South Atlantic Anomaly

The earth's magnetic dipole field is offset from its centre by about 500 km. As a result of this, the inner Van Allen belt is on one side closer to the earth's surface. This region is named the South Atlantic Anomaly (SAA) and it covers a part of South America and the southern Atlantic Ocean: it lies roughly between latitudes 5 and 40 degrees South, and between longitudes 0 and 80 degrees West -- the precise strength, shape and size of the SAA varies with the seasons.

This dip in the earth's magnetic field allows charged particles and cosmic rays to reach lower into the atmosphere. Low-orbiting satellites, such as ERS-2 and ENVISAT, pass daily through the inner radiation belt in the SAA-region. Upon passing the inner belt, charged particles may impact on the detector, causing higher-than-normal radiance values, which in turn decreases the quality of the measurements (i.e. the signal-to-noise ration, of earthshine spectra), notably in the UV.

This reduction of the signal-to-noise also affects the retrievals of SO₂ slant and vertical columns: in the SAA region the variations in slant and vertical column values are much higher than elsewhere. This shows up clearly, for example, in the monthly average, as the image below illustrates. The decreased signal-to-noise may result in artifacts in the SO₂ vertical columns.

The monthly average SO2 vertical column for May 2010. Note the increased noise in the SO2 vertical column over South America and a part of the Southern Atlantic Ocean, which can be attributed to the South Atlantic Anomaly.

 
SO₂ data derived from OMI measurement seem to be affected less by the SAA than GOME-2 and SCIAMACHY. Probably this is because of the way the SO₂ is computed: as a kind of "by-product" of the ozone retrieval, and the ozone retrieval is less sensitive to the SAA due to the strong ozone absorption signals in the measurements.

Another difference is that SCIAMACHY and GOME-2 are scanning instruments whereas OMI is an imaging instrument. A scanning instrument observes the individual ground pixels along an east-west swath one at a time and for each of these pixels charged particles will hit the detector in a different place and affect the measurements differently for each ground pixel. An imaging instrument sees an entire swath width in one go and there the detector is hit in several places by charged particles, affecting only a few ground pixels.

Example of SO2 vertical column data based on SCIAMACHY (top), GOME-2 (centre) and OMI (bottom) data on 2 January 2008. The SO2 plume is related to the eruption of the Llaima volcano (Chile), marked by a blue triangle in plots, on 1 January. The top and centre plots show a lot of noise in the SO2 signal due to the interference caused by the SAA; on the bottom the OMI data is clearly much less affected.

 
Because of these artifacts in the SO₂ data, neither the Volcanic SO₂ nor the Air Quality SO₂ Service has a geographic region defined in the area of the South Atlantic Anomaly for which plots are made and shown on the website. To benefit the analysis and to help avoid issuing false notifications of an exceptional SO₂ concentration due to the SAA, a "dummy region" of 40 by 40 degrees, centred around (-45.0,-25.0), is defined and monitored hidden from data users. This "SAA-region" overlaps with regions "Northern Chile" and "Central Chile" of the Volcanic SO₂ Service. As the following picture show, the "SAA-region" covers only the central part of the SAA, but for the moment this seems good enough.

Image of the South Atlantic Anomaly from data collected by the X-ray detector of ROSAT. The image is taken from the ROSAT page on the South Atlantic Anomaly.