Observations and Simulations of a Termination Shock in an Eruptive Solar Flare as a Possible Particle Accelerator

Abstract:

A termination shock has been often invoked in the standard model for eruptive solar flares as a possible driver for particle acceleration. It is hypothesized as a standing shock wave generated by super-magnetosonic reconnection outflows impinging upon dense, newly-reconnected magnetic loops during the flare energy release process. However, such shock wave has largely remained a theoretical concept inferred from model predictions due to the lack of observational evidence. Here we present observations of a termination shock in a solar flare and trace its morphology and dynamics using high-cadence radio imaging spectroscopy enabled by the upgraded Karl G. Jansky Very Large Array. The observed properties of the shock, including its location, morphology, and dynamics, are well-reproduced by magnetohydrodynamics simulations in a standard Kopp-Pneuman-type reconnection geometry for two-ribbon flares. We further show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.