Cosmic noise absorption at South Pole Station during magnetic impulse events

Abstract

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 occurred 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 magnetic local time (MLT) and 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 farther from SP Station, and thus out of the range of the riometer measurement, than those which show absorption. This is suggested by the distribution of their H component values which peaks at 40–70 nT, the lowest‐amplitude range considered. The high percentage of absorption/MIE coincidences indicates that the MIE mechanism often leads to the precipitation of energetic electrons but that on rare occasions may result in the temporary reduction or cessation of preexisting precipitation.