Electron Acceleration from Expanding Magnetic Vortices During Reconnection with a Guide Field
Electron Acceleration from Expanding Magnetic Vortices During Reconnection with a Guide Field
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DOI：
10.3847/1538-4357/ab5d3b
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发表时间：
2020-01-20
影响因子：
4.9
通讯作者：
Zank, G. P.
中科院分区：
文献类型：
--
作者：
Che, H.;Zank, G. P.
磁重联被认为是太阳风和太阳耀斑中具有幂律谱的高能电子加速的原因。然而，最近对重联中电子加速的主要机制，即撕裂不稳定性引起的多岛加速的研究表明，这种机制在轻度相对论性重联加速中存在“注入问题”。在本文中，我们研究了一种新型的重联加速，在强引导场的磁重联过程中，当电流片达到电子惯性长度尺度时，驱动电子开尔文-亥姆霍兹不稳定性（EKHI）。在EKHI产生的旋涡的诱导下，电子被随机电场加速，迅速膨胀，并产生与W- α成正比的幂律电子能谱f (W)，指数α类似于3.5 （W为电子动能，f (W)为能量分布函数）。结果表明，该机制为二阶费米加速过程，指标α = (1 + 4a(2)D/R)/2，其中a = B-g/B-0，由感应电场的空间尺度D与涡旋的空间尺度R之比和导场B-g与渐近磁场B-0之比决定。
Magnetic reconnection is believed to be responsible for the acceleration of energetic electrons with a power-law spectrum in the solar wind and solar flares. However, recent studies of the leading mechanism of electron acceleration in reconnection, namely the acceleration by tearing instability induced multi-islands, demonstrates that this mechanism suffers from an "injection problem" for mildly relativistic reconnection acceleration. In this paper, we investigate a new type of reconnection acceleration in which an electron Kelvin-Helmholtz instability (EKHI) is driven as the current sheet reaches electron inertial length scales during magnetic reconnection with a strong guide field. Electrons are accelerated by stochastic electric fields, induced by the EKHI generated vortices that expand rapidly, and a power-law electron energy spectrum f (W) proportional to W-alpha with index alpha similar to 3.5 is produced (W is the electron kinetic energy and f (W) is the energy distribution function). We show that the mechanism is a second-order Fermi acceleration process, and the index alpha = (1 + 4a(2)D/R)/2 where a = B-g/B-0, which is determined by the ratio of the spatial scale of the inductive electric field D to that of vortices R and the ratio of guide field B-g to asymptotic magnetic field B-0.