Particle Acceleration at a Flare Termination Shock: Effect of Large-scale Magnetic Turbulence

TL;DR

This study models how large-scale magnetic turbulence influences particle acceleration at solar flare termination shocks, showing electrons can reach MeV energies rapidly, aligning with observed X-ray emissions.

Contribution

It introduces a numerical simulation framework incorporating turbulence effects to analyze particle acceleration at solar flare termination shocks, revealing significant electron energization.

Findings

Electrons can reach a few MeV within 100 ion cyclotron periods.

Approximately 10% of electrons are accelerated above 15 keV with sufficient turbulence.

Protons are accelerated less efficiently, with steeper energy spectra.

Abstract

We investigate the acceleration of charged particles (both electrons and protons) at collisionless shocks predicted to exist in the vicinity of solar flares. The existence of standing termination shocks has been examined by flare models and numerical simulations e.g., Shibata,Forbes. We study electron energization by numerically integrating the equations of motion of a large number of test-particle electrons in the time-dependent two-dimensional electric and magnetic fields generated from hybrid simulations (kinetic ions and fluid electron) using parameters typical of the solar flare plasma environment. The shock is produced by injecting plasma flow toward a rigid piston. Large-scale magnetic fluctuations -- known to exist in plasmas and known to have important effects on the nonthermal electron acceleration at shocks -- are also included in our simulations. For the parameters characteristic of the flaring region, our calculations suggest that the termination shock formed in the reconnection outflow region (above post-flare loops) could accelerate electrons to a kinetic energy of a few MeV within 100 ion cyclotron periods, which is of the order of a millisecond. Given a sufficient turbulence amplitude level ($\delta B^2/B_0^2 \sim 0.3$), about 10% of thermal test-particle electrons are accelerated to more than 15 keV. We find that protons are also accelerated, but not to as high energy in the available time and the energy spectra are considerably steeper than that of the electrons for the parameters used in our simulations. Our results are qualitatively consistent with the observed hard X-ray emissions in solar flares.