D-REGION OBSERVATIONS OF POLAR-CAP ABSORPTION EVENTS DURING THE EISCAT OPERATION IN 1981-1989

During the years 1981-1989, 71 solar proton events altogether were observed. Dividing the events into strong, p.f.u. > 1000 (p.f.u.-proton flux measured at geosynchronous satellite orbit in units of (cm2 s sr)-1), medium, 100 < p.f.u. < 1000 and weak events, p.f.u. < 100, only the strong and medium events have a considerable effect on the lower ionosphere. The mean daily absorption at 30 MHz (A), measured in the auroral zone, is > 2 dB during strong events, < 2 dB during medium events and < 1 dB during weak events. The most active year during the EISCAT operation was 1989 when 23 solar proton events were observed including six strong events. Diurnal variation of the electron density in the D-region during PCA is a function of the solar zenith angle. However, south of L = 5 a minimum in absorption is observed during the noon hours. During sunrise the absorption increases simultaneously with solar elevation angle, but during sunset there is about 2 h delay between the decrease of absorption and the solar elevation angle.