The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis
The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis
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DOI：
10.5194/angeo-22-3039-2004
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发表时间：
2004-09
影响因子：
1.9
通讯作者：
B. Lavraud;T. Phan;M. Dunlop;M. Taylor;P. Cargill;J. Bosqued;I. Dandouras;H. Rème;J. Sauvaud;C. Escoubet;A. Balogh;A. Fazakerley
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作者：
B. Lavraud;T. Phan;M. Dunlop;M. Taylor;P. Cargill;J. Bosqued;I. Dandouras;H. Rème;J. Sauvaud;C. Escoubet;A. Balogh;A. Fazakerley
我们报告的观察三个高海拔的尖点交叉的集群航天器稳定向北IMF条件下。本研究的重点是外尖点及其边界。在极向边缘的尖点，大的向下喷流存在，他们的特点是由对流速度的黎明-黄昏分量相反的IMF B y方向和逐渐演变（速度过滤器效应）对应的注射部位位于高纬度磁层顶尾部的尖点，随后向日对流。当从极向边缘移动到外部尖点时，由于上述等离子体流的镜像和散射，等离子体逐渐变得停滞。即使当IMF B y很大且钟角约为90°时，在这里研究的所有事件中都发现了这样一个停滞区（停滞外尖点：SEC）的存在。SEC磁鞘边界出现作为一个空间结构，具有指向内的磁场的正常分量，根据该地区和磁鞘（与北向场）之间的可能的连接。这个边界通常有一个缓慢的deHoffmann-Teller速度，并且朝向太阳和向下，与从高纬度磁层顶附近的位置传播的不连续性兼容。虽然切向应力平衡并不总是满足，SEC磁鞘边界可能是一个旋转的不连续性。就在这个边界之外，存在明显的亚阿尔芬等离子体耗尽层（PDL）。这些结果都与尖点尾部高纬磁层顶存在一个近稳定重联点的事实相一致。我们建议，外部不连续性（和整个区域）的稳定性是由亚Alfvenic PDL的存在保持。然而，检查的电子数据显示，存在的加热电子传播平行于磁场（向上）外的SEC磁鞘边界。这似乎与它们的来源是北方叶重联网站不一致。最后，在高纬度磁层顶的定义是重新审视。将SEC-磁鞘边界定义为磁层顶将导致对“外尖点”的错误命名。
We report on the observation of three high-altitude cusp crossings by the Cluster spacecraft under steady northward IMF conditions. The focus of this study is on the exterior cusp and its boundaries. At the poleward edge of the cusp, large downward jets are present; they are characterized by a dawn-dusk component of the convection velocity opposite to the IMF B y direction and a gradual evolution (velocity filter effect) corresponding to an injection site located at the high-latitude magnetopause tailward of the cusp, with subsequent sunward convection. As one moves from the poleward edge into the exterior cusp proper, the plasma gradually becomes stagnant as the result of the mirroring and scattering of the aforementioned plasma flows. The existence of such a stagnant region (Stagnant Exterior Cusp: SEC) is found in all events studied here even when the IMF B y is large and the clock angle is ~90°. The SEC-magnetosheath boundary appears as a spatial structure that has a normal component of the magnetic field pointing inward, in accordance with a probable connection between the region and the magnetosheath (with northward field). This boundary generally has a deHoffmann-Teller velocity that is slow and oriented sunward and downward, compatible with a discontinuity propagating from a location near the high-latitude magnetopause. Although the tangential stress balance is not always satisfied, the SEC-magnetosheath boundary is possibly a rotational discontinuity. Just outside this boundary, there exists a clear sub-Alfvenic plasma depletion layer (PDL). These results are all consistent with the existence of a nearly steady reconnection site at the high-latitude magnetopause tailward of the cusp. We suggest that the stability of the external discontinuity (and of the whole region) is maintained by the presence of the sub-Alfvenic PDL. However, examination of the electron data shows the presence of heated electrons propagating parallel to the magnetic field (upward) just outside of the SEC-magnetosheath boundary. This appears inconsistent with their source being the northern lobe reconnection site. Finally, the definition of the magnetopause at high latitudes is revisited. To define the SEC-magnetosheath boundary as the magnetopause would lead to the misnaming of the "exterior cusp".