ENHANCED MUF PROPAGATION OF HF RADIO-WAVES IN THE AURORAL-ZONE

Four high frequency propagation paths were monitored from a transmitte r located within the polar cap by four receivers located variously wit hin the polar cap and al sub-auroral latitudes. Of these paths, one wa s contained entirely within the polar cap at all times, two were trans -auroral at all times, and one varied from trans-auroral during the da y to polar cap during the night. Fourteen frequencies within the HF ba nd were transmitted each hour for the duration of two 24 day experimen tal campaigns during the summer of 1988 and the winter of 1989. From a n analysis of the received signals the confidence of signal recognitio n and signal strength were determined. During geomagnetically undistur bed periods the propagation behaviour resembled that of mid-latitude p aths. During geomagnetically disturbed times, however, night-time prop agation occurred on frequencies up to and sometimes over 10 MHz above the undisturbed night-time MUF, for periods of 2 to 6 h. These feature s appeared on the trans-auroral paths only and were attributed to E re gion (and occasionally F region) enhancement by auroral precipitation. APEs (auroral E propagation events) occurred on over 50% of nights. T he occurrence of APEs also coincided with ionospheric storm periods wh en the HF band available for propagation was otherwise significantly n arrowed due to a depletion of the F region electron density. Copyright (C) 1996 Elsevier Science Ltd.