Observation of a large density dropout across the magnetic field at 600 km altitude during the 6-7 April 2000 magnetic storm

The first Republic of China Satellite, ROCSAT-1 at 600 km altitude observed two groups of equatorial spread-F (ESF) plasma depletion structures located in either side of the midnight meridian during the 6-7 April 2000 magnetic storm recovery phase. In the group of postmidnight ESF irregularities, a large density dropout was observed in two consecutive ROCSAT orbits. This large density dropout structure is similar to the density depletion event observed by Defense Meteorological Satellite Program (DMSP) F9 spacecraft during the 13-14 March 1989 magnetic storm [Greenspan et al., 1991] except that the ROCSAT orbit traverses the magnetic field in the east-west direction. The postmidnight group of depletions together with the large density dropout was observed to drift westward with the background plasma at ∼170 m s^_1. We examine the properties of the large density dropout through the plasma data taken from the drift meter (DM) and retarding potential analyzer (RPA) sensors in three consecutive orbits. The apparent widening of the density depletion is explained by the different locations of the satellite within the depletion region. The extreme low-density level inside the upwelling depletion indicates that of the nighttime F-peak passed over the ROCSAT orbit at 600 km. The existence of NO⁺ ions inside the dropout is consistent with such an elevation of the F-peak.