Particle Acceleration due to the Interaction of Current Sheets with a Shock Wave

Abstract:

Zank et al. 2015 developed a model that describes the acceleration of charged particles via both diffusive shock acceleration and acceleration by magnetic reconnection-related processes downstream of the shock to explain e.g., the origin of anomalous cosmic rays at the heliospheric termination shock. Specifically, they considered downstream particle energization via the magnetic island anti-reconnection electric field and magnetic island contraction due to magnetic reconnection. The model results in a gradual increase in the energetic particle flux downstream of a shock wave. Zhao et al. 2019a verified that this model explains well the particle flux observed by Voyager 2 downstream of the termination shock. Similar energetic particle behavior has been observed by Ulysses at an interplanetary shock [Zhao et al. 2019b]. This indicates that particle acceleration due to magnetic reconnection downstream of a shock might be quite common throughout the heliosphere. However, it is not clear how magnetic reconnection over a range of scales occurs in the downstream region of a shock wave. One possibility is that reconnection downstream can be triggered because of the interaction between a shock wave and the heliospheric current sheet. The region surrounding the heliospheric current sheet appears to be filled with small-scale magnetic field structures that may well interact with a shock propagating in this region. We study the interaction of current sheets and structures using a 2D hybrid simulation. We introduce multiple current sheets upstream perpendicular to the plasma flow satisfying the force-free condition. Although the current sheets are stable in the upstream flow, they are compressed after crossing the shock wave and become unstable due to the tearing instability and extensive multiple and highly dynamical magnetic reconnection events are initiated. As a result, the electromagnetic fields become highly turbulent downstream. The downstream proton energy spectrum shows heated thermal plasma and non-thermal particles. The particle flux downstream exhibits a gradual increase as shown in Zank et al. 2015, with the onset of the increase corresponding to where magnetic reconnection occurs downstream. We provide a detailed analysis of a particle trajectory to understand how particles are accelerated.