Suppression of energetic electron transport in flares by double layers

TL;DR

This study uses particle-in-cell simulations to demonstrate how double layers form at the contact of hot and cold electron populations during solar flares, effectively confining energetic electrons and reducing heat flux.

Contribution

It reveals a new mechanism where double layers suppress electron transport and confine energetic electrons in flares, driven by ion-electron streaming instabilities.

Findings

Double layers form at the contact of hot and cold electrons.

Double layers significantly reduce heat flux between regions.

Electron confinement lasts much longer than transit times.

Abstract

During flares and coronal mass ejections, energetic electrons from coronal sources typically have very long lifetimes compared to the transit times across the systems, suggesting confinement in the source region. Particle-in-cell simulations are carried out to explore the mechanisms of energetic electron transport from the corona to the chromosphere and possible confinement. We set up an initial system of pre-accelerated hot electrons in contact with ambient cold electrons along the local magnetic field, and let it evolve over time. Suppression of transport by a nonlinear, highly localized electrostatic electric field (in the form of a double layer) is observed after a short phase of free-streaming by hot electrons. The double layer (DL) emerges at the contact of the two electron populations. It is driven by an ion-electron streaming instability due to the drift of the back-streaming return current electrons interacting with the ions. The DL grows over time and supports a significant drop in temperature and hence reduces heat flux between the two regions that is sustained for the duration of the simulation. This study shows transport suppression begins when the energetic electrons start to propagate away from a coronal acceleration site. It also implies confinement of energetic electrons with kinetic energies less than the electrostatic energy of the DL for the DL lifetime, which is much longer than the electron transit time through the source region.