Top side ionospheric spread F and particle precipitation in the day side magnetospheric clefts

At high latitudes, top side ionospheric spread F is observed over a very large latitude range. Within this spread F region there is a severe top side irregularity zone (stiz) in which irregularities have larger amplitudes than elsewhere. Top side ionograms obtained within the stiz exhibit very severe frequency spread (≳1 MHz) at zero range. The stiz occurs up to altitudes of at least 3000 km. Its equatorward edge is quite abrupt and is a good reflector of HF radio waves. Echoes from the stiz boundary have been observed by satellites as far as 2500 km equatorward. Comparisons of simultaneous top side sounder and particle data indicate that on the day side of the earth the equatorward boundaries of the stiz and of ≲ 300-eV electron precipitation coincide. It is clear that the ionizing and heating effects of the particle precipitation produce irregularities, but other processes may also contribute. Usually, the stiz extends beyond the poleward boundary of the particle precipitation. This feature is probably due to magnetospheric convection transporting irregularities poleward of the region of production.