NIGHTTIME D-REGION ELECTRON-DENSITY VARIATIONS OBSERVED IN THE SOUTH-ATLANTIC GEOMAGNETIC ANOMALY IN ASSOCIATION WITH SOLAR-PROTON EVENTS

Very low frequency signals (VLF) received in Atibaia (Brazil) during the occurrence of the two major solar-proton events of August 1972 and July 1974 were analysed. Phase and amplitude changes observed at night were introduced in the model given by,Wait and Spies (1964) properly adapted to the ionospheric conditions of the South Atlantic Geomagnetic Anomaly (SAGA) region. Electron density disturbed profiles for ''L'' shell intervals 1.15 < L < 2.0 and 1.08 < L < 1.15 were deduced. The results highlight the great amount of ionization produced at low ''L'' values due to the precipitation of energetic particles in the SAGA, mainly at night. Some discrepancies among the electron density profiles proposed by different authors are discussed.