New approach for determining the normal of the bow shock based on Cluster four‐point magnetic field measurements
New approach for determining the normal of the bow shock based on Cluster four‐point magnetic field measurements
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DOI：
10.1029/2006ja011699
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发表时间：
2007-03
影响因子：
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通讯作者：
C. Shen;M. Dunlop;Xinlin Li;Z. Liu;A. Balogh;Tielong Zhang;C. Carr;Q. Shi;Z. Chen
中科院分区：
文献类型：
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作者：
C. Shen;M. Dunlop;Xinlin Li;Z. Liu;A. Balogh;Tielong Zhang;C. Carr;Q. Shi;Z. Chen
我们介绍了一种基于四点磁场测量的新方法来确定地球弓形激波的法线。该方法基于法向与磁压力或磁强度梯度方向相反（激波前缘内）的假设，得到局部弓形激波法向。这个梯度可以从星团的四点磁场测量中推断出来。应用该方法计算了2002年3月至4月和2004年2月至3月28次星系团交叉事件的弓形激波法向，并与常用的最小方差分析和共平面定理进行了比较。此外，我们还比较了在局部平面和边界等速（三角剖分）的假设下，四种航天器定时分析得到的法向。已知最小方差分析常常因为弓形激波的时间变化而失败，共面分析常常因为等离子体扰动导致下游磁场存在超调或测定不佳而失败。我们发现，梯度分析与定时分析最接近，并且，总的来说，得到的激波方向符合预期的弓形激波几何形状。梯度法对拟稳定激波是有效的，它不需要满足弓形激波的共面或等速条件。它的精度取决于这种稳定性和冲击的相对空间尺度，与航天器结构相比，冲击的相对空间尺度应该很大。然而，该方法的一个特点是，它提供了一个正常时间点的估计，允许在穿越每个冲击事件时根据行为监测航天器采样的性质。
[1] We introduce a new approach to determine the normal of Earth’s bow shock, based on four-point magnetic field measurements. The method obtains the local bow shock normal, based on the assumption that the normal is in the opposite direction to the gradient of the magnetic pressure or magnetic strength (within the shock front). This gradient can be deduced from the four-point magnetic field measurements of Cluster. Applying the method, we calculate the normal of the bow shock for 28 Cluster crossing events from March to April of 2002 and February to March of 2004 and compare the results for the boundary orientation to commonly known methods, i.e., minimum variance analysis and the coplanarity theorem. In addition, we compare results of the normal obtained by four spacecraft timing analysis, under the assumptions of local planarity and constant velocity of the boundary (triangulation). It is known that minimum variance analysis often fails because of the temporal variation of the bow shock and coplanarity analysis often fails because of the existence of an overshoot or poor determination of the downstream magnetic field due to plasma disturbance. We find that the gradient analysis agrees most closely with the timing analysis, and, in general, the shock orientations obtained fit the expected bow shock geometry. The gradient method is valid for quasi-stable shocks and does not require the conditions of coplanarity or constant velocity of the bow shock to be satisfied. Its accuracy depends on this stability and the relative spatial scale of the shock, which should be large compared to the spacecraft configuration. A feature of the method, however, is that it provides an estimate of the normal point by point in time, allowing the nature of the spacecraft sampling to be monitored in terms of behavior as each shock event is traversed.