Field-aligned current systems in the magnetospheric ground state

Using magnetic field or particle data obtained by the Magsat, DMSP-F6, and DMSP-F7 satellites, we have investigated characteristics of the magnetospheric plasma environment and field-aligned currents (FACs) during prolonged geomagnetic quiescence (Kp = 0) associated with extremely low values of the interplanetary magnetic field (|By| ≤ 1.5 nT, −0.5 nT ≤ Bz ≤ 1.5 nT). We refer to this as the “ground state” of the magnetosphere. The large-scale FACs exhibit the traditional region 1/region 2 current pattern and the dayside region 0 (traditional cusp) current system. The median region 1 current intensity at 835-km altitude is 135 nT (with half of the values contained in the range 126–156 nT) in the 0800–1600 magnetic local time (MLT) region excluding the cusp. These are much smaller than the region 1 peak intensities of 740 nT for the |AL| ≥ 100 nT case and 350 nT for the |AL| < 100 nT case as statistically determined by Iijima and Potemra [1978]. In addition, the region 1/region 2 FAC systems tend to degenerate into smaller-scale FACs at local times separated from the dayside toward morning and evening hours. The median region 1 intensity is 70 nT (with one-half the values between 52 and 96 nT) in the 2200–0200 MLT sector. In the prenoon and postnoon sectors, region O currents with intensities comparable to region 1 continue to be paired with region 1 currents. The region 0 and region 1 currents on the dayside extend in the azimuthal direction, are balanced in current intensity with a typical value of 140 nT in the 0915–1000 MLT sector, and are collocated with boundary layer particles. These observations have provided the first opportunity to study the basic flow patterns of field-aligned currents during the magnetospheric ground state.