On the Onset Time and Background Ionospheric Conditions of Postsunset Equatorial Plasma Bubble

Abstract

AbstractOnset time and conditions of the postsunset equatorial plasma bubble (EPB) observed by the Gadanki Ionospheric Radar Interferometer are presented and discussed in the light of collocated digisonde observations of background ionosphere. Observations show two important findings: (a) onset time of EPB is linked with the sunset terminator and it varies between the low latitude E region sunset and apex F region sunset with a tendency of EPB onset to be close to low latitude E region sunset, and (b) higher height of the bottomside F layer (by 50–60 km) and vertical plasma drift (by 10–30 m s−1) observed over Gadanki are found to be associated with the EPBs growing in a narrow longitude zone of ±1° centered on Gadanki than those outside this zone. The early onset of EPB coinciding with the low latitude E region sunset is attributed to the early predominance of the F region dynamo leading to prereversal enhancement (PRE) of zonal electric field, consistent with theoretical expectation. Delay in the onset of EPB is attributed to the effect of flux tube integrated Pedersen conductivity owing to low latitude sporadic E activity and the F region meridional neutral wind that could change the F region plasma density along the flux tube through field‐aligned movement of plasma. Observations also show that ionosonde observations from a single location cannot be used to predict the growth of EPB at other location, including those close to the station but away from the station by about 120 km.