Electron Acceleration at a Coronal Shock Propagating Through a Large-scale Streamer-like Magnetic Field

TL;DR

This study uses test-particle simulations to explore how large-scale coronal magnetic fields influence electron acceleration at outward-propagating shocks, revealing the importance of shock curvature and magnetic topology in acceleration efficiency.

Contribution

It demonstrates the role of large-scale streamer-like magnetic fields and shock curvature in determining electron acceleration regions and efficiency during coronal shock propagation.

Findings

Electron acceleration shifts from shock flanks to the nose with altitude.

Magnetic traps at the top of closed field lines enhance acceleration.

Electron energy spectra follow a power-law, hardening then softening.

Abstract

With a test-particle simulation, we investigate the effect of large-scale coronal magnetic fields on electron acceleration at an outward-propagating coronal shock with a circular front. The coronal field is approximated by an analytical solution with a streamer-like magnetic field featured by partially open magnetic field and a current sheet at the equator atop the closed region. We show that the large-scale shock-field configuration, especially the relative curvature of the shock and the magnetic field line across which the shock is sweeping, plays an important role in the efficiency of electron acceleration. At low shock altitudes, when the shock curvature is larger than that of magnetic field lines, the electrons are mainly accelerated at the shock flanks; at higher altitudes, when the shock curvature is smaller, the electrons are mainly accelerated at the shock nose around the top of closed field lines. The above process reveals the shift of efficient electron acceleration region along the shock front during its propagation. It is also found that in general the electron acceleration at the shock flank is not so efficient as that at the top of closed field since at the top a collapsing magnetic trap can be formed. In addition, we find that the energy spectra of electrons is power-law like, first hardening then softening with the spectral index varying in a range of -3 to -6. Physical interpretations of the results and implications on the study of solar radio bursts are discussed.