Probing Proton versus Electron Heating and Energization during Magnetic Reconnection

TL;DR

This study uses simulations to investigate how electrons and protons are heated and energized during magnetic reconnection, revealing that protons gain more energy initially and continue to do so due to their larger mass and specific dynamics.

Contribution

The paper demonstrates how proton and electron heating differ during magnetic reconnection, highlighting the role of initial conditions and mass ratios in energy gain mechanisms.

Findings

Protons gain more energy than electrons when upstream temperatures are equal.

Proton energy gain scales with $m_iC_A^2$, exceeding electron energy gain.

Protons' non-thermal energy content surpasses that of electrons.

Abstract

The mechanisms controlling the relative heating and energization of electrons and protons during magnetic reconnection are explored. Simulations are carried out with the kglobal model, which produces bulk heating and the extended powerlaw distributions of both species that have been documented in observations. The simulations have been carried out with a range of proton-to-electron mass ratios and upstream temperatures to isolate the factors that control energy gain. The simulations reveal that when the upstream temperatures of the two species are equal, the proton heating and energization exceeds that of electrons and that this is a consequence of the much larger energy gain of protons on their first entry into the reconnection exhaust. The effective energy gain of protons on exhaust entry scales as $m_iC_A^2$ since the protons counterstream at the Alfv\'en speed $C_A$ while the initial electron energy gain is smaller by the factor $(\beta_{e0}m_e/m_i)^{1/2}$. Since Fermi reflection during flux rope merger dominates energy gain in large-scale reconnecting systems and the rate of energy gain is proportional to energy, protons continue to gain energy faster than electrons for the duration of the simulations, leading to temperature increments of protons exceeding that of electrons and the non-thermal energy content of protons also exceeding that of electrons.