Magnetohydrodynamic Simulations of Reconnection and Particle Acceleration: Three-Dimensional Effects

TL;DR

This study uses three-dimensional magnetohydrodynamic simulations to analyze particle acceleration during magnetic reconnection, highlighting the significance of guide fields and 3D effects for efficient acceleration mechanisms.

Contribution

It demonstrates the importance of three-dimensional effects and guide fields in particle acceleration during magnetic reconnection, extending previous 2D models.

Findings

Parallel velocity component increases exponentially in reconnection zones.

Particle acceleration is mainly through first order Fermi process in magnetic islands.

Saturation of parallel acceleration occurs in 2D models without guide fields.

Abstract

The magnetic fields can change their topology through a process known as magnetic reconnection. This process in not only important for understanding the origin and evolution of the large-scale magnetic field, but is seen as a possibly efficient particle accelerator producing cosmic rays mainly through the first order Fermi process. In this work we study the properties of particle acceleration in reconnection zones and show that the velocity component parallel to the magnetic field of test particles inserted in nearly non-resistive magnetohydrodynamic (MHD) domains of reconnection without including kinetic effects, such as pressure anisotropy, the Hall term, or anomalous effects, increases exponentially. Also, the acceleration of the perpendicular component is always possible in such models. We have found that within contracting magnetic islands or current sheets the particles accelerate predominantly through the first order Fermi process, as previously described, while outside the current sheets and islands the particles experience mostly drift acceleration due to magnetic fields gradients. Considering two dimensional MHD models without a guide field, we find that the parallel acceleration stops at some level. This saturation effect is however removed in the presence of an out-of-plane guide field or in three dimensional models. Therefore, we stress the importance of the guide field and fully three dimensional studies for a complete understanding of the process of particle acceleration in astrophysical reconnection environments.