AURORAL ELECTROJET EFFECT ON THE GLOBAL OCCURRENCE PATTERN OF LARGE-SCALE TRAVELING IONOSPHERIC DISTURBANCES

Hourly values of the virtual height enhancements (DELTA-h'F) of the F-region, from 15 ionosonde stations in both hemispheres, were compared with the corresponding hourly values of the auroral electrojet (AE) index for the year of sunspot maximum (1980). It is evident that surges in the auroral electrojet generate trains of large scale travelling ionospheric disturbances (LSTIDs) in both hemispheres, which subsequently propagate equatorwards causing substantial height enhancements (DELTA-h'F) from sub-auroral to equatorial latitudes. A high degree of correlation exists between the current surges (represented by the variability of AE-index) and DELTA-h'F in the night-time sector, with an average correlation coefficient of 0.7 and on some occasions reaching 0.9. The optimum correlation between these two different parameters exists for an average time lag of 1.6 h between the onset of an auroral surge and the corresponding value of DELTA-h'F in mid-latitudes. The ratio of the distance (from the auroral oval to a mid-latitude location) of travel of LSTIDs to the time lag corresponds to an average equatorward velocity (500 m s-1) of the disturbance. The correlation between AE and DELTA-h'F depends strongly on solar illumination condition and is absent in the daytime sector. The amplitude of LSTIDs, as represented by the magnitude of DELTA-h'F, tends to be largest in sub-auroral locations near the source region. It rapidly decreases in mid-latitudes and remains fairly constant from mid to low latitudes, with a slight tendency to increase at the equator.