Mapping the South Atlantic Anomaly continuously over 27 years

Abstract

The South Atlantic Anomaly (SAA) is a region of reduced magnetic intensity where the inner radiation belt makes its closest approach to the Earth's surface. Satellites in low-Earth orbit pass though the SAA periodically, exposing them to several minutes of strong radiation each time, creating problems for scientific instruments, human safety, and single event upsets (SEU). For the first time, we are able track the SAA movement continuously over 27 years, using overlapping satellites in similar orbits with similar instruments. The Defense Meteorological Satellite Program (DMSP) spacecraft have been carrying the Special Sensor J (SSJ) precipitating energetic particle spectrometers since 1982. The instruments are susceptible to MeV electrons and protons that pass through the spacecraft skin and instrument case and get counted. This “backgroundˮ is easily identified and we use it to map the movement of the SAA. Comparison with energetic particle data from the Energetic Particle Telescope (EPT) instrument on the Proba-V spacecraft indicates that the best match with the SSJ data occurs in the energy range above about 2.6 MeV for electrons and above about 29 MeV for protons. The peak flux and extent of the SAA from both the SSJ and EPT instruments are nearly identical in longitude while in latitude, the peak EPT flux is 5° south of the peak SSJ flux. However, the shapes of the SAA in latitude and the locations of the outer radiation belts are nearly identical. We find that the SAA moves 0.06° N/yr and 0.28° W/yr. We also find a difference with the movement and location of the SAA from the contamination by high energy particles on the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on DMSP F16 (Shaefer et al., 2016). However, the SSUSI instrument is located on the opposite side/bottom of the spacecraft and Shaefer et al. (2016) estimated that most of the particle noise pulses in the SSUSI instrument are produced by protons greater than about 45 MeV. Thus the contamination in the SSUSI instrument is produced by a different population of particles that the contamination in the SSJ/5 instrument which would lead to differences in movement and location of the SAA. While this study focuses on the SAA movement on a yearly basis, further analysis will allow us to investigate the movement on shorter time scales, the variation of the flux intensity, the spatial extent of the SAA, and the dynamics of the outer radiation belt.