Particle acceleration by fluctuating electric fields at a magnetic field null point

TL;DR

This study investigates how fluctuating electric fields near magnetic null points in collisionless solar plasma can accelerate particles, revealing energy distributions relevant to solar flares and noise storms.

Contribution

It numerically demonstrates particle acceleration mechanisms at magnetic null points due to fluctuating electric fields, highlighting energy distribution differences based on electric field parameters.

Findings

Protons can reach gamma-ray energies within 1 second.

Electrons and ions often have different energy distribution forms.

Bimodal particle energy distributions are common.

Abstract

Particle acceleration consequences from fluctuating electric fields superposed on an X-type magnetic field in collisionless solar plasma are studied. Such a system is chosen to mimic generic features of dynamic reconnection, or the reconnective dissipation of a linear disturbance. We explore numerically the consequences for charged particle distributions of fluctuating electric fields superposed on an X-type magnetic field. Particle distributions are obtained by numerically integrating individual charged particle orbits when a time varying electric field is superimposed on a static X-type neutral point. This configuration represents the effects of the passage of a generic MHD disturbance through such a system. Different frequencies of the electric field are used, representing different possible types of wave. The electric field reduces with increasing distance from the X-type neutral point as in linear dynamic magnetic reconnection. The resulting particle distributions have properties that depend on the amplitude and frequency of the electric field. In many cases a bimodal form is found. Depending on the timescale for variation of the electric field, electrons and ions may be accelerated to different degrees and often have energy distributions of different forms. Protons are accelerated to $\gamma$-ray producing energies and electrons to and above hard X-ray producing energies in timescales of 1 second. The acceleration mechanism is possibly important for solar flares and solar noise storms but is also applicable to all collisionless plasmas.