We analyze the ROCSAT-1 IPEI data collected between March and June 1999 to study the statistical features of the ion vertical drifts at equatorial and tropical latitudes. The dependencies of ion vertical drifts on local time, longitude and geomagnetic field configuration, as well as geomagnetic activity are examined. The variations of the equatorial vertical drifts near the dawn and dusk terminators are of particular interest. From this preliminary study, we have shown that the overall local-time characteristics of the quiet-time equatorial vertical drift patterns derived from IPEI are in good agreement with those observed by other satellites and groundbased instruments. More importantly, several new results due to the unique 35 degrees orbital inclination of ROCSAT-1 and the 100% duty-cycle operation of IPEI are found. These include: (a) enhanced upward ion drifts to a critical level of 30-60 m/s at post-sunset hours strongly correlate with the occurrence of rising bubbles in the pre-midnight local time sector; (b) large (> 300 m/s) downward ion drifts are most often found near sunrise and at longitudes where the geomagnetic field has greatest variations; (c) the statistical drift patterns strongly depend on the hemispheres at the equatorial anomaly latitudes, This north-south asymmetry may result from seasonal effects and/or from differences in geomagnetic field configuration.
