Geotail observations of magnetosheath flow near the magnetopause, using wind as a solar wind monitor
Geotail observations of magnetosheath flow near the magnetopause, using wind as a solar wind monitor
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使用风作为太阳风监测器对磁层顶附近的磁鞘流进行 Geotail 观测
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
S. Kokubun
中科院分区：
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作者：
S. Petrinec;T. Mukai;A. Nishida;Tatsundo Yamamoto;T. Nakamura;S. Kokubun
我们已经研究了几个Geotail通过地球下游的磁鞘区，当风航天器观测到的太阳风条件稳定。我们发现在磁层顶附近，磁鞘等离子体的整体速度强烈地依赖于磁场相对于局部速度矢量的局部方向。在一组观测中，当磁鞘磁场和速度矢量对齐（平行或反平行）时，观测到的磁鞘速度最慢，当磁场垂直于磁鞘速度时，速度更大。然而，磁鞘等离子体的速度没有观察到超过太阳风等离子体。垂直流动的速度增加是由于等离子体的加速，通过磁场线张力，磁鞘等离子体被拖动周围的磁层。磁鞘等离子体速度的最大增强可以通过考虑磁流体动力学（MHD）关系的中间模式来定量估计。一个简单的模型的速度增强的推导，我们发现非常好的协议之间的Geotail磁鞘观测和这个模型。第二组观测结果与第一组相似，除了当磁鞘速度和磁场矢量彼此平行时，磁鞘流高于矢量彼此反平行或垂直时，甚至大于太阳风的速度。这种行为不能用无阻理想MHD理论解释，其他解释进行了调查。
We have examined several Geotail passes through the magnetosheath region downstream of the Earth, when the solar wind conditions as observed by the Wind spacecraft were steady. We have found that near the magnetopause the bulk speed of the magnetosheath plasma is strongly dependent upon the local direction of the magnetic field with respect to the local velocity vector. In one set of observations, the observed magnetosheath speed is slowest when the magnetosheath magnetic field and velocity vectors are aligned (either parallel or antiparallel), and the speed is larger when the magnetic field is oriented perpendicular to the magnetosheath velocity. However, the speed of the magnetosheath plasma is not observed to exceed that of the solar wind plasma. The enhanced speed for perpendicular flows is due to acceleration of the plasma by means of magnetic field line tension, as the magnetosheath plasma is dragged around the magnetosphere. The maximum enhancement in magnetosheath plasma speed can be quantitatively estimated by a consideration of the intermediate mode of the magnetohydrodynamic (MHD) relations. A simple model for the velocity enhancement is derived, and we find extremely good agreement between the Geotail magnetosheath observations and this model. The second set of observations are similar to the first, except that when the magnetosheath velocity and magnetic field vectors are parallel to one another, the magnetosheath flow is higher than when the vectors are antiparallel or perpendicular to each other, and is even larger than the solar wind speed. This behavior cannot be explained by nonresistive ideal MHD theory, and other explanations are investigated.