空间各向异性密度结构驱动的韦伯不稳定性
Weibel instability driven by spatially anisotropic density structures
摘要：
对伽马射线暴余辉的观测表明（伽玛暴），冲击后的磁场被强烈放大到冲击压缩值的100倍左右。韦伯不稳定性在产生磁场中起着重要的作用。然而，最近对均匀等离子体中无碰撞冲击的模拟表明，由韦伯不稳定性产生的磁场迅速衰减。在星际和星周介质中一定存在着密度涨落。在相对论激波的下游区域，密度起伏被各向异性压缩。本文利用二维粒子模拟方法研究了具有空间各向异性密度分布的正负电子等离子体中的韦伯不稳定性。我们发现，大的磁场保持较长的时间由空间各向异性密度结构驱动的韦伯不稳定性。粒子各向异性地从高密度区逃逸，从而产生温度各向异性，使韦伯不稳定性变得不稳定。我们的模拟结果表明，由各向异性密度结构驱动的韦伯不稳定性可以产生足够大的磁场，它们可以覆盖足够大的区域来解释伽玛暴的余辉发射。
英文摘要：
Observations of afterglows of gamma-ray bursts suggest (GRBs) that post-shock magnetic fields are strongly amplified to about 100 times the shock-compressed value. The Weibel instability appears to play an important role in generating of the magnetic field. However, recent simulations of collisionless shocks in homogeneous plasmas show that the magnetic field generated by the Weibel instability rapidly decays. There must be some density fluctuations in interstellar and circumstellar media. The density fluctuations are anisotropically compressed in the downstream region of relativistic shocks. In this paper, we study the Weibel instability in electron--positron plasmas with the spatially anisotropic density distributions by means of two-dimensional particle-in-cell simulations. We find that large magnetic fields are maintained for a longer time by the Weibel instability driven by the spatially anisotropic density structure. Particles anisotropically escape from the high density region, so that the temperature anisotropy is generated and the Weibel instability becomes unstable. Our simulation results suggest that the Weibel instability driven by an anisotropic density structure can generate sufficiently large magnetic fields and they can cover sufficiently large regions to explain the afterglow emission of GRBs.
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