Open Access

Unusual ionospheric absorption characterizing energetic electron precipitation into the South Atlantic Magnetic Anomaly

  • Masanori Nishino1Email author,
  • Kazuo Makita2,
  • Kiyofumi Yumoto3,
  • Fabiano S. Rodrigues4,
  • Nelson J. Schuch5 and
  • Mangalathayil A. Abdu4
Earth, Planets and Space201454:BF03352438

Received: 7 December 2001

Accepted: 25 September 2002

Published: 21 June 2014


An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (20 h LT) near the maximum depression (D st −164 nT) and the following positive excursion during the strong geomagnetic storm on September 22–23, 1999. The unusual absorption that has short time-duration of 30–40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100–150 km) appearing from the zenith to the low-latitude part of the IRIS field-of-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA ionosphere, associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (250 m/s) of the sheet absorption, precipitating electrons are estimated to be 20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station.