Electron acceleration from contracting magnetic islands during reconnection

Abstract

A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields 'reconnect' and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1-3), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects. In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons. Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting 'magnetic islands' that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls--the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations.

Energetic electron acceleration by unsteady magnetic reconnection

“…b, Magnified view of the ion diffusion region and the reconnection jet. Secondary islands and turbulence around the X-line have been found in simulations 18,20,28,29 . Changes in the inflow speed may produce the two reconnection jets 22 that have been observed by Cluster in the downstream region at [−17.2, −6.9, 0.0] R E .…”

“…2f). This implies other field-aligned acceleration processes near the reconnection X-line (for example, localized Fermi acceleration by contracting magnetic islands 18 ). These processes occur primarily during the decline of the plasma jets.…”

Observations of electron vorticity in the inner plasma sheet

“…Simulations further show that much of this flow is composed of bi-directional beams that are formed by reflections off of localized potentials or mirroring off of magnetic islands (Onsager et al, 1991;Drake et al, 2006;Egedal et al, 2010). The presence of bi-direction beams in the vicinity of reconnection in the plasma sheet has been observationally reported by a number of authors (e.g., Nakamura et al, 2006;Åsnes et al, 2008).…”

Energetic electrons associated with magnetic reconnection in the sheath of interplanetary coronal mass ejection

“…As the primary acceleration mechanism, electrons are accelerated by the reconnection electric field in the X-line region, and then may be further accelerated in the pileup region owing to curvature and gradient drifts along the electric field [9,26]. In addition, electrons can be accelerated by contracting magnetic islands via Fermi acceleration [24].…”

“…Various acceleration mechanisms have been proposed to interpret the origin of the energetic electrons during magnetic reconnection with or without guide field [9,[24][25][26]. As the primary acceleration mechanism, electrons are accelerated by the reconnection electric field in the X-line region, and then may be further accelerated in the pileup region owing to curvature and gradient drifts along the electric field [9,26].…”