Cosmic noise absorption at South Pole Station during magnetic impulse events

We report the results of an examination of the records of cosmic radio nois e absorption in the ionosphere during times of previously identified magnet ic impulse events (MIEs) at South Pole (SP) Station, Antarctica. Approximat ely 80% of the 153 MIEs that occurred in the interval from January to Decem ber 1986 were accompanied by impulsive increases in riometer absorption; a much smaller fraction (similar to 4%) showed temporary decreases from a pre vailing enhanced level. As is the case for high-latitude MIE events, genera lly, those associated with absorption exhibit a large morning peak at simil ar to 1000 magnetic local time (MLT) and a smaller afternoon peak at simila r to 1400 MLT and occur most commonly at the equinoxes. Approximately 20% o f the MIE events were not accompanied by an absorption response. These case s occurred preferentially in the afternoon hours. MIEs in the no response c lass may have occurred farther from SP Station, and thus out of the range o f the riometer measurement, than those which show absorption. This is sugge sted by the distribution of their H component values which peaks at 40-70 n T, the lowest-amplitude range considered. The high percentage of absorption /MIE coincidences indicates that the MIE mechanism often leads to the preci pitation of energetic electrons but that on rare occasions may result in th e temporary reduction or cessation of preexisting precipitation.