Summary
The onset of auroral substorms is invariably associated with the generation of large-scale ionospheric disturbances (LSTID’s) which modify the virtual height (h’F) of the F-region, primarily at sub-auroral locations. Five sub-auroral ionosonde stations, located in three largely different diurnal sectors: Australian-Asian (Hobart-Yakutsk), Indian Ocean (Kerguelen) and European (Uppsala-Dourbes) were used to monitor the aurorally-generated LSTID’s over a sunspot cycle (1980–85). The virtual height enhancements (Dh’F) were analysed in conjunction with auroral substorm onsets deduced from the auroral electrojet index AE. Significant statistical results were obtained from the analysis of auroral-ionospheric data obtained during 473 disturbed days (for AE-index 400nT). It is evident that substorm onsets are most frequent in a time interval: 1100–1900UT, peaking at 1500UT. Since LSTID’s are strongly attenuated during daytime, due to the ion drag, the most advantageous geographical location for monitoring the disturbances is in the Australian-Asian sector which is then well into the night-time sector of the Earth, with the northern station Yakutsk being closest to the auroral source region. The other westward-displaced sectors are progressively less suitable for monitoring of LSTID’s, as the propagation of LSTID’s occurs in the sunlit sector of the Earth. Symmetrical propagation of LSTID’s in both hemispheres is observed mainly in the equinoctial months.