Field‐aligned current signatures in the near‐tail region: 1. ISEE observations in the plasma sheet boundary layer
Field‐aligned current signatures in the near‐tail region: 1. ISEE observations in the plasma sheet boundary layer
复制标题
DOI：
10.1029/ja093ia09p09709
复制
发表时间：
1988-09
影响因子：
--
通讯作者：
S. Ohtani;S. Kokubun;R. Elphic;C. Russell
中科院分区：
文献类型：
--
作者：
S. Ohtani;S. Kokubun;R. Elphic;C. Russell
利用ISEE磁强计的观测数据，研究了近尾区场向电流的特征。我们把注意力集中在磁场方向的阶梯状变化，这被解释为航天器穿越场向电流片。对1979年3月22日两次亚暴的个案研究表明，近尾区的场向电流特征与低海拔观测结果一致，等离子体片边界层中的电流可被认为是大尺度电流系统（区域1系统）的一部分。还研究了场向电流的统计特性。向地流和向尾流倾向于分别在午夜后和午夜前观测到，正如从区域1系统中的电流方向分布所预期的那样。目前的结果似乎与Elphic等人（1985）的结果不一致。这种明显的差异是由于现场对齐的电流签名的识别的差异。他们分析了等离子体片边界层边缘的场向电流，而我们在本分析中研究大尺度电流。因此，它建议，具有相反极性的小规模场向电流嵌入极向的区域1系统。这两项研究的结果被认为是相辅相成的。电流强度与地磁活动和距地径向距离的关系表明，磁尾存在两个场向电流源。一个位于RXY =（X²GSM +X²GSM）1/2 ≥ 15 Re区域，另一个离地球较近，RXY < 15 Re。远源电流持续存在，电流强度几乎与磁层活动无关。另一方面，近地电流源在亚暴期间起着积极的作用。
Signatures of field-aligned currents in the near-tail region are examined by using ISEE magnetometer data. We focus our attention on steplike changes of magnetic field direction, which are interpreted as the spacecraft crossing of field-aligned current sheets. Individual case studies of two substorms on March 22, 1979 (CDAW 6 events), indicate that the field-aligned current signatures in the near-tail region are consistent with observations at lower altitudes and that the currents in the plasma sheet boundary layer can be identified as parts of the large-scale current system, the region 1 system. Statistical properties of the field-aligned currents are also studied. Earthward and tailward currents tend to be observed in postmidnight and in premidnight, respectively, as expected from the distribution of current direction in the region 1 system. The present result seems to be inconsistent with that of Elphic et al. (1985). This apparent discrepancy is due to the difference in identification of field-aligned current signatures. They have analyzed field-aligned currents at the edge of the plasma sheet boundary layer, while we examine large-scale currents in the present analysis. It is thus suggested that small-scale field-aligned currents with opposite polarities are imbedded poleward of the region 1 system. The results of these two studies are considered to be complementary to each other. The dependence of the current intensity on the geomagnetic activity and the radial distance from the Earth indicates that there are two field-aligned current sources in the magnetotail. One is in the region RXY = (X²GSM +X²GSM)1/2 ≥ 15 Re, and the other is nearer to the Earth, RXY < 15 Re. The distant current source persistently exists and the current intensity is almost independent of the activity of the magnetosphere. On the other hand, the near-earth current source plays an active role during substorms.