Universal time effects on plasma convection in the geomagnetic frame

Abstract

Coordinate frames for description of plasma convection in the ionosphere at high latitudes, together with the transformations between the frames, are presented for a centred dipole magnetic field. The Earth's corotation velocity is shown to depend on universal time in the geomagnetic frame; the dependence is most marked at the solstices. The UT-dependent corotation electric field (in the geomagnetic frame) is added to a magnetospheric convection electric field model that is UT-independent in the geomagnetic frame and plasma velocities are calculated. Under these conditions, the plasma velocity observed in the magnetic frame sufficiently poleward of the plasmapause does not vary significantly with UT. In the transplasmapause region where the convection and corotation electric fields become comparable, the plasma velocity at a point varies markedly with UT. The resulting plasma trajectories, along a fixed equipotential, have markedly different shapes if they start at different universal times. Trajectories starting from the same location but at different times can be very different. The UT variations can cause the plasma to make multiple traversais of the polar cap boundary and can cause the formation of eddies in convection paths, thus affecting plasma densities; there may be UT-dependence of the latitude of the dusk and evening mid-latitude trough. The ‘last closed equipotential’ is time-dependent even for a steady magnetospheric convection electric field. Thus the plasmapause may be located in a region of plasma with behaviour intermediate between that in the plasmasphere and that at higher latitudes, even under steady conditions. Plasmatails may form even in the absence of a disturbance. The conclusions are not critically dependent on the forms assumed for the convection electric field and for the magnetic field.