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(searched for: doi:10.3847/1538-4357/abf42d)
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J. Hong, M. Carlsson, M. D. Ding
Published: 3 May 2022
Astronomy & Astrophysics, Volume 661; https://doi.org/10.1051/0004-6361/202142839

Abstract:
Context. Radiative losses in the chromosphere are very important for the energy balance of the Sun. There have been efforts to make simple lookup tables for chromospheric radiative losses in the quiet Sun. During solar flares, the atmospheric conditions are quite different, and the currently available recipe is constructed from semi-empirical models. How these recipes work in flare conditions remains to be evaluated. Aims. We aim to construct an approximate recipe of chromospheric radiative losses for solar flares. Methods. We tabulate the optically thin radiative loss, escape probability, and ionization fraction using a grid of flare models from radiative hydrodynamic simulations as our dataset. Results. We provide new lookup tables to calculate chromospheric radiative losses for flares. Compared with previous recipes, our recipe provides a better approximation of the detailed radiative losses for flares.
, David E. Siskind,
The Astrophysical Journal, Volume 927; https://doi.org/10.3847/1538-4357/ac4784

Abstract:
We examine Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE) data to better understand solar flare irradiance, and how that irradiance may vary for large events. We measure scaling laws relating Geostationary Orbital Environmental Satellites (GOES) flare classes to irradiance in 21 lines measured with SDO/EVE, formed across a wide range of temperatures, and find that this scaling depends on the line-formation temperature. We extrapolate these irradiance values to large events, exceeding X10. In order to create full spectra, however, we need a physical model of the irradiance. We present the first results of a new physical model of solar flare irradiance, NRLFLARE, that sums together a series of flare loops to calculate the spectral irradiance ranging from the X-rays through the far-UV (≈0 to 1250 Å), constrained only by GOES/X-ray Sensors observations. We test this model against SDO/EVE data. The model spectra and time evolution compares well in high-temperature emission, but cooler lines show large discrepancies. We speculate that the discrepancies are likely due to both a nonuniform cross-section of the flaring loops as well as opacity effects. We then show that allowing the cross-sectional area to vary with height significantly improves agreement with observations, and is therefore a crucial parameter needed to accurately model the intensity of spectral lines, particularly in the transition region from 4.7logT6.
The Astrophysical Journal, Volume 926; https://doi.org/10.3847/1538-4357/ac4028

Abstract:
We analyze the structure and evolution of ribbons from the M7.3 SOL2014-04-18T13 flare using ultraviolet images from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), magnetic data from the SDO/Helioseismic and Magnetic Imager, hard X-ray (HXR) images from the Reuven Ramaty High Energy Solar Spectroscopic Imager, and light curves from the Fermi/Gamma-ray Burst Monitor, in order to infer properties of coronal magnetic reconnection. As the event progresses, two flare ribbons spread away from the magnetic polarity inversion line. The width of the newly brightened front along the extension of the ribbon is highly intermittent in both space and time, presumably reflecting nonuniformities in the structure and/or dynamics of the flare current sheet. Furthermore, the ribbon width grows most rapidly in regions exhibiting concentrated nonthermal HXR emission, with sharp increases slightly preceding the HXR bursts. The light curve of the ultraviolet emission matches the HXR light curve at photon energies above 25 keV. In other regions the ribbon-width evolution and light curves do not temporally correlate with the HXR emission. This indicates that the production of nonthermal electrons is highly nonuniform within the flare current sheet. Our results suggest a strong connection between the production of nonthermal electrons and the locally enhanced perpendicular extent of flare ribbon fronts, which in turn reflects the inhomogeneous structure and/or reconnection dynamics of the current sheet. Despite this variability, the ribbon fronts remain nearly continuous, quasi-one-dimensional features. Thus, although the reconnecting coronal current sheets are highly structured, they remain quasi-two-dimensional and the magnetic energy release occurs systematically, rather than stochastically, through the volume of the reconnecting magnetic flux.
Yan Xu, Xu Yang, Graham S. Kerr, , , , Wenda Cao,
The Astrophysical Journal, Volume 924; https://doi.org/10.3847/2041-8213/ac447c

Abstract:
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.
, , C Beichman, , H Harakawa, K W Hodapp, M Ishizuka, S Jacobson, M Konishi, T Kotani, et al.
Monthly Notices of the Royal Astronomical Society, Volume 509, pp 2969-2978; https://doi.org/10.1093/mnras/stab3107

Abstract:
Studies of planetary systems of stars in star-forming regions and young clusters open a window on the formative stages of planetary evolution. We obtained high-cadence high-resolution infrared spectroscopy of the solar-mass Taurus association-member V1298 Tau during a transit of its 10R⊕-size ‘b’ planet. We measured the systemic radial velocity (RV) and find that the kinematics of V1298 Tau suggest an affiliation with a ≳6 Myr-old subgroup. A comparison of V1298 Tau and the nearby, co-moving star 2M0405 with stellar evolution models suggests an age of ∼10–25 Myr. We measured the projected spin-orbit angle of ‘b’ as $\lambda =15_{-16}^{+15}$ and $\lambda = 2_{-4}^{+12}$ degrees using the apparent RV shift and change in line profile, respectively, induced by the transient occultation of the rotating star by the planet. These values indicate a prograde orbit like that of the interior ‘c’ planet of V1298 Tau and point to a co-planar multiplanet system that formed within a disc. We also measured variation in the strength of the 1083 nm triplet of neutral orthohelium as a probe of any extended/escaping atmosphere around ‘b’. We detect a steady decrease in absorption over the transit that appears to arise from the star or its planetary system. While this variation could be ascribed to ‘b’ or possibly to the immediately preceding transit of ‘d’, we cannot rule out that this is due to rapid variation in the stellar disc-integrated flux in the triplet. The amplitude of variation (∼0.04 nm) is consistent with moderate estimates of atmospheric escape driven by XUV radiation from the central star.
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