Instability of toroidal magnetic field in jets and plerions
Instability of toroidal magnetic field in jets and plerions
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DOI：
10.1086/305119
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发表时间：
1998-01-20
影响因子：
4.9
通讯作者：
Begelman, MC
中科院分区：
文献类型：
--
作者：
Begelman, MC
天体物理喷流和脉冲星供给的超新星遗迹（plerions）预计将发展高度有组织的磁结构为主的同心环的环形举行，B-phi。有人认为，这种结构可以解释一些射流的偏振特性，并有助于通过磁张力的横向约束。同心环向场几何也是Rees-Gunn蟹状星云模型的核心，蟹状星云是典型的plerion，并导致蟹状星云脉冲星的风必须具有弱磁场的推论。然而，众所周知，磁压力和气体压力之间的这种平衡，受控聚变文献中的“平衡Z箍缩”，容易受到破坏性局部不稳定性的影响，即使当磁场较弱和/或边界条件（例如，致密的外部介质）减慢或抑制全局模式。为了确定天体物理条件下Z箍缩不稳定性的增长率，我导出了一个色散关系，它适用于相对论流体的喷流和plerions可能组成的，在理想的磁流体力学（MHD）的限制。主要的不稳定性是扭结（m = 1）和捏（m = 0）模式。前者通常占主导地位，破坏同心场结构，并可能将系统推向更混乱的状态，其中平均场强与半径无关（并且场的电阻耗散可能会增强）。我估计的时间尺度上的场结构很可能被重新安排，并与这些距离沿着相对论射流和半径从中央脉冲星在plerion.I的结论是，中心宗旨的Rees-Gunn模型的蟹状星云，存在一个同心环形举行以及脉冲星风的终止冲击波之外，是物理上不现实的。在这个假设消失后，就没有动力学的理由来断定脉冲星风携带的磁能通量比动能通量弱得多。放弃里斯和古恩的主要结论将解决脉冲星风理论中一个长期存在的难题。
Astrophysical jets and pulsar-fed supernova remnants (plerions) are expected to develop highly organized magnetic structures dominated by concentric loops of toroidal held, B-phi. It has been argued that such structures could explain the polarization properties of some jets and contribute to their lateral confinement through magnetic tension forces. A concentric toroidal field geometry is also central to the Rees-Gunn model for the Crab Nebula, the archetypal plerion, and leads to the deduction that the Crab pulsar's wind must have a weak magnetic field. Yet this kind of equilibrium between magnetic and gas pressure forces, the "equilibrium Z-pinch" of the controlled fusion literature, is well known to be susceptible to disruptive localized instabilities, even when the magnetic field is weak and/or boundary conditions (e.g., a dense external medium) slow or suppress global modes. Thus, the magnetic field structures imputed to the interiors of jets and plerions are unlikely to persist for very longTo determine the growth rates of Z-pinch instabilities under astrophysical conditions, I derive a dispersion relation that is valid for the relativistic fluids of which jets and plerions may be composed, in the ideal magnetohydrodynamics (MHD) limit. The dominant instabilities are kink (m = 1) and pinch (m = 0) modes. The former generally dominate, destroying the concentric field structure and probably driving the system toward a more chaotic state in which the mean field strength is independent of radius (and in which resistive dissipation of the field may be enhanced). I estimate the timescales over which the field structure is likely to be rearranged and relate these to distances along relativistic jets and radii from the central pulsar in a plerion.I conclude that the central tenet of the Rees-Gunn model for the Crab Nebula, the existence of a concentric toroidal held well outside the pulsar wind's termination shock, is physically unrealistic. With this assumption gone, there is no dynamical reason to conclude that the magnetic energy flux carried by the pulsar wind is much weaker than the kinetic energy flux. Abandoning the principal conclusion of Rees & Gunn would resolve a long-standing puzzle in pulsar wind theory.