Current disruption in the Earth's magnetosphere: Observations and models
Current disruption in the Earth's magnetosphere: Observations and models
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DOI：
10.1029/96ja00079
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发表时间：
1996-06
期刊：
影响因子：
--
通讯作者：
A. Lui
中科院分区：
文献类型：
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作者：
A. Lui
简要回顾了地球磁层电流扰动的观测和模型。在接近电流中断开始时，交叉尾电流片显示出电流密度的快速增长，向黄昏离子体速度的大幅飙升接近离子热速度，等离子体压力及其各向同性的增加，等离子体β的上升，以及电流片厚度的减小到与热离子回旋半径相当的长度尺度。在电流中断期间，存在（1）局部磁场和电场的大变化，（2）在宽频率范围内的显著的磁场和电波动，（3）磁场对准的逆流电子束，（4）垂直于磁场的离子束，以及（5）交叉尾电流的减少量与生长阶段期间建立的量相似。观测结果进一步表明，南北磁场分量局部反转的区域不一定是强烈粒子碰撞的场所。中断活动的遥感显示，至少有一些电流中断是不引起的扰动从尾部向地球传播超过10 RE下游。所涉及的时间尺度与离子回旋周期相当或更短。因此，电流扰动具有磁流体机制之外的空间和时间尺度。电流中断的几种模型进行了简要讨论。两个角色被认为是跨场电流不稳定性提出的电流中断。它既可以为传统观点所主张的磁重联提供异常电阻率，也可以在亚暴膨胀的初始阶段单独引起磁层的全球变化。后者的作用是详细说明的不稳定性可能会修改显着的局部电流密度和任何这样的过程将改变力平衡的电流片，并引起一个有效的等离子体和能量传输在全球范围内。此外，这样的过程可以产生场向电流，其强度与亚暴膨胀开始时极光破裂弧的强度相当。这种情况导致了一个新的重点，除了磁重联，快速转换的磁能到粒子的能量在磁尾系统可能会发生没有一个磁性X线或分界线发挥关键作用的能量转换。
Observations and models of current disruption in the Earth's magnetosphere are briefly reviewed. At the approach of current disruption onset, the cross‐tail current sheet shows a rapid growth in the current density, a large upsurge in the duskward ion bulk speed to nearly the ion thermal speed, an increase in the plasma pressure and its isotropy, a rise in the plasma beta, and a decrease in the current sheet thickness to a length scale comparable to the thermal ion gyroradius. During current disruption, there are (1) large changes in the local magnetic and electric fields, (2) significant magnetic and electric fluctuations over a broad frequency range, (3) magnetic field‐aligned counterstreaming electron beams, (4) ion energization perpendicular to the magnetic field, and (5) reduction in the cross‐tail current by an amount similar to that built up during the growth phase. Observations further indicate that regions of local reversal of the north‐south magnetic field component are not necessarily sites of intense particle energization. Remote sensing of disruption activities shows that at least some current disruptions are not caused by a disturbance propagating earthward from the tail beyond 10 RE downstream. The timescale involved is comparable to or shorter than the ion gyroperiod. Current disruption thus has spatial and temporal scales outside the MHD regime. Several models for current disruption are briefly discussed. Two roles are considered for the cross‐field current instability proposed for current disruption. It can provide anomalous resistivity for magnetic reconnection as advocated by the traditional viewpoint or act singly to instigate global changes of the magnetosphere during the initial substorrn expansion phase. The latter role is elaborated by showing that the instability may modify significantly the local current density and any such process will alter the force equilibrium in the current sheet and give rise to an efficient plasma and energy transport on a global scale. Furthermore, such a process can generate field‐aligned current with intensity comparable to those associated with an auroral breakup arc at substorrn expansion onset. This scenario leads to a new emphasis that in addition to magnetic reconnection, rapid conversion of magnetic energy into particle energy in magnetotail systems may take place without a magnetic X line or separatrix playing the key role in energy conversion.