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Ionospheric absorption associated with a great geomagnetic storm on July 15-16, 2000 (the "Bastille Day storm") was observed in the Brazilian geomagnetic anomaly using a two-dimensional 4 × 4 imaging riometer (IRIS). In the afternoon of July 15, weak absorption (∼0.2 dB) was observed during the initial phase of the storm; large spatial-scale absorption exceeded the IRIS field of view (330×330 km). During the sharp magnetic decrease in the main phase of the storm, absorption was intensified (<0.5 dB) in the evening, showing a sheet structure with ∼150 km latitudinal width and >330 km longitudinal elongation. Subsequently, absorption was intensified (∼1 dB), having a small spatial-scale (∼150 km) in the background sheet structure and a pronounced westward drift (∼ 570 ms −1). In association with large magnetic fluctuations in the Bz component of the interplanetary magnetic field (IMF), the ground magnetic variation in the night sector showed large positive swings during the initial to main phases of the storm. With the subsequent southward turning of the IMF Bz, the ground magnetic variation in the evening sector showed rapid storm development. Particle fluxes measured by a geosynchronous satellite (L =∼6.6) demonstrated large enhancements of low-energy protons (50-400 keV) and probably electrons (50- 225 keV) during the storm's initial phase. Particle fluxes from the low-altitude NOAA satellite (∼870 km) indicated the invasion of low-energy particles into the region of L < 2 during the main phase of the storm. These results indicate that low-energy particles injected into the outer radiation belt in association with frequent and strong substorm occurrences, were transported into the inner radiation belt through direct convective access by the storm-induced electric fields during the storm's development. These particles then precipitated into the ionosphere over the Brazilian geomagnetic anomaly. Notably, the most intense absorption could be dominantly caused by proton precipitation with energies of ∼40 keV.

Energetic particle precipitation in the Brazilian geomagnetic anomaly during the “Bastille Day storm” of July 2000