A global study of large scale travelling ionospheric disturbances (TIDS) Following a step-like onset of auroral substorms in both hemispheres

Abstract

A study has been undertaken on the conjugate effects in the generation and propagation of large scale ionospheric disturbances (LSTIDs), as deduced from the virtual height ( h′ F) rises obtained from ionosonde chains in the Japanese and Australian longitudinal sectors. Hitherto, the results of such studies have revealed the presence of simultaneous trains of LSTIDs propagating at night-time over a large range of southern and northern latitudes, following the onset of step-like auroral particle precipitation events in the conjugate locations. In the present case, the onset of an intense auroral substorm ( K p -index = 7−) occurred in the morning when the Japanese and Australian stations were at different solar illumination conditions unlike in the previous cases when both chains of ionosonde stations were in the night sector. Only the Japanese and other Asian stations which were in the same longitudinal sector and at night-time, showed the presence of pronounced LSTIDs. Simultaneously, all the ionosonde stations positioned in eastern Australia, which were then in daytime, showed that no detectable height rises were present in the sunlit portion of the Earth. The wavefronts of LSTIDs had a large longitudinal extent, spanning the night sectors of the Northern and Southern Hemispheres, as deduced from the h′ F hourly data obtained from 43 ionosonde stations. LSTIDs moved with a considerable velocity of 1.3 km s −1, from high to low latitudes, originating at auroral latitudes ( L-value > 6). It is suggested that a substantial increase in the ionization density in the F-region during daytime, inferred from an increase in the critical frequency (ƒ0 F2) of the layer for the stations in the sunlit sector of the Earth, was a dominant factor in the attenuation of large scale gravity waves responsible for the generation of LSTIDs.