Longitudinal structure of the vertical E × B drift and ion density seen from ROCSAT‐1

We investigate the longitudinal distribution of the vertical E × B drift velocity and ion density in the low‐latitude ionosphere using the first Republic of China Satellite (ROCSAT‐1) data acquired during 1999–2004. The ROCSAT‐1 observations during daytime demonstrate the presence of the longitudinally periodic patterns of the vertical E × B drift and plasma density on the topside F region (600 km). The four longitude sectors where the peaks in the plasma density are found are coincident with the peaks in the E × B drift. This observation may indicate the association of the large‐scale longitudinal density structure with the daytime E ‐region dynamo electric field. The density structure exists before the occurrence of the pre‐reversal enhancement (PRE) and therefore the PRE is not directly related to this phenomenon.