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° corrected geomagnetic latitude). Some effects of the high-latitude ionosphere on polar radio paths were documented as early as the 1930s, but intensive investigations of this ionospheric global region started during the International Geophysical Year (IGY) 1957-1959. Starting in the mid-1960s it was realized that high-latitude ionospheric anomalies could 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. Salient past results are briefly reviewed, and developments in this field for the period 1970-91 are presented