Multi-hierarchy simulation of Riemann problem for reconnection exhausts

TL;DR

This study uses a multi-hierarchy simulation combining MHD and PIC methods to investigate slow shock formation in magnetic reconnection, supporting the viability of Petschek reconnection in collisionless-collisional systems.

Contribution

It demonstrates that Petschek-like reconnection can occur in systems with kinetic effects, even when slow shock formation is suppressed in the kinetic domain.

Findings

Slow shocks form near the switch-off limit in MHD domain despite suppression in PIC domain.

Slow shock formation promotes plasma isotropization within the PIC domain.

Petschek-like reconnection remains viable in collisionless-collisional systems.

Abstract

Magnetic reconnection drives a wide range of astrophysical plasma phenomena, including solar flares, by converting magnetic energy into plasma energy through changes in magnetic field topology. Petschek reconnection is a magnetohydrodynamic (MHD) model in which magnetic field lines reconnect within a localized diffusion region, and a pair of switch-off slow shocks forms outside this region, enabling efficient energy conversion. Whether this picture remains valid when kinetic effects are included remains an open question. In this study, we examine the formation and properties of slow shocks associated with reconnection exhausts by solving a two-dimensional Riemann problem using a multi-hierarchy framework that couples MHD and particle-in-cell (PIC) simulations. We find that a slow shock close to the switch-off limit forms in the MHD domain even when slow shock formation is suppressed in the PIC domain, and that this behavior is insensitive to the size of the PIC domain. The formation of the slow shock further promotes plasma isotropization within the PIC domain. These results suggest that Petschek-like reconnection remains viable in collisionless-collisional systems, such as solar flares, where temperature anisotropy appears to be relaxed far from the reconnection region.