Electron Acceleration via Trapping inside Ion Mirror-mode Structures within A Large-scale Magnetic Flux Rope

TL;DR

This paper reveals that magnetic mirror structures inside flux ropes can trap energetic electrons, enabling continuous acceleration and forming power-law energy distributions, thus overcoming previous limitations of finite flux rope contraction.

Contribution

It introduces a novel mechanism where magnetic mirror structures inside flux ropes facilitate sustained electron acceleration, demonstrated through magnetospheric observations.

Findings

Magnetic mirror structures prevent electron escape in flux ropes.

Energetic electrons form power-law energy distributions.

Continuous acceleration occurs within mirror structures near flux rope centers.

Abstract

Fermi acceleration is believed as a crucial process for the acceleration of energetic electrons within flux ropes (FRs) during magnetic reconnection. However, in finite-length FRs with a large core field, the finite contracting and the escaping of electrons along the axis can significantly limit the efficiency of Fermi acceleration. Using observations from the Magnetospheric Multiscale mission in the magnetotail, we demonstrate that magnetic mirror structures inside the FR can effectively prevent the escape of energetic electrons and overcome the limitation of finite contraction. Energetic electrons were produced and formed a power-law energy distribution in these mirror structures. By evaluating the acceleration rates, we show that these energetic electrons can be continuously accelerated within the mirror structures near the central region of the FR. These results unveil a novel mechanism that is universally applicable to electron acceleration within FRs in space, laboratory, and astrophysical plasmas.