Suppression of Energetic Electron Transport in Flares by Double Layers

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. Particlein-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 that 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.

Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause

“…However, MMS observations also show that strong fluctuations in B with frequencies between~1 Hz and~10 Hz are often present in asymmetric magnetic reconnection [Ergun et al, 2016a]. Similar characteristics also have been reported in 3D simulations of magnetic reconnection [Li et al, 2012;Roytershteyn et al, 2013;Egedal et al, 2015;Lapenta et al, 2015;Price et al, 2016]. In the observations, however, the B fluctuations are often accompanied by large-amplitude (~100 mV/m), parallel electric fields (E || ) that appear as nonlinear structures [Ergun et al, 2016a].…”

The role of three-dimensional transport in driving enhanced electron acceleration during magnetic reconnection

“…An additional issue is that these simulations have periodic boundary conditions that prevent particle loss from the system. Solar observations suggest that electrons are confined in regions of energy release in the corona 3 ; possible mechanisms for this confinement include mirroring and double layers 54 . This could suggest that particle loss is not an important concern.…”

Magnetospheric Multiscale observations of large‐amplitude, parallel, electrostatic waves associated with magnetic reconnection at the magnetopause

“…The direction of a net potential is such that it would act to retard electrons moving from the magnetosheath into the magnetosphere. Such electric fields in the form of double layers have been reported in simulations relevant to solar magnetic reconnection [ Li et al ., ; Egedal et al ., ]. Such a potential can alter the electron and ion distributions in the electron diffusion region and, possibly affect the reconnection process.…”

“…The 2‐D Vlasov simulation results indicate that a parallel potential acts to retard magnetosheath electrons from an in‐rush into the magnetosphere, preserving the quasi‐neutral plasma. Such parallel electric fields have been reported from laboratory experiments [e. g. Carr et al ., ] and from several simulations of magnetic reconnections [ Li et al ., ; Egedal et al ., ], albeit these simulations did not include a cold population. The same electric fields appear to accelerate cold electrons from the magnetosphere toward the reconnection site.…”