Quiet time plasma sheet ion pressure contribution to Birkeland currents


  • Simon Wing,

  • Patrick T. Newell


Birkeland currents transport magnetic tangential stress resulting from J × B forces, which, in the plasma sheet, are balanced by the pressure gradient, ∇p. However, derivation of nightside Birkeland currents from ∇p observationally has not been possible because pressure must be known everywhere in the plasma sheet at high resolution, which in situ satellites have been unable to provide. Recently, a method of inferring plasma sheet temperature, density, and pressure from low-altitude satellites was developed. The quiet time Birkeland currents (or J//) are computed from the pressure profile derived from DMSP F8, F9, F10, and F11 data for the year 1992 and magnetic field from a modified Tsyganenko [1989] magnetic field model. Our results show that (1) the region l Birkeland currents exhibit a dawn-dusk asymmetry which can be explained by the dawn-dusk asymmetry in the plasma pressure arising if the near-Earth plasma sheet ions are supplied largely by the deep tail plasma sheet and LLBL ions undergoing E × B earthward and gradient/curvature duskward motions; (2) the average region 1 J// is −0.6 and 0.7 nA/m2 near the neutral sheet (negative current density indicates the currents flow out of the ionosphere and positive means into the ionosphere); and (3) the current system tailward/poleward of the region 1 has the opposite polarities from those in region 1 and apparently is generated from the midnight pressure maximum. These results are fairly consistent with those previously obtained with in situ magnetometer measurements.