Super-Fermi Acceleration in Multiscale MHD Reconnection

TL;DR

This study reveals a super-Fermi acceleration mechanism in 2D MHD plasmoid reconnection, significantly boosting particle energization rates and maximum energies, especially in large plasmoids relevant to solar flares.

Contribution

It introduces a novel super-Fermi acceleration process in multiscale MHD reconnection, demonstrating enhanced energization rates and energy gains dependent on plasmoid size.

Findings

Energization rate $ o ext{energy}$ is drastically enhanced over standard models.

Maximum particle energy gain increases with plasmoid size, by factors of 10 or more.

Power law indices vary with plasmoid size and drift effects, affecting acceleration efficiency.

Abstract

We investigate the Fermi acceleration of charged particles in 2D MHD anti-parallel plasmoid reconnection, finding a drastic enhancement in energization rate $\dot{\varepsilon}$ over a standard Fermi model of $\dot{\varepsilon} \sim \varepsilon$. The shrinking particle orbit width around a magnetic island due to $\vec{E}\times\vec{B}$ drift produces a $\dot{\varepsilon}_\parallel \sim \varepsilon_\parallel^{1+1/2\chi}$ power law with $\chi \sim 0.75$. The increase in the maximum possible energy gain of a particle within a plasmoid due to the enhanced efficiency increases with the plasmoid size, and is by multiple factors of 10 in the case of solar flares and much more for larger plasmas. Including effects of the non-constant $\vec{E}\times\vec{B}$ drift rates leads to further variation of power law indices from $\gtrsim 2$ to $\lesssim 1$, decreasing with plasmoid size at the time of injection.