On the Short‐Range Longitudinal Variation of Pre‐Reversal Enhancement of Vertical Plasma Drift and Associated Implications

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Pre‐reversal enhancement of vertical plasma drift (PRE) is a unique feature of the equatorial ionosphere and plays an important role in defining the state and stability of the evening ionosphere. In this paper, we investigate short‐range longitudinal variations of PRE and its consequence on the longitudinal configuration of the equatorial bottomside F layer and the equatorial plasma bubble (EPB) phenomenon. We use simultaneous observations made using three longitudinally separated ionosondes located close to the magnetic equator and spread over a longitude zone of 24° in the Asian sector to study longitudinal variations in the vertical plasma drift, Vz and the height of the F layer base. The peak Vz as well as the temporal pattern of Vz show large longitudinal variations and the maximum difference in the peak Vz between two stations is found to be in the range of 29–49 m s−1, which is remarkable. The large longitudinal difference in Vz is attributed to localized polarization process, which acts additively to the large‐scale PRE process. The longitudinal variation in Vz could result in both positive and negative tilt of the F layer base, which is shown to have a considerable role on the growth of the Rayleigh‐Taylor instability for the formation of EPB. The plausible origins of longitudinal variation in PRE and its consequences are discussed.