CSES satellite reveals spatiotemporal evolution of high-energy particles in the South Atlantic anomaly during Solar Cycle 25

The South Atlantic Anomaly (SAA) is the region within Earth's radiation belts characterized by the highest particle flux and the weakest geomagnetic field. Its existence poses significant risks to the electronic equipment and communication systems of low-Earth-orbit satellites, as well as to the health of astronauts. Based on six years of observational data from the China Seismo-Electromagnetic Satellite (CSES), a research team from the Institute of High Energy Physics, Chinese Academy of Sciences, in collaboration with the National Institute of Natural Hazards, conducted a systematic analysis of the spatiotemporal evolution of the geomagnetic field and high-energy protons in the SAA region during the ascending phase of Solar Cycle 25. The study revealed the dynamic variation mechanisms of the SAA and its coupling relationship with solar activity. The findings provide new empirical evidence for understanding radiation belt particle transport and the long-term evolution of Earth's geomagnetic field, and were published in the top-tier Chinese journal Science China: Earth Sciences.

Results show that the center of the SAA proton flux has continued drifting westward and northward over the past five years. Based on a double-Gaussian fitting model, the daytime northward drift rate of the proton flux center is (0.29 ± 0.12)°/year, while the westward drift rates are (0.36 ± 0.08)°/year during the day and (0.33 ± 0.10)°/year at night. The drift of lower-energy protons is slightly faster than that of higher-energy ones, indicating an energy-dependent response to geomagnetic inhomogeneity. Comparisons with the International Geomagnetic Reference Field (IGRF-13) model show that the drift direction of the SAA magnetic center is consistent with that of the proton flux center. In addition, the study reveals a distinct spatial structure: low-energy protons (2.0–10.0 MeV) exhibit a characteristic double-peak distribution, while higher-energy protons (10.0–20.0 MeV) display a single-peak pattern—consistent with long-term NOAA/MEPED satellite observations.

Combined CSES/HPM observations and IGRF-13 model results indicate a pronounced east–west hemispheric asymmetry in global geomagnetic field strength: while the magnetic field in the Eastern Hemisphere is strengthening, that in the Western Hemisphere is weakening, leading to a further reduction in the magnetic shielding capability over the SAA. This weakening correlates with the CSES-measured proton flux variations, showing that between 2019 and 2024, proton flux in the core region of the SAA (L = 1.2–1.5) increased significantly, whereas the flux in the outer region (L > 1.5) decreased. These variations suggest two competing mechanisms: enhanced solar activity suppresses proton flux in the outer inner-belt region (L > 1.5), while the gradual weakening of the local geomagnetic field in the SAA core allows protons to accelerate and precipitate deeper into lower L-shells (L = 1.2–1.5). Boundary identification and quantitative analysis further show that the area of the SAA proton flux decreased by approximately (6.09 ± 1.03)% between 2019 and 2024, with an average annual shrinkage rate of –(4.33 ± 0.73) × 10⁵ km²/year. This area reduction shows a significant negative correlation with the F10.7 solar radio flux, highlighting the critical role of solar activity in modulating the SAA’s evolution.

With its high-precision High Energy Particle Package (HEPP) and Magnetic Field Measurement Instrument (HPM), the CSES satellite provides unprecedented high-quality data for studying the dynamic evolution of the SAA. This research not only offers important insights for spacecraft orbit design and radiation protection but also provides key observational evidence for understanding the long-term evolution of Earth’s magnetic field and radiation belt dynamics.

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