Low-latitude boundary layer and generation of field-aligned currents

Abstract

Expressions for electric radial and azimuthal currents flowing respectively across and along the low-latitude boundary layer are derived from equation of motion for isotropic plasma. In case of usual simplified suggestions the radial and azimuthal currents are easily expressed in terms of the azimuthal and radial gradients of plasma pressure and velocity in LLBL. Estimations of plasma pressure and velocity gradients, based on the spacecraft measurements, show that radial currents in the morning sector flow from outer border towards the inner one, the current intensity being increased as plasma flows towards the dawn-dusk meridian; in the evening sector their direction is reversed. The inner surface of the boundary layer is such a border where the plasma velocity tangential component changes direction from antisunward in the boundary layer to sunward in the inner magnetosphere. Therefore there are optimal conditions on the inner border for generation of field-aligned currents. The radial currents flowing across the boundary layer are regarded to be closed by the field-aligned currents into the polar cap ionosphere. Increase of the solar wind pressure on the subsolar magnetosphere gives rise to azimuthal gradient of the plasma pressure in LLBL, as a result the intensity of radial currents in LLBL and field-aligned currents (FAC) in region 1 increases. This mechanism easily explains the close connection observed between the polar ionosphere disturbances and variations of the solar wind pressure.