Field‐aligned currents in the inner magnetosphere: Control by geomagnetic activity

The occurrence rate of field‐aligned currents measured in the Earth's inner magnetosphere by ISEE 1 and 2 is examined for three levels of geomagnetic activity, | AL | ≤ 35 nT, 35 nT < | AL |≤ 135 nT, and | AL | > 135 nT, using observations from 180 orbits during the period from January 1978 to February 1980. Maps of the occurrence rate of these currents in L ‐value/LT space are provided at each of the three levels of activity. There is a clear distinction of the field‐aligned current (FAC) occurrence rate between the three levels of activity. At low activity, FACs are observed at high L values and near dawn/dusk and midnight regions, with the dawn/dusk FACs perhaps being associated with the viscous solar wind‐magnetospheric interaction along the flanks of the magnetopause. At intermediate levels of activity, FACs are observed with greater frequency at lower L values and throughout a larger range of local times, indicating internal stress buildup in the magnetosphere. When auroral activity levels are high, magnetospheric internal stresses become even greater, leading to widespread occurrence of FACs in the Earth's inner magnetosphere. Field‐aligned currents in the magnetosphere show as much range in strength as the currents flowing in the auroral ionosphere and vary in concert with these currents.