This paper was published as Journal of Atmospheric and Solar-Terrestrial Physics, 1998, 60 (6), pp. 617-629. It is available from http://www.sciencedirect.com/science/journal/13646826. Doi: 10.1016/S1364-6826(98)00005-4Metadata only entryFour high frequency propagation paths were provided by a transmitter located within the polar cap and four receivers located variously within the polar cap and at sub-auroral latitudes. Of these paths, one was contained entirely within the polar cap at all times, two were trans-auroral at all times, and one varied from trans-auroral during the day to polar cap during the night. Fourteen frequencies within the HF band were transmitted each hour for the duration of two 24 day experimental campaigns during the summer of 1988 and the winter of 1989. The received signals were analysed to determine signal recognition and signal strength. The mean quiet-time behaviour of the propagation was determined and compared with four periods during which the ionosphere was perturbed by (relatively mild) geomagnetic storm conditions. The daily occurrence of correctly recognised signals decreased during the disturbed periods, both on polar cap and trans-auroral paths. Several geophysical causes for this decrease are identified, including a decrease in F-region electron density, decreasing the MOF, as a consequence of negative storm effects and an extended mid latitude trough. On polar cap paths, an increase in the LOF caused by mild PCA events also decreased the available HF band. Storm-time auroral-E makes propagation possible on frequencies higher than expected during the night on trans-auroral paths, mitigating to some extent the degradation in propagation. The storm-time decrease in the available HF band is quantified, falling to 0.3–0.5 of undisturbed levels even during relatively small geomagnetic storms. During the two experimental campaigns, 50% of days are identified as being storm-disturbed
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