High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements
High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements
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来自原位磁层簇 EDI 测量的北向 IMF 期间的高纬度等离子体对流
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
C. Kletzing
中科院分区：
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作者：
M. Förster;S. Haaland;G. Paschmann;J. Quinn;R. Torbert;H. Vaith;C. Kletzing
在这项研究中，我们研究了北向IMF期间高纬度对流单元结构的统计、系统变化。利用六年半（2001 年 2 月至 2007 年 7 月）对北极和南极盖区上空的 Cluster/EDI 电子漂移观测进行 1 分钟平均值，并将空间分布的测量结果映射到磁纬度/MLT 网格中电离层水平的公共参考平面，我们根据各种 IMF 条件获得了规则的漂移模式。我们重点关注向北 IMF 期间的特殊条件，其中磁纬度 >80° 的叶单元以及中央极冠上空的相反（向阳）对流是除了低纬度主要对流单元之外的永久特征。它们是由于尖点区域磁层顶边界极向的重联过程造成的。绘制的 EDI 数据在极冠的中心部分具有特别好的覆盖范围，因此这些模式及其对各种太阳风条件的依赖性在统计意义上得到了很好的验证。平均而言，在 IMF 近北方向期间，4 单元对流模式显示为规则结构，并有向负时钟角小幅偏移的趋势。这些高纬度对流焦点的位置在磁纬79°至85°和MLT 09:00–15:00之间。 MLT 位置大约在 11:30 MLT 左右对称 ±2 小时，即从中午到中午前略有偏移，而高纬度单元的最大（最小）电势位于较高磁纬度，接近其最大电势差，北（南）半球时钟角为 ≈−10° 至 −15°。随着距 ≈IMF B z + 的钟角距离的增加，从 4 单元型经由 3 单元逐渐过渡到常见的 2 单元对流型，在此过程中，其中一个中等规模的高纬度日侧单元减少并消失，而另一个则增强并在接近众所周知的 IMF B y 主导配置时与相反的同极性主单元合并，形成大型“圆形”对流单元。在时钟角度相反转动的情况下以及在南半球，会发生相反的情况，各个细胞的角色互换。随着 IMF 矢量幅度的增加，高纬度日侧单元变得更加明显。
In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes >80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00–15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ≈−10° to −15° clock angle for the North (South) Hemisphere. With increasing clock angle distances from ≈IMF B z +, a gradual transition occurs from the 4-cell pattern via a 3-cell to the common 2-cell convection pattern, in the course of which one of the medium-scale high-latitude dayside cells diminishes and disappears while the other intensifies and merges with the opposite main cell of the same polarity to form the large "round-shaped" convection cell when approaching a well-known IMF B y -dominated configuration. Opposite scenarios with interchanged roles of the respective cells occur for the opposite turning of the clock angle and at the Southern Hemisphere. The high-latitude dayside cells become more pronounced with increasing magnitude of the IMF vector.