It is generally understood that quasi-trapped particles in the inner radiation belt sink to the South Atlantic Magnetic Anomaly (SAMA) which is characterized by a global minimum in the Earth’s total magnetic field intensity. Possible aeronomic effects by particle precipitation into the SAMA region were reviewed so far (e.g. Paulikas, 1975; Gledhill, 1976: Pinto and Gonzalez, 1989).
On the ground-based observations in early years, a broad-beam 30 MHz riometer was sed to investigate ionization effects induced by precipitation of energetic electrons. Associated with a sudden commencement (SC) followed by a strong geomagnetic storm, the riometer absorption at Atibaia in Brazil indicated an eastward drift of about 100 m/s (Abdu et al., 1973). They interpreted that energetic electrons near 30 keV precipitating into the ionospheric E- or D-region of 90-100 km were subjected to an E x B drift in the plasmasphere.
Recently Obara et al. (2000) displayed that around the maximum depression (Dst ~ -200 nT) in the strong geomagnetic storm on May 4, 1998, electrons in 30-1100 keV energy range on the NOAA satellite observations were evidently distributed in the inner radiation belt of L < 1.5. This result indicates that continuous ground observations using the riometer could be useful for understanding of dynamics in the inner radiation belt and precipitation processes of energetic electrons into the SAMA. However, there remain still major uncertainties on a single-beam riometer; temporal and spatial changes of precipitations from magnetically quiet to disturbed periods.
Aiming at continuous, long-term investigations of temporal and spatial changes of energetic particle precipitation in the SAMA from magnetically quiet and disturbed periods, we installed newly an imaging riometer (IRIS) at the INPE-SSO (Instituto Nacional de Pesquisas Espaciais, Southern Space Research Observatory) near Santa Maria (SMR) in Brazil, where the total intensity of the geomagnetic field is near the global minimum of 22,855 nT in 2000 (Private communication, Trivedi). The IRIS has a great advantage to measure spatial scale, shape and motion of ionospheric absorption.
We revealed from continuous observations that the IRIS detected unusual ionospheric absorption characterizing energetic electron precipitation into the SAMA during the strong magnetic storm (minimum Dst= -164 nT) on September 22-23, 1999 (Nishino et al., 2002). Furthermore we observed westward drift of ionospheric absorption associated with the great geomagnetic storm (minimum Dst = -301 nT) on July 15-16, 2000, called as the “Bastille Day Storm”. Comparing the absorption features with magnetic field data and particle data from the NOAA satellite, we have revealed that the absorption characterize energetic particle precipitation into the Brazilian geomagnetic anomaly. However, we could not measure spatial scale of the absorption area, because the absorption area extended beyond the IRIS field-of-view, and also could not predict large-scale traveling of the absorption in South Atlantic region. Therefore we installed a second IRIS in Punta Arenas in Chile which located at higher-latitude apart from SMR and a third IRIS in Concepcion in Chile which located at westward apart from SMR (Nishino et al., 2004). Thus South America RIometer NETwork (SARINET) composed by the three IRISs was established. Ionospheric absorption data obtained by the SARINET could provide valuable information not only about particle precipitation in the SAMA region but also about ionospheric disturbances associated with geomagnetic storms and/or atmospheric turbulences occurring in the southern hemisphere.
Recently we installed another IRIS at Kakioka in Japan in order to investigate ionospheric disturbances in the northern hemisphere, which could compare with ones in the southern hemisphere. Ionospheric absorption features during the strong magnetic storm in December 2006 showed a uniform and large-scale structure, and also a striped structure, which might relate with large-scale and middle-scale traveling ionospheric disturbances (LSTID and MSTID, respectively). Absorption images observed by the SMR-IRIS in South America during the December storm showed a discrete structure (Nishino et al., 2007). Coordinated IRIS observations by the SARINET in the southern hemisphere and at Kakioka in the northern hemisphere would contribute for studies of ionospheric disturbances in low and middle latitudes.