Electron Heating in Low Mach Number Perpendicular Shocks. II. Dependence on the Pre-shock Conditions
Electron Heating in Low Mach Number Perpendicular Shocks. II. Dependence on the Pre-shock Conditions
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DOI：
10.3847/1538-4357/aab6ad
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发表时间：
2017-12
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，激波后的等离子体是双温的，质子比电子热。在这项工作中，一系列的第二，我们调查的效率不可逆电子加热垂直低马赫数冲击波，通过二维粒子在细胞模拟。我们考虑等离子体β值（热压力和磁压力的比值）在4 μ βp0 μ 32范围内，音速马赫数（激波速度与激波前声速的比值）在2 μ Ms μ 5范围内，适用于星系团激波。如论文中所示，磁场放大诱导的冲击压缩的预冲击场，或由强质子回旋和镜像模式伴随着质子温度各向异性的松弛，可以驱动电子温度各向异性超过阈值的电子哨声不稳定性。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。我们发现，冲击后电子温度Te 2超过绝热预期的量（这里，Te 0是冲击前的温度），它仅微弱地依赖于我们已经探索过的范围4 <$βp0 <$32内的等离子体β，以及质子与电子的质量比（对于我们的基准，测量到的系数为0.044，并且我们估计对于实际质量比，它将降低到0.03）。我们的研究结果具有重要的意义，目前和未来的观测星系团冲击的无线电波段（同步辐射和Sunyaev-Zel'dovich效应）和X射线频率。
Recent X-ray observations of merger shocks in galaxy clusters have shown that the post-shock plasma is two-temperature, with the protons being hotter than the electrons. In this work, the second of a series, we investigate the efficiency of irreversible electron heating in perpendicular low Mach number shocks, by means of two-dimensional particle-in-cell simulations. We consider values of plasma beta (the ratio of thermal and magnetic pressures) in the range 4 ≲ βp0 ≲ 32, and sonic Mach number (the ratio of shock speed to pre-shock sound speed) in the range 2 ≲ Ms ≲ 5, as appropriate for galaxy cluster shocks. As shown in Paper , magnetic field amplification—induced by shock compression of the pre-shock field, or by strong proton cyclotron and mirror modes accompanying the relaxation of proton temperature anisotropy—can drive the electron temperature anisotropy beyond the threshold of the electron whistler instability. The growth of whistler waves breaks the electron adiabatic invariance, and allows for efficient entropy production. We find that the post-shock electron temperature Te2 exceeds the adiabatic expectation by an amount (here, Te0 is the pre-shock temperature), which depends only weakly on the plasma beta over the range 4 ≲ βp0 ≲ 32 that we have explored, as well as on the proton-to-electron mass ratio (the coefficient of ≃0.044 is measured for our fiducial , and we estimate that it will decrease to ≃0.03 for the realistic mass ratio). Our results have important implications for current and future observations of galaxy cluster shocks in the radio band (synchrotron emission and Sunyaev–Zel’dovich effect) and at X-ray frequencies.