Plasma compression in magnetic reconnection regions in the solar corona

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 ($\ge 4$) 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.

arXiv commentAccepted for publication in the Astrophysical Journal, 36 pages, 11 figures, 1 table

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• Large-scale Compression Acceleration during Magnetic Reconnection in a Low-$\beta$ Plasma
• Reflection of fast magnetosonic waves near magnetic reconnection region
• Effects of a Velocity Shear on Double Current Sheet Systems: Explosive Reconnection and Particle Acceleration
• Compression Acceleration of Protons and Heavier Ions at the Heliospheric Current Sheet
• Energetic Nonthermal Electrons within the Above-the-looptop Regions in Solar Flares: Acceleration, Feedback, and Quasi-Periodic Pulsations
• High $\beta$ magnetic reconnection at different altitudes in the cool low solar atmosphere
• Analysis of Short-term Solar Activity Variability and Estimating the Timings of the Next Enhanced Bursts