[1] The occurrence statistics of equatorial plasma bubbles (EPBs) obtained from lowinclination orbit satellites are significantly affected by the way the data are sampled and the way that the EPBs are counted. To resolve the discrepancy between the EPB occurrence frequency determined by ground-based observations and in situ sampling of plasma density from spacecraft, we have developed a new EPB detection method that minimizes the dependence of the EPB occurrence rate on the data processing method. The global EPB distribution maps are created by analyzing the measurements of the ion density from the first Republic of China satellite (ROCSAT-1) during March 1999 to June 2004. The EPB occurrence probability obtained using our new EPB detection method is a few times greater than that obtained using the conventional method. Our results are comparable to the ground observations. The good agreement of the global EPB distribution with the global morphology of the evening prereversal enhancement (PRE) of vertical ion velocity supports the notion that the PRE is an important factor on a global scale in the generation of EPBs. However, the generation of EPBs is not guaranteed by the occurrence of an intense PRE. Other mechanisms, in addition to the PRE, should be considered as an explanation for the occurrence of EPBs on the topside.

[1] We investigate the seasonal, longitudinal, local time (LT), and altitudinal variations of the F region morphology at low latitudes using data from the first Republic of China satellite (ROCSAT-1), Global Ultraviolet Imager (GUVI), on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, and the Defense Meteorological Satellite Program (DMSP) F13 and F15 satellites. Signatures of the longitudinally periodic plasma density structure emerge before 0900 LT. The wave structure is established before noon and further amplified in the afternoon. The amplitudes of the wave structure start to diminish in the evening. The wave-4 structure is clearly distinguishable during equinox and northern hemisphere summer. During northern hemisphere winter, the density structure can be characterized to either wave-4 or wave-3 structure owing to marginal separation of the two peaks in 180°-300°E. Observations of similar density structures from ROCSAT-1 (600 km) and DMSP (840 km) at 0930 and 1800 LT indicate the extension of the wave structure to altitudes greater than 840 km. The daytime wave structure persists into the night during the equinoxes but is significantly modified during the solstices. The modification is more significant at higher altitudes and is attributed to the effects of interhemispheric winds and the prereversal enhancement. The formation of the wavelike density structure in the morning and its temporal evolution in the afternoon show a close association with the vertical E Â B drift. We conclude that the E Â B drift during 0900-1200 LT determines the formation of the wavelike density structure.

[1] The measurements of vertical ion velocity from the first Republic of China satellite (ROCSAT-1) provide a unique database for the development of an annually and longitudinally high-resolution vertical plasma drift model in the equatorial ionosphere. Currently, the ROCSAT-1-based empirical vertical drift models are available for three seasons: equinox and solstices. However, the vertical drift patterns are not precisely divided by the three seasons. A monthly vertical drift model with high longitudinal resolution is desirable to accurately model the low-latitude ionosphere and to identify the coupling between the ionosphere and atmospheric tide. Here we introduce an empirical vertical drift model derived by using the ROCSAT-1 data in three solar flux conditions (F 10.7 < 150, 130 < F 10.7 < 200, and F 10.7 > 180) under Kp 3 + . The local time, day of the year, and longitude of the model are binned by 15 min, 1 month, and 10°, respectively, under each solar flux condition. Our vertical drift model is validated by comparing the model with the measurements of the vertical drift velocity at the Jicamarca Radio Observatory. The characteristics and variability of the vertical drift are briefly discussed.

[1] We investigate the annual and local time variations of the wave-4 structures in the plasma density and vertical drift in the low-latitude F region by analyzing the measurements from the first Republic of China satellite (ROCSAT-1) and conducting simulations with the Global Ionosphere and Plasmasphere (GIP) model. The GIP model uses apex magnetic coordinates with International Geomagnetic Reference Field (IGRF) for magnetic field, neutral wind from HWM-07, and thermospheric parameters from the NRLMSISE-00 model. In order to understand how the vertical drifts relate to the longitudinal structure of the topside ionosphere, we apply the equatorial vertical drifts observed from ROCSAT-1 to drive the GIP model. The model well reproduces the longitudinal structure in electron density, and the magnitudes of electron density are comparable with ROCSAT-1 measurement at 600 km. The ROCSAT-1 observations of the vertical drift and plasma density show maximum amplitudes of their wave-4 components in July-September and minimum amplitudes in December-February. An eastward shift of the wave-4 components with increasing local time is observed in both the density and the vertical drift. The GIP model density showed similar annual and local time variations of the wave-4 component. Since the model uses the observed equatorial vertical E Â B drift as an input, the results indicate the vertical drifts are essential in the formation and evolution of the longitudinal wave-4 density structure. The amplitude of the eastward propagating diurnal tide (DE3) at 110 km shows similar annual and local time variations as the F region parameters, supporting the link between the DE3 tide, vertical E Â B drift, and F region plasma density on a global scale.

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