Particle energisation in a collapsing magnetic trap model: the relativistic regime

TL;DR

This paper extends the analysis of particle energisation in collapsing magnetic traps to the relativistic regime, revealing quantitative differences in particle orbits and energies compared to non-relativistic models, which are crucial for understanding solar flare acceleration.

Contribution

It introduces a relativistic guiding centre approach to particle orbits in collapsing magnetic traps, highlighting key differences from non-relativistic models at high energies.

Findings

Relativistic and non-relativistic orbits agree at mildly relativistic energies.

Relativistic energies are systematically lower than non-relativistic energies.

Relativistic mirror points are higher than non-relativistic ones.

Abstract

In solar flares, a large number of charged particles is accelerated to high energies. By which physical processes this is achieved is one of the main open problems in solar physics. It has been suggested that during a flare, regions of the rapidly relaxing magnetic field can form a collapsing magnetic trap (CMT) and that this trap may contribute to particle energisation.} In this Research Note we focus on a particular analytical CMT model based on kinematic magnetohydrodynamics. Previous investigations of particle acceleration for this CMT model focused on the non-relativistic energy regime. It is the specific aim of this Research Note to extend the previous work to relativistic particle energies. Particle orbits were calculated numerically using the relativistic guiding centre equations. We also calculated particle orbits using the non-relativistic guiding centre equations for comparison. For mildly relativistic energies the relativistic and non-relativistic particle orbits mainly agree well, but clear deviations are seen for higher energies. In particular, the final particle energies obtained from the relativistic calculations are systematically lower than the energies reached from the corresponding non-relativistic calculations, and the mirror points of the relativistic orbits are systematically higher than for the corresponding non-relativistic orbits. While the overall behaviour of particle orbits in CMTs does not differ qualitatively when using the relativistic guiding centre equations, there are a few systematic quantitative differences between relativistic and non-relativistic particle dynamics.