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

Abstract

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. Prominent depletion structures (bubbles) that are characterized by large-amplitude density decrease (N/No ≤ 1%) with rapid horizontal ion drift (600~800 m/s) are found to cluster in the 03:00 ~ 04:30 local time sector and at magnetic latitudes 14° ~ 20° S when the storm was in its early recovery phase. These presunrise bubbles are positively correlated to the enhanced eastward electric fields of greater than 1~2 mV/m, which were in response to the storm-time disturbances resulting from the in-phase contributions of the prompt penetration magnetospheric and the long lasting ionospheric disturbance dynamo electric fields. Further analyses of the field-aligned and crossfield ion drifts within the depletions reveal that bubble plasma were driven by the eastward polarization electric fields to move upward, but these upward velocities were compensated by large downward field-aligned diffusive motions. These features confirm that the disturbance electric fields produced during a great magnetic storm can significantly affect the occurrence timing and spatial extent of severe plasma irregularities in low-latitude ionosphere. The spatial dimensions of the pre-sunrise irregularities may exceed the large region observed by the 35° inclined circular orbiting ROCSAT-1.