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 (K_p = 0) associated with extremely low values of the interplanetary magnetic field (⌊B_y⌋ ≤ 1.5 nT, -0.5 nT ≤ B_z ≤ 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 0 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.