Effects of the May 2024 Solar Storm on the Earth's Radiation Belts Observed by CALET on the International Space Station

In May 2024, extraordinary solar activity triggered a powerful solar storm, impacting Earth and producing the extreme geomagnetic storm of May 10-11, the most intense since 2003. This had significant effects on the magnetosphere, leading to the creation of a new long-lasting component of relativistic electrons and to flux changes in the South-Atlantic Anomaly. Here we present radiation-belt observations made by the Calorimetric Electron Telescope (CALET) on the International Space Station. Specifically, we took advantage of the count rates from three layers of the CALET charge detector and imaging calorimeter. We show that the new electron storage ring extended to energies in the multi-MeV range and down to L=2.2, well below the nominal slot-region barrier of L=2.8, and persisted for several months, depending on energy. The evolution of the new radiation-belt configuration over time was characterized by estimating the decay rates as a function of energy and L.

💡 Research Summary

The paper presents a comprehensive analysis of the impact of the extreme May 2024 solar storm on Earth’s radiation belts, using data from the Calorimetric Electron Telescope (CALET) aboard the International Space Station. The storm, which peaked on May 10‑11, 2024, produced a G5‑class geomagnetic disturbance (Kp = 9, Dst = ‑412 nT), the strongest since 2003. The authors exploit count‑rate measurements from three CALET detector layers—CHDX (>1.5 MeV electrons / >17 MeV protons), CHDY (>3.4 MeV electrons / >37 MeV protons), and IMC4X (>8.2 MeV electrons / >52 MeV protons)—to monitor variations as a function of McIlwain L‑shell.

Key observations include the emergence, immediately after the storm’s main phase, of a pronounced, long‑lasting enhancement of counts in a narrow L‑shell band from L ≈ 2.2 to 3.2. Because the detector thresholds are well above typical trapped‑proton energies at these L‑values, the authors attribute the signal to ultra‑relativistic electrons (≥1.5 MeV). This “electron storage ring” extends below the traditionally impenetrable barrier at L ≈ 2.8, indicating that the extreme compression and erosion of the plasmasphere allowed local wave‑particle acceleration (e.g., via plasmaspheric hiss and chorus) at relatively low L‑shells.

Temporal decay analysis reveals energy‑dependent lifetimes. At L ≈ 2.2, electron lifetimes increase with energy, whereas at L ≈ 3 the lifetimes decrease with energy, suggesting a transition from hiss‑dominated scattering (efficient for ≤1 MeV electrons) to mechanisms that preferentially remove lower‑energy particles at higher L. The new belt persisted for several months, with higher‑energy electrons surviving longer, a behavior not previously documented for the 2003 Halloween storm, where the slot‑region filling lasted only about a month.

In parallel, the study documents significant changes in the South‑Atlantic Anomaly (SAA). CALET’s proton‑sensitive channels show a sharpening of the SAA’s eastern edge and a depletion in the slot region (L ≈ 2.0‑2.2). These features are linked to the injection of solar energetic protons during the May 10‑11 SEP event, which penetrated deeper due to the storm‑induced weakening of geomagnetic shielding. The authors corroborate these observations with measurements from PROBA‑V/EPT and REPTile‑2, confirming that both electrons and protons were redistributed by the storm.

Methodologically, the paper demonstrates that CALET, despite its primary astrophysics mission, can serve as a valuable space‑weather monitor in low‑Earth orbit. By using multiple energy thresholds and contextual data (GOES proton fluxes, OMNI solar‑wind parameters, Dst index), the authors effectively separate electron‑dominated from proton‑dominated signatures, even though the instrument cannot intrinsically discriminate particle species.

The findings have several implications: (1) they refine our understanding of how extreme geomagnetic storms can breach the slot region and create long‑lived relativistic electron populations at low L‑shells; (2) they highlight the role of energy‑dependent loss processes (hiss, chorus, curvature scattering) in shaping electron lifetimes; (3) they reveal that SEP‑driven proton injections can modify the SAA geometry, affecting radiation exposure for low‑Earth‑orbit assets; and (4) they provide a benchmark dataset for improving radiation‑belt models and for designing more robust satellite shielding. Overall, the study underscores the importance of continuous, multi‑energy monitoring of the near‑Earth radiation environment, especially during periods of extreme solar activity.