Stochastic Fermi Energization of Coronal Plasma during explosive magnetic energy release

TL;DR

This study models how particles in the solar corona gain energy through stochastic interactions with magnetic irregularities formed during explosive magnetic energy releases, using a 3D simulation and Fokker-Planck analysis.

Contribution

It introduces a 3D simulation framework for particle energization via Alfvénic Scatterers and estimates transport coefficients to solve the Fokker-Planck equation for energy distribution.

Findings

Particles are heated and accelerated, forming a high-energy tail.

The energy distribution depends on the particles' mean free path.

The model reproduces expected energization effects in coronal plasma.

Abstract

The aim of this study is to analyze the interaction of charged particles (ions and electrons) with randomly formed particle scatterers (e.g.\ large scale local "magnetic fluctuations" or "coherent magnetic irregularities"), using the set up proposed initially by \cite{Fermi49}. These scatterers are formed by the explosive magnetic energy release and propagate with the Alfv\'en speed along the irregular magnetic fields. They are large scale local fluctuations ($\delta B/B \approx 1$), randomly distributed inside the unstable magnetic topology and will here be called {\bf Alfv\'enic Scatterers (AS)}. We constructed a 3D grid on which a small fraction of randomly chosen grid points are acting as AS. In particular, we study how a large number of test particles evolve inside a collection of AS, analyzing the evolution of their energy distribution and their escape time distribution. We use a well established method to estimate the transport coefficients directly from the trajectories of the particles. Using the estimated transport coefficients and solving the Fokker-Planck (FP) equation numerically, we can recover the energy distribution of the particles. We have shown that the Stochastic Fermi Energization (SFE) of mildly relativistic and relativistic plasma can heat and accelerate the tail of the ambient particle distribution as predicted by \citet{Parker58} and \citet{Ramaty79}. The temperature of the hot plasma and the tail of the energetic particles depend on the mean free path ($\lambda_{\rm sc}$) of the particles between the scatterers inside the energization volume.