Effect of upstream rotational field on the formation of magnetic depressions in a quasi-perpendicular shock downstream
Effect of upstream rotational field on the formation of magnetic depressions in a quasi-perpendicular shock downstream
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DOI：
10.1029/2004ja010818
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发表时间：
2005-04
影响因子：
--
通讯作者：
K. Tsubouchi;H. Matsumoto
中科院分区：
文献类型：
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作者：
K. Tsubouchi;H. Matsumoto
[1]利用一维混合模拟方法研究了行星际旋转磁场与地球弓形激波（准垂直和超临界状态）之间的相互作用。在进入磁鞘后，与密度峰值相关的磁凹陷结构被称为瞬态密度事件（TDE），并从RF区域扩大。相反的MHD的观点，其中这样的抗磁结构是夹在两个缓慢的或随时间变化的中间冲击，TDE产生机制是强烈相关的粒子动力学的影响。在TDE中，平行于环境磁场的质子温度Tp <$p增加到各向同性，而下游的普通激波由强各向异性质子组成（Tp <$p/Tp <$p> 1）。这种平行加热是由于施加的RF将垂直质子运动强制转换为平行运动。由此产生的强烈的平行/反平行流在前缘和后缘处产生场梯度，其充当镜像力并降低磁场强度。在这种动力学意义上，由MHD理论预测的慢模或阿尔芬模不足以被视为这种结构的边缘。与正常镜像模式过程类似，粒子被集中并被捕获在弱场区域内，导致密度累积。被捕获的粒子失去能量，并在发散的镜面点上反弹时冷却。因此，TDE结构几乎不塌陷，并通过磁鞘持续存在。与镜像不稳定性相比，外场涨落强迫的各向同性更能有效地产生这种磁凹陷。因此，准垂直激波的存在下，大的温度各向异性的产生，是一个合适的情况下的TDE时，与旋转场的相互作用已经发生。这种相互作用模型也适用于太阳风中磁洞的形成机制。
[1] One-dimensional hybrid simulations are performed to investigate the interaction between an interplanetary rotational magnetic field (RF) and the terrestrial bow shock (a quasi-perpendicular and supercritical regime). A magnetic depression structure called a transient density event (TDE) that is anticorrelated to the density peak is built up and is enlarged from the RF region after its entry into the magnetosheath. Contrary to the MHD view, in which such a diamagnetic structure is sandwiched by two slow or time-dependent intermediate shocks, the TDE generation mechanism is strongly associated with effects of particle kinetics. Within the TDE, the proton temperature parallel to the ambient magnetic field Tp∥ increases to be isotropic, while the ordinary shock downstream consists of strong anisotropic protons (Tp⊥/Tp∥ > 1). This parallel heating is due to enforced conversion of the perpendicular proton motion into a parallel one by the imposed RF. The resultant intense parallel/antiparallel flows generate the field gradient at the leading and trailing edges, which act as a mirror force and reduce the magnetic intensity. In this kinetic sense, slow or Alfven modes predicted by MHD theories are inadequate for regarding as edges of such a structure. Similar to the normal mirror mode process, particles are concentrated and trapped within the weak field region, leading to a density buildup. The trapped particles lose their energy and undergo cooling as they are bounced at the diverging mirror points. Accordingly, the TDE structure hardly collapses and endures through the magnetosheath. Compared with the mirror instability, isotropization forced by external field fluctuations works more efficiently to produce such a magnetic depression. Thus the presence of the quasi-perpendicular shock, where large temperature anisotropy is generated, is one of the suitable situations for the TDE when interaction with the rotational field has taken place. This interaction model may also be applicable to the magnetic hole formation mechanism in the solar wind.