Abstract
A topside sounding experiment by the PPS system on board the EXOS-C (Ohzora) satellite made successful observations in the polar ionosphere. From analyses of the sonograms the following results have been obtained.
i) Associated with the particle precipitation into the polar cusp region, small scale irregularities causing the spread F phenomena with electron density enhancement have been generated.
ii) In the polar cap ionosphere, there are also small-scale irregularities of the electron density causing the spread F phenomena. These irregularities have also been associated with an enhancement of the electron density. The origin of these irregularities in the polar cap is in the polar cusp region, and the irregularities are transported into the polar cap region by convection from the cusp region.
iii) In the midnight polar cap region, an electron density depression region, called a plasma hole, has been sometimes observed. The magnetic activity index corresponding to detection of the plasma hole always indicates a very low value. This is interpreted to occur as the result of a reduced plasma convection in the polar cap region. While the polar cap plasma escapes continuously due to the effect of the polar wind and losses through the attachment process, the supply is insufficient at the ionospheric level with the lack of convection, causing the plasma hole.
