AURORAL AND POLAR-CAP IONOSPHERIC EFFECTS ON RADIO PROPAGATION

Disturbances in the auroral and polar-cap ionosphere can have profound effects on radio signals traversing the high-latitude ionosphere (defined here as that latitudinal region poleward of approximately 55-degrees corrected geomagnetic latitude). Some effects of the high latitude ionosphere on polar radio paths were documented as early as the 1930's, but intensive investigations of this ionospheric global region started during the International Geophysical Year (IGY) 1957-1959. Starting in the mid-1960's it was realized that high latitude ionospheric anomalies could, in fact, affect radio signals from ELF through VHF on terrestrial and earth-space paths, especially during sunspot maximum, disturbed periods. A considerable effort has been made in the last two decades to model and predict the salient parameters of the auroral and polar-cap ionosphere. A somewhat lesser effort has been devoted to develop HF propagation programs which included high-latitude ionospheric effects. The purpose of this paper is to briefly review salient past results and present new developments in this field for the period 1970-1991.