH43C2587L Altcode:
Solar prominences and coronal rain are intimately related phenomena,
both involving cool material at chromospheric temperatures within the
hot corona and both playing important roles as part of the return flow
of the chromosphere-corona mass cycle. At the same time, they exhibit
distinct morphologies and dynamics not yet well understood. Quiescent
prominences consist of numerous long-lasting, filamentary downflow
threads, while coronal rain is more transient and falls comparably
faster along well-defined curved paths. We report here a novel, hybrid
prominence-coronal rain complex in an arcade-fan geometry observed
by SDO/AIA and IRIS, which provides new insights to the underlying
physics of such contrasting behaviors. We found that the supra-arcade
fan region hosts a prominence sheet consisting of meandering threads
with broad line widths. As the prominence material descends to the
arcade, it turns into coronal rain sliding down coronal loops with
line widths 2-3 times narrower. This contrast suggests that distinct
local plasma and magnetic conditions determine the fate of the cool
material, a scenario supported by our magnetic field extrapolations
from SDO/HMI. Specifically, the supra-arcade fan (similar to those
in solar flares) is likely situated in a current sheet, where the
magnetic field is weak and the plasma-beta could be close to unity, thus
favoring turbulent flows like those prominence threads. In contrast,
the underlying arcade has a stronger magnetic field and most likely a
low-beta environment, such that the material is guided along magnetic
field lines to appear as coronal rain. We will discuss the physical
implications of these observations beyond the phenomena of prominences
and coronal rain.
---------------------------------------------------------
Title: Joint SDO and IRIS Observations of a Novel, Hybrid
Prominence-Coronal Rain Complex
Authors: Liu, Wei; Antolin, Patrick; Sun, Xudong; Gao, Lijia; Vial,
Jean-Claude; Gibson, Sarah; Okamoto, Takenori; Berger, Thomas;
Uitenbroek, Han; De Pontieu, Bart
2016usc..confE..99L Altcode:
Solar prominences and coronal rain are intimately related phenomena,
both involving cool material at chromospheric temperatures within the
hot corona and both playing important roles as part of the return flow
of the chromosphere-corona mass cycle. At the same time, they exhibit
distinct morphologies and dynamics not yet well understood. Quiescent
prominences consist of numerous long-lasting, filamentary downflow
threads, while coronal rain is more transient and falls comparably
faster along well-defined curved paths. We report here a novel, hybrid
prominence-coronal rain complex in an arcade-fan geometry observed
by SDO/AIA and IRIS, which provides new insights to the underlying
physics of such contrasting behaviors. We found that the supra-arcade
fan region hosts a prominence sheet consisting of meandering threads
with broad line widths. As the prominence material descends to the
arcade, it turns into coronal rain sliding down coronal loops with
line widths 2-3 times narrower. This contrast suggests that distinct
local plasma and magnetic conditions determine the fate of the cool
material, a scenario supported by our magnetic field extrapolations
from SDO/HMI. Specifically, the supra-arcade fan (similar to those
in solar flares; e.g., McKenzie 2013) is likely situated in a current
sheet, where the magnetic field is weak and the plasma-beta could be
close to unity, thus favoring turbulent flows like those prominence
threads. In contrast, the underlying arcade has a stronger magnetic
field and most likely a low-beta environment, such that the material
is guided along magnetic field lines to appear as coronal rain. We
will discuss the physical implications of these observations beyond
prominence and coronal rain.
---------------------------------------------------------
Title: Modeling Observed Decay-less Oscillations as Resonantly
Enhanced Kelvin-Helmholtz Vortices from Transverse MHD Waves and
Their Seismological Application
Authors: Antolin, P.; De Moortel, I.; Van Doorsselaere, T.; Yokoyama,
T.
2016ApJ...830L..22A Altcode: 2016arXiv160909716A
In the highly structured solar corona, resonant absorption is an
unavoidable mechanism of energy transfer from global transverse MHD
waves to local azimuthal Alfvén waves. Due to its localized nature,
direct detection of this mechanism is extremely difficult. Yet, it is
the leading theory explaining the observed fast damping of the global
transverse waves. However, at odds with this theoretical prediction
are recent observations that indicate that in the low-amplitude regime
such transverse MHD waves can also appear decay-less, a still unsolved
phenomenon. Recent numerical work has shown that Kelvin-Helmholtz
instabilities (KHI) often accompany transverse MHD waves. In this work,
we combine 3D MHD simulations and forward modeling to show that for
currently achieved spatial resolution and observed small amplitudes,
an apparent decay-less oscillation is obtained. This effect results
from the combination of periodic brightenings produced by the KHI
and the coherent motion of the KHI vortices amplified by resonant
absorption. Such an effect is especially clear in emission lines forming
at temperatures that capture the boundary dynamics rather than the core,
and reflects the low damping character of the local azimuthal Alfvén
waves resonantly coupled to the kink mode. Due to phase mixing, the
detected period can vary depending on the emission line, with those
sensitive to the boundary having shorter periods than those sensitive
to the loop core. This allows us to estimate the density contrast at
the boundary.
---------------------------------------------------------
Title: Global Sausage Oscillation of Solar Flare Loops Detected by
the Interface Region Imaging Spectrograph
Authors: Tian, Hui; Young, Peter R.; Reeves, Katharine K.; Wang,
Tongjiang; Antolin, Patrick; Chen, Bin; He, Jiansen
2016ApJ...823L..16T Altcode: 2016arXiv160501963T
An observation from the Interface Region Imaging Spectrograph
reveals coherent oscillations in the loops of an M1.6 flare on 2015
March 12. Both the intensity and Doppler shift of Fe xxi 1354.08 Å
show clear oscillations with a period of ∼25 s. Remarkably similar
oscillations were also detected in the soft X-ray flux recorded by
the Geostationary Operational Environmental Satellites (GOES). With
an estimated phase speed of ∼2420 km s<SUP>-1</SUP> and a derived
electron density of at least 5.4 × 10<SUP>10</SUP> cm<SUP>-3</SUP>,
the observed short-period oscillation is most likely the global
fast sausage mode of a hot flare loop. We find a phase shift of
∼π/2 (1/4 period) between the Doppler shift oscillation and the
intensity/GOES oscillations, which is consistent with a recent forward
modeling study of the sausage mode. The observed oscillation requires
a density contrast between the flare loop and coronal background of a
factor ≥42. The estimated phase speed of the global mode provides a
lower limit of the Alfvén speed outside the flare loop. We also find
an increase of the oscillation period, which might be caused by the
separation of the loop footpoints with time.
---------------------------------------------------------
Title: IRIS Observations of a Novel, Hybrid Prominence-Coronal
Rain Complex
Authors: Liu, Wei; Antolin, Patrick; Sun, Xudong
2016SPD....47.0402L Altcode:
Solar prominences and coronal rain are intimately related phenomena,
both involving cool material at chromospheric temperatures within the
hot corona and both playing important roles as part of the return flow
of the chromosphere-corona mass cycle. At the same time, they exhibit
distinct morphologies and dynamics not yet well understood. Quiescent
prominences consist of numerous long-lasting, filamentary downflow
threads, while coronal rain is more transient and falls comparably
faster along well-defined curved paths. We report here a novel, hybrid
prominence-coronal rain complex in an arcade-fan geometry observed
by IRIS and SDO/AIA, which provides new insights to the underlying
physics of such contrasting behaviors. We found that the supra-arcade
fan region hosts a prominence sheet consisting of meandering threads
with broad Mg II k/h line widths. As the prominence material descends to
the arcade, it turns into coronal rain sliding down coronal loops with
line widths 2-3 times narrower. This contrast suggests that distinct
local plasma and magnetic conditions determine the fate of the cool
material, a scenario supported by our magnetic field extrapolations from
SDO/HMI. Specifically, the supra-arcade fan (similar to those in solar
flares; e.g., McKenzie 2013) is likely situated in a current sheet,
where the magnetic field is weak and the plasma-beta could be close to
unity, thus favoring turbulent flows like those prominence threads. In
contrast, the underlying arcade has a stronger magnetic field and
most likely a low-beta environment, such that the material is guided
along magnetic field lines to appear as coronal rain. We will discuss
the implications of these novel results for future observations e.g.,
with DKIST.
---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
---------------------------------------------------------
Title: Forward modelling of optically thin coronal plasma with the
FoMo tool
Authors: Van Doorsselaere, Tom; Antolin, Patrick; Yuan, Ding;
Reznikova, Veronika; Magyar, Norbert
2016FrASS...3....4V Altcode:
The FoMo code was developed to calculate the EUV emission from optically
thin coronal plasmas. The input data for FoMo consists of the coronal
density, temperature and velocity on a 3D grid. This is translated to
emissivity on the 3D grid, using CHIANTI data. Then, the emissivity is
integrated along the line-of-sight to calculate the emergent spectral
line that could be observed by a spectrometer. Moreover, the code
has been extended to model also the radio emission from plasmas with
a population of non-thermal particles. In this case, also optically
thick plasmas may be modelled. The radio spectrum is calculated over
a large wavelength range, allowing for the comparison with data from
a wide range of radio telescopes.
---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.
---------------------------------------------------------
Title: Combining IRIS/Hinode Observations and Modeling: a Pathfinder
for Coronal Heating
Authors: Antolin, P.; Okamoto, J.; De Pontieu, B.
2015AGUFMSH13C2451A Altcode:
The combination of imaging and spectroscopic instruments with multiple
temperature diagnostics at high spatial, temporal and spectral
resolution can allow to recover the 3D plasma flow and thermodynamic
evolution associated with specific coronal heating mechanisms. Although
very hard considering the complexity of the solar atmosphere, this
approach is becoming possible now through combination of instruments
such as IRIS and Hinode, and with proper guiding from advanced numerical
simulations and forward modeling. In this talk I will review recent
examples of this approach, focusing on a particular, recently published,
case study, that serves as a pathfinder in the search for the dominant
coronal heating mechanism. In this case, resonant absorption, a long
hypothesised wave-related energy conversion mechanism is spotted
in action for the first time, and is characterised by a peculiar 3D
motion of the plasma. With the help of 3D MHD numerical simulations and
forward modeling the observational signatures of resonant absorption
are characterised, matching very well the observational results. The
process through which this mechanism can lead to observed significant
heating in the solar corona is further identified: the resonant
flow becomes turbulent following dynamic instabilities and heats
the plasma. I will show how this resonance + instability process is
expected in different scenarios of the solar atmosphere (the corona,
prominences and spicules) and can potentially explain several observed
features that remain so far unexplained.
---------------------------------------------------------
Title: Resonant Absorption of Transverse Oscillations and Associated
Heating in a Solar Prominence. II. Numerical Aspects
Authors: Antolin, P.; Okamoto, T. J.; De Pontieu, B.; Uitenbroek,
H.; Van Doorsselaere, T.; Yokoyama, T.
2015ApJ...809...72A Altcode: 2015arXiv150609108A
Transverse magnetohydrodynamic (MHD) waves are ubiquitous in
the solar atmosphere and may be responsible for generating the
Sun’s million-degree outer atmosphere. However, direct evidence
of the dissipation process and heating from these waves remains
elusive. Through advanced numerical simulations combined with
appropriate forward modeling of a prominence flux tube, we provide
the observational signatures of transverse MHD waves in prominence
plasmas. We show that these signatures are characterized by a
thread-like substructure, strong transverse dynamical coherence,
an out-of-phase difference between plane-of-the-sky motions and
line-of-sight velocities, and enhanced line broadening and heating
around most of the flux tube. A complex combination between resonant
absorption and Kelvin-Helmholtz instabilities (KHIs) takes place
in which the KHI extracts the energy from the resonant layer and
dissipates it through vortices and current sheets, which rapidly
degenerate into turbulence. An inward enlargement of the boundary
is produced in which the turbulent flows conserve the characteristic
dynamics from the resonance, therefore guaranteeing detectability of
the resonance imprints. We show that the features described in the
accompanying paper through coordinated Hinode and Interface Region
Imaging Spectrograph observations match the numerical results well.
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Title: Resonant Absorption of Transverse Oscillations and Associated
Heating in a Solar Prominence. I. Observational Aspects
Authors: Okamoto, Takenori J.; Antolin, Patrick; De Pontieu, Bart;
Uitenbroek, Han; Van Doorsselaere, Tom; Yokoyama, Takaaki
2015ApJ...809...71O Altcode: 2015arXiv150608965O
Transverse magnetohydrodynamic waves have been shown to be ubiquitous
in the solar atmosphere and can, in principle, carry sufficient energy
to generate and maintain the Sun’s million-degree outer atmosphere
or corona. However, direct evidence of the dissipation process of these
waves and subsequent heating has not yet been directly observed. Here we
report on high spatial, temporal, and spectral resolution observations
of a solar prominence that show a compelling signature of so-called
resonant absorption, a long hypothesized mechanism to efficiently
convert and dissipate transverse wave energy into heat. Aside
from coherence in the transverse direction, our observations show
telltale phase differences around 180° between transverse motions
in the plane-of-sky and line-of-sight velocities of the oscillating
fine structures or threads, and also suggest significant heating from
chromospheric to higher temperatures. Comparison with advanced numerical
simulations support a scenario in which transverse oscillations trigger
a Kelvin-Helmholtz instability (KHI) at the boundaries of oscillating
threads via resonant absorption. This instability leads to numerous
thin current sheets in which wave energy is dissipated and plasma is
heated. Our results provide direct evidence for wave-related heating
in action, one of the candidate coronal heating mechanisms.
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Title: Forward Modeling of Standing Slow Modes in Flaring Coronal
Loops
Authors: Yuan, D.; Van Doorsselaere, T.; Banerjee, D.; Antolin, P.
2015ApJ...807...98Y Altcode: 2015arXiv150407475Y
Standing slow-mode waves in hot flaring loops are exclusively observed
in spectrometers and are used to diagnose the magnetic field strength
and temperature of the loop structure. Owing to the lack of spatial
information, the longitudinal mode cannot be effectively identified. In
this study, we simulate standing slow-mode waves in flaring loops and
compare the synthesized line emission properties with Solar Ultraviolet
Measurements of Emitted Radiation spectrographic and Solar Dynamics
Observatory/Atmospheric Imaging Assembly imaging observations. We find
that the emission intensity and line width oscillations are a quarter
period out of phase with Doppler shift velocity in both time and spatial
domain, which can be used to identify a standing slow-mode wave from
spectroscopic observations. However, the longitudinal overtones could
only be measured with the assistance of imagers. We find emission
intensity asymmetry in the positive and negative modulations this is
because the contribution function pertaining to the atomic emission
process responds differently to positive and negative temperature
variations. One may detect half periodicity close to the loop
apex, where emission intensity modulation is relatively small. The
line-of-sight projection affects the observation of Doppler shift
significantly. A more accurate estimate of the amplitude of velocity
perturbation is obtained by de-projecting the Doppler shift by a
factor of 1-2θ/π rather than the traditionally used {cos}θ . If a
loop is heated to the hotter wing, the intensity modulation could be
overwhelmed by background emission, while the Doppler shift velocity
could still be detected to a certain extent.
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Title: The Multithermal and Multi-stranded Nature of Coronal Rain
Authors: Antolin, P.; Vissers, G.; Pereira, T. M. D.; Rouppe van der
Voort, L.; Scullion, E.
2015ApJ...806...81A Altcode: 2015arXiv150404418A
We analyze coordinated observations of coronal rain in loops,
spanning chromospheric, transition region (TR), and coronal
temperatures with sub-arcsecond spatial resolution. Coronal rain
is found to be a highly multithermal phenomenon with a high degree
of co-spatiality in the multi-wavelength emission. EUV darkening
and quasi-periodic intensity variations are found to be strongly
correlated with coronal rain showers. Progressive cooling of coronal
rain is observed, leading to a height dependence of the emission. A
fast-slow two-step catastrophic cooling progression is found, which
may reflect the transition to optically thick plasma states. The
intermittent and clumpy appearance of coronal rain at coronal heights
becomes more continuous and persistent at chromospheric heights
just before impact, mainly due to a funnel effect from the observed
expansion of the magnetic field. Strong density inhomogeneities of
0\buildrel{\prime\prime}\over{.} 2-0\buildrel{\prime\prime}\over{.} 5
are found, in which a transition from temperatures of 10<SUP>5</SUP>
to 10<SUP>4</SUP> K occurs. The 0\buildrel{\prime\prime}\over{.}
2-0\buildrel{\prime\prime}\over{.} 8 width of the distribution
of coronal rain is found to be independent of temperature. The
sharp increase in the number of clumps at the coolest temperatures,
especially at higher resolution, suggests that the bulk distribution
of the rain remains undetected. Rain clumps appear organized in
strands in both chromospheric and TR temperatures. We further find
structure reminiscent of the magnetohydrodynamic (MHD) thermal mode
(also known as entropy mode), thereby suggesting an important role of
thermal instability in shaping the basic loop substructure. Rain core
densities are estimated to vary between 2 × 10<SUP>10</SUP> and 2.5×
{{10}<SUP>11</SUP>} cm<SUP>-3</SUP>, leading to significant downward
mass fluxes per loop of 1-5 × 10<SUP>9</SUP> g s<SUP>-1</SUP>, thus
suggesting a major role in the chromosphere-corona mass cycle.
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Title: Hα and EUV Observations of a Partial CME
Authors: Christian, Damian J.; Jess, David B.; Antolin, Patrick;
Mathioudakis, Mihalis
2015ApJ...804..147C Altcode: 2015arXiv150303982C
We have obtained Hα high spatial and time resolution observations
of the upper solar chromosphere and supplemented these with
multi-wavelength observations from the Solar Dynamics Observatory
(SDO) and the Hinode Extreme-ultraviolet Imaging Spectrometer. The Hα
observations were conducted on 2012 February 11 with the Hydrogen-Alpha
Rapid Dynamics Camera instrument at the National Solar Observatory’s
Dunn Solar Telescope. Our Hα observations found large downflows
of chromospheric material returning from coronal heights following a
failed prominence eruption. We have detected several large condensations
(“blobs”) returning to the solar surface at velocities of ≈200 km
s<SUP>-1</SUP> in both Hα and several SDO Atmospheric Imaging Assembly
band passes. The average derived size of these “blobs” in Hα is 500
by 3000 km<SUP>2</SUP> in the directions perpendicular and parallel to
the direction of travel, respectively. A comparison of our “blob”
widths to those found from coronal rain, indicate that there are
additional, smaller, unresolved “blobs” in agreement with previous
studies and recent numerical simulations. Our observed velocities and
decelerations of the “blobs” in both Hα and SDO bands are less
than those expected for gravitational free-fall and imply additional
magnetic or gas pressure impeding the flow. We derived a kinetic energy
of ≈2 orders of magnitude lower for the main eruption than a typical
coronal mass ejection, which may explain its partial nature.
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Title: First High-resolution Spectroscopic Observations of an Erupting
Prominence Within a Coronal Mass Ejection by the Interface Region
Imaging Spectrograph (IRIS)
Authors: Liu, Wei; De Pontieu, Bart; Vial, Jean-Claude; Title, Alan
M.; Carlsson, Mats; Uitenbroek, Han; Okamoto, Takenori J.; Berger,
Thomas E.; Antolin, Patrick
2015ApJ...803...85L Altcode: 2015arXiv150204738L
Spectroscopic observations of prominence eruptions associated with
coronal mass ejections (CMEs), although relatively rare, can provide
valuable plasma and three-dimensional geometry diagnostics. We report
the first observations by the Interface Region Imaging Spectrograph
mission of a spectacular fast CME/prominence eruption associated with
an equivalent X1.6 flare on 2014 May 9. The maximum plane-of-sky and
Doppler velocities of the eruption are 1200 and 460 km s<SUP>-1</SUP>,
respectively. There are two eruption components separated by ∼200
km s<SUP>-1</SUP> in Doppler velocity: a primary, bright component
and a secondary, faint component, suggesting a hollow, rather than
solid, cone-shaped distribution of material. The eruption involves
a left-handed helical structure undergoing counterclockwise (viewed
top-down) unwinding motion. There is a temporal evolution from upward
eruption to downward fallback with less-than-free-fall speeds and
decreasing nonthermal line widths. We find a wide range of Mg ii k/h
line intensity ratios (less than ∼2 expected for optically-thin
thermal emission): the lowest ever reported median value of 1.17
found in the fallback material, a comparably high value of 1.63 in
nearby coronal rain, and intermediate values of 1.53 and 1.41 in
the two eruption components. The fallback material exhibits a strong
(\gt 5σ ) linear correlation between the k/h ratio and the Doppler
velocity as well as the line intensity. We demonstrate that Doppler
dimming of scattered chromospheric emission by the erupted material
can potentially explain such characteristics.
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Title: Observational Signatures of Waves and Flows in the Solar Corona
Authors: De Moortel, I.; Antolin, P.; Van Doorsselaere, T.
2015SoPh..290..399D Altcode: 2014SoPh..tmp..133D; 2015arXiv151001030D
Propagating perturbations have been observed in extended coronal loop
structures for a number of years, but the interpretation in terms of
slow (propagating) magneto-acoustic waves and/or as quasi-periodic
upflows remains unresolved. We used forward-modelling to construct
observational signatures associated with a simple slow magneto-acoustic
wave or periodic flow model. Observational signatures were computed
for the 171 Å Fe IX and the 193 Å Fe XII spectral lines. Although
there are many differences between the flow and wave models, we did
not find any clear, robust observational characteristics that can be
used in isolation (i.e. that do not rely on a comparison between the
models). For the waves model, a relatively rapid change of the average
line widths as a function of (shallow) line-of-sight angles was found,
whereas the ratio of the line width amplitudes to the Doppler velocity
amplitudes is relatively high for the flow model. The most robust
observational signature found is that the ratio of the mean to the
amplitudes of the Doppler velocity is always higher than one for the
flow model. This ratio is substantially higher for flows than for
waves, and for the flows model used in the study is exactly the same
in the 171 Å Fe IX and the 193 Å Fe XII spectral lines. However,
these potential observational signatures need to be treated cautiously
because they are likely to be model-dependent.
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Title: Observational Evidence of Resonant Absorption in Oscillating
Prominence
Authors: Okamoto, J.; Antolin, P.; De Pontieu, B.; Uitenbroek, H.;
Van Doorsselaere, T.; Yokoyama, T.
2014AGUFMSH12A..05O Altcode:
Coronal heating and the acceleration of the solar wind are unsolved
problems in solar physics. The propagation of Alfven waves along
magnetic field lines is one of the candidate mechanisms for
carrying energy to large distances from the surface and heat the
coronal plasma. However, the dissipation process is still unclear
in observational aspects.The new NASA's solar physics satellite IRIS
(Interface Region Imaging Spectrograph) provides spectral information of
plasma in the chromosphere and transition region with high-spatial and
high-temporal resolution. Hence, we performed observations of a limb
prominence to find evidence and clues of dissipation in collaboration
with Hinode/SOT and SDO/AIA.In our observations, we found a clear
evidence of resonant absorption that takes place on the surface of
the oscillating prominence flux tubes. This mechanism facilitates
the onset of the Kelvin-Helmholtz instability, which deforms the thin
tube's boundaries and generates thin current sheets and turbulence,
leading to dissipation of the wave energy into heat. In this talk, we
will show the observed phenomena and discuss the dissipation mechanism
compared with numerical simulations of an oscillating prominence.
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Title: Unresolved Fine-scale Structure in Solar Coronal Loop-tops
Authors: Scullion, E.; Rouppe van der Voort, L.; Wedemeyer, S.;
Antolin, P.
2014ApJ...797...36S Altcode: 2014arXiv1409.1920S
New and advanced space-based observing facilities continue to lower
the resolution limit and detect solar coronal loops in greater
detail. We continue to discover even finer substructures within
coronal loop cross-sections, in order to understand the nature of
the solar corona. Here, we push this lower limit further to search
for the finest coronal loop substructures, through taking advantage
of the resolving power of the Swedish 1 m Solar Telescope/CRisp
Imaging Spectro-Polarimeter (CRISP), together with co-observations
from the Solar Dynamics Observatory/Atmospheric Image Assembly
(AIA). High-resolution imaging of the chromospheric Hα 656.28 nm
spectral line core and wings can, under certain circumstances, allow
one to deduce the topology of the local magnetic environment of the
solar atmosphere where its observed. Here, we study post-flare coronal
loops, which become filled with evaporated chromosphere that rapidly
condenses into chromospheric clumps of plasma (detectable in Hα)
known as a coronal rain, to investigate their fine-scale structure. We
identify, through analysis of three data sets, large-scale catastrophic
cooling in coronal loop-tops and the existence of multi-thermal,
multi-stranded substructures. Many cool strands even extend fully
intact from loop-top to footpoint. We discover that coronal loop
fine-scale strands can appear bunched with as many as eight parallel
strands within an AIA coronal loop cross-section. The strand number
density versus cross-sectional width distribution, as detected by CRISP
within AIA-defined coronal loops, most likely peaks at well below 100
km, and currently, 69% of the substructure strands are statistically
unresolved in AIA coronal loops.
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Title: First High-resolution Spectroscopic Observations by IRIS
of a Fast, Helical Prominence Eruption Associated with a Coronal
Mass Ejection
Authors: Liu, W.; De Pontieu, B.; Okamoto, T. J.; Vial, J. C.; Title,
A. M.; Antolin, P.; Berger, T. E.; Uitenbroek, H.
2014AGUFMSH11D..04L Altcode:
High-resolution spectroscopic observations of prominence eruptions and
associated coronal mass ejections (CMEs) are rare but can provide
valuable plasma and energy diagnostics. New opportunities have
recently become available with the advent of the Interface Region
Imaging Spectrograph (IRIS) mission equipped with high resolution of
0.33-0.4 arcsec in space and 1 km/s in velocity, together with the
Hinode Solar Optical Telescope of 0.2 arcsec spatial resolution. We
report the first result of joint IRIS-Hinode observations of a
spectacular prominence eruption occurring on 2014-May-09. IRIS
detected a maximum redshift of 450 km/s, which, combined with the
plane-of-sky speed of 800 km/s, gives a large velocity vector of 920
km/s at 30 degrees from the sky plane. This direction agrees with the
source location at 30 degrees behind the limb observed by STEREO-A
and indicates a nearly vertical ejection. We found two branches of
redshifts separated by 200 km/s appearing in all strong lines at
chromospheric to transition-region temperatures, including Mg II k/h,
C II, and Si IV, suggesting a hollow, rather than solid, cone in the
velocity space of the ejected material. Opposite blue- and redshifts
on the two sides of the prominence exhibit corkscrew variations both
in space and time, suggestive of unwinding rotations of a left-handed
helical flux rope. Some erupted material returns as nearly streamline
flows, exhibiting distinctly narrow line widths (~10 km/s), about
50% of those of the nearby coronal rain at the apexes of coronal
loops, where the rain material is initially formed out of cooling
condensation. We estimate the mass and kinetic energy of the ejected
and returning material and compare them with those of the associated
CME. We will discuss the implications of these observations for CME
initiation mechanisms.
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Title: Simulating the in Situ Condensation Process of Solar
Prominences
Authors: Xia, C.; Keppens, R.; Antolin, P.; Porth, O.
2014ApJ...792L..38X Altcode: 2014arXiv1408.4249X
Prominences in the solar corona are a hundredfold cooler and denser
than their surroundings, with a total mass of 10<SUP>13</SUP> up
to 10<SUP>15</SUP> g. Here, we report on the first comprehensive
simulations of three-dimensional, thermally and gravitationally
stratified magnetic flux ropes where in situ condensation to a
prominence occurs due to radiative losses. After a gradual thermodynamic
adjustment, we witness a phase where runaway cooling occurs while
counter-streaming shearing flows drain off mass along helical field
lines. After this drainage, a prominence-like condensation resides
in concave upward field regions, and this prominence retains its
overall characteristics for more than two hours. While condensing,
the prominence establishes a prominence-corona transition region where
magnetic field-aligned thermal conduction is operative during the
runaway cooling. The prominence structure represents a force-balanced
state in a helical flux rope. The simulated condensation demonstrates a
right-bearing barb, as a remnant of the drainage. Synthetic images at
extreme ultraviolet wavelengths follow the onset of the condensation,
and confirm the appearance of horns and a three-part structure for the
stable prominence state, as often seen in erupting prominences. This
naturally explains recent Solar Dynamics Observatory views with
the Atmospheric Imaging Assembly on prominences in coronal cavities
demonstrating horns.
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Title: Detection of Supersonic Downflows and Associated Heating
Events in the Transition Region above Sunspots
Authors: Kleint, L.; Antolin, P.; Tian, H.; Judge, P.; Testa, P.;
De Pontieu, B.; Martínez-Sykora, J.; Reeves, K. K.; Wuelser, J. P.;
McKillop, S.; Saar, S.; Carlsson, M.; Boerner, P.; Hurlburt, N.; Lemen,
J.; Tarbell, T. D.; Title, A.; Golub, L.; Hansteen, V.; Jaeggli, S.;
Kankelborg, C.
2014ApJ...789L..42K Altcode: 2014arXiv1406.6816K
Interface Region Imaging Spectrograph data allow us to study the solar
transition region (TR) with an unprecedented spatial resolution of
0.”33. On 2013 August 30, we observed bursts of high Doppler shifts
suggesting strong supersonic downflows of up to 200 km s<SUP>-1</SUP>
and weaker, slightly slower upflows in the spectral lines Mg II h
and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated
with brightenings in the slitjaw images (SJIs). The bursty behavior
lasts throughout the 2 hr observation, with average burst durations
of about 20 s. The locations of these short-lived events appear to
be the umbral and penumbral footpoints of EUV loops. Fast apparent
downflows are observed along these loops in the SJIs and in the
Atmospheric Imaging Assembly, suggesting that the loops are thermally
unstable. We interpret the observations as cool material falling
from coronal heights, and especially coronal rain produced along the
thermally unstable loops, which leads to an increase of intensity
at the loop footpoints, probably indicating an increase of density
and temperature in the TR. The rain speeds are on the higher end of
previously reported speeds for this phenomenon, and possibly higher
than the free-fall velocity along the loops. On other observing days,
similar bright dots are sometimes aligned into ribbons, resembling
small flare ribbons. These observations provide a first insight into
small-scale heating events in sunspots in the TR.
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Title: Fine Strand-like Structure in the Solar Corona from
Magnetohydrodynamic Transverse Oscillations
Authors: Antolin, P.; Yokoyama, T.; Van Doorsselaere, T.
2014ApJ...787L..22A Altcode: 2014arXiv1405.0076A
Current analytical and numerical modeling suggest the existence of
ubiquitous thin current sheets in the corona that could explain the
observed heating requirements. On the other hand, new high resolution
observations of the corona indicate that its magnetic field may tend
to organize itself in fine strand-like structures of few hundred
kilometers widths. The link between small structure in models and the
observed widths of strand-like structure several orders of magnitude
larger is still not clear. A popular theoretical scenario is the
nanoflare model, in which each strand is the product of an ensemble
of heating events. Here, we suggest an alternative mechanism for
strand generation. Through forward modeling of three-dimensional
MHD simulations we show that small amplitude transverse MHD waves
can lead in a few periods time to strand-like structure in loops in
EUV intensity images. Our model is based on previous numerical work
showing that transverse MHD oscillations can lead to Kelvin-Helmholtz
instabilities that deform the cross-sectional area of loops. While
previous work has focused on large amplitude oscillations, here we
show that the instability can occur even for low wave amplitudes
for long and thin loops, matching those presently observed in the
corona. We show that the vortices generated from the instability are
velocity sheared regions with enhanced emissivity hosting current
sheets. Strands result as a complex combination of the vortices and
the line-of-sight angle, last for timescales of a period, and can be
observed for spatial resolutions of a tenth of loop radius.
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Title: Evidence of Magnetic Reconnection Involving Partially Ionized
Coronal Rain near Null Points Observed by SDO/AIA and IRIS
Authors: Liu, Wei; Antolin, Patrick; Sun, Xudong; Berger, Thomas E.
2014shin.confE..50L Altcode:
Coronal rain is cool, partially ionized material formed in the hot,
fully ionized corona. We report a newly discovered class of coronal
rain formed near cusp-shaped portions of coronal loops, indicative
of topological null points. We present evidence of cross-field flows
associated with magnetic reconnection near such null points from
SDO/AIA and IRIS observations, investigate the responsible magnetic
environment, and infer clues to where and when catastrophic cooling
take place to produce coronal rain. We also discuss the implications
of such a cooling process for the enigmatic coronal heating mechanisms
(e.g., Antolin et al. 2010) and compare transient coronal rain and
persistent prominence downflows.
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Title: IRIS Observations of Coronal Rain and Prominences: Return
Flows of the Chromosphere-Corona Mass Cycle
Authors: Liu, Wei; Berger, Thomas; Antolin, Patrick; Schrijver, Karel
2014AAS...22431303L Altcode:
It has recently been recognized that a mass cycle (e.g., Berger
et al. 2011; McIntosh et al. 2012) between the hot, tenuous solar
corona and the cool, dense chromosphere underneath it plays an
important role in the mass budget and dynamic evolution of the solar
atmosphere. Although the corona ultimately loses mass through the solar
wind and coronal mass ejections, a fraction of its mass returns to the
chromosphere in coronal rain, downflows of prominences, and other as-yet
unidentified processes. We present here analysis of joint observations
of IRIS, SDO/AIA, and Hinode/SOT of such phenomena. By utilizing the
wide temperature coverage (logT: 4 - 7) provided by these instruments
combined, we track the coronal cooling sequence (e.g., Schrijver 2001;
Liu et al. 2012; Berger et al. 2012) leading to the formation of such
material at transition region or chromospheric temperatures (logT: 4 -
5) in the million-degree corona. We compare the cooling times with those
expected from the radiative cooling instability. We also measure the
kinematics and densities of such downflows and infer their mass fluxes,
which are compared to the upward mass fluxes into the corona, e.g.,
those associated with spicules and flux emergence. Special attention is
paid to coronal rain formed near cusp-shaped portions of coronal loops,
funnel-shaped prominences at dips of coronal loops, and their respective
magnetic environments. With the information about where and when such
catastrophic cooling events take place, we discuss the implications for
the enigmatic coronal heating mechanisms (e.g., Antolin et al. 2010).
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Title: Forward Modeling of Gyrosynchrotron Intensity Perturbations
by Sausage Modes
Authors: Reznikova, V. E.; Antolin, P.; Van Doorsselaere, T.
2014ApJ...785...86R Altcode:
To determine the observable radio signatures of the fast sausage
standing wave, we examine gyrosynchrotron (GS) emission modulation
using a linear three-dimensional magnetohydrodynamic model of a plasma
cylinder. Effects of the line-of-sight angle and instrumental resolution
on perturbations of the GS intensity are analyzed for two models:
a base model with strong Razin suppression and a low-density model
in which the Razin effect was unimportant. Our finding contradicts
previous predictions made with simpler models: an in-phase variation
of intensity between low (f < f <SUB>peak</SUB>) and high (f >
f <SUB>peak</SUB>) frequencies is found for the low-density model and
an anti-phase variation for the base model in the case of a viewing
angle of 45°. The spatially inhomogeneous character of the oscillating
emission source and the spatial resolution of the model are found to
have a significant effect on the resulting intensity.
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Title: Prominence Formation and Destruction
Authors: Xia, Chun; Antolin, Patrick; Keppens, Rony
2014IAUS..300..468X Altcode:
In earlier work, we demonstrated the in-situ formation of a quiescent
prominence in a sheared magnetic arcade by chromospheric evaporation
and thermal instability in a multi-dimensional MHD model. Here,
we improve our setup and reproduce the formation of a curtain-like
prominence from first principles, while showing the coexistence of the
growing, large-scale prominence with short-lived dynamic coronal rain
in overlying loops. When the localized heating is gradually switched
off, the central prominence expands laterally beyond the range of its
self-created magnetic dips and falls down along the arched loops. The
dipped loops recover their initially arched shape and the prominence
plasma drains to the chromosphere completely.
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Title: Coronal rain observed with IRIS
Authors: Antolin, Patrick; Katsukawa, Yukio; De Pontieu, Bart; Kleint,
Lucia; Pereira, Tiago
2014cosp...40E.105A Altcode:
New IRIS observations in upper chromospheric and TR lines show abundance
of coronal rain in active regions. The wide range of spectral lines in
which it is observed together with co-observations in cool chromospheric
lines with SOT and SST show clearly that coronal rain has a broad
multi-thermal character. This picture agrees well with the thermal
instability scenario in which the plasma cools down catastrophically
from coronal temperatures. A statistical analysis of the line widths
in the rain provides estimates of the non-thermal line broadening and
temperature. Mainly, we find Gaussian-like distributions of non-thermal
line broadening between 0 and 17 km/s with a peak at 7 km/s and a small
upper tail spanning up to 25 km/s. We also report on short-lived heating
events in umbrae and penumbrae at the end of thermally unstable coronal
loops. Bursts of high redshifts up to 200 km/s in TR lines are found,
accompanied by milder blue shifts. The bright dots sometimes display
coherent structure into a "string of pearls" with striking similarity
to flare ribbons, suggesting a strong heating correlation between the
loops. We discuss these results within the coronal rain scenario.
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Title: Simulations of gyrosynchrotron microwave emission from an
oscillating magnetic loop
Authors: Kuznetsov, Alexey; Reznikova, Veronika; Van Doorsselaere,
Tom; Antolin, Patrick
2014cosp...40E1717K Altcode:
Radio observations of solar flares often reveal various periodic
or quasi-periodic oscillations. Most likely, these oscillations
are caused by MHD oscillations of flaring loops which modulate the
radio emission via variations of the magnetic field and electron
concentration. We perform numerical simulations of gyrosynchrotron
radiation from a toroidal-shaped magnetic loop containing sausage-mode
MHD oscillations. Different parameters of the loop and MHD oscillations
and different loop orientations are considered. The simulation results
are compared with the observations of the Nobeyama Radioheliograph.
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Title: Forward modeling of gyrosynchrotron emission perturbations
by sausage mode
Authors: Reznikova, Veronika; Van Doorsselaere, Tom; Antolin, Patrick
2014cosp...40E2741R Altcode:
The modulation of the GS emission by fast sausage MHD oscillations
was modeled for typical flaring parameters. For the first time a 3D
model was adapted for this purpose and variations of the angle between
the magnetic field vector and the line-of-sight have been taken into
account. The variation of the thermodynamic quantities are found by
linearizing the perturbed ideal MHD equations about the magnetostatic
equilibrium. Effects of line-of-sight angle and instrumental reso-
lution on perturbations of gyrosynchrotron intensity are analyzed for
two models: the base model with the strong Razin suppression and the
low density model in which the Razin effect was inessential at all
examined frequencies. Results obtained for phase relations between low
(f < fpeak) and high (f > fpeak) frequency emission oscillation
contradict to previous predictions made with models without spatial
resolution and assuming inhomogeneous emitting source.
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Title: Fine strand-like structure in the corona from MHD transverse
oscillations
Authors: Antolin, Patrick; Yokoyama, Takaaki; Van Doorsselaere, Tom
2014cosp...40E.104A Altcode:
Current analytical and numerical modelling suggest the existence
of ubiquitous thin current sheets in the corona that could explain
the observed line broadening and heating requirements. On the other
hand, new high resolution observations of the corona indicate that
its magnetic field may tend to organise itself in fine strand-like
structures of a few hundred kilometres widths. The link between small
structure in models and the observed widths of strand-like structure
several orders of magnitude larger is still not clear. A popular
theoretical scenario is the nanoflare model, in which each strand
is the product of an ensemble of heating events. Here, we suggest an
alternative mechanism for strand generation. Through forward modelling
of 3D MHD simulations we show that if a loop has initially a monolithic
structure, even a small amplitude transverse MHD wave can lead in a
few periods time to strand-like structure in EUV intensity images. Our
model is based on previous numerical work showing that transverse MHD
oscillations can lead to Kelvin-Helmholtz instabilities that deform the
cross-sectional area of loops. While previous work has focused on large
amplitude oscillations, here we show that the instability can occur
even for low wave amplitudes, matching those presently observed in the
corona. Through forward modelling we show that the roll-ups generated
from the instability are velocity sheared regions with enhanced
emissivity and line broadening hosting current sheets. Strand-like
structure results as a complex combination of the roll-ups and the
line-of-sight angle, can last over relatively long timescales and can
be observed for spatial resolutions discerning a tenth of a loop radius.
---------------------------------------------------------
Title: Are Giant Tornadoes the Legs of Solar Prominences?
Authors: Wedemeyer, Sven; Scullion, Eamon; Rouppe van der Voort, Luc;
Bosnjak, Antonija; Antolin, Patrick
2013ApJ...774..123W Altcode: 2013arXiv1306.2661W
Observations in the 171 Å channel of the Atmospheric Imaging Assembly
of the space-borne Solar Dynamics Observatory show tornado-like
features in the atmosphere of the Sun. These giant tornadoes appear
as dark, elongated, and apparently rotating structures in front of
a brighter background. This phenomenon is thought to be produced
by rotating magnetic field structures that extend throughout the
atmosphere. We characterize giant tornadoes through a statistical
analysis of properties such as spatial distribution, lifetimes,
and sizes. A total number of 201 giant tornadoes are detected in a
period of 25 days, suggesting that, on average, about 30 events are
present across the whole Sun at a time close to solar maximum. Most
tornadoes appear in groups and seem to form the legs of prominences,
thus serving as plasma sources/sinks. Additional Hα observations with
the Swedish 1 m Solar Telescope imply that giant tornadoes rotate as
a structure, although they clearly exhibit a thread-like structure. We
observe tornado groups that grow prior to the eruption of the connected
prominence. The rotation of the tornadoes may progressively twist
the magnetic structure of the prominence until it becomes unstable
and erupts. Finally, we investigate the potential relation of giant
tornadoes to other phenomena, which may also be produced by rotating
magnetic field structures. A comparison to cyclones, magnetic tornadoes,
and spicules implies that such events are more abundant and short-lived
the smaller they are. This comparison might help to construct a power
law for the effective atmospheric heating contribution as a function
of spatial scale.
---------------------------------------------------------
Title: Line-of-sight geometrical and instrumental resolution effects
on intensity perturbations by sausage modes
Authors: Antolin, P.; Van Doorsselaere, T.
2013A&A...555A..74A Altcode: 2013arXiv1303.6147A
Context. Diagnostics of magnetohydrodynamic (MHD) waves in the solar
atmosphere is a topic that often encounters interpretation problems,
partly because of the high complexity of the solar atmospheric
medium. Forward modelling can significantly guide interpretation,
bridging the gap between numerical simulations and observations,
and increasing the reliability of mode identification for applying
MHD seismology. <BR /> Aims: We determine the characteristics of the
fast MHD sausage mode in the corona on the modulation of observable
quantities, such as line intensity and spectral line broadening. Effects
of the line-of-sight angle and of spatial, temporal, and spectral
resolutions are considered. <BR /> Methods: We take a cylindrical
tube that simulates a loop in a low-β coronal environment with an
optically thin background and let it oscillate with the fast sausage
mode. A parametric study is performed. <BR /> Results: Longitudinal
structuring of the intensity modulation is obtained and set by the
nodal structure of the radial velocity. The modulation is strongly
dependent on the contribution function of the spectral line. Under the
assumption of equilibrium ionisation, the intensity variation can be
very low (≲4% for Fe ix 171) or significant (35% for Fe xii 193). Most
of this variation disappears when considering the radiative relaxation
times of the ions, due to the fast timescales of the sausage mode in the
corona. Regardless of the ionisation state of the plasma, the variation
in spectral line broadening can be significant, even for low intensity
modulation. The nature of this broadening is not thermal but is mostly
turbulent. This places spectrometers in clear advantage over imaging
instruments for the detection of the sausage mode. The modulation
of all quantities can considerably decrease with the line-of-sight
angle with respect to the perpendicular to the tube axis. The spatial
and temporal resolution are the main factors affecting modulation,
erasing longitudinal structuring when these are on the order of the
mode's wavelength or the mode's period, placing high constraints on
instrumentation. Significant variability in all quantities can still
be obtained when viewing at an angle of up to 30°, with pixel size
resolutions up to one-third of the mode's wavelength, or temporal
resolution of one fifth of the mode's period. Modulation is only weakly
dependent on spectral resolution due to the mode's inherent symmetry.
---------------------------------------------------------
Title: Statistical seismology of transverse waves in the solar corona
Authors: Verwichte, E.; Van Doorsselaere, T.; White, R. S.; Antolin, P.
2013A&A...552A.138V Altcode:
Context. Observations show that transverse oscillations commonly occur
in solar coronal loops. The rapid damping of these waves has been
attributed to resonant absorption. The oscillation characteristics
carries information of the structuring of the corona. However,
self-consistent seismological methods that extract information
from individual oscillations are limited because there are fewer
observables than unknown parameters in the model, and the problem
is underdetermined. Furthermore, it has been shown that one-to-one
comparisons of the observed scaling of period and damping times with
wave damping theories are misleading. <BR /> Aims: We aim to investigate
whether seismological information can be gained from the observed
scaling laws in a statistical sense. <BR /> Methods: A statistical
approach is used whereby scaling laws are produced by forward modelling
using distributions of values for key loop cross-sectional structuring
parameters. We study two types of observations: 1) transverse loops
oscillations as seen mainly with TRACE and SDO and 2) running transverse
waves seen with the Coronal Multichannel Polarimeter (CoMP). <BR />
Results: We demonstrate that the observed period-damping time scaling
law does provide information about the physical damping mechanism,
if observations are collected from as wide range of periods as
possible and a comparison with theory is performed in a statistical
sense. The distribution of the ratio of damping time over period,
i.e. the quality factor, has been derived analytically and fitted to
the observations. A minimum value for the quality factor of 0.65 has
been found. From this, a constraint linking the ranges of possible
values for the density contrast and inhomogeneity layer thickness is
obtained for transverse loop oscillations. If the layer thickness is
not constrained, then the density contrast is at most equal to 3. For
transverse waves seen by CoMP, it is found that the ratio of maximum
to minimum values for these two parameters has to be less than 2.06;
i.e., the sampled values for the layer thickness and Alfvén travel
time come from a relatively narrow distribution. <BR /> Conclusions:
Now that more and more transverse loop oscillations have been analysed,
a statistical approach to coronal seismology becomes possible. Using
the observed data cloud, we have found restrictions to the loop's
density contrast and inhomogeneity layer thickness. Surprisingly, for
running waves, narrow distributions for loop parameters have been found.
---------------------------------------------------------
Title: On-Disk Coronal Rain
Authors: Antolin, Patrick; Vissers, Gregal; Rouppe van der Voort, Luc
2012SoPh..280..457A Altcode: 2012SoPh..tmp...78A; 2012arXiv1203.2077A
Small and elongated, cool and dense blob-like structures are being
reported with high resolution telescopes in physically different regions
throughout the solar atmosphere. Their detection and the understanding
of their formation, morphology, and thermodynamical characteristics can
provide important information on their hosting environment, especially
concerning the magnetic field, whose understanding constitutes a major
problem in solar physics. An example of such blobs is coronal rain, a
phenomenon of thermal non-equilibrium observed in active region loops,
which consists of cool and dense chromospheric blobs falling along
loop-like paths from coronal heights. So far, only off-limb coronal
rain has been observed, and few reports on the phenomenon exist. In
the present work, several data sets of on-disk Hα observations with
the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar
Telescope (SST) are analyzed. A special family of on-disk blobs is
selected for each data set, and a statistical analysis is carried out
on their dynamics, morphology, and temperature. All characteristics
present distributions which are very similar to reported coronal rain
statistics. We discuss possible interpretations considering other
similar blob-like structures reported so far and show that a coronal
rain interpretation is the most likely one. The chromospheric nature
of the blobs and the projection effects (which eliminate all direct
possibilities of height estimation) on one side, and their small sizes,
fast dynamics, and especially their faint character (offering low
contrast with the background intensity) on the other side, are found
as the main causes for the absence until now of the detection of this
on-disk coronal rain counterpart.
---------------------------------------------------------
Title: Implications for Coronal Heating from Coronal Rain
Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort,
L.; Vissers, G.; Hansteen, V.
2012ASPC..454..171A Altcode:
Coronal rain is a phenomenon above active regions in which cool plasma
condensations fall down from coronal heights. Numerical simulations of
loops have shown that such condensations can naturally form in the case
of footpoint concentrated heating through the “catastrophic cooling”
mechanism. In this work we analize high resolution limb observations in
Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of
SST and derive statistical properties. We further investigate the link
between coronal rain and the coronal heating mechanisms by performing
1.5-D MHD simulations of a loop subject to footpoint heating and to
Alfvén waves generated in the photosphere. It is found that if a loop
is heated predominantly from Alfvén waves coronal rain is inhibited
due to the characteristic uniform heating they produce. Hence coronal
rain can point both to the spatial distribution of the heating and to
the agent of the heating itself, thus acting as a marker for coronal
heating mechanisms.
---------------------------------------------------------
Title: A Sharp Look at Coronal Rain with Hinode/SOT and SST/CRISP
Authors: Antolin, P.; Carlsson, M.; Rouppe van der Voort, L.;
Verwichte, E.; Vissers, G.
2012ASPC..455..253A Altcode: 2012arXiv1202.0787A
The tropical wisdom that when it is hot and dense we can expect
rain might also apply to the Sun. Indeed, observations and numerical
simulations have showed that strong heating at footpoints of loops,
as is the case for active regions, puts their coronae out of thermal
equilibrium, which can lead to a phenomenon known as catastrophic
cooling. Following local pressure loss in the corona, hot plasma
locally condenses in these loops and dramatically cools down to
chromospheric temperatures. These blobs become bright in Hα and
Ca ii H in time scales of minutes, and their dynamics seem to be
subject more to internal pressure changes in the loop rather than to
gravity. They thus become trackers of the magnetic field, which results
in the spectacular coronal rain that is observed falling down coronal
loops. In this work we report on high resolution observations of coronal
rain with the Solar Optical Telescope (SOT) on Hinode and CRISP at
the Swedish Solar Telescope (SST). A statistical study is performed in
which properties such as velocities and accelerations of coronal rain
are derived. We show how this phenomenon can constitute a diagnostic
tool for the internal physical conditions inside loops. Furthermore, we
analyze transverse oscillations of strand-like condensations composing
coronal rain falling in a loop, and discuss the possible nature of the
wave. This points to the important role that coronal rain can play in
the fields of coronal heating and coronal seismology.
---------------------------------------------------------
Title: Implications for coronal heating and magnetic field topology
from coronal rain observations
Authors: Antolin, Patrick
2012PhDT........99A Altcode:
According to tropical wisdom, when the atmosphere feels hot and dense we
can expect rain. Such thinking may also apply to the Sun, as this thesis
explains. The presented new high-resolution observations with the Solar
Optical Telescope (SOT) of Hinode and the CRISP spectropolarimeter at
the Swedish 1-m Solar Telescope (SST) show a picture of the Sun in which
coronal rain seems to be a far more common phenomenon of active regions
(the hot and dense regions in the solar atmosphere) than previously
thought. Coronal rain, a phenomenon of thermal instability in plasmas
for the case of coronal loops, is composed of small cool and dense blobs
observed in chromospheric lines such as Hα or Ca II H, rapidly forming
and falling down from coronal heights along loop-like paths. Apart from
suggesting its ubiquitous character, in this thesis the importance
of coronal rain is highlighted in 3 different ways. First, its
potential as a marker for coronal heating mechanisms is shown. More
specifically, through numerical simulations the effects of Alfvén waves
(a strong coronal heating candidate) on the thermal stability of loops
is treated. The results indicate that coronae heated through shock
heating from mode conversion of Alfvén waves cannot exhibit coronal
rain, thus suggesting that this mechanism may not be important for
the heating of active region coronae. Second, the role it plays in
coronal seismology is shown. Transverse MHD oscillations in loops are
put in evidence by coronal rain in observations with Hinode/SOT, thus
offering a way to estimate the coronal magnetic field strength, one
of the hardest physical quantities to measure accurately, yet lying at
the root of most solar and heliospheric physics. Third, due to the very
small sizes of the blobs of which it is composed of, it also serves as
a probe for the internal structure and local thermodynamic conditions
in loops. In the obtained picture with CRISP of the SST coronal
loops appear with constant area cross-sections along their lengths,
multi-stranded and unbraided. Furthermore, a significant fraction of
strands in the loops show a coherent thermodynamic evolution, thus
imposing several constraints on coronal loop modeling. The mass flux
raining down is shown to be significant, as compared to the estimated
mass injected into the corona from spicules.
---------------------------------------------------------
Title: Observing the Fine Structure of Loops through High-resolution
Spectroscopic Observations of Coronal Rain with the CRISP Instrument
at the Swedish Solar Telescope
Authors: Antolin, P.; Rouppe van der Voort, L.
2012ApJ...745..152A Altcode: 2011arXiv1112.0656A
Observed in cool chromospheric lines, such as Hα or Ca II H, coronal
rain corresponds to cool and dense plasma falling from coronal
heights. Considered as a peculiar sporadic phenomenon of active
regions, it has not received much attention since its discovery
more than 40 years ago. Yet, it has been shown recently that a
close relationship exists between this phenomenon and the coronal
heating mechanism. Indeed, numerical simulations have shown that
this phenomenon is most likely due to a loss of thermal equilibrium
ensuing from a heating mechanism acting mostly toward the footpoints of
loops. We present here one of the first high-resolution spectroscopic
observations of coronal rain, performed with the CRisp Imaging Spectro
Polarimeter (CRISP) instrument at the Swedish Solar Telescope. This
work constitutes the first attempt to assess the importance of coronal
rain in the understanding of the coronal magnetic field in active
regions. With the present resolution, coronal rain is observed to
literally invade the entire field of view. A large statistical set is
obtained in which dynamics (total velocities and accelerations), shapes
(lengths and widths), trajectories (angles of fall of the blobs),
and thermodynamic properties (temperatures) of the condensations
are derived. Specifically, we find that coronal rain is composed of
small and dense chromospheric cores with average widths and lengths of
~310 km and ~710 km, respectively, average temperatures below 7000 K,
displaying a broad distribution of falling speeds with an average of
~70 km s<SUP>-1</SUP>, and accelerations largely below the effective
gravity along loops. Through estimates of the ion-neutral coupling in
the blobs we show that coronal rain acts as a tracer of the coronal
magnetic field, thus supporting the multi-strand loop scenario, and
acts as a probe of the local thermodynamic conditions in loops. We
further elucidate its potential in coronal heating. We find that
the cooling in neighboring strands occurs simultaneously in general
suggesting a similar thermodynamic evolution among strands, which can
be explained by a common footpoint heating process. Constraints for
coronal heating models of loops are thus provided. Estimates of the
fraction of coronal volume with coronal rain give values between 7%
and 30%. Estimates of the occurrence time of the phenomenon in loops
set times between 5 and 20 hr, implying that coronal rain may be a
common phenomenon, in agreement with the frequent observations of cool
downflows in extreme-ultraviolet lines. The coronal mass drain rate
in the form of coronal rain is estimated to be on the order of 5 ×
10<SUP>9</SUP> g s<SUP>-1</SUP>, a significant quantity compared to
the estimate of mass flux into the corona from spicules.
---------------------------------------------------------
Title: Transverse Oscillations of Loops with Coronal Rain Observed
by Hinode/Solar Optical Telescope
Authors: Antolin, P.; Verwichte, E.
2011ApJ...736..121A Altcode: 2011arXiv1105.2175A
The condensations composing coronal rain, falling down along
loop-like structures observed in cool chromospheric lines such as
Hα and Ca II H, have long been a spectacular phenomenon of the solar
corona. However, considered a peculiar sporadic phenomenon, it has not
received much attention. This picture is rapidly changing due to recent
high-resolution observations with instruments such as the Hinode/Solar
Optical Telescope (SOT), CRISP of the Swedish 1-m Solar Telescope, and
the Solar Dynamics Observatory. Furthermore, numerical simulations
have shown that coronal rain is the loss of thermal equilibrium
of loops linked to footpoint heating. This result has highlighted
the importance that coronal rain can play in the field of coronal
heating. In this work, we further stress the importance of coronal rain
by showing the role it can play in the understanding of the coronal
magnetic field topology. We analyze Hinode/SOT observations in the Ca
II H line of a loop in which coronal rain puts in evidence in-phase
transverse oscillations of multiple strand-like structures. The periods,
amplitudes, transverse velocities, and phase velocities are calculated,
allowing an estimation of the energy flux of the wave and the coronal
magnetic field inside the loop through means of coronal seismology. We
discuss the possible interpretations of the wave as either standing or
propagating torsional Alfvén or fast kink waves. An estimate of the
plasma beta parameter of the condensations indicates a condition that
may allow the often observed separation and elongation processes of the
condensations. We also show that the wave pressure from the transverse
wave can be responsible for the observed low downward acceleration of
coronal rain.
---------------------------------------------------------
Title: Coronal Rain as a Marker for Coronal Heating Mechanisms
Authors: Antolin, P.; Shibata, K.; Vissers, G.
2010ApJ...716..154A Altcode: 2009arXiv0910.2383A
Reported observations in Hα, Ca II H, and K or other chromospheric
lines of coronal rain trace back to the days of the Skylab
mission. Corresponding to cool and dense plasma, coronal rain is often
observed falling down along coronal loops in active regions. A physical
explanation for this spectacular phenomenon has been put forward
thanks to numerical simulations of loops with footpoint-concentrated
heating, a heating scenario in which cool condensations naturally
form in the corona. This effect has been termed "catastrophic cooling"
and is the predominant explanation for coronal rain. In this work, we
further investigate the link between this phenomenon and the heating
mechanisms acting in the corona. We start by analyzing observations of
coronal rain at the limb in the Ca II H line performed by the Hinode
satellite, and derive interesting statistical properties concerning
the dynamics. We then compare the observations with 1.5-dimensional
MHD simulations of loops being heated by small-scale discrete events
concentrated toward the footpoints (that could come, for instance,
from magnetic reconnection events), and by Alfvén waves generated at
the photospheric level. Both our observation and simulation results
suggest that coronal rain is a far more common phenomenon than
previously thought. Also, we show that the structure and dynamics of
condensations are far more sensitive to the internal pressure changes
in loops than to gravity. Furthermore, it is found that if a loop is
predominantly heated from Alfvén waves, coronal rain is inhibited due
to the characteristic uniform heating they produce. Hence, coronal
rain may not only point to the spatial distribution of the heating
in coronal loops but also to the agent of the heating itself. We thus
propose coronal rain as a marker for coronal heating mechanisms.
---------------------------------------------------------
Title: The Role Of Torsional Alfvén Waves in Coronal Heating
Authors: Antolin, P.; Shibata, K.
2010ApJ...712..494A Altcode: 2009arXiv0910.0962A
In the context of coronal heating, among the zoo of magnetohydrodynamic
(MHD) waves that exist in the solar atmosphere, Alfvén waves receive
special attention. Indeed, these waves constitute an attractive
heating agent due to their ability to carry over the many different
layers of the solar atmosphere sufficient energy to heat and maintain
a corona. However, due to their incompressible nature these waves
need a mechanism such as mode conversion (leading to shock heating),
phase mixing, resonant absorption, or turbulent cascade in order
to heat the plasma. Furthermore, their incompressibility makes their
detection in the solar atmosphere very difficult. New observations with
polarimetric, spectroscopic, and imaging instruments such as those on
board the Japanese satellite Hinode, or the Crisp spectropolarimeter of
the Swedish Solar Telescope or the Coronal Multi-channel Polarimeter,
are bringing strong evidence for the existence of energetic Alfvén
waves in the solar corona. In order to assess the role of Alfvén
waves in coronal heating, in this work we model a magnetic flux tube
being subject to Alfvén wave heating through the mode conversion
mechanism. Using a 1.5 dimensional MHD code, we carry out a parameter
survey varying the magnetic flux tube geometry (length and expansion),
the photospheric magnetic field, the photospheric velocity amplitudes,
and the nature of the waves (monochromatic or white-noise spectrum). The
regimes under which Alfvén wave heating produces hot and stable coronae
are found to be rather narrow. Independently of the photospheric wave
amplitude and magnetic field, a corona can be produced and maintained
only for long (>80 Mm) and thick (area ratio between the photosphere
and corona >500) loops. Above a critical value of the photospheric
velocity amplitude (generally a few km s<SUP>-1</SUP>) the corona can
no longer be maintained over extended periods of time and collapses due
to the large momentum of the waves. These results establish several
constraints on Alfvén wave heating as a coronal heating mechanism,
especially for active region loops.
---------------------------------------------------------
Title: Signatures of Coronal Heating Mechanisms
Authors: Antolin, P.; Shibata, K.; Kudoh, T.; Shiota, D.; Brooks, D.
2010ASSP...19..277A Altcode: 2010mcia.conf..277A; 2009arXiv0903.1766A
Alfvén waves created by sub-photospheric motions or by magnetic
reconnection in the low solar atmosphere seem good candidates for
coronal heating. However, the corona is also likely to be heated more
directly by magnetic reconnection, with dissipation taking place
in current sheets. Distinguishing observationally between these
two heating mechanisms is an extremely difficult task. We perform
1.5-dimensional MHD simulations of a coronal loop subject to each
type of heating and derive observational quantities that may allow
these to be differentiated. This work is presented in more detail in
Antolin et al. (2008).
---------------------------------------------------------
Title: Alfvén Wave and Nanoflare Reconnection Heating: How to
Distinguish Them Observationally?
Authors: Antolin, P.; Shibata, K.; Kudoh, T.; Shiota, D.; Brooks, D.
2009ASPC..415..247A Altcode:
Alfvén waves can dissipate their energy by means of nonlinear
mechanisms, and constitute good candidates to heat and maintain the
solar corona to the observed few million degrees. Another appealing
candidate is nanoflare reconnection heating, in which energy is released
through many small magnetic reconnection events. Distinguishing the
observational features of each mechanism is an extremely difficult
task. By setting up a 1.5D MHD model of a loop we test both heating
mechanisms and derive observational quantities. The obtained coronae
differ in many aspects; for instance, in the flow patterns along
the loop, flow velocities, and the simulated intensity profile that
Hinode/XRT would observe. The heating events in the loop exhibit
power-law distributions in frequency, whose indexes differ considerably
depending on the heating mechanism and its location along the loop. We
thus test the observational signatures of the power-law index as a
diagnostic tool for the above coronal heating mechanisms.
---------------------------------------------------------
Title: Predicting observational signatures of coronal heating by
Alfvén waves and Nanoflares
Authors: Antolin, Patrick
2009PhDT.......196A Altcode:
The subject of this thesis is the coronal heating problem, a long
standing problem not only in solar physics but in astrophysics, since
it is addressed to all stars that possess a corona. The Sun, a middle
aged main sequence star of class G2V, has been unveiling many mysteries
to us in the last century, especially since the advent of the space
era. More than 70 years ago a very hot temperature component in the
corona was discovered, reaching temperatures as high as a few million
degrees. Such a hot corona came as a surprise to astrophysicists,
since it seemed to contradict the second law of thermodynamics being
200 times hotter than the underlying photosphere, the source of its
energy. Since then the coronal heating problem has spawned an active
research community in solar physics that aims to unveil yet another
mystery. <P />This thesis has as purpose to shed some light into the
fascinating subject of coronal heating. In the first chapter we give
an introduction to the field, in which we discuss the main heating
candidate mechanisms: Alfvén wave heating and nanoflare-reconnection
heating. Predicting unique observational signatures of each heating
mechanism which would allow their distinction during observations
is the main purpose of this thesis and the subject of the second
chapter. In this chapter we investigate the thermodynamic properties
of a corona in a magnetic flux tube obtained, separately, with the two
heating mechanisms. We derive a series of observational features which
may allow the clear distinction between the two heating mechanisms
during observations. In chapter 3 we further investigate the role of
Alfvén wave heating in the solar atmosphere. We concentrate our study
on magnetic flux tubes (loops), which are closed magnetic structures
which populate the solar atmosphere. In the considered model Alfvén
waves are generated at the footpoints of a loop and can dissipate
their energy mainly through the mode conversion mechanism. A parameter
survey is conducted where different parameters of the loop are varied,
such as the loop's length, the loop expansion factor, the magnetic
field strength at the footpoints of the loop (in the photosphere),
and the properties of the Alfvén waves generated in the photosphere
(a monochromatic and a white noise spectrum are considered). In chapter
4 we link the coronal heating problem to an observational phenomenon
known as coronal rain. We start first by reporting limb observations
of coronal rain with Hinode/SOT in the Ca II H line. We then attempt
to reproduce the phenomenon in simulations. For this, the mechanism of
catastrophic cooling is considered and results are compared with the
reported observations. Alfvén waves are then generated in the loop
and the effect on the thermal stability of the corona is studied. We
show that coronal rain is intimately linked with the underlying coronal
heating mechanism, and thus can help pointing out the coronal heating
agent.
---------------------------------------------------------
Title: Predicting Observational Signatures of Coronal Heating by
Alfvén Waves and Nanoflares
Authors: Antolin, P.; Shibata, K.; Kudoh, T.; Shiota, D.; Brooks, D.
2008ApJ...688..669A Altcode:
Alfvén waves can dissipate their energy by means of nonlinear
mechanisms, and constitute good candidates to heat and maintain the
solar corona to the observed few million degrees. Another appealing
candidate is nanoflare reconnection heating, in which energy is released
through many small magnetic reconnection events. Distinguishing the
observational features of each mechanism is an extremely difficult
task. On the other hand, observations have shown that energy release
processes in the corona follow a power-law distribution in frequency
whose index may tell us whether small heating events contribute
substantially to the heating or not. In this work we show a link
between the power-law index and the operating heating mechanism in
a loop. We set up two coronal loop models: in the first model Alfvén
waves created by footpoint shuffling nonlinearly convert to longitudinal
modes which dissipate their energy through shocks; in the second model
numerous heating events with nanoflare-like energies are input randomly
along the loop, either distributed uniformly or concentrated at the
footpoints. Both models are based on a 1.5-dimensional MHD code. The
obtained coronae differ in many aspects; for instance, in the flow
patterns along the loop and the simulated intensity profile that
Hinode XRT would observe. The intensity histograms display power-law
distributions whose indexes differ considerably. This number is found
to be related to the distribution of the shocks along the loop. We
thus test the observational signatures of the power-law index as a
diagnostic tool for the above heating mechanisms and the influence of
the location of nanoflares.
---------------------------------------------------------
Title: Predicting observational signatures of coronal heating by
Alfvén waves and nanoflares
Authors: Antolin, Patrick; Shibata, Kazunari; Kudoh, Takahiro; Shiota,
Daiko; Brooks, David
2008IAUS..247..279A Altcode: 2007IAUS..247..279A
Alfvén waves can dissipate their energy by means of nonlinear
mechanisms, and constitute good candidates to heat and maintain the
solar corona to the observed few million degrees. Another appealing
candidate is the nanoflare-reconnection heating, in which energy is
released through many small magnetic reconnection events. Distinguishing
the observational features of each mechanism is an extremely difficult
task. On the other hand, observations have shown that energy release
processes in the corona follow a power law distribution in frequency
whose index may tell us whether small heating events contribute
substantially to the heating or not. In this work we show a link
between the power law index and the operating heating mechanism in
a loop. We set up two coronal loop models: in the first model Alfvén
waves created by footpoint shuffling nonlinearly convert to longitudinal
modes which dissipate their energy through shocks; in the second model
numerous heating events with nanoflare-like energies are input randomly
along the loop, either distributed uniformly or concentrated at the
footpoints. Both models are based on a 1.5-D MHD code. The obtained
coronae differ in many aspects, for instance, in the simulated intensity
profile that Hinode/XRT would observe. The intensity histograms display
power law distributions whose indexes differ considerably. This number
is found to be related to the distribution of the shocks along the
loop. We thus test the observational signatures of the power law index
as a diagnostic tool for the above heating mechanisms and the influence
of the location of nanoflares.
