Modelling studies of ionospheric convection in northern and southern polar regions

The realistic model of Quegan et al. has been used to investigate the convection paths of ionospheric plasma at 300 km altitude, for different polar cap radii and in both hemispheres. Taking the Northern magnetic dip pole to be at a co-latitude of 11° and the Southern magnetic dip pole at a co-latitude of 23°, these paths are presented in a Sun-Earth frame, with the position of the Earth's axis fixed as it is on 21 March, as polar plots centred on the magnetic pole. There are marked hemispheric differences between 13 and 23 L.T., particularly near the stagnation region at 18 to 21 L.T., but only minor differences between 00 and 12 L.T., when the radius of the polar cap exceeds 12°. For a smaller polar cap, the differences between the hemispheres are small at all local times. The time taken to perform a complete circuit is most dependent on the polar cap radius, and most variable - between 15 and 36 h - for convection paths starting near 60° latitude. The time that plasma convecting from noon to near midnight across the Northern polar cap spends within the 10° co-latitude circle increases from 6 h, for a polar cap radius of 10°, to 11.5 h at 17°. These results are compared and contrasted with other model calculation results and with some ground-based and satellite observations of plasma densities at high latitudes.