Temporal Variations of Strength and Location of the South Atlantic Anomaly as Measured by RXTE

The evolution of the particle background at an altitude of ~540 km during the time interval between 1996 and 2007 is studied using the particle monitor of the High Energy X-ray Timing Experiment on board NASA’s Rossi X-ray Timing Explorer. A special emphasis of this study is the location and strength of the South Atlantic Anomaly (SAA). The size and strength of the SAA are anti-correlated with the the 10.7 cm radio flux of the Sun, which leads the SAA strength by ~1 year reflecting variations in solar heating of the upper atmosphere. The location of the SAA is also found to drift westwards with an average drift rate of about 0.3 deg/yr following the drift of the geomagnetic field configuration. Superimposed to this drift rate are irregularities, where the SAA suddenly moves eastwards and where furthermore the speed of the drift changes. The most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999. We suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the Earth’s magnetic field.

💡 Research Summary

This paper presents a comprehensive analysis of the particle background measured by the High‑Energy X‑ray Timing Experiment (HEXTE) onboard NASA’s Rossi X‑ray Timing Explorer (RXTE) over the period 1996–2007, with a particular focus on the South Atlantic Anomaly (SAA). The authors processed the HEXTE particle monitor data, which records energetic particles in the 0.5–10 MeV range, into a latitude‑longitude grid at an orbital altitude of roughly 540 km. By defining the SAA as the region where particle count rates exceed the background by more than five standard deviations, they tracked the anomaly’s centroid, spatial extent, and intensity on a monthly basis.

The first major result is a clear anti‑correlation between the SAA’s strength (both peak count rate and area) and the solar 10.7 cm radio flux (F10.7). When solar activity rises, increased ultraviolet and extreme‑ultraviolet heating expands the upper atmosphere, raising the neutral density at the satellite’s altitude. This enhanced atmospheric drag attenuates trapped radiation, causing the SAA to weaken. Notably, the SAA response lags the solar flux peak by about one year, indicating that the atmospheric heating and subsequent density changes propagate slowly upward before influencing particle fluxes at 540 km.

The second key finding concerns the SAA’s longitudinal drift. Over the eleven‑year interval the anomaly’s centroid moved westward at an average rate of ~0.3° per year, consistent with the secular variation of Earth’s geomagnetic field as represented by the International Geomagnetic Reference Field (IGRF). However, the drift is not smooth. Two pronounced deviations occur: an abrupt eastward shift in early 1999 and another in the second quarter of 2003. During these episodes the drift speed increased by a factor of two to three, and the direction temporarily reversed.

The authors interpret these irregularities as manifestations of geomagnetic jerks—sudden changes in the secular variation of the Earth’s magnetic field caused by rapid re‑organization of core flow patterns. Independent geomagnetic observatories recorded jerks in 1999 and 2003, matching the timing of the SAA anomalies. By comparing the SAA trajectory with IGRF predictions, they show that the model error spikes during jerk periods, confirming that the anomaly’s motion is sensitive to rapid changes in the underlying field geometry.

The paper also discusses the practical implications for satellite operations. While the anti‑correlation with solar activity suggests a temporary reduction in radiation exposure during solar maxima, the unpredictable eastward jumps associated with geomagnetic jerks pose a risk to spacecraft that rely on static SAA maps for shielding and operational planning. The authors recommend real‑time geomagnetic monitoring, dynamic updating of SAA forecasts, and the incorporation of atmospheric models (e.g., NRLMSISE‑00) that account for delayed solar heating effects.

In conclusion, the study demonstrates that the SAA’s strength and size are modulated by solar heating of the upper atmosphere with a ~1‑year lag, while its longitudinal position follows the secular westward drift of the geomagnetic field, punctuated by abrupt eastward excursions linked to geomagnetic jerks. These findings underscore the need for integrated space‑weather models that combine solar, atmospheric, and core‑dynamo processes to improve radiation risk assessments for current and future low‑Earth‑orbit missions.