Synergy of stochastic and systematic energization of plasmas during turbulent reconnection

TL;DR

This paper investigates how the combined effects of large-scale magnetic disturbances and Unstable Current Sheets (UCSs) in turbulent reconnection lead to efficient particle acceleration, explaining rapid energization and formation of super-hot populations.

Contribution

It is the first analysis of electron and ion energization in a large-scale environment combining stochastic and systematic acceleration mechanisms.

Findings

Stochastic (second order Fermi) acceleration heats particles.

Systematic (first order Fermi) acceleration forms high-energy tails.

Synergy explains fast, impulsive particle acceleration and super-hot populations.

Abstract

The important characteristic of turbulent reconnection is that it combines large scale magnetic disturbances $(\delta B/B \sim 1)$ with randomly distributed Unstable Current Sheets (UCSs). Many well known non linear MHD structures (strong turbulence, current sheet(s), shock(s)) lead asymptotically to the state of turbulent reconnection. We analyze in this article, for the first time, the energization of electrons and ions in a {\bf large scale} environment that {\bf combines} large amplitude disturbances propagating with sub-Alfv\'enic speed with UCSs. The magnetic disturbances interact stochastically (second order Fermi) with the charged particles and they play a crucial role in the heating of the particles, while the UCS interact systematically (first order Fermi) and play a crucial role in the formation of the high energy tail. The synergy of stochastic and systematic acceleration provided by the mixture of magnetic disturbances and UCSs influences the energetics of the thermal and non-thermal particles, the power law index, and the time the particles remain inside the energy release volume. We show that this synergy can explain the observed very fast and impulsive particle acceleration and the slightly delayed formation of a super-hot particle population.