Magnetic Reconnection during the Post-impulsive Phase of a Long-duration Solar Flare: Bidirectional Outflows as a Cause of Microwave and X-Ray Bursts

Sijie Yu, Bin Chen, Katharine K. Reeves, Dale E. Gary, Sophie Musset, Gregory D. Fleishman, Gelu M. Nita, Lindsay Glesener

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Magnetic reconnection plays a crucial role in powering solar flares, production of energetic particles, and plasma heating. However, where the magnetic reconnections occur, how and where the released magnetic energy is transported, and how it is converted to other forms remain unclear. Here we report recurring bidirectional plasma outflows located within a large-scale plasma sheet observed in extreme-ultraviolet emission and scattered white light during the post-impulsive gradual phase of the X8.2 solar flare on 2017 September 10. Each of the bidirectional outflows originates in the plasma sheet from a discrete site, identified as a magnetic reconnection site. These reconnection sites reside at very low altitudes (<180 Mm, or 0.26 R o˙) above the top of the flare arcade, a distance only <3% of the total length of a plasma sheet that extends to at least 10 R o˙. Each arrival of sunward outflows at the loop-top region appears to coincide with an impulsive microwave and X-ray burst dominated by a hot source (10-20 MK) at the loop top and a nonthermal microwave burst located in the loop-leg region. We propose that the reconnection outflows transport the magnetic energy released at localized magnetic reconnection sites outward in the form of kinetic energy flux and/or electromagnetic Poynting flux. The sunward-directed energy flux induces particle acceleration and plasma heating in the post-flare arcades, observed as the hot and nonthermal flare emissions.

Original languageEnglish (US)
Article number17
JournalAstrophysical Journal
Issue number1
StatePublished - Sep 1 2020

Bibliographical note

Funding Information:
EOVSA operation is supported by NSF grant AST- 1910354. S.Y. and B.C. are supported by NSF grants AGS-1654382, AGS-1723436, AST-1735405, and NASA grant 80NSSC20K1318 to NJIT. K.R. is supported by NSF grant AGS-1923365 to SAO and grant 80NSSC18K0732 from NASA to SAO. D.E.G., G.D.F., and G.M.N. are supported by NSF grant AGS-1817277 and NASA grants 80NSSC18K0667, 80NSSC19K0068, and 80NSSC18K1128 to NJIT. G.M.N. is also supported by NSF grant AGS- 1743321 to NJIT. The work was supported partly by NASA DRIVE Science Center grant 80NSSC20K0627. We thank Dr. Chengcai Shen for his inspiring MHD modeling on solar flares. We thank the anonymous referee, who provided constructive comments to improve the paper. We thank the SDO/AIA team for providing the EUV data. We thank the RHESSI and Fermi-GBM team for providing the hard X-ray data. We also thank the SOHO/LASCO and MLSO/K-Cor teams for providing white-light data.

Publisher Copyright:
© 2020. The American Astronomical Society. All rights reserved..


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