Intra-annual variation of wave number 4 structure of vertical E x B drifts in the equatorial ionosphere seen from ROCSAT-1

This paper studies the intra-annual variation of the wave number 4 structure in the equatorial vertical E x B drifts at high solar flux level based on of the ROCSAT-1 observations during the interval from 1999 to 2004. It is found that the daytime longitudinal structure is significant in northern summer and early autumn but weak in northern winter. This is consistent with the intra-annual variations of the tidal mode DE3. At night, the intra-annual variation of the wave number 4 structure shows some differences from that in daytime, which may relate to different dynamo mechanisms operating in different local time. We also found that the wave number 4 structure mainly shifts eastward during daytime in most of months, which is coincident with the eastward propagation of the DE3 tide. However, it is largely disturbed near sunrise and sunset. We attribute this disturbance as the jumping of the main dynamo region between E and F layers, as well as the different dynamo mechanisms in different dynamo regions.