The ROCSAT-1 IPEI preliminary results: Vertical ion drift statistics

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 acti vity are examined. The variations of the equatorial vertical drifts near th e dawn and dusk terminators are of particular interest. From this prelimina ry 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 instrume nts. More importantly, several new results due to the unique 35 degrees orb ital 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 r ising 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 whe re the geomagnetic field has greatest variations; (c) the statistical drift patterns strongly depend on the hemispheres at the equatorial anomaly lati tudes, This north-south asymmetry may result from seasonal effects and/or f rom differences in geomagnetic field configuration.