[1] This paper studies the intra-annual variation of the wave number 4 structure in the equatorial vertical E Â 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.Citation: Ren, Z., W. Wan, L. Liu, and J. Xiong (2009), Intra-annual variation of wave number 4 structure of vertical E Â B drifts in the equatorial ionosphere seen from ROCSAT-1,

During November 11–16, 2003, the interplanetary magnetic field (IMF) Bz oscillated between northward and southward directions, which suggests discontinuous magnetic reconnection associated with the multiple pulses‐like reconnection electric field. The Jicamarca incoherent scatter radar (ISR) measurements of ionospheric zonal electric field showed similar fluctuations during this period. The high correlation coefficient of 0.71 between the reconnection electric field and equatorial zonal electric field during 125 hours suggests that the interplanetary electric field (IEF) pulsively penetrated into the equatorial ionosphere due to the discontinuous magnetic reconnection. It is implied that the short lifetime (<3 hours) dawn‐dusk IEF pulses can penetrate into ionosphere without shielding, in other words, they may exhibit the “shielding immunity”. The averaged penetration efficiency is about 0.136 and highly local time‐dependent. Furthermore, the intense AU and AL indices imply that the multiple electric field penetration is associated with a “High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA).”