Vol 11, No 10

Observing the reconnection region in a transequatorial loop system

Rui Liu, Tong-Jiang Wang, Jeongwoo Lee, Guillermo Stenborg, Chang Liu, Sung-Hong Park, Hai-Min Wang


Abstract A vertical current sheet is a crucial element in many flare/coronal mass ejection (CME) models. For the first time, Liu et al. reported a vertical current sheet directly imaged during the flare rising phase with the EUV Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). As a follow-up study, here we present the comprehensive analysis and detailed physical interpretation of the observation. The current sheet formed due to the gradual rise of a transequatorial loop system. As the loop legs approached each other, plasma flew at ~6 km s−1 into a local area where a cusp-shaped flare loop subsequently formed and the current sheet was seen as a bright, collimated structure of global length (≥ 0.25R⊙) and macroscopic width ((5–10)×103 km), extending from 50Mm above the flaring loop to the border of the EIT field of view (FOV). The reconnection rate in terms of the Alfv´en Mach number is estimated to be only 0.005–0.009, albeit a halo CME was accelerated from ~400 km s−1 to ~1300 km s−1 within the coronagraph FOV. Drifting pulsating structures at metric frequencies were recorded during the impulsive phase, implying tearing of the current sheet in the high corona. A radio Type III burst occurred when the current sheet was clearly seen in EUV, indicative of accelerated electrons beaming upward from the upper tip of the current sheet. A cusp-shaped dimming region was observed to be located above the post-flare arcade during the decay phase in EIT; both the arcade and the dimming expanded with time. With the Coronal Diagnostic Spectrometer (CDS) aboard SOHO, a clear signature of chromospheric evaporation was seen during the decay phase, i.e., the cusp-shaped dimming region was associated with plasma upflows detected with EUV hot emission lines, while the post-flare loop was associated with downflows detected with cold lines. This event provides a comprehensive view of the reconnection geometry and dynamics in the solar corona.


Keywords Sun: coronal mass ejections— Sun: flares— Sun: corona

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