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 m agnetospheric plasma environment and field-aligned currents (FACs) dur ing prolonged geomagnetic quiescence (K-p = 0) associated with extreme ly low values of the interplanetary magnetic field (\B-y\ less than or equal to 1.5 nT, -0.5 nT less than or equal to B-z less than or equal to 1.5 nT). We refer to this as the ''ground state'' of the magnetosp here. The large-scale FACs exhibit the traditional region 1/region 2 c urrent pattern and the dayside region 0 (traditional cusp) current sys tem. The median region 1 current intensity at 835-km altitude is 135 n T (with half of the values contained in the range 126-156 nT) in the 0 800-1600 magnetic local time (MLT) region excluding the cusp. These ar e much smaller than the region 1 peak intensities of 740 nT for the \A L\ greater than or equal to 100 nT case and 350 nT for the \AL\ < 100 nT case as statistically determined by lijima and Potemra [1978], In a ddition, the region 1/region 2 FAC systems tend to degenerate into sma ller-scale FACs at local times separated from the dayside toward morni ng 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. T he region 0 and region 1 currents on the dayside extend in the azimuth al direction, are balanced in current intensity with a typical value o f 140 nT in the 0915-1000 MLT sector, and are collocated with boundary layer particles. These observations have provided the first opportuni ty to study the basic flow patterns of field-aligned currents during t he magnetospheric ground state.