STATISTICAL NATURE OF THE MAGNETOTAIL CURRENT IN THE NEAR-EARTH REGION

ISEE 1 magnetic field data for 1978-1987 are examined to obtain the av erage distribution of the magnetotail current in the region -20 less t han or equal to X less than or equal to-6 R-E, /Y/ less than or equal to 7 R-E. The whole data set is divided into three subsets according t o different geomagnetic conditions, that is, AL greater than or equal to 50 nT, -200 less than or equal to AL<-50 nT, and AL<-200 nT. Magnet ic field values are averaged in each cell with a volume of 1x1x1 R-E(3 ), and the corresponding current densities are calculated from the ave rage tail field. The radial distribution of the neutral sheet current for disturbed periods is characterized by intense currents near geosta tionary altitudes. In the midnight sector, it reaches as large as simi lar to 140 mA/m, twice that for quiet periods. It is found that the ne utral sheet current is maximized in the region -3<Y<6 R-E; exhibiting clear dawn-dusk asymmetry. There is no evident indication for the curr ent disruption in the near-Earth magnetotail, except for a similar to 20 mA/m depression in the current intensity for disturbed periods near the midnight meridian within X similar to-7 R-E. Currents parallel to the average magnetic field are also derived. They are regarded as the average field-aligned currents (FACs) in this paper. It is shown that irrespective of variable geomagnetic activity, large-scale FACs are d istributed in the conventional pattern, that is, region 1 and region 2 currents. The total intensity of FACs in the 2100-0300 MLT sector is found to increase rapidly with increasing X in the region X>-15 R-E. T his implies that the main source of FACs in the midnight sector is loc ated in the near-Earth magnetotail. A new model of the magnetotail cur rent system for disturbed periods is proposed on the basis of our find ings.