2.5D MHD Simulation Of The Evolution Of Magnetic Islands Toward Flare Loops

On the day-side of the earth magnetopause, the existence of magnetic reconnection has been proven by in-situ observations. Interacting with the solar wind, the loop-like magnetic field on magnetopause breaks and releases energy to accelerate particles that result in magnetic substorms. In analogy with this process, during solar eruptions, the reconnection downflow carrying multiple magnetic islands can also induce magnetic reconnection on the loop top.To investigate the loop top magnetic reconnection (LTMR) processes, we perform high-resolution 2.5-dimensional MHD simulations of a post-eruption current-sheet (PECS) under the high-Lundquist-number coronal environment. The fast reconnection scheme triggered by the plasmoid instability produces various downflow magnetic islands. It’s found that LTMRs are of frequent occurrence when the small-scale magnetic islands encounter the loop-top field, and the LTMR rates are of the same order as in the PECS. The LTMR plays a key role in the formation of the flare loop. In the early phase, the flare loop is enlarged via the accumulation of multiple small-scale plasmoids merging with each other via reconnection. The flare loop finally evolves into a multi-layer structure involving three notably distinct parts, the stable cool kernel with high density, the low-density hot loop showing quasi-periodic oscillations, and the above-loop-top region where LTMR happens. Steady magnetic loop structure and supersonic downflow are important conditions for the formation of terminal shocks (TSs). According to our simulations, the violent plasmoid instability of PECS shatters the downflow into pieces. Although supersonic downflows do form, they are relatively short and can only interact with the above-loop-top region. Meanwhile, hit by intermittent downflows and magnetic islands, the above-loop-top region shows significantly anisotropic turbulent characteristics, which further hampers the formation of TSs. Our results thus imply that the LTMR might provide another important mechanism for particle acceleration and loop-top heating, and it can be more important than TSs, especially for high Lundquist-number conditions.