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(searched for: doi:10.3847/1538-4357/ac01eb)
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Yuchuan Wu, , , Liyun Zhang, Jianrong Shi, , Hongpeng Lu, Yu Xu, Haifeng Wang
The Astrophysical Journal, Volume 928; https://doi.org/10.3847/1538-4357/ac5897

Abstract:
Stellar flares are characterized by sudden enhancement of electromagnetic radiation in stellar atmospheres. So far, much of our understanding of stellar flares has come from photometric observations, from which plasma motions in flare regions could not be detected. From the spectroscopic data of LAMOST DR7, we have found one stellar flare that is characterized by an impulsive increase followed by a gradual decrease in the Hα line intensity on an M4-type star, and the total energy radiated through Hα is estimated to be of the order of 1033 erg. The Hα line appears to have a Voigt profile during the flare, which is likely caused by Stark pressure broadening due to the dramatic increase in electron density and/or opacity broadening due to the occurrence of strong nonthermal heating. Obvious enhancement has been identified in the red wing of the Hα line profile after the impulsive increase in the Hα line intensity. The red-wing enhancement corresponds to plasma moving away from the Earth at a velocity of 100–200 km s−1. According to our current knowledge of solar flares, this red-wing enhancement may originate from: (1) flare-driven coronal rain, (2) chromospheric condensation, or (3) a filament/prominence eruption either with nonradial backward propagation or with strong magnetic suppression. The total mass of the moving plasma is estimated to be of the order of 1015 kg.
Hechao Chen, Hui Tian, Leping Li, Hardi Peter, Lakshmi Pradeep Chitta, Zhenyong Hou
Published: 15 March 2022
Astronomy & Astrophysics, Volume 659; https://doi.org/10.1051/0004-6361/202142093

Abstract:
Context. Plasma loops or plumes rooted in sunspot umbrae often harbor downflows with speeds of 100 km s−1. These downflows are supersonic at transition region temperatures of ∼0.1 MK. The source of these flows is not well understood. Aims. We aim to investigate the source of sunspot supersonic downflows (SSDs) in active region 12740 using simultaneous spectroscopic and imaging observations. Methods. We identified SSD events from multiple raster scans of a sunspot by the Interface Region Imaging Spectrograph, and we calculated the electron densities, mass fluxes, and velocities of these SSDs. The extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the EUV Imager onboard the Solar Terrestrial Relations Observatory were employed to investigate the origin of these SSDs and their associated coronal rain. Results. Almost all the identified SSDs appear at the footpoints of sunspot plumes and are temporally associated with the appearance of chromospheric bright dots inside the sunspot umbra. Dual-perspective EUV imaging observations reveal a large-scale closed magnetic loop system spanning the sunspot region and a remote region. We observed that the SSDs are caused by repeated coronal rain that forms and flows along these closed magnetic loops toward the sunspot. One episode of coronal rain clearly indicates that reconnection near a coronal X-shaped structure first leads to the formation of a magnetic dip. Subsequently, hot coronal plasma catastrophically cools from ∼2 MK in the dip region via thermal instability. This results in the formation of a transient prominence in the dip, from which the cool gas mostly slides into the sunspot along inclined magnetic fields under the gravity. This drainage process manifests as a continuous rain flow, which lasts for ∼2 h and concurrently results in a nearly steady SSD event. The total mass of condensation (1.3 × 1014 g) and condensation rate (1.5 × 1010 g s−1) in the dip region were found to be sufficient to sustain this long-lived SSD event, which has a mass transport rate of 0.7 − 1.2 × 1010 g s−1. Conclusions. Our results demonstrate that coronal condensation in magnetic dips can result in the quasi-steady sunspot supersonic downflows.
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