polito

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Author name code: polito
ADS astronomy entries on 2022-09-14
author:Polito, V.

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Title: Diagnostics of non-Maxwellian electron distributions in solar
active regions from Fe XII lines observed by Hinode/EIS and IRIS
Authors: Del Zanna, G.; Polito, V.; Dudík, J.; Testa, P.; Mason,
H. E.; Dzifčáková, E.
2022arXiv220707026D Altcode:
We present joint Hinode/EIS and IRIS observations of Fe XII lines
in active regions, both on-disk and off-limb. We use an improved
calibration for the EIS data, and find that the 192.4 A / 1349 A
observed ratio is consistent with the values predicted by CHIANTI and
the coronal approximation in quiescent areas, but not in all active
region observations, where the ratio is often lower than expected
by up to a factor of about two. We investigate a number of physical
mechanisms that could affect this ratio, such as opacity and absorption
from cooler material. We find significant opacity in the EIS Fe XII
193 and 195 A lines, but not in the 192.4 A line, in agreement with
previous findings. As we cannot rule out possible EUV absorption by
H, He and He II in the on-disk observations, we focus on an off-limb
observation where such absorption is minimal. After considering these,
as well as possible non-equilibrium effects, we suggest that the most
likely explanation for the observed low Fe XII 192.4 A / 1349 A ratio
is the presence of non-Maxwellian electron distributions in the active
regions. This is in agreement with previous findings based on EIS and
IRIS observations independently.

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Title: Blueshifted Si IV 1402.77 Å Line Profiles in a Moving Flare
Kernel Observed by IRIS
Authors: Lörinčík, Juraj; Dudík, Jaroslav; Polito, Vanessa
2022ApJ...934...80L Altcode:
We analyze the spectra of a slipping flare kernel observed during
the 2015 June 22 M6.5-class flare by the Interface Region Imaging
Spectrograph (IRIS). During the impulsive and peak phases of the flare,
loops exhibiting an apparent slipping motion along the ribbons were
observed in the 131 Å channel of SDO/AIA. The IRIS spectrograph
slit observed a portion of the ribbons, including a moving kernel
corresponding to a flare loop footpoint in Si IV, C II, and Mg II at a
very-high 1 s cadence. The spectra observed in the kernel were mostly
redshifted and exhibited pronounced red wings, as typically observed
in large flares. However, in a small region in one of the ribbons, the
Si IV 1402.77 Å line was partially blueshifted, with the corresponding
Doppler velocity ∣v D∣ exceeding 50 km s-1. In
the same region, the C II 1334.53, 1335.66, and 1335.71 Å lines were
weakly blueshifted (∣v D∣ &lt; 20 km s-1)
and showed pronounced blue wings, which were also observed in the Mg
II k 2796.35 Å as well as the Mg II triplet 2798.75 and 2798.82 Å
lines. Using high-cadence AIA observations we found that the region
where the blueshifts occurred corresponds to the accelerating kernel
front as it moved through a weak field region. The IRIS observations
with high resolution allowed us to capture the acceleration of the
kernel under the slit for the first time. The unique observations of
blueshifted chromospheric and TR lines provide new constraints for
current models of flares.

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Title: Blueshifted Si IV 1402.77Å line profiles in a moving flare
kernel observed by IRIS
Authors: Lörinčík, Juraj; Dudík, Jaroslav; Polito, Vanessa
2022arXiv220610114L Altcode:
We analyze spectra of a slipping flare kernel observed during the 2015
June 22 M6.5-class flare by the Interface Region Imaging Spectrograph
(IRIS). During the impulsive and peak phases of the flare, loops
exhibiting an apparent slipping motion along the ribbons were observed
in the 131Å channel of SDO/AIA. The IRIS spectrograph slit observed
a portion of the ribbons, including a moving kernel corresponding to
a flare loop footpoint in Si IV, C II, and Mg II at a very-high 1 s
cadence. The spectra observed in the kernel were mostly redshifted
and exhibited pronounced red wings, as typically observed in large
flares. However, in a small region in one of the ribbons, the Si IV
1402.77Å line was partially blueshifted, with the corresponding Doppler
velocity |v_{D}| exceeding 50 km s$^{-1}$. In the same region, the C
II 1334.53Å, 1335.66Å and 1335.71Å lines were weakly blueshifted
(|v_{D}| &lt; 20 km s$^{-1}$) and showed pronounced blue wings,
which were observed also in the Mg II k 2796.35Å as well as the
Mg II triplet 2798.75Å and 2798.82Å lines. Using high-cadence AIA
observations we found that the region where the blueshifts occurred
corresponds to the accelerating kernel front as it moved through a
weak-field region. The IRIS observations with high resolution allowed
us to capture the acceleration of the kernel under the slit for the
first time. The unique observations of blueshifted chromospheric and
TR lines provide new constrains for current models of flares.

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Title: Diagnostics of Non-Maxwellian Electron Distributions in
Solar Active Regions from Fe XII Lines Observed by the Hinode
Extreme Ultraviolet Imaging Spectrometer and Interface Region
Imaging Spectrograph
Authors: Del Zanna, G.; Polito, V.; Dudík, J.; Testa, P.; Mason,
H. E.; Dzifčáková, E.
2022ApJ...930...61D Altcode:
We present joint Hinode Extreme Ultraviolet Imaging Spectrometer
(EIS) and Interface Region Imaging Spectrograph (IRIS) observations
of Fe XII lines in active regions, both on-disk and off-limb. We use
an improved calibration for the EIS data, and find that the 192.4
Å/1349 Å observed ratio is consistent with the values predicted by
CHIANTI and the coronal approximation in quiescent areas, but not in
all active-region observations, where the ratio is often lower than
expected by up to a factor of about two. We investigate a number of
physical mechanisms that could affect this ratio, such as opacity and
absorption from cooler material. We find significant opacity in the EIS
Fe XII 193 and 195 Å lines, but not in the 192.4 Å line, in agreement
with previous findings. As we cannot rule out possible EUV absorption by
H, He, and He II in the on-disk observations, we focus on an off-limb
observation where such absorption is minimal. After considering these,
as well as possible nonequilibrium effects, we suggest that the most
likely explanation for the observed low Fe XII 192.4 Å/1349 Å ratio
is the presence of non-Maxwellian electron distributions in the active
regions. This is in agreement with previous findings based on EIS and
IRIS observations independently.

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

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Title: 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 (&lt;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). 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: Multi-passband Observations of a Solar Flare over the He I
10830 Å line
Authors: Xu, Yan; Yang, Xu; Kerr, Graham S.; Polito, Vanessa; Sadykov,
Viacheslav M.; Jing, Ju; Cao, Wenda; Wang, Haimin
2022ApJ...924L..18X Altcode: 2021arXiv211209949X
This study presents a C3.0 flare observed by the Big Bear Solar
Observatory/Goode Solar Telescope (GST) and Interface Region Imaging
Spectrograph (IRIS) on 2018 May 28 around 17:10 UT. The Near-Infrared
Imaging Spectropolarimeter of GST was set to spectral imaging mode to
scan five spectral positions at ±0.8, ±0.4 Å and line center of He I
10830 Å. At the flare ribbon's leading edge, the line is observed to
undergo enhanced absorption, while the rest of the ribbon is observed
to be in emission. When in emission, the contrast compared to the
preflare ranges from about 30% to nearly 100% at different spectral
positions. Two types of spectra, "convex" shape with higher intensity at
line core and "concave" shape with higher emission in the line wings,
are found at the trailing and peak flaring areas, respectively. On the
ribbon front, negative contrasts, or enhanced absorption, of about
~10%-20% appear in all five wavelengths. This observation strongly
suggests that the negative flares observed in He I 10830 Å with
mono-filtergram previously were not caused by pure Doppler shifts of
this spectral line. Instead, the enhanced absorption appears to be a
consequence of flare-energy injection, namely nonthermal collisional
ionization of helium caused by the precipitation of high-energy
electrons, as found in our recent numerical modeling results. In
addition, though not strictly simultaneous, observations of Mg II
from the IRIS spacecraft, show an obvious central reversal pattern
at the locations where enhanced absorption of He I 10830 Å is seen,
which is consistent with previous observations.

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Title: The origin of underdense plasma downflows associated with
magnetic reconnection in solar flares
Authors: Shen, Chengcai; Chen, Bin; Reeves, Katharine K.; Yu, Sijie;
Polito, Vanessa; Xie, Xiaoyan
2022NatAs...6..317S Altcode: 2021arXiv211111407S; 2022NatAs.tmp...29S; 2022NatAs.tmp...23S
Magnetic reconnection is a universal process that powers explosive
energy-release events such as solar flares, geomagnetic substorms
and some astrophysical jets. A characteristic feature of magnetic
reconnection is the production of fast reconnection outflow jets near
the plasma Alfvén speeds1,2. In eruptive solar flares,
dark finger-shaped plasma downflows moving toward the flare arcade
have been commonly regarded as the principal observational evidence
for such reconnection-driven outflows3,4. However, they
often show a speed much slower than that expected in reconnection
theories5,6, challenging the reconnection-driven
energy-release scenario in standard flare models. Here we present
a three-dimensional magnetohydrodynamics model of solar flares. By
comparing the model predictions with the observed plasma downflow
features, we conclude that these dark downflows are self-organized
structures formed in a turbulent interface region below the
flare termination shock where the outflows meet the flare arcade,
a phenomenon analogous to the formation of similar structures in
supernova remnants. This interface region hosts a myriad of turbulent
flows, electron currents and shocks, crucial for flare energy release
and particle acceleration.

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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
(&lt;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: Spectroscopy of ribbon fronts as diagnostics of energy release
during solar flares
Authors: Polito, Vanessa; Kerr, Graham; Sadykov, Viacheslav; Xu, Yan
2021AGUFMSH23B..02P Altcode:
Lower atmospheric spectra have been observed to show peculiar
profiles characterized by increased absorption (in He I with GST,
Xu et al. 2016) as well as broad and reversed profiles (e.g. in Mg
II and CII with IRIS, Panos et al. 2018, 2021a, 2021b) at the leading
edge of ribbons during the impulsive phase of flares. In this work, we
explore the correlation between spectral lines formed across different
layers of the atmosphere in multiple flares, as observed by IRIS and
GST. We find that the locations where the increased He I absorption
or typical broad reverse IRIS chromospheric profiles are observed are
also regions where transition region and coronal emission as well as
chromospheric evaporation are weaker. Comparison of these observations
with predictions from advanced hydrodynamic models provides tight
constraints into the flare heating models.

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Title: A Novel Integral Field Spectrograph Design for taking
High-Cadence Spectral Solar Images: SNIFS
Authors: Knoer, Vicki; Chamberlin, Phillip; Daw, Adrian; Gong, Qian;
Milligan, Ryan; Polito, Vanessa; Schmit, Donald
2021AGUFMSH55B1837K Altcode:
Many features on the sun such as flares and nanoflares are highly
dynamic and change over the course of seconds. This is at least an order
of magnitude faster than our current ability to 2D spectrally image the
sun. This difference in time scale has made it difficult to study some
of the sun's faster-changing features. The newly designed Solar eruptioN
Integral Field Spectrograph (SNIFS) is an extreme ultraviolet (EUV)
integral field spectrograph which will be able to take spectral images
of the sun at a 1 second time cadence. The game-changing innovations
which allow a faster cadence include a fast-readout CMOS detector
and an array of mirrorlets to focus the incoming light into a square
array spatial pixels, the spectrum for each of which will be measured
simultaneously. The optical path is doubled in order to view both active
network and flaring sun. This new optical design will allow high-cadence
spectral imaging of the sun which will contribute to our understanding
of energy and mass transport in the chromosphere and transition region.

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Title: The Formation and Lifetime of Outflows in a Solar Active Region
Authors: Brooks, David H.; Harra, Louise; Bale, Stuart D.; Barczynski,
Krzysztof; Mandrini, Cristina; Polito, Vanessa; Warren, Harry P.
2021ApJ...917...25B Altcode: 2021arXiv210603318B
Active regions are thought to be one contributor to the slow solar
wind. Upflows in EUV coronal spectral lines are routinely observed at
their boundaries, and provide the most direct way for upflowing material
to escape into the heliosphere. The mechanisms that form and drive these
upflows, however, remain to be fully characterized. It is unclear how
quickly they form, or how long they exist during their lifetimes. They
could be initiated low in the atmosphere during magnetic flux emergence,
or as a response to processes occurring high in the corona when the
active region is fully developed. On 2019 March 31 a simple bipolar
active region (AR 12737) emerged and upflows developed on each side. We
used observations from Hinode, SDO, IRIS, and Parker Solar Probe (PSP)
to investigate the formation and development of the upflows from the
eastern side. We used the spectroscopic data to detect the upflow,
and then used the imaging data to try to trace its signature back to
earlier in the active region emergence phase. We find that the upflow
forms quickly, low down in the atmosphere, and that its initiation
appears associated with a small field-opening eruption and the onset
of a radio noise storm detected by PSP. We also confirmed that the
upflows existed for the vast majority of the time the active region
was observed. These results suggest that the contribution to the solar
wind occurs even when the region is small, and continues for most of
its lifetime.

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Title: Observation of bi-directional jets in a prominence
Authors: Hillier, A.; Polito, V.
2021A&A...651A..60H Altcode:
Quiescent prominences host a large range of flows, many driven
by buoyancy, which lead to velocity shear. The presence of these
shear flows could bend and stretch the magnetic field resulting
in the formation of current sheets which can lead to magnetic
reconnection. Though this has been hypothesised to occur in
prominences, with some observations that are suggestive of
this process, clear evidence has been lacking. In this paper
we present observations performed on June 30, 2015 using the
Interface Region Imaging Spectrograph Si IV and Mg II slit-jaw
imagers of two bi-directional jets that occur inside the body of
the prominence. Such jets are highly consistent with what would be
expected from magnetic reconnection theory. Using this observation,
we estimate that the prominence under study has an ambient field
strength in the range of 4.5−9.2 G with `turbulent' field strengths
of 1 G. Our results highlight the ability of gravity-driven flows
to stretch and fold the magnetic field of the prominence, implying
that locally, the quiescent prominence field can be far from a
static, force-free magnetic field. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935774/olm">https://www.aanda.org</A>

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Title: The Crucial Role Of Non-thermal Electrons In Solar
Flare-induced Dimming Of He I 10830
Authors: Kerr, G.; Xu, Y.; Allred, J.; Polito, V.; Sadykov, V.; Huang,
N.; Wang, H.
2021AAS...23830307K Altcode:
While solar flares are predominantly characterised by an intense
broadband enhancement to the solar radiative output, certain
spectral lines and continua will, in theory, exhibit flare-induced
dimmings. Observations of orthohelium spectral transitions (He I 10830Å
and the He I D3 lines near 5876A) have shown evidence of such dimming
in some weak flares, usually followed by enhanced emission. It has been
suggested that the presence of non-thermal collisional ionisation of
helium by the electron beam, followed by recombinations to orthohelium,
is responsible for overpopulating the orthohelium levels leading to
stronger absorption. However it has not been possible observationally
to preclude the possibility of overpopulating orthohelium via enhanced
photoionisation of He I by EUV irradiance from the flaring corona
followed by recombinations. Here we present radiation hydrodynamics
simulations of non-thermal electron beam-driven flares where (1) both
non-thermal collisional ionisation of Helium and coronal irradiance
are included, and (2) only coronal irradiance is included. A grid of
simulations covering a range of total energies deposited by the electron
beam, and a range of non-thermal electron beam low-energy cutoff values,
were simulated. For each simulation the He I 10830A line was forward
modelled. In order to obtain flare-induced dimming of the He I 10830A
line it was necessary for non-thermal collisional ionisations to be
present. Further, the effect was more prominent in flares with harder
non-thermal electron spectrum (larger low-energy cutoff values) and
longer lived in weaker flares and flares with a more gradual energy
deposition timescale. These results demonstrate the usefulness of
orthohelium line emission as a diagnostic of flare energy transport.

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Title: Multi-passband Observations Of A Negative Flare Near He I
10830 Å
Authors: Xu, Y.; Yang, X.; Kerr, G.; Polito, V.; Jing, J.; Cao, W.;
Wang, H.
2021AAS...23830305X Altcode:
This study presents a C3.0 flare observed by the BBSO/GST and
IRIS, on 2018-May-28 around 17:10 UT. The Near Infrared Imaging
Spectropolarimeter (NIRIS) was set to spectral imaging mode to scan
five spectral positions at ±0.8 Å, ±0.4 Å and line center of He I
10830 Å. Negative contrasts of around 10%, appear in all of the five
wavelengths, with a weak dependence of these wavelengths. This means
that the line is undergoing enhanced absorption at these times. The
observations confirm that the negative flares observed in He I 10830 Å
with mono-filtergram previously were not caused by pure Doppler shifts
of this spectral line. Instead, the enhanced absorption is a consequence
of nonthermal ionization of helium following precipitation of high
energy electrons, as found in recent numerical modeling results. In
addition, though not strictly simultaneously, the IRIS observations show
clear central reversals in Mg II lines and strong Doppler shifts in C
II and Mg II lines at the locations where enhanced absorption in He I
10830 Å is occurring, consistent with previous observations and the
modeling. In other locations the Mg II profiles appear as single peaked.

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Title: He I 10830 Å Dimming during Solar Flares. I. The Crucial
Role of Nonthermal Collisional Ionizations
Authors: Kerr, Graham S.; Xu, Yan; Allred, Joel C.; Polito, Vanessa;
Sadykov, Viacheslav M.; Huang, Nengyi; Wang, Haimin
2021ApJ...912..153K Altcode: 2021arXiv210316686K
While solar flares are predominantly characterized by an intense
broadband enhancement to the solar radiative output, certain
spectral lines and continua will, in theory, exhibit flare-induced
dimmings. Observations of transitions of orthohelium He I λλ 10830
Å and the He I D3 lines have shown evidence of such dimming, usually
followed by enhanced emission. It has been suggested that nonthermal
collisional ionization of helium by an electron beam, followed by
recombinations to orthohelium, is responsible for overpopulating
those levels, leading to stronger absorption. However, it has not been
possible observationally to preclude the possibility of overpopulating
orthohelium via enhanced photoionization of He I by EUV irradiance from
the flaring corona followed by recombinations. Here we present radiation
hydrodynamics simulations of nonthermal electron-beam-driven flares
where (1) both nonthermal collisional ionization of helium and coronal
irradiance are included, and (2) only coronal irradiance is included. A
grid of simulations covering a range of total energies deposited by
the electron beam and a range of nonthermal electron-beam low-energy
cutoff values were simulated. In order to obtain flare-induced dimming
of the He I 10830 Å line, it was necessary for nonthermal collisional
ionization to be present. The effect was more prominent in flares with
larger low-energy cutoff values and longer lived in weaker flares and
flares with a more gradual energy deposition timescale. These results
demonstrate the usefulness of orthohelium line emission as a diagnostic
of flare energy transport.

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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: Future perspectives in solar hot plasma observations in the
soft X-rays
Authors: Corso, Alain Jody; Del Zanna, Giulio; Polito, Vanessa
2021ExA...tmp...49C Altcode: 2021arXiv210505549C
The soft X-rays (SXRs: 90-150 Å) are among the most interesting
spectral ranges to be investigated in the next generation of solar
missions due to their unique capability of diagnosing phenomena
involving hot plasma with temperatures up to 15 MK. Multilayer
(ML) coatings are crucial for developing SXR instrumentation, as
so far they represent the only viable option for the development
of high-efficiency mirrors in the this spectral range. However, the
current standard MLs are characterized by a very narrow spectral band
which is incompatible with the science requirements expected for a SXR
spectrometer. Nevertheless, recent advancement in the ML technology has
made the development of non-periodic stacks repeatable and reliable,
enabling the manufacturing of SXR mirrors with a valuable efficiency
over a large range of wavelengths. In this work, after reviewing the
state-of-the-art ML coatings for the SXR range, we investigate the
possibility of using M-fold and aperiodic stacks for the development
of multiband SXR spectrometers. After selecting a possible choice
of key spectral lines, some trade-off studies for an eight-bands
spectrometer are also presented and discussed, giving an evaluation
of their feasibility and potential performance.

---------------------------------------------------------
Title: Solar Flare Arcade Modelling using Field-Aligned Flare
Simulations: Bridging the gap from 1D to 3D
Authors: Kerr, G. S.; Allred, J. C.; Polito, V.
2020AGUFMSH0500009K Altcode:
Solar flares are 3D phenomenon but modelling a flare in 3D, including
many of the important processes in the chromosphere, is a computational
challenge. Accurately modelling the chromosphere is important, even
if the transition region and corona are the areas of interest, due to
the flow of energy, mass, and radiation through the interconnected
layers. We present a solar flare arcade model, that aims to bridge
the gap between 1D and 3D modelling. Our approach is limited to
the synthesis of optically thin emission. Using observed active
region loop structures in a 3D domain we graft simulated 1D flare
atmospheres onto each loop, synthesise the emission and then project
that emission onto to the 2D observational plane. Emission from SDO/AIA,
GOES/XRS, and IRIS/SG Fe XXI 1354.1A was forward modelled. We analyse
the temperatures, durations, mass flows, and line widths associated
with the flare, finding qualitative agreement but certain quantitative
differences. Compared to observations, the Doppler shifts are of similar
magnitude but decay too quickly. They are not as ordered, containing
a larger amount of scatter compared to observations. The duration of
gradual phase emission from GOES and AIA emission is also too short. Fe
XXI lines are broadened, but not sufficiently. These findings suggest
that additional physics is required in our model. The arcade model
that we show here as a proof-of-concept can be extended to investigate
other lines and global aspects of solar flares, providing a means to
better test the coronal response to models of flare energy injection.

---------------------------------------------------------
Title: The Solar eruptioN Integral Field Spectrograph (SNIFS)
Sounding Rocket
Authors: Chamberlin, P. C.; Schmit, D. J.; Daw, A. N.; Polito, V.;
Gong, Q.; Milligan, R. O.
2020AGUFMSH056..03C Altcode:
The lower solar atmosphere is temporally dynamic and spatially
inhomogeneous, and it is becoming increasingly clear that this
complex activity must be measured and quantified if we are to fully
understand how mass and energy are transported into the corona. The
Solar eruptioN Integral Field Spectrograph (SNIFS) sounding rocket is
designed to break new ground by using a unique set of capabilities to
probe the most vexingly complex region of the solar atmosphere, the
chromosphere. Hydrogen Lyman-alpha (Ly-α; 121.6 nm) is the brightest
line in the solar UV spectrum, it is energetically one of the most
important. Using radiation transfer models, we can use the observed line
profiles to reconstruct the transit of these photon through the solar
atmosphere and understand the plasma from which they came. SNIFS will
observe not only Ly-ɑ, but the nearby Si III and O V emissions, two
transition regions lines that allow us to observe how the chromosphere
connects with upper atmosphere. The SNIFS rocket mission has a primary
objective to explore the energetics and dynamics of chromosphere using
a next-generation solar spectral imager. SNIFS will be the first
of its kind: a solar ultraviolet integral field spectrograph (IFS;
Chamberlin and Gong, 2016). SNIFS technology will revolutionize solar
observations by obtaining high cadence 3D information simultaneously:
two spatial dimensions and one spectral dimensions.SNIFS utilizes a
novel optical design to simultaneously observe a 32 x 32 arcsec field
of view with 0.45 arcsec pixels, with a spectral resolution of 66mÅ
and at 1 s cadence. The SNIFS design employs, for the first time in
a spaceflight instrument as a technology development, a 72x72 element
2D array of reflecting and focusing mirrorlets, allowing IFS concepts
to move down into EUV wavelengths. This mirrorlet array is placed
at the imaging plane of the telescope, similar to the location of
a slit in a traditional imaging slit-spectrometer design. After the
mirrorlet in the optical path, a focusing grating will then produce
a high-resolution spectrum for each spatial element defined by the
mirrorlet elements. SNIFS's IFS technology is truly a game-changing
instrument needed for future solar physics missions, and was recently
selected and funded by NASA to fly in Spring of 2024.

---------------------------------------------------------
Title: Hot Plasma Flows and Oscillations in the Loop-top Region
During the 2017 September 10 X8.2 Solar Flare
Authors: Reeves, Katharine K.; Polito, Vanessa; Chen, Bin; Galan,
Giselle; Yu, Sijie; Liu, Wei; Li, Gang
2020ApJ...905..165R Altcode: 2020arXiv201012049R
In this study, we investigate motions in the hot plasma above the
flare loops during the 2017 September 10 X8.2 flare event. We examine
the region to the south of the main flare arcade, where there is data
from the Interface Region Imaging Spectrograph (IRIS) and the Extreme
ultraviolet Imaging Spectrometer (EIS) on Hinode. We find that there are
initial blueshifts of 20-60 km s-1 observed in this region
in the Fe XXI line in IRIS and the Fe XXIV line in EIS, and that the
locations of these blueshifts move southward along the arcade over the
course of about 10 minutes. The cadence of IRIS allows us to follow
the evolution of these flows, and we find that at each location where
there is an initial blueshift in the Fe XXI line, there are damped
oscillations in the Doppler velocity with periods of ∼400 s. We
conclude that these periods are independent of loop length, ruling
out magnetoacoustic standing modes as a possible mechanism. Microwave
observations from the Expanded Owens Valley Solar Array (EOVSA)
indicate that there are nonthermal emissions in the region where the
Doppler shifts are observed, indicating that accelerated particles
are present. We suggest that the flows and oscillations are due to
motions of the magnetic field that are caused by reconnection outflows
disturbing the loop-top region.

---------------------------------------------------------
Title: IRIS Observations of the Low-atmosphere Counterparts of Active
Region Outflows
Authors: Polito, Vanessa; De Pontieu, Bart; Testa, Paola; Brooks,
David H.; Hansteen, Viggo
2020ApJ...903...68P Altcode: 2020arXiv201015945P
Active region (AR) outflows have been studied in detail since
the launch of Hinode/EIS and are believed to provide a possible
source of mass and energy to the slow solar wind. In this work, we
investigate the lower atmospheric counterpart of AR outflows using
observations from the Interface Region Imaging Spectrograph (IRIS). We
find that the IRIS Si IV, C II&gt; and Mg II transition region (TR)
and chromospheric lines exhibit different spectral features in the
outflows as compared to neighboring regions at the footpoints ("moss")
of hot AR loops. The average redshift of Si IV in the outflow region
(≍5.5 km s-1) is smaller than typical moss (≍12-13
km s-1) and quiet Sun (≍7.5 km s-1) values,
while the C II line is blueshifted (≍-1.1-1.5 km s-1),
in contrast to the moss where it is observed to be redshifted by
about ≍2.5 km s-1. Further, we observe that the low
atmosphere underneath the coronal outflows is highly structured, with
the presence of blueshifts in Si IV and positive Mg II k2 asymmetries
(which can be interpreted as signatures of chromospheric upflows)
which are mostly not observed in the moss. These observations show a
clear correlation between the coronal outflows and the chromosphere
and TR underneath, which has not been shown before. Our work strongly
suggests that these regions are not separate environments and should
be treated together, and that current leading theories of AR outflows,
such as the interchange reconnection model, need to take into account
the dynamics of the low atmosphere.

---------------------------------------------------------
Title: Solar Flare Arcade Modeling: Bridging the Gap from 1D to 3D
Simulations of Optically Thin Radiation
Authors: Kerr, Graham S.; Allred, Joel C.; Polito, Vanessa
2020ApJ...900...18K Altcode: 2020arXiv200713856K
Solar flares are 3D phenomena, but modeling a flare in 3D, including
many of the important processes in the chromosphere, is a computational
challenge. Accurately modeling the chromosphere is important, even if
the transition region and corona are the areas of interest, due to
the flow of energy, mass, and radiation through the interconnected
layers. We present a solar flare arcade model that aims to bridge
the gap between 1D and 3D modeling. Our approach is limited to
the synthesis of optically thin emission. Using observed active
region loop structures in a 3D domain, we graft simulated 1D flare
atmospheres onto each loop, synthesize the emission, and then project
that emission onto the 2D observational plane. Emission from SDO/AIA,
GOES/XRS, and IRIS/SG Fe XXI λ1354.1 was forward modeled. We analyze
the temperatures, durations, mass flows, and line widths associated
with the flare, finding qualitative agreement but certain quantitative
differences. Compared to observations, the Doppler shifts are of similar
magnitude but decay too quickly. They are not as ordered, containing
a larger amount of scatter compared to observations. The duration of
gradual phase emission from GOES and AIA emission is also too short. Fe
XXI lines are broadened, but not sufficiently. These findings suggest
that additional physics is required in our model. The arcade model
that we show here as a proof of concept can be extended to investigate
other lines and global aspects of solar flares, providing a means to
better test the coronal response to models of flare energy injection.

---------------------------------------------------------
Title: Solar Flare Energy Partitioning and Transport -- the Impulsive
Phase (a Heliophysics 2050 White Paper)
Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian,
Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay;
Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh
S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito,
Vanessa; Qiu, Jiong; Ryan, Daniel F.
2020arXiv200908400K Altcode:
Solar flares are a fundamental component of solar eruptive events (SEEs;
along with solar energetic particles, SEPs, and coronal mass ejections,
CMEs). Flares are the first component of the SEE to impact our
atmosphere, which can set the stage for the arrival of the associated
SEPs and CME. Magnetic reconnection drives SEEs by restructuring the
solar coronal magnetic field, liberating a tremendous amount of energy
which is partitioned into various physical manifestations: particle
acceleration, mass and magnetic-field eruption, atmospheric heating,
and the subsequent emission of radiation as solar flares. To explain
and ultimately predict these geoeffective events, the heliophysics
community requires a comprehensive understanding of the processes that
transform and distribute stored magnetic energy into other forms,
including the broadband radiative enhancement that characterises
flares. This white paper, submitted to the Heliophysics 2050 Workshop,
discusses the flare impulsive phase part of SEEs, setting out the
questions that need addressing via a combination of theoretical,
modelling, and observational research. In short, by 2050 we must
determine the mechanisms of particle acceleration and propagation,
and must push beyond the paradigm of energy transport via nonthermal
electron beams, to also account for accelerated protons &amp; ions
and downward directed Alfven waves.

---------------------------------------------------------
Title: Solar Flare Energy Partitioning and Transport -- the Gradual
Phase (a Heliophysics 2050 White Paper)
Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian,
Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay;
Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh
S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito,
Vanessa; Qiu, Jiong; Ryan, Daniel F.
2020arXiv200908407K Altcode:
Solar flares are a fundamental component of solar eruptive events
(SEEs; along with solar energetic particles, SEPs, and coronal
mass ejections, CMEs). Flares are the first component of the SEE
to impact our atmosphere, which can set the stage for the arrival
of the associated SEPs and CME. Magnetic reconnection drives SEEs
by restructuring the solar coronal magnetic field, liberating a
tremendous amount of energy which is partitioned into various physical
manifestations: particle acceleration, mass and magnetic-field eruption,
atmospheric heating, and the subsequent emission of radiation as solar
flares. To explain and ultimately predict these geoeffective events,
the heliophysics community requires a comprehensive understanding of
the processes that transform and distribute stored magnetic energy
into other forms, including the broadband radiative enhancement that
characterises flares. This white paper, submitted to the Heliophysics
2050 Workshop, discusses the flare gradual phase part of SEEs, setting
out the questions that need addressing via a combination of theoretical,
modelling, and observational research. In short, the flare gradual phase
persists much longer than predicted so, by 2050, we must identify the
characteristics of the significant energy deposition sustaining the
gradual phase, and address the fundamental processes of turbulence
and non-local heat flux.

---------------------------------------------------------
Title: Solar Flare Arcade Modelling: Bridging the gap from 1D to 3D
Simulations of Optically Thin Radiation
Authors: Kerr, G. S.; Allred, J.; Polito, V.
2020SPD....5121106K Altcode:
Solar flares are 3D phenomenon but modelling a flare in 3D, including
many of the important processes in the chromosphere, is a computational
challenge. Accurately modelling the chromosphere is important, even
if the transition region and corona are the areas of interest, due to
the flow of energy, mass, and radiation through the interconnected
layers. We present a solar flare arcade model, that aims to bridge
the gap between 1D and 3D modelling. Our approach is limited to the
synthesis of optically thin emission. Using observed active region
loop structures in a 3D domain we graft simulated 1D flare atmospheres
onto each loop, synthesise the emission and then project that emission
onto to the 2D observational plane. Emission from SDO/AIA, GOES/XRS,
and IRIS/SG Fe XXI 1354.1Å was forward modelled. We analyse the
temperatures, durations, mass flows, and line widths associated with
the flare, finding qualitative agreement but certain quantitative
differences. Compared to observations, the Doppler shifts are of similar
magnitude but decay too quickly. They are not as ordered, containing
a larger amount of scatter compared to observations. The duration of
gradual phase emission from GOES and AIA emission is also too short. Fe
XXI lines are broadened, but not sufficiently. These findings suggest
that additional physics is required in our model. The arcade model
that we show here as a proof-of-concept can be extended to investigate
other lines and global aspects of solar flares, providing a means to
better test the coronal response to models of flare energy injection.

---------------------------------------------------------
Title: Roadmap on cosmic EUV and x-ray spectroscopy
Authors: Smith, Randall; Hahn, Michael; Raymond, John; Kallman, T.;
Ballance, C. P.; Polito, Vanessa; Del Zanna, Giulio; Gu, Liyi; Hell,
Natalie; Cumbee, Renata; Betancourt-Martinez, Gabriele; Costantini,
Elisa; Corrales, Lia
2020JPhB...53i2001S Altcode:
Cosmic EUV/x-ray spectroscopists, including both solar and astrophysical
analysts, have a wide range of high-resolution and high-sensitivity
tools in use and a number of new facilities in development for
launch. As this bandpass requires placing the spectrometer beyond
the Earth's atmosphere, each mission represents a major investment
by a national space agency such as NASA, ESA, or JAXA, and more
typically a collaboration between two or three. In general justifying
new mission requires an improvement in capabilities of at least an
order of magnitude, but the sensitivity of these existing missions
are already taxing existing atomic data quantity and accuracy. This
roadmap reviews the existing missions, showing how in a number of areas
atomic data limits the science that can be performed. The missions that
will be launched in the coming Decade will without doubt require both
more and improved measurements of wavelengths and rates, along with
theoretical calculations of collisional and radiative cross sections
for a wide range of processes.

---------------------------------------------------------
Title: IRIS Observations of Short-term Variability in Moss Associated
with Transient Hot Coronal Loops
Authors: Testa, Paola; Polito, Vanessa; De Pontieu, Bart
2020ApJ...889..124T Altcode: 2019arXiv191008201T
We observed rapid variability (≲60 s) at the footpoints of transient,
hot (∼8-10 MK) coronal loops in active region cores, with the
Interface Region Imaging Spectrograph (IRIS). The high spatial (∼0"33)
and temporal (≲5-10 s) resolution of IRIS is often crucial for the
detection of this variability. We show how, in combination with 1D RADYN
loop modeling, these IRIS spectral observations of the transition region
(TR) and chromosphere provide powerful diagnostics of the properties of
coronal heating and energy transport (thermal conduction or nonthermal
electrons, NTEs). Our simulations of nanoflare-heated loops indicate
that emission in the Mg II triplet can be used as a sensitive diagnostic
for nonthermal particles. In our events, we observe a large variety
of IRIS spectral properties (intensity, Doppler shifts, broadening,
chromospheric/TR line ratios, Mg II triplet emission) even for
different footpoints of the same coronal events. In several events,
we find spectroscopic evidence for NTEs (e.g., TR blueshifts and Mg
II triplet emission), suggesting that particle acceleration can occur
even for very small magnetic reconnection events, which are generally
below the detection threshold of hard X-ray instruments that provide
direct detection of emission of nonthermal particles.

---------------------------------------------------------
Title: Loop-top oscillations observed with IRIS spatially correlated
with nonthermal emission from EOVSA in the September 10 2017 X8 Flare
Authors: Reeves, K.; Polito, V.; Galan, G.; Yu, S.; Chen, B.; Liu,
W.; Li, G.
2019AGUFMSH13D3416R Altcode:
he September 10 2017 X8 flare was a spectacular limb event complete with
a fast coronal mass ejection, a fully global EUV wave, a bright flare
loop arcade, and strong emission ranging from microwave to white-light
continuum and gamma-rays. We examine the IRIS Fe XXI data from this
event. Fe XXI is a coronal line that is formed at about 10 MK. The IRIS
pointing was just south of the main cusp-shaped loop structure visible
in AIA, but it did capture most of the flare arcade on the limb. We find
that the majority of the emission in the loops is slightly red shifted,
with speeds of about 20 km/s, probably due to chromospheric evaporation
and an inclined viewing angle. During the period from 16:05 - 16:15
UT, we find that faint blue-shifted regions appear at the top of the
flare loops, indicating plasma flows of 20-60 km/s. After the initial
blue shift at each location, the Fe XXI Doppler velocities exhibit a
damped oscillation with a period of about 40 sec. Interestingly, in
the minutes before the blue-shifted loop-top emission was observed,
the Expanded Owens Valley Solar Array observed nonthermal microwave
emission at the same location above the loop-tops, possibly indicative
of particle acceleration there. We will discuss possible mechanisms
for these observations.

---------------------------------------------------------
Title: Diagnostics of nanoflare heating in active region core loops
from chromospheric and transition region observations and modeling
Authors: Testa, P.; Polito, V.; De Pontieu, B.; Reale, F.; Graham, D.
2019AGUFMSH13B..07T Altcode:
Rapid variability at the footpoints of active region coronal loops
has been observed (Testa et al. 2013, 2014), and provides powerful
diagnostics of the properties of coronal heating and energy transport
(e.g., Testa et al. 2014, Polito et al. 2018, Reale et al. 2019, Testa
et al. 2019). We will present results of our detailed analysis of
a dozen of IRIS/AIA observations of footpoints brightenings associated
with coronal heating, and will present the distribution of the observed
properties (e.g., duration of brightenings, intensity ratios, Doppler
shifts, non-thermal broadening,..). We will discuss the properties
of coronal heating as inferred from the coupling of these high
spatial, spectral, and temporal resolution chromospheric/transition
region/coronal observations, with modeling. We will also
present results of a new algorithm we have developed for an automatic
detection of these footpoint brightenings in AIA observations (Graham
et al. 2019), which will allow us, in our next step, to significantly
expand the number of events detected, and build more robust statistics
of the properties of nanoflares in active region loops.

---------------------------------------------------------
Title: Can superposition of evaporative flows explain broad IRIS Fe
XXI line profiles during flares?
Authors: Polito, V.; Testa, P.; De Pontieu, B.
2019AGUFMSH44A..07P Altcode:
The observation of the high-temperature (&gt;10MK) IRIS Fe XXI 1354A
line with the Interface Region Imaging Spectrograph (IRIS) has provided
significant insights into the chromospheric evaporation process in
flares. In particular, the line is often observed to be completely
blueshifted, in contrast to previous observations at lower spatial
and spectral resolution, and in agreement with predictions from
theoretical models. Interestingly, the line is also observed to be
mostly symmetric and significantly broader than expected from thermal
motions (assuming the peak formation temperature of the ion is in
equilibrium). One popular interpretation for the non-thermal broadening
is the superposition of flows from different loop strands. In this work,
we test this scenario by forward-modelling the Fe XXI line profile
assuming different possible observational scenarios using hydrodynamic
simulations of multi-thread flare loops with the 1D RADYN code. Our
results indicate that the superposition of flows alone cannot easily
reproduce both the symmetry and the significant broadening of the line
and that some other physical process, such as turbulence, or a much
larger ion temperature than previously expected, likely needs to be
invoked in order to explain the observed profiles.

---------------------------------------------------------
Title: Can the Superposition of Evaporative Flows Explain Broad Fe
XXI Profiles during Solar Flares?
Authors: Polito, Vanessa; Testa, Paola; De Pontieu, Bart
2019ApJ...879L..17P Altcode:
The observation of the high-temperature (≳10 MK) Fe XXI 1354.1
Å line with the Interface Region Imaging Spectrograph has provided
significant insights into the chromospheric evaporation process in
flares. In particular, the line is often observed to be completely
blueshifted, in contrast to previous observations at lower spatial
and spectral resolution, and in agreement with predictions from
theoretical models. Interestingly, the line is also observed to be
mostly symmetric and significantly broader than expected from thermal
motions (assuming the peak formation temperature of the ion is in
equilibrium). One popular interpretation for the nonthermal broadening
is the superposition of flows from different loop strands. In this
work, we test this scenario by forward-modeling the Fe XXI line profile
assuming different possible observational scenarios using hydrodynamic
simulations of multi-thread flare loops with the 1D RADYN code. Our
results indicate that the superposition of flows alone cannot easily
reproduce both the symmetry and the significant broadening of the line
and that some other physical process, such as turbulence, or a much
larger ion temperature than previously expected, likely needs to be
invoked in order to explain the observed profiles.

---------------------------------------------------------
Title: Possible Signatures of a Termination Shock in the 2014 March
29 X-class Flare Observed by IRIS
Authors: Polito, Vanessa; Galan, Giselle; Reeves, Katharine K.;
Musset, Sophie
2018ApJ...865..161P Altcode:
The standard model of flares predicts the existence of a fast-mode
magnetohydrodynamic shock above the looptops, also known as termination
shock (TS), as the result of the downward-directed outflow reconnection
jets colliding with the closed magnetic loops. A crucial spectral
signature of a TS is the presence of large Doppler shifts in the spectra
of high-temperature lines (≥10 MK), which has been rarely observed
so far. Using high-resolution observations of the Fe XXI line with the
Interface Region Imaging Spectrograph (IRIS), we detect large redshifts
(≈200 km s-1) at the top of the bright looptop arcade of
the X1-class flare on 2014 March 29. In some cases, the redshifts are
accompanied by faint simultaneous Fe XXI blueshifts of about -250 km
s-1. The values of red and blueshifts are in agreement with
recent modeling of Fe XXI spectra downflow of the reconnection site and
previous spectroscopic observations with higher temperature lines. The
locations where we observe the Fe XXI shifts are co-spatial with 30-70
keV hard X-ray sources detected by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), indicating that nonthermal electrons
are located above the flare loops. We speculate that our results are
consistent with the presence of a TS in flare reconnection models.

---------------------------------------------------------
Title: Broad Non-Gaussian Fe XXIV Line Profiles in the Impulsive
Phase of the 2017 September 10 X8.3-class Flare Observed by Hinode/EIS
Authors: Polito, Vanessa; Dudík, Jaroslav; Kašparová, Jana;
Dzifčáková, Elena; Reeves, Katharine K.; Testa, Paola; Chen, Bin
2018ApJ...864...63P Altcode: 2018arXiv180709361P
We analyze the spectra of high-temperature Fe XXIV lines observed by
the Hinode/Extreme-Ultraviolet Imaging Spectrometer (EIS) during the
impulsive phase of the X8.3-class flare on 2017 September 10. The
line profiles are broad, show pronounced wings, and clearly depart
from a single-Gaussian shape. The lines can be well fitted with κ
distributions, with values of κ varying between ≈1.7 and 3. The
regions where we observe the non-Gaussian profiles coincide with
the location of high-energy (≈100-300 keV) hard X-ray (HXR) sources
observed by RHESSI, suggesting the presence of particle acceleration or
turbulence, also confirmed by the observations of nonthermal microwave
sources with the Expanded Owens Valley Solar Array at and above the HXR
loop-top source. We also investigate the effect of taking into account
κ distributions in the temperature diagnostics based on the ratio of
the Fe XXIII λ263.76 and Fe XXIV λ255.1 EIS lines. We found that
these lines can be formed at much higher temperatures than expected
(up to log(T[K]) ≈ 7.8) if departures from Maxwellian distributions
are taken into account. Although larger line widths are expected because
of these higher formation temperatures, the observed line widths still
imply nonthermal broadening in excess of 200 km s-1. The
nonthermal broadening related to HXR emission is better interpreted
by turbulence than by chromospheric evaporation.

---------------------------------------------------------
Title: Observations of the Kelvin-Helmholtz Instability Driven by
Dynamic Motions in a Solar Prominence
Authors: Hillier, Andrew; Polito, Vanessa
2018ApJ...864L..10H Altcode: 2018arXiv180802286H
Prominences are incredibly dynamic across the whole range of their
observable spatial scales, with observations revealing gravity-driven
fluid instabilities, waves, and turbulence. With all of these complex
motions, it would be expected that instabilities driven by shear in
the internal fluid motions would develop. However, evidence of these
have been lacking. Here we present the discovery in a prominence,
using observations from the Interface Region Imaging Spectrograph,
of a shear flow instability, the Kelvin-Helmholtz sinusoidal-mode of
a fluid channel, driven by flows in the prominence body. This finding
presents a new mechanism through which we can create turbulent motions
from the flows observed in quiescent prominences. The observation of
this instability in a prominence highlights their great value as a
laboratory for understanding the complex interplay between magnetic
fields and fluid flows that play a crucial role in a vast range of
astrophysical systems.

---------------------------------------------------------
Title: Microwave Spectral Imaging of Bi-Directional Magnetic
Reconnection Outflow Region of the 2017 Sep 10 X8.2 Flare
Authors: Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Krucker,
Sam; Nita, Gelu M.; Dennis, Brian R.; Yu, Sijie; Kuroda, Natsuha;
Reeves, Katharine K.; Polito, Vanessa; Shih, Albert
2018shin.confE.211C Altcode:
The newly commissioned Expanded Owens Valley Solar Array (EOVSA)
obtained microwave spectral imaging of the spectacular eruptive solar
flare on 2017 September 10 in 2.5-18 GHz. During the early impulsive
phase of the flare ( 15:54 UT), An elongated microwave source appears
to connect the top of the flare arcade to the bottom of the erupting
magnetic flux rope. Multi-frequency images reveal that the source
bifurcates into two parts: One is located at and above the hard X-ray
looptop source, and another located behind the flux rope. They appear to
follow closely with the bi-directional reconnection downflow and upflow
region as inferred from the SDO/AIA EUV images. The spatially resolved
spectra of this microwave source show characteristics of gyrosynchrotron
radiation, suggesting the presence of high-energy (100s of keV to MeV)
electrons throughout the bi-directional reconnection outflow region. We
derive physical parameters of the source region, and discuss their
implications in magnetic energy release and electron acceleration.

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Title: Observations of a shear-flow instability driven by dynamic
prominence motions
Authors: Hillier, Andrew; Polito, . V.
2018cosp...42E1460H Altcode:
Prominences are incredibly dynamic across the whole range of their
observable spatial scales, with observations revealing gravity-driven
fluid instabilities, waves, and turbulence. With all these complex
motions, it would be expected that instabilities driven by shear in the
fluid motions contained in the prominence body would develop. However,
evidence of these have been lacking. Here we present the discovery in
a prominence, using observations from the Interface Region Imaging
Spectrograph (IRIS), of a shear flow instability, a mode of the
Kelvin-Helmholtz instability that makes streams of fluid develop
serpentine patterns, driven by transonic motions in the prominence
body. This finding presents a new mechanism through which we can
create turbulence from the flows observed in quiescent prominences. The
observation of this instability in a prominence highlights their great
value as a laboratory for understanding the complex interplay between
magnetic fields and fluid flows that play a crucial role in a vast
range of astrophysical systems.