EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION VOLUME 83 NUMBER 15, 9 APRIL 2002
[Excerpt:] On 15 June 2001,at 3:56 UT, the Moderate Resolution Imaging Spectroradiometer (MODIS), one of NASA's prime remote sensing instruments on the TERRA spacecraft, was rendered inoperative while on the western flank of the South Atlantic Anomaly (SAA). The SAA is a vaguely-defined, oval-shaped geographic region centered roughly off the east coast of Brazil where the Earth’s geomagnetic field is relatively weak at all altitudes. Space science and engineering projects have frequently given insufficient attention to the serious problems of working in this geographic area. Here we illustrate that MODIS is just one of many spacecraft that have had problems, and that these problems are expected to get worse.
The dipole component of the geomagnetic field is tilted about 12 degrees relative to the Earth’s rotational axis and is offset away from the Earth's center in the direction of the northwestern Pacific Ocean. This weakens the geomagnetic field at the diametrically opposite point in the South Atlantic and causes Van Allen radiation to concentrate there at lower altitudes.This radiation has been encountered by satellites in Low Earth Orbit (LEO).
The hazard of this region is little recognized outside of segments of the space science community but nearly all spacecraft crossing this area at altitudes of 100 km to 1000 km have been damaged or degraded in performance.
Fig. 1. Shown are geomagnetic total field contours at 1,338 km altitude (after IGRF1995), TOPEX anomaly sites (yellow dots), the location of the recent MODIS failure (star), and the location of two ground-based geomagnetic observatories in or near the SAA (blue squares).
Fig. 2. Flux of trapped electrons with energy greater than 300 KEV is shown as measured by NOAA-10, 11-20 March 1989. Color bar shows flux per square centimeter per second per steradian. Triangles show locations of operational anomalies on NOAA 11 from September 1988 to August 1999, with upward pointing on the ascending past and downward pointing on the ascending past. (Figure courtesy of H. Sauer and D. Wilkinson.)
Copyright 2002 J. L. Ahern