Comment on "Nonideal Fields Solve the Injection Problem in Relativistic Reconnection"

TL;DR

This paper critically examines claims that electric fields in E>B regions dominate particle acceleration in relativistic reconnection, finding that these regions are not the primary source of injection energy and that previous conclusions may be flawed.

Contribution

The study re-analyzes simulation data to show that E>B regions are not the main contributors to particle injection and challenges prior assumptions about their role in acceleration.

Findings

E>B regions contribute minimally to injection energy

Pre-acceleration occurs mainly outside E>B regions

Excluding E>B fields does not significantly affect injection

Abstract

Recently, Sironi (PRL, 128, 145102; S22) reported the correlation between particles accelerated into high energy and their crossings of regions with electric field larger than magnetic field (E>B regions) in kinetic simulations of relativistic magnetic reconnection. They claim that electric fields in E>B regions (for a vanishing guide field) dominate in accelerating particles to the injection energy. S22 presented test-particle simulations showing that if particle energies are reset to low energies in E>B regions, efficient injection is suppressed. This Comment re-examines these claims by analyzing a simulation resembling the reference case in S22. We show that during crossings E>B acceleration only contributes a small fraction to the injection energy as E>B regions only host particles for a short duration. The energization before any E>B crossings has a comparable contribution, indicating E>B regions are not unique in pre-accelerating particles. A new test-particle simulation shows that zero-outing electric fields in E>B regions does not strongly influence the injection. We suggest that the procedure used in S22 to exclude E>B acceleration partly removes acceleration outside E>B regions, leading to a false conclusion.