Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism
Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism
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DOI：
10.3847/1538-4357/aa9b82
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发表时间：
2017-10
期刊：
影响因子：
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通讯作者：
Xinyi Guo;L. Sironi;R. Narayan
中科院分区：
文献类型：
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作者：
Xinyi Guo;L. Sironi;R. Narayan
最近对星系团合并激波的X射线观测表明，激波后的等离子体有两种温度，质子比电子热。通过二维粒子模拟，我们研究了低马赫数垂直激波中电子不可逆加热的物理，对于音速马赫数为3和等离子体β为16的代表性情况。我们发现电子熵产生需要两个基本成分：（1）电子温度各向异性，由场放大耦合到绝热不变性引起;（2）打破电子绝热不变性本身的机制。在冲击中，场放大发生在两个主要位置：在冲击斜坡处，密度压缩导致冻结场的增加;以及更远的下游，冲击驱动的质子温度各向异性产生强质子回旋和镜像模式。场放大引起的电子温度各向异性超过了电子哨声不稳定性的阈值。哨声波的增长打破了电子绝热不变性，并允许有效的熵产生。对于我们的参考运行，冲击后的电子温度超过绝热预期，导致电子与质子的温度比。我们发现，电子加热效率显示只有一个弱的依赖于质量比（小于下降，因为我们增加的质量比从到）。我们开发了一个电子不可逆加热的分析模型，并表明它是在很好的协议与我们的模拟结果。
Recent X-ray observations of merger shocks in galaxy clusters have shown that the postshock plasma has two temperatures, with the protons hotter than the electrons. By means of two-dimensional particle-in-cell simulations, we study the physics of electron irreversible heating in low-Mach-number perpendicular shocks, for a representative case with sonic Mach number of 3 and plasma beta of 16. We find that two basic ingredients are needed for electron entropy production: (1) an electron temperature anisotropy, induced by field amplification coupled to adiabatic invariance; and (2) a mechanism to break the electron adiabatic invariance itself. In shocks, field amplification occurs at two major sites: at the shock ramp, where density compression leads to an increase of the frozen-in field; and farther downstream, where the shock-driven proton temperature anisotropy generates strong proton cyclotron and mirror modes. The electron temperature anisotropy induced by field amplification exceeds the threshold of the electron whistler instability. The growth of whistler waves breaks the electron adiabatic invariance and allows for efficient entropy production. For our reference run, the postshock electron temperature exceeds the adiabatic expectation by , resulting in an electron-to-proton temperature ratio of . We find that the electron heating efficiency displays only a weak dependence on mass ratio (less than drop, as we increase the mass ratio from up to ). We develop an analytical model of electron irreversible heating and show that it is in excellent agreement with our simulation results.