Energization of halo electrons at a high beta low Mach number quasi-perpendicular shock

High beta and low Mach number shocks are present in various circumstances in space such as pickup ion mediated heliospheric termination shock, cosmic ray modified sub-shcok of a supernova remnant shock, galaxy cluster merger shock, etc. Recently, it has been revealed from numerical simulations that this type of collisionless shock can efficiently accelerate electrons. We previously showed that some of the upstream incident thermal electrons are accelerated through the mechanism called shock drift acceleration and reflected back toward upstream. For appropriate parameters, accelerated electrons can have relativistic energy after the reflection. However, the region of parameter space where the mechanism works is limited. Here, we examine the possibility that such high beta and low Mach number shocks can preferentially energize the electrons having already non-thermal energies which are preaccelerated through some unknown mechanisms. We perform two-dimensional full particle-in-cell simulation of a high beta and low Mach number quasi-perpendicular shock. In addition to the self-consistent plasma electrons and ions, test electrons whose temperature is one order higher than background upstream self-consistent electrons are introduced. We assume that these halo electrons are sufficiently tenuous so that they do not affect electromagnetic fields. We found that the halo electrons are well energized, even though background self-consistent electrons are not. Mechanism and efficiency of energization of the halo electrons are discussed.