Auroral radio absorption as an indicator of magnetospheric electrons and of conditions in the disturbed auroral D-region

Abstract

In a previous paper we demonstrated a method by which the auroral radio absorption measured with a riometer can be predicted from energetic electron measurements at geosynchronous orbit. The present paper enquires to what extent the process can be inverted: what levels of magnetospheric electron flux, and of D-region production rate, electron density and incremental absorption, are predicted by a given measurement of radio absorption and what reliance can be placed on such predictions? Using data from 45 precipitation features recorded with riometers in Scandinavia and at geosynchronous orbit with GEOS-2, it is shown that electron fluxes in the ranges 20–40,40–80 and 80–160 keV increase with increasing absorption and can be predicted to better than 50% for absorption events of 2 dB or greater. Electrons above 160 keV show little or no correlation with absorption. D-region production rates and electron densities can be predicted to within factors of 2 and √2, respectively. It is more difficult to specify the height of the absorbing region because of uncertainly in the profile of the effective recombination coefficient. Having regard to other data, an α eff profile is proposed which satisfies rocket and incoherent scatter data as well as the present calculations. It is shown that any day-night variation in auroral absorption is associated with a change of spectrum rather than a change of recombination coefficient.