Kinetic modeling of particle acceleration in a solar null point reconnection region

TL;DR

This paper models particle acceleration in solar null-point reconnection regions using combined MHD and kinetic simulations, revealing how electrons gain energy and form non-thermal distributions during solar flares.

Contribution

It introduces a novel multi-scale simulation approach bridging macroscopic and kinetic scales in the solar corona, demonstrating electron acceleration mechanisms in 3D null-point regions.

Findings

Electrons are accelerated mainly by a systematic electric field in the current sheet.

A non-thermal electron population with a power-law index of about -1.78 forms.

Simulations achieve unprecedented resolution with up to 135 billion particles.

Abstract

The primary focus of this paper is on the particle acceleration mechanism in solar coronal three-dimensional reconnection null-point regions. Starting from a potential field extrapolation of a Solar and Heliospheric Observatory (SOHO) magnetogram taken on 2002 November 16, we first performed magnetohydrodynamics (MHD) simulations with horizontal motions observed by SOHO applied to the photospheric boundary of the computational box. After a build-up of electric current in the fan-plane of the null-point, a sub-section of the evolved MHD data was used as initial and boundary conditions for a kinetic particle-in-cell model of the plasma. We find that sub-relativistic electron acceleration is mainly driven by a systematic electric field in the current sheet. A non-thermal population of electrons with a power-law distribution in energy forms in the simulated pre-flare phase, featuring a power-law index of about -1.78. This work provides a first step towards bridging the gap between macroscopic scales on the order of hundreds of Mm and kinetic scales on the order of cm in the solar corona, and explains how to achieve such a cross-scale coupling by utilizing either physical modifications or (equivalent) modifications of the constants of nature. With their exceptionally high resolution - up to 135 billion particles and 3.5 billion grid cells of size 17.5 km - these simulations offer a new opportunity to study particle acceleration in solar-like settings.