Storm time plasma irregularities in the pre-dawn hours observed by the low-latitude ROCSAT-1 satellite at 600 km altitude

Large scale ion density depletions were detected in the nighttime sector by ROCSAT-1 for over 10 hours during the 22 October 1999 geomagnetic storm. P rominent depletion structures (bubbles) that are characterized by large-amp litude density decrease (N/N-o less than or equal to 1%) with rapid horizon tal ion drift (600 similar to 800 m/s) are found to cluster in the 03:00 si milar to 04:30 local time sector and at magnetic latitudes 14 degrees simil ar to 20 degrees S when the storm was in its early recovery phase. These pr e-sunrise bubbles are positively correlated to the enhanced eastward electr ic fields of greater than 1 similar to2 mV/m, which were in response to the storm-time disturbances resulting from the in-phase contributions of the p rompt penetration magnetospheric and the long lasting ionospheric disturban ce dynamo electric fields. Further analyses of the field-aligned and cross- field ion drifts within the depletions reveal that bubble plasma were drive n by the eastward polarization electric fields to move upward, but these up ward velocities were compensated by large downward field-aligned diffusive motions. These features confirm that the disturbance electric fields produc ed during a great magnetic storm can significantly affect the occurrence ti ming and spatial extent of severe plasma irregularities in low-latitude ion osphere. The spatial dimensions of the pre-sunrise irregularities may excee d the large region observed by the 35 degrees inclined circular orbiting RO CSAT-1.