Significant depletions of the ionospheric plasma density at middle latitudes: A possible signature of equatorial spread F bubbles near the plasmapause

摘要

Plasma density irregularities in the equatorial F region ionosphere are generally called equatorial spread F (ESF). Large-scale plasma bubbles associated with ESF processes are generated in the bottomside F region and penetrate the F peak to the topside F region. It is still not fully understood how high ESF bubbles can reach. In this paper, we present the measurements of deep depletions of the ionospheric plasma density at 840-km altitude with the Defense Meteorological Satellite Program (DMSP) satellites in the evening sector on 29 October 2003. The plasma depletions occurred after an interval of strong southward interplanetary magnetic field (IMF). A prominent feature of the observations is that the plasma depletions occurred at very high magnetic latitudes (MLAT), as well as at equatorial latitudes. The equatorial depletions were identified as plasma bubbles. The depletions at middle latitudes (20°–46° MLAT) were separated from the conventional ionospheric midlatitude trough and identified as the extension of ESF flux tubes into these latitudes. It implies that the plasma bubble, which caused the emptied flux tube at 46° MLAT, reached the altitude of 6800 km over the magnetic equator. The observations may represent the highest altitude/latitude of plasma bubbles that has ever been reported in the literature. For the plasma depletions at even higher latitudes (47°–49° MLAT) near the subauroral polarization stream (SAPS)/midlatitude trough, there are two candidate generation mechanisms. One is the extension of ESF flux tubes into these latitudes, implying that the plasma bubbles reached the altitude of 8400 km over the magnetic equator. Because the SAPS/midlatitude trough corresponds to the plasmapause at the plasmaspheric heights, the observations suggest that ESF plasma bubbles may have reached the plasmapause. Another mechanism is a field-aligned, low-density structure between the plasmasphere and ionosphere, and the plasma structure is generated through the strong electric field in the subauroral polarization stream region.