Injection of thermal and suprathermal seed particles into coronal shocks of varying obliquity

TL;DR

This paper analyzes how seed particle injection efficiency at coronal shocks varies with shock obliquity, seed population type, and downstream diffusion, highlighting the importance of detailed distribution functions for realistic modeling.

Contribution

It provides a comprehensive analysis of injection processes at coronal shocks, incorporating analytical, semi-analytical, and Monte Carlo methods to assess effects of seed populations and shock parameters.

Findings

Injection bias favors small shock-normal angles for thermal seeds.

Downstream isotropisation methods influence injection angle dependence.

Cross-shock potential and downstream diffusion significantly affect injection efficiency.

Abstract

Context: Diffusive shock acceleration in the solar corona can accelerate solar energetic particles to very high energies. Acceleration efficiency is increased by entrapment through self-generated waves, which is highly dependent on the amount of accelerated particles. This, in turn, is determined by the efficiency of particle injection into the acceleration process. Aims: We present an analysis of the injection efficiency at coronal shocks of varying obliquity. We assessed injection through reflection and downstream scattering, including the effect of a cross-shock potential. Both quasi-thermal and suprathermal seed populations were analysed. We present results on the effect of cross-field diffusion downstream of the shock on the injection efficiency. Methods: Using analytical methods, we present applicable injection speed thresholds that were compared with both semi-analytical flux integration and Monte Carlo simulations, which do not resort to binary thresholds. Shock-normal angle and shock-normal velocity were varied to assess the injection efficiency with respect to these parameters. Results: We present evidence of a significant bias of thermal seed particle injection at small shock-normal angles. We show that downstream isotropisation methods affect the theta-dependence of this result. We show a non-negligible effect caused by the cross-shock potential, and that the effect of downstream cross-field diffusion is highly dependent on boundary definitions. Conclusions: Our results show that for Monte Carlo simulations of coronal shock acceleration a full distribution function assessment with downstream isotropisation through scatterings is necessary to realistically model particle injection. Based on our results, seed particle injection at quasi-parallel coronal shocks can result in significant acceleration efficiency, especially when combined with varying field-line geometry.