Electron heating during magnetic reconnection: A simulation scaling study

Abstract

1Electron bulk heating during magnetic reconnection with symmetric inflow conditions is examined using kinetic particle-in-cell (PIC) simulations. Inflowing plasma parameters are varied over a wide range of conditions, and the increase of electron temperature is measured in the exhaust well downstream of the x-line. The degree of electron heating is well correlated with the inflowing Alfvén speed c Ar based on the reconnecting magnetic field through the relation ∆T e = 0.033 m i c 2 Ar , where ∆T e is the increase in electron temperature. For the range of simulations performed, the heating shows almost no correlation with inflow total temperature T tot = T i + T e or plasma β. An out-of-plane (guide) magnetic field of similar magnitude to the reconnecting field does not affect the total heating, but it does quench perpendicular heating, with almost all heating being in the parallel direction. These results are qualitatively consistent with a recent statistical survey of electron heating in the dayside magnetopause (Phan et al, Geophys. Res. Lett., 40, doi:10.1002/grl.50917, 2013), which also found that ∆T e was proportional to the inflowing Alfvén speed. The net electron heating varies very little with distance downstream of the x-line.The simulations show at most a very weak dependence of electron heating on the ion to electron mass ratio. In the antiparallel reconnection case, the largely parallel heating is eventually isotropized downstream due a scattering mechanism such as stochastic particle motion or instabilities. The simulation size is large enough to be directly relevant to reconnection in the Earth's magnetosphere, and the present findings may prove to be universal in nature with applications to the solar wind, the solar corona, and other astrophysical plasmas. The study highlights key properties that must be satisfied by an electron heating mechanism: (1) Preferential heating in the parallel direction; (2) Heating proportional to m i c 2 Ar ; (3) At most a weak dependence on electron mass; and (4) An exhaust electron temperature that varies little with distance from the x-line. a) shay@udel.edu 2

Electron and Proton Heating in Transrelativistic Magnetic Reconnection

“…Although the initial plasma β i spans more than two orders of magnitude, and the initial temperature ratio an order of magnitude, even the most extreme values of M T e,tot differ by no more than a factor of ∼ 4. The value of M T e,tot in our β i 0.5 simulations, for which electrons stay nonrelativistic both in the upstream and in the downstream, is ∼ 0.04, which is consistent with the results of nonrelativistic reconnection by Shay et al (2014) for mass ratio m i /m e = 25.…”

“…2 As shown by Shay et al (2014), the electron heating efficiency in non-relativistic reconnection is expected to decrease with increasing mass ratio; in Sections 4.4 and 4.6, we present the dependence of the electron and proton heating fractions in trans-relativistic reconnection on m i /m e , up to the physical value.…”

“…In this paper, we focus exclusively on the outflow (i.e., before the the plasma reaches the primary island; see also Shay et al (2014), in the context of non-relativistic reconnection), shown by the green region between the two wavefronts in Fig. 2.…”

“…Electron heating by reconnection in the non-relativistic limit (σ i 1) has been studied extensively, both theoretically and by means of PIC simulations, in the context of the solar wind, Earth's magnetotail, and laboratory plasmas (Hoshino et al 2001;Jaroschek et al 2004;Loureiro et al 2013;Schoeffler et al 2011Schoeffler et al , 2013Shay et al 2014;Dahlin et al 2014;Daughton et al 2014;Li et al 2015;Haggerty et al 2015;Numata & Loureiro 2015;Le et al 2016;Li et al 2017). Though less commonly studied, relativistic reconnection (i.e., σ i 1) in electron-proton plasmas has also received some attention in recent years Guo et al 2016).…”

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

“…We propose the following explanation: in order for a charged particle to accelerate multiple times, it must propagate upstream against the Alfvénic outflow that ejects plasma β ≪ 1, reconnection heats electrons to an appreciable fraction of the available magnetic energy density 47,48 : ∆T e ≈ 0.03m e c 2 Ae , corresponding to v th,e /c A ≈ 0.06m i /m e ≈ 10 so that essentially all reconnection-heated electrons will satisfy the criterion, independent of the initial temperature. In contrast, ions are typically sub-Alfénic and would require an injection mechanism (e.g.…”