磁星中非轴对称无力磁场的非理想演化
Nonideal evolution of nonaxisymmetric, force-free magnetic fields in a magnetar
摘要：
Braithwaite及其合作者最近的磁流体动力学数值计算支持了有关致密星磁场起源的“化石场”假设，并表明化石场的电阻演化可以解释磁星磁场的重组和衰变。在这里，这些调查结果进行建模分析，允许恒星磁场放松通过准静态序列的非轴对称，无力状态，类比spheromak松弛实验，从一个随机场。在没有电阻率的情况下，无力模式接近能量均分的假设下，数值计算的输出被半定量地恢复：场沉降到一个连接的极线-环形配置，随着时间的推移，它膨胀并变得更加环形。如果磁场根据$α$ -类发电机的平均场机制通过在小尺度和大尺度之间交换螺旋度而演化，则达到定性相似（但不相同）的终态，该机制产生于初始随机场产生的波动电动势。在磁星的背景下，讨论了不可能将无力的内部场与潜在的外部场相匹配。
英文摘要：
Recent numerical magnetohydrodynamic calculations by Braithwaite and collaborators support the `fossil field' hypothesis regarding the origin of magnetic fields in compact stars and suggest that the resistive evolution of the fossil field can explain the reorganisation and decay of magnetar magnetic fields. Here, these findings are modelled analytically by allowing the stellar magnetic field to relax through a quasistatic sequence of nonaxisymmetric, force-free states, by analogy with spheromak relaxation experiments, starting from a random field. Under the hypothesis that the force-free modes approach energy equipartition in the absence of resistivity, the output of the numerical calculations is semiquantitatively recovered: the field settles down to a linked poloidal-toroidal configuration, which inflates and becomes more toroidal as time passes. A qualitatively similar (but not identical) end state is reached if the magnetic field evolves by exchanging helicity between small and large scales according to an $α$-dynamo-like, mean-field mechanism, arising from the fluctuating electromotive force produced by the initial random field. The impossibility of matching a force-free internal field to a potential exterior field is discussed in the magnetar context.
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