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

¹ Department of Physics and AstronomyUniversity of Turku, 20014 Turku, Finland
e-mail: markus.battarbee@utu.fi
² Department of Physics, University of Helsinki, 00014 Helsinki, Finland
³ Jeremiah Horrocks Institute, University of Central Lancashire, Lancashire PR/2, UK
⁴ The Blackett Laboratory, Imperial College, London SW7 2AZ, UK

Received: 22 February 2013
Accepted: 30 August 2013

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 θ_Bn and shock-normal velocity V_s 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 θ_Bn-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.