Particle acceleration in turbulence and weakly stochastic reconnection.

Area of Science:

• Plasma Physics
• Astrophysical Plasmas
• Computational Astrophysics

Background:

• Particle acceleration is a key process in astrophysical plasmas, influencing phenomena from solar flares to cosmic rays.
• Magnetic reconnection, a fundamental process in plasma physics, is believed to be a primary driver of particle energization.
• Previous studies have explored particle acceleration in simplified magnetic reconnection models, but the role of turbulence requires further investigation.

Purpose of the Study:

• To analyze the energy distribution evolution of test particles in three-dimensional (3D) magnetohydrodynamic (MHD) simulations of magnetic reconnection.
• To compare particle acceleration mechanisms in different magnetic reconnection configurations, including turbulent and non-turbulent scenarios.
• To determine the dominant particle acceleration processes and their dependence on reconnection topology and turbulence.

Main Methods:

• Utilized 3D MHD simulations to model various magnetic reconnection configurations.
• Injected test particles into the simulations to track their energy evolution.
• Analyzed particle acceleration mechanisms, distinguishing between first-order and second-order Fermi processes.

Main Results:

• In a single Sweet-Parker topology, particles accelerated via a first-order Fermi process.
• Turbulence within the current sheet significantly enhanced the acceleration rate, maintaining a first-order Fermi process.
• In MHD turbulence without current sheets, particle acceleration was reduced, approaching a second-order Fermi process.

Conclusions:

• Turbulent magnetic reconnection is a highly efficient particle acceleration mechanism, significantly outperforming simpler reconnection models.
• The presence of turbulence leads to a robust first-order Fermi acceleration process by enabling multiple scatterings within reconnecting magnetic fluxes.
• Particle acceleration in pure MHD turbulence is less efficient and follows a second-order Fermi process, highlighting the importance of reconnection dynamics.