Reduction of Ion Heating During Magnetic Reconnection by Large-Scale Effective Potentials

TL;DR

This study uses simulations and analytical methods to show that large-scale potentials during magnetic reconnection influence how energy is divided between electrons and ions, with the potential's magnitude depending on upstream conditions.

Contribution

It reveals that a large-scale parallel electric field and its potential control electron and ion heating, with the potential magnitude depending on upstream electron temperature.

Findings

The potential confines electrons, enhancing their heating.

The potential slows ions, affecting their energy gain.

The energy partition depends on upstream electron temperature.

Abstract

The physical processes that control the partition of released magnetic energy between electrons and ions during reconnection is explored through particle-in-cell simulations and analytical techniques. We demonstrate that the development of a large-scale parallel electric field and its associated potential controls the relative heating of electrons and ions. The potential develops to restrain heated exhaust electrons and enhances their heating by confining electrons in the region where magnetic energy is released. Simultaneously the potential slows ions entering the exhaust below the Alfv\'enic speed expected from the traditional counterstreaming picture of ion heating. Unexpectedly, the magnitude of the potential and therefore the relative partition of energy between electrons and ions is not a constant but rather depends on the upstream parameters and specifically the upstream electron normalized temperature (electron beta). These findings suggest that the fraction of magnetic energy converted into the total thermal energy may be independent of upstream parameters.