The Maximum Particle Energy Gain During Magnetic Reconnection

TL;DR

This paper investigates the maximum energy particles can attain during magnetic reconnection, revealing that the number of flux rope mergers, which depends on system size, governs the energy limit.

Contribution

It provides an analytical and simulation-based explanation for how system size and flux rope mergers determine maximum particle energy during reconnection.

Findings

Maximum particle energy is regulated by flux rope mergers.

Larger systems produce more mergers and higher maximum energies.

Fermi reflection dominates particle energy gain in repeated mergers.

Abstract

The factors that control the maximum energy attained by protons and electrons during magnetic reconnection are investigated analytically and using large-scale simulations with the \textit{kglobal} model. Previous work revealed that a strong ambient guide field strongly impacts particle energy gain during reconnection, suppressing energy gain from Fermi reflection by increasing the radius of curvature of reconnected field lines. However, previous simulations have also shown that the maximum energy gain increases with the system size. The physical basis for this result has not been explored. We perform simulations that vary the effective system size over a large range to isolate the processes determining the maximum energy gain. The maximum energy $W_{max}$ is regulated by the number of magnetic-island mergers that occur, as multiple flux ropes that form at early time repeatedly merge until the largest approaches the system scale. Fermi reflection in these repeated mergers dominates particle energy gain. The number of mergers is linked to the effective system size -- larger systems produce a larger number of flux ropes and more mergers. That $W_{max}$ is linked to the number of flux rope mergers has implications for understanding why particle-in-cell simulations only produce powerlaw distributions of energetic particles with a limited range in energy.