Large-scale electron acceleration by parallel electric fields during magnetic reconnection

Abstract

Reconnection is the process by which stress in the field of a magnetized plasma is reduced by a topological rearrangement of its magnetic-field lines. The process is often accompanied by an explosive release of magnetic energy and is implicated in a range of astrophysical phenomena 1. In the Earth's magnetotail, reconnection energizes electrons up to hundreds of keV (ref. 2) and solar-flare events can channel up to 50% of the magnetic energy into the electrons, resulting in superthermal populations in the MeV range 3-5. Electron energization is also fundamentally important to astrophysical applications 6 yielding a window into the extreme environments. Here we show that during reconnection powerful energization of electrons by magnetic-field-aligned electric field (E) can occur over spatial scales that hugely exceed previous theories and simulations 7. In our kinetic simulation E is supported by non-thermal and strongly anisotropic features in the electron distributions not permitted in standard fluid formulations, but routinely observed by spacecraft in the Earth's magnetosphere. This allows for electron energization in spatial regions that exceed the regular d e-scale electron-diffusion region by at least three orders of magnitude. Over the past decade, spacecraft have made detailed measurements in the vicinity of reconnection sites in the Earth's magnetotail. In particular, electron distribution functions, f , have been obtained in situ; as an example, in Fig. 1 we consider measurements by the Cluster mission from the much studied event of 1 October 2001 (ref. 8). Previous analyses have established that Cluster 3 passed through a reconnection region as shown in the schematic representation of Fig. 1a (ref. 9), and the distribution in Fig. 1b is typical for the reconnection inflow region, where the electrons are heated in the parallel direction up to 1 keV (from about 100 eV), whereas no energization is observed for the perpendicular direction. In contrast, for the distribution in Fig. 1c observed in the exhaust region, the electrons are heated in both the parallel and the perpendicular directions all the way up to 14 keV. This type of distribution is common in the exhaust close to the reconnection region and is referred to as a flat-top distribution because f (ε) is nearly constant (flat) for a large range of energies (here, for ε between 1 keV and 14 keV; ref. 10). Analysis of spacecraft data as well as kinetic simulations shows that the moderate parallel energization of electrons in the inflow region is caused by the acceleration potential Φ = ∞ x E dl, where spatial integration, dl, from the location, x, is carried along magnetic-field lines to the ambient plasma 11. Thus, eΦ is the energy gained from the parallel electric fields E by electrons entering the reconnection region in a straight shot along a magnetic-field line.

The energetics of relativistic magnetic reconnection: ion-electron repartition and particle distribution hardness

“…There, magnetic energy conversion occurs as the plasma flows through the complex structure of collisionless nonlinear waves (slow shock(s), compound wave, and rotational wave). Instabilities and parallel electric fields in these regions can produce thermal and non-thermal electrons (Drake et al 2005;Egedal et al 2009Egedal et al , 2012. This shock structure has been investigated in the non-relativistic case (Liu et al 2012;Higashimori & Hoshino 2012).…”

The Roles of Fluid Compression and Shear in Electron Energization during Magnetic Reconnection

“…Previous reconnection studies have identified that particles are accelerated close to the reconnection X-point (Hoshino et al 2001;Drake et al 2005;Fu et al 2006;Oka et al 2010;Egedal et al 2012Egedal et al , 2015Wang et al 2016), in contracting magnetic islands (Drake et al 2006;Oka et al 2010), and also in island-merging regions (Oka et al 2010;Liu et al 2011;Drake et al 2013;Nalewajko et al 2015). At the X-points, particles get accelerated by streaming along the nonideal electric field.…”

Particle acceleration during magnetic reconnection in a low-beta pair plasma

“…Hoshino et al [9] showed that several processes can occur in a single reconnection layer -in the X-line region [50,51] and along the separatrix region [52,53], particles can get accelerated in the nonideal electric field and then further accelerated due to grad-B drift and the curvature drift in the magnetic pileup region [54], where the electric field is mostly ideal E = −v×B/c. Drake et al [11] have further developed the Fermi mechanism inside the magnetic islands as particles get bounced at two ends of islands repeatedly [12].…”