Universal time effect in the occurrences of large-scale ionospheric disturbances

Abstract

The auroral storm-related enhancements in the virtual height ( Δh′ F) of the F-region were analysed for a period of 3 years (1980–1982), using hourly ionosonde data from six sub-auroral stations positioned in a geographic longitude range 4–284°E. The night-time height enhancements, due to the magnetic storm-generated large-scale ionospheric disturbances (LSTIDs), followed (with an average time delay of 0.6 h) peaks of auroral surges deduced from the hourly auroral electrojet ( AE) index. Since the average auroral activity had a distinct diurnal distribution, peaking between 13:00 and 19:00 U.T., the most distinct height rises tended to occur for the stations which were then at night-time, i.e. in the Australian-Asian sector. Other stations, West of this sector and towards the sunlit region of the Earth, showed progressively smaller LSTID activity. Thus, the average height enhancements were smaller in the European sector and were very much reduced in the North American sector which was in full daylight during the period of maximum auroral activity. The largest hemispheric differences in ΔhF in the Australian-Asian sector were in the solstitial periods; the height rises tended to be larger and of a longer duration in the winter hemisphere. In the equinoctial months the height rises in the Australian-Asian sector were approximately of the same magnitude and duration in both hemispheres. The morphology in the occurrence of LSTIDs confirms previous results on the strong effect of solar illumination conditions on the propagation of LSTIDs.