The Onset of Ion Heating During Magnetic Reconnection with a Strong Guide Field

TL;DR

This study uses particle-in-cell simulations to investigate ion heating during magnetic reconnection with a strong guide field, revealing a threshold-dependent pickup mechanism that favors perpendicular ion heating and its relevance to solar corona observations.

Contribution

It demonstrates how ion pickup behavior during guide field reconnection leads to distinct perpendicular heating patterns and establishes a scaling law for ion heating based on ambient magnetic parameters.

Findings

Perpendicular ion heating dominates during guide field reconnection.

A threshold in magnetic field strength triggers sharp ion heating onset.

Ion heating scales with ambient parameters and is relevant to solar corona observations.

Abstract

The onset of the acceleration of ions during magnetic reconnection is explored via particle-in-cell simulations in the limit of a strong ambient guide field that self-consistently and simultaneously follow the motions of protons and $\alpha$ particles. Heating parallel to the local magnetic field during reconnection with a guide field is strongly reduced compared with the reconnection of anti-parallel magnetic fields. The dominant heating of thermal ions during guide field reconnection results from pickup behavior of ions during their entry into reconnection exhausts and dominantly produces heating perpendicular rather than parallel to the local magnetic field. Pickup behavior requires that the ion transit time across the exhaust boundary (with a transverse scale of the order of the ion sound Larmor radius) be short compared with the ion cyclotron period. This translates into a threshold in the strength of reconnecting magnetic field that favors the heating of ions with high mass-to-charge. A simulation with a broad initial current layer produces a reconnecting system in which the amplitude of the reconnecting magnetic field just upstream of the dissipation region increases with time as reconnection proceeds. The sharp onset of perpendicular heating when the pickup threshold is crossed is documented. A comparison of the time variation of the parallel and perpendicular ion heating with that predicted based on the strength of the reconnecting field establishes the scaling of ion heating with ambient parameters both below and above the pickup threshold. The relevance to observations of ion heating in the solar corona is discussed.