MAGNETIC AMPLIFICATION BY MAGNETIZED COSMIC RAYS IN SUPERNOVA REMNANT SHOCKS
MAGNETIC AMPLIFICATION BY MAGNETIZED COSMIC RAYS IN SUPERNOVA REMNANT SHOCKS
复制标题
超新星遗迹激波中磁化宇宙射线的磁放大
DOI：
10.1088/0004-637x/717/2/1054
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
A. Spitkovsky
中科院分区：
文献类型：
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作者：
M. Riquelme;A. Spitkovsky
超新星遗迹激波中同步加速器边缘的X射线观测显示了有效的电子加速和强磁场放大的证据（上游和下游介质之间的系数为100）。这种放大可能是由于激波加速粒子或宇宙射线（CR）驱动的等离子体不稳定性，因为它们在激波之前传播。一个候选过程是由“未磁化”CR的电流引起的宇宙射线电流驱动（CRCD）不稳定性（即，其拉莫尔半径远大于CRCD模式的长度尺度的CR）平行于上游磁场传播。粒子模拟（PIC）已经表明，在星系SNR中，放大场对CR的反作用将限制这种不稳定性的放大因子小于1000（不包括激波处的额外场压缩）。在本文中，我们研究的可能性进一步放大驱动附近的冲击“磁化”的CR，其拉莫尔半径小于长度尺度的字段，以前被放大的CRCD不稳定性。我们发现，额外的放大可以发生由于一个新的不稳定性，驱动的CR电流垂直的领域，我们的术语的垂直电流驱动的不稳定性（PCDI）。我们推导出这种不稳定性的增长率，并使用PIC模拟，研究其非线性演化。我们表明，PCDI的最大放大率是由CR电流的中断，发生时，CR拉莫尔半径在放大的字段变得可比的长度尺度的不稳定性。我们发现，在靠近冲击波的区域，PCDI的增长规模小于CRCD不稳定性的规模，因此，它会导致更大的放大场（放大因子高达1.45）。PCDI的一个可能的观测特征是放大场对激波速度B2 v2sh的特征依赖性，这与CRCD不稳定性单独作用的B2 v3sh形成对比。我们的研究结果加强了CR驱动SNR冲击中观察到的磁场放大的重要部分的想法。
X-ray observations of synchrotron rims in supernova remnant (SNR) shocks show evidence of efficient electron acceleration and strong magnetic field amplification (a factor of ∼100 between the upstream and downstream medium). This amplification may be due to plasma instabilities driven by shock-accelerated particles or cosmic rays (CRs), as they propagate ahead of the shocks. One candidate process is the cosmic ray current-driven (CRCD) instability caused by the electric current of “unmagnetized” CRs (i.e., CRs whose Larmor radii are much larger than the length scale of the CRCD modes) propagating parallel to the upstream magnetic field. Particle-in-cell (PIC) simulations have shown that the back-reaction of the amplified field on CRs would limit the amplification factor of this instability to less than ∼10 in galactic SNRs (not including the additional field compression at the shock). In this paper, we study the possibility of further amplification driven near shocks by “magnetized” CRs, whose Larmor radii are smaller than the length scale of the field that was previously amplified by the CRCD instability. We find that additional amplification can occur due to a new instability, driven by the CR current perpendicular to the field, which we term the perpendicular current-driven instability (PCDI). We derive the growth rate of this instability and, using PIC simulations, study its non-linear evolution. We show that the maximum amplification of PCDI is determined by the disruption of CR current, which happens when CR Larmor radii in the amplified field become comparable to the length scale of the instability. We find that, in regions close to the shock, PCDI grows on scales smaller than the scales of the CRCD instability, and, therefore, it results in larger amplification of the field (amplification factor up to ∼45). One possible observational signature of PCDI is the characteristic dependence of the amplified field on the shock velocity, B2 ∝ v2sh, which contrasts with the one corresponding to the CRCD instability acting alone, B2 ∝ v3sh. Our results strengthen the idea of CRs driving a significant part of the magnetic field amplification observed in SNR shocks.