In situ observations of reconnection Hall magnetic fields at Mars: Evidence for ion diffusion region encounters
In situ observations of reconnection Hall magnetic fields at Mars: Evidence for ion diffusion region encounters
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DOI：
10.1029/2009ja014544
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发表时间：
2009-11
影响因子：
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通讯作者：
J. Halekas;J. Eastwood;D. Brain;T. Phan;M. Øieroset;R. Lin
中科院分区：
文献类型：
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作者：
J. Halekas;J. Eastwood;D. Brain;T. Phan;M. Øieroset;R. Lin
[1]我们提出了火星全球勘测者测量的双极平面外磁场在火星磁层电流片。这些特征与来自模拟和地面观测的无碰撞磁重联的预测相匹配，并且可以类似地指示差分离子和电子运动以及由此产生的磁X线附近的霍尔电流系统。因此，这些观测结果可能代表了穿过或非常接近火星重联扩散区的情况。在400公里的高度与明确的电流片方向发现的28个事件，26个具有与扩散区附近霍尔场的预期极性一致的磁场。对于这些事件，我们发现霍尔场与主场的平均比值为0.51 ± 0.13，正常场与主场的平均比值（重联率）为0.16 ± 0.09，与地球观测的重联一致。这些事件并不总是与地壳磁场的位置或与IMF反转相关，表明磁场悬垂单独（也许由高太阳风动压增强）可能会产生能够重新连接的电流片。对于某些事件，我们观察到场对准的电子，可能携带关闭霍尔电流回路的平行电流。电流片周围的电子分布通常表明与碰撞散逸层的磁连接。对于X线向太阳的交叉点，我们通常在电流片处观察到电子通量最小值，这与由此产生的闭合磁结构一致。对于十字路口antisunward的X线，我们没有观察到通量最小值，一致的领域线开放下游。无碰撞重连，如果在火星上很常见，可能代表着一个重大的大气损失过程。
[1] We present Mars Global Surveyor measurements of bipolar out-of-plane magnetic fields at current sheets in Mars' magnetosphere. These signatures match predictions from simulations and terrestrial observations of collisionless magnetic reconnection, and could similarly indicate differential ion and electron motion and the resulting Hall current systems near magnetic X lines. Thus, these observations may represent passages through or very near reconnection diffusion regions at Mars. Out of 28 events found at 400 km altitude with well-defined current sheet orientations, 26 have magnetic fields consistent with the expected polarities of Hall fields near diffusion regions. For these events, we find an average ratio of Hall field to main field of 0.51 ± 0.13, and an average ratio of normal to main field (reconnection rate) of 0.16 ± 0.09, consistent with terrestrial observations of reconnection. These events do not consistently correlate with the location of crustal fields or with IMF reversals, indicating that magnetic field draping alone (perhaps enhanced by high solar wind dynamic pressure) may generate current sheets capable of reconnection. For some events, we observe field-aligned electrons that may carry parallel currents that close the Hall current loop. Electron distributions around current sheets often indicate magnetic connection to the collisional exosphere. For crossings sunward of the X line, we usually observe an electron flux minimum at the current sheet, consistent with the resulting closed magnetic structure. For crossings antisunward of the X line, we do not observe flux minima, consistent with field lines open downstream. Collisionless reconnection, if common at Mars, could represent a significant atmospheric loss process.