Self-constrained Magnetic Reconnection of a Solar Flare Cospatial with a Nonejected Filament

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Abstract
For flares associated with eruptive filaments, the filament normally rises above the postflare loops. Here, we report an unusual event in which the postflare loops instead overlie a filament that remains nonejected throughout the flare. This event was simultaneously captured by the Chinese H α Solar Explorer (CHASE), the Solar Extreme-UltraViolet Imager (SUVI) on board Fengyun-4C, the Hard X-ray Imager (HXI) on board the Advanced Space-based Solar Observatory (ASO-S) and the Solar Dynamics Observatory (SDO). Extreme ultraviolet (EUV) observations provided by FY4C/SUVI and SDO show that two hot loop systems form at the flare onset. The western end of the northern loop system undergoes a southwestward drifting motion with gradually increasing shear, whereas the southern loop system progressively converges toward and eventually overlies the northern one. Meanwhile, three-dimensional magnetic field extrapolations reveal an increase in both the twist of the filament-supporting magnetic structure and its poloidal magnetic field after the flare. Taken together, these observations suggest that the flare is driven by a self-constraining zipper-type magnetic reconnection, in which highly sheared arcades reconnect to form a series of overlying flare arcades and an underlying flux rope. This reconnection continuously strengthens the magnetic confinement of the filament, preventing its outward eruption, while facilitating substantial plasma drainage along the reconnected magnetic field revealed by the CHASE H α spectroscopic observations.

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