DEPENDENCE OF FIELD-ALIGNED CURRENTS ON THE ORIENTATION OF THE INTERPLANETARY MAGNETIC FIELD

Field-aligned currents （FACs） measured in the inner magnetosphere by theISEE-1 and -2 spacecraft are investigated as a function of the Interplanetary Mag-netic Field （IMF） orientation and the level of geomagnetic activity, as indicatedby the AL index. It is found that～75% of the FACs observed are associated withsouthward IMF. The remaining 25% occur during the period of IMF turning fromsouthward to northward- Very few are associated with sustained intervals of north-ward IMF. Approximately 75% of the FACs occur when the AL index is < -150 nT.The current intensity and density are higher for both southward IMF and high gecrmagnetic activity. These results show that the occurrence of FACs within the innermagnetosphere is controlled mainly by the IMF, with the actual occurrence being aconsequence of enhanced electrodynamic coupling between. the solar wind and themagnetosphere- ionosphere system.