Author name code: polito ADS astronomy entries on 2022-09-14 author:Polito, V. ------------------------------------------------------------------------ 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. Bibcode: 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. Title: Blueshifted Si IV 1402.77 Å Line Profiles in a Moving Flare Kernel Observed by IRIS Authors: Lörinčík, Juraj; Dudík, Jaroslav; Polito, Vanessa Bibcode: 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∣ < 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. Title: Blueshifted Si IV 1402.77Å line profiles in a moving flare kernel observed by IRIS Authors: Lörinčík, Juraj; Dudík, Jaroslav; Polito, Vanessa Bibcode: 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}| < 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. Title: Spectroscopy of ribbon fronts as diagnostics of energy release during solar flares Authors: Polito, Vanessa; Kerr, Graham; Sadykov, Viacheslav; Xu, Yan Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. Title: Observation of bi-directional jets in a prominence Authors: Hillier, A.; Polito, V. Bibcode: 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 https://www.aanda.org 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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> 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 Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2019AGUFMSH44A..07P Altcode: The observation of the high-temperature (>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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Title: Observations of a shear-flow instability driven by dynamic prominence motions Authors: Hillier, Andrew; Polito, . V. Bibcode: 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.