Field‐aligned current signatures in the near‐tail region: 1. ISEE observations in the plasma sheet boundary layer

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 R XY = ( X² GSM +X² GSM ) 1/2 ≥ 15 Re , and the other is nearer to the Earth, R XY < 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.