Observations of polar cusp and polar cap ionospheric irregularities and formation of ionospheric holes using topside sounder on board EXOS-C (Ohzora) satellite

Results are presented on the analysis of topside sounding measurements in the polar ionosphere by the PPS instrument on board the EXOS-C (Ohzora) satellite. It is found that, associated with the particle precipitation into the polar-cusp region, small-scale irregularities were generated, causing the spread F phenomena with electron density enhancement. In the polar cap ionosphere, there are also small-scale irregularities of the electron density causing the spread F phenomena, the origin of which is in the polar-cusp region. In the midnight polar cap region, an electron density depression region (a 'plasma hole') was sometimes observed, with a low value of the magnetic activity index. The plasma hole is interpreted to occur as the result of a reduced plasma convection in the polar cap region.