Ionospheric disturbances during low-latitude auroral events and their association with magnetospheric processes

Ionospheric disturbances associated with low-latitude auroral events that occurred during geomagnetic storms on October 21, 1989, and May 10, 1992, are investigated from measurements of the total electron content (TEC) by the U.S. Navy Navigation Satellite System (NNSS) and ionosonde data from the Japanese meridian chain. Features of the ionospheric disturbances are then associated with the progression of geomagnetic disturbances during the storms. After the onset of thr: main phase of each storm, anomalous TEC distributions characterized by depressed TEC distributions on the high-latitude side (> 35 degrees geographic latitude) and an enhanced equatorial ionization anomaly (EFA) on the low-latitude side (< 35 degrees geographic latitude) were observed over Japan both by the NNSS and by the meridian chain of ionosonde stations. This enhancement of the ELA suggests the penetration of magnetospheric electric fields during the storms. Corresponding to the anomalous TEC decreases in the northern part of Japan, ionosonde stations in this region observed specific disturbances of ionospheric variation characterized by simultaneous decreases of f(0)F(2) and increases of h'F. These features of ionospheric variations on the high-latitude side can be attributed to the upward escape of ionospheric plasma caused by the heating and evacuation mechanisms, which are induced by depletion of the plasmasphere and the resulting access of ring current particles to low latitudes. After the appearance of the characteristic ionospheric variations mentioned above, low-latitude aurorae were observed at the maximum development stage of the Dst, associated with the recovery of midlatitude geomagnetic horizontal (H) components. Prior to the appearance of the low-latitude aurorae, magnetic disturbances in the auroral region increased with the decrease of midlatitude H components, as a natural consequence of magnetic storms. However, they showed a recovery before the start of the low-latitude aurorae, while the magnitude of the low-latitude magnetic field continued to decrease. These observed features of the magnetic disturbances suggest that the position of the auroral oval shifted toward the equator before the appearance of the low-latitude aurorae. Consequently, these observations are consistent with the plasmapause and auroral zone moving toward the equator before the onset of low-latitude aurorae. We conclude that the convection electric field penetrating toward low latitudes causes the enhancement of the EIA, the shrinkage of the plasmasphere, and the penetration of high-energy particles toward the low-latitude region.