Particle-in-cell simulations of particle energization via shock drift acceleration from low Mach number quasi-perpendicular shocks in solar flares

TL;DR

This study uses 2D particle-in-cell simulations to investigate how low Mach number, high beta shocks in solar flares can accelerate electrons via shock drift acceleration, aligning with observed X-ray spectra.

Contribution

It presents the first detailed PIC simulation analysis of electron acceleration at low Mach number, high beta quasi-perpendicular shocks in solar flare environments.

Findings

Electrons above a certain energy threshold are shock-drift-accelerated.

Simulated spectral indices match observed X-ray spectra from RHESSI.

Results with reduced mass ratios can be scaled to realistic plasma conditions.

Abstract

Low Mach number, high beta fast mode shocks can occur in the magnetic reconnection outflows of solar flares. These shocks, which occur above flare loop tops, may provide the electron energization responsible for some of the observed hard X-rays and contemporaneous radio emission. Here we present new 2D particle-in-cell simulations of low Mach number/high beta quasi-perpendicular shocks. The simulations show that electrons above a certain energy threshold experience shock-drift-acceleration. The transition energy between the thermal and non-thermal spectrum and the spectral index from the simulations are consistent with some of the X-ray spectra from RHESSI in the energy regime of $E\lesssim 40\sim 100$ keV. Plasma instabilities associated with the shock structure such as the modified-two-stream and the electron whistler instabilities are identified using numerical solutions of the kinetic dispersion relations. We also show that the results from PIC simulations with reduced ion/electron mass ratio can be scaled to those with the realistic mass ratio.