Nonthermal electron acceleration in turbulent post-flare coronal loops

TL;DR

This study uses advanced simulations to analyze how turbulence in post-flare coronal loops accelerates electrons, revealing stochastic Fermi-like processes as key to producing nonthermal electron populations observed in solar flares.

Contribution

It introduces a numerical framework with energy-conserving guiding-centre equations to quantify electron acceleration mechanisms in turbulent post-flare loops, highlighting the role of Fermi-like stochastic acceleration.

Findings

Electrons develop suprathermal energy distributions compatible with X-ray emissions.

Perpendicular gradient effects dominate electron energization via stochastic Fermi-like acceleration.

Trapped electrons in turbulent magnetic structures experience the strongest acceleration.

Abstract

The generation of nonthermal electrons during solar flares plays a critical role in energy transport from the corona to the chromosphere, producing regions of observed intense X-ray emission. Turbulence in post-flare loops, particularly from Kelvin-Helmholtz instabilities (KHI), has been suggested and investigated as a mechanism for trapping and accelerating electrons in such scenarios. Starting from past results, we aim to characterize the energization process of electrons trapped in a turbulent post-flare looptop, quantifying the contributions of different acceleration mechanisms, and establishing a coherent numerical framework for describing particle energetics. We perform test-particle simulations with the guiding-centre approximation on top of a 2.5D magnetohydrodynamic model of a time-evolving post-flare coronal looptop. We implement an improved formulation of the guiding-centre equations that explicitly conserves energy, enabling a consistent analysis of electron acceleration in the turbulent plasma. We find that, in the plasma turbulence inside the looptop, electrons develop suprathermal energy distributions with tails compatible with hard X-ray emission. The dominant energization channel arises from perpendicular gradient effects in the form of second-order Fermi-like stochastic acceleration, while curvature effects are dominant for particles on long trajectories. Statistical correlations with the measured particle pitch angle confirm that the strongest acceleration occurs for electrons trapped in bouncing motions within turbulent magnetic structures. Our results provide an understanding of how KHI-induced turbulence in coronal looptops produces and sustains populations of trapped nonthermal electrons. We dissect and clarify the relative roles of different magnetic effects and the emergence of stochastic Fermi-like energization.