The Dynamical Behavior of Reconnection-driven Termination Shocks in Solar Flares: Magnetohydrodynamic Simulations

In eruptive solar flares, termination shocks (TSs), formed when high-speed reconnection outflows collide with closed dense flaring loops, are believed to be one of the possible candidates for plasma heating and particle acceleration. In this work, we perform resistive magnetohydrodynamic simulations in a classic Kopp-Pneuman flare configuration to study the formation and evolution of TSs, and analyze in detail the dynamic features of TSs and variations of the shock strength in space and time. This research focuses on the fast reconnection phase when plasmoids form and produce small-scale structures inside the flare current sheet. It is found that the TS emerges once the downward outflow colliding with closed magnetic loops becomes super-magnetosonic, and immediately becomes highly dynamical. The morphology of a TS can be flat, oblique, or curved depending on the detailed interactions between the outflows/plasmoids and the highly dynamic plasma in the looptop region. The TS becomes weaker when a plasmoid is crossing through, or may even be destroyed by well developed plasmoids and then re-constructed above the plasmoids. We also perform detailed statistical analysis on important physical quantities along and across the shock front. The density and temperature ratios range from 1 to 3 across the TS front, and the pressure ratio typically has larger values up to 10. We show that weak guide fields do not strongly affect the Mach number and compression ratios, and the TS length becomes slightly larger in the case with thermal conduction.