Auroral absorption of cosmic radio noise

Abstract

Inconsistencies in previous work on the association of auroras with ionospheric absorption of cosmic radio noise are discussed and attributed to a number of experimental factors. Such "auroral absorption" events observed at the Australian National Antarctic Research Expeditions station at Mawson may be classified into four types: (i) day-time events, (ii) weak night-time ionospheric absorption, (iii) sudden ionospheric absorption (SIA), (iv) slowly varying ionospheric absorption (SVIA). During SIA events, peaks of absorption and auroral (lambda5577) intensity are simultaneous and the absorption is limited to the luminous regions of the sky. During SVIA events, the absorption is more widespread and there is little correlation between absorption fluctuations and auroral intensity fluctuations; the ratio of absorption to intensity may be of the order of 100 times greater than in SIA events. The variation observed in the absorption/intensity ratio from event to event, and within single events, is attributed to changes in the incident electron energy spectrum. Spectra varying as exp(- e/E) arc considered a.nd a range in beta from 5 to 24 keV is required to explain SIA events; this is in good agreement with rocket data. Consideration of the day/night ratio, two-frequency measurements, and simultaneity of auroral intensity and riometer absorption loads to a lower limit to the height; of the absorbing region varying from 80 to 95 km, depending on the electron energy spectrum. Comparison of riometer and Hbeta photometer records suggests that protons arc important in explaining SVIA events and could in fact be solely responsible.