Investigating the Role of Solar Flare Termination Shocks in Initiating Ground Level Enhancements

The solar flare is the primary source of eruptions that generate space weather. Its high-speed jet is believed to produce the potential termination shock (TS) at the apex of the magnetic flux loop. Within the solar atmosphere, it becomes particularly intriguing to explore the fundamental mechanisms responsible for the initial acceleration of particles and their role in the generation of solar energetic particles (SEPs), extending to the phenomenon known as ground level enhancement (GLE). This study focuses on uncovering the relationship between GLE events and the flare-TS. To achieve this, we employ a Dynamic Monte Carlo (DMC) simulation technique to model the behavior of the flare-TS. In this theoretical framework, thermal particles that are part of the high-speed outflow from magnetic reconnection events penetrate the shock front at the loop top. Through numerous cycles of interaction with the TS, these particles undergo successive energy gains. Consequently, our simulation reveals details of the energy spectral structure. Besides the standard power-law with a hard index below 2 MeV, the emergence of a “bump-on-tail” structure between 2 and 20 MeV is observed in the simulated accelerated protons. Additionally, the efficiency of the TS acceleration dependent on the speed of the input bulk flow suggests a potential SEPs source for boosting GLEs. Based on these findings, we suggest that the termination shock acceleration mechanism serves as an initial source of energetic particles, which would lead to GLEs directly or seed the subsequent interplanetary processes for GLEs indirectly.