Magnetic Reconnection During the Post-Impulsive Phase of a Long-Duration Solar Flare: Bi-Directional Outflows as a Cause of Microwave and X-ray Bursts

Abstract

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 bi-directional 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 bi-directional 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 $R_{\odot }$) 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 $R_{\odot }$. Each arrival of sunward outflows at the looptop region appears to coincide with an impulsive microwave and X-ray burst dominated by a hot source (10-20 MK) at the looptop, which is immediately followed by a nonthermal microwave burst located in the loopleg 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.

19 pages, 11 figures, accepted for publication in ApJ

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• Strongest coronal magnetic fields in solar cycles 23-24: probing, statistics, and implications
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• Observations of Thomson scattering from a loop-prominence system
• The corona of a fully convective star with a near-polar flare
• Numerical Investigation of Efficient Electron Acceleration at an Unsteady Solar Flare Loop-Top
• Microwave Imaging Spectroscopy Diagnosis of the Slow-rise Precursor of a Major Solar Eruption
• Spatial extent of the magnetic influence of an active region on the solar atmosphere
• Decay of a photospheric transient filament at the boundary of a pore and the chromospheric response
• Forward Modeling of Solar Coronal Magnetic Field Measurements Based on a Magnetic-field-induced Transition in Fe X
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