Cosmic Noise Absorption at South Pole Station During Magnetic Impulse Events

We report the results of an examination of the records of cosmic radio noise absorption in the ionosphere during times of previously identified magnetic impulse events (MIEs) at South Pole (SP) station, Antarctica. Approximately 80% of the 153 MIEs that ocurred in the interval from January to December 1986 were accompanied by impulsive increases in riometer absorption; a much smaller fraction (~4%) showed temporary decreases from a prevailing enhanced level. As is the case for high-latitude MIE events generally, those associated with absorption exhibit a large morning peak at 1000 MLT, a smaller afternoon peak at ~ 1400 MLT, and occur most commonly at the equinoxes. Approximately 20% of the MIE events were not accompanied by an absorption response. These cases occurred preferentially in the afternoon hours. MIEs in the no response class may have occurred further from SP station, and thus out of the range of the riometer measurement, than those which show absorption. This is suggested b their distribution of H-component values which peaks at 40-70nT, the lowest amplitude range considered. The high percentage of absorption/MIE coincedences indicates that MIE mechanism often leads to the precipitaion of energy electrons, but that on rare occasions may result in its temporary reduction of cessation.