Plasma compression in magnetic reconnection regions in the solar corona

TL;DR

This study investigates plasma compression during magnetic reconnection in the solar corona, showing that strong compressions can occur under certain conditions, potentially influencing particle acceleration mechanisms.

Contribution

It provides numerical evidence that plasma compression in reconnection regions depends on cooling processes and plasma beta, advancing understanding of particle acceleration in solar phenomena.

Findings

Strong plasma compressions (≥4) can form with efficient cooling.

Radiative cooling significantly affects compression in low-temperature regions.

Maximum compression occurs in low-beta plasma near magnetic nulls.

Abstract

It has been proposed that particles bouncing between magnetized flows converging in a reconnection region can be accelerated by the first order Fermi mechanism. Analytical considerations of this mechanism have shown that the spectral index of accelerated particles is related to the total plasma compression within the reconnection region similarly to the case of diffusive shock acceleration mechanism. As a first step to investigate the efficiency of Fermi acceleration in reconnection regions in producing hard energy spectra of particles in the solar corona, we explore the degree of plasma compression that can be achieved at reconnection sites. In particular, we aim to determine the conditions for the strong compressions to form. Using a two-dimensional resistive MHD numerical model we consider a set of magnetic field configurations where magnetic reconnection can occur including a Harris current sheet, a force-free current sheet, and two merging flux ropes. Plasma parameters are taken to be characteristic of the solar corona. Numerical simulations show that strong plasma compressions ($\geq 4$) in the reconnection regions can form when the plasma heating due to reconnection is efficiently removed by fast thermal conduction or radiative cooling process. The radiative cooling process which is negligible in the typical 1 MK corona can play an important role in the low corona/transition region. It is found that plasma compression is expected to be strongest in low-beta plasma $\beta \sim 0.01-0.07$ at reconnection magnetic nulls.