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

Abstract: 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.

• The paper documents the formation and persistence of a new relativistic electron storage ring in Earth's slot region after the May 2024 superstorm.
• It employs CALET/ISS energy-resolved measurements with cross-calibration from GOES, EPT, and REPTile-2 to capture an order-of-magnitude increase in electron fluxes.
• The study provides critical insights for refining space weather models and risk assessments for satellites by elucidating energy- and L-dependent electron decay trends.

Effects of the May 2024 Solar Storm on the Earth's Radiation Belts: Insights from CALET/ISS Observations

Introduction

The May 2024 solar storm, driven by the merger of NOAA active regions 13664 and 13668, represents one of the most intense geomagnetic disturbances in recent decades, with a minimum Dst of $-412$ nT and a planetary K-index (Kp) of 9. This superstorm provided a critical opportunity to study the ring current and relativistic electron dynamics in Earth's inner magnetosphere. Leveraging the long-term, high-energy monitoring capacity of the Calorimetric Electron Telescope (CALET) aboard the International Space Station, this study systematically characterizes the unprecedented formation and persistence of a new relativistic electron storage ring in the aftermath of the event (2602.03990).

Experimental Overview and Instrumentation

CALET, residing in low Earth orbit (LEO) onboard the ISS, is equipped with the charge detector (CHD) and imaging calorimeter (IMC), enabling energy-resolved measurements of electrons and protons above key thresholds ($>1.5$, $>3.4$, $>8.2$ MeV for electrons). Despite the integral nature and species non-discrimination in CALET's channels, contextual cross-calibration with GOES, REPTile-2 (CIRBE), and PROBA-V/EPT allowed robust association of count rate enhancements to relativistic electrons, specifically within the $L = 2.2$--$3.2$ shell domain.

Radiation Belt Response to the May 2024 Geomagnetic Superstorm

The sequence of CME impacts, culminating in the May 10--11 interval, produced a severe erosion of the plasmasphere and significant dayside compression of the magnetopause, triggering deep transport of outer radiation belt electrons into classical slot regions. CALET detected an intense, narrow enhancement in $>1.5$~MeV and mult-MeV electron fluxes, representing an order-of-magnitude increase in slot region intensity, most pronounced for $L$ between 2.2 and 3.2 and persisting up to five months post-event, depending on energy. This phenomenon was reminiscent of the exceptional "storage ring" episode observed during the 2003 Halloween storm, but with significantly greater longevity.

Of particular note, multi-MeV electron fluxes extended well below the canonical slot region boundary at $L=2.8$, approaching the stable inner belt. Observational concurrence by EPT and CIRBE/REPTile-2 confirmed the spatial and energetic characteristics and the protracted presence of this population.

Temporal Evolution and Decay Analysis

The study provided a detailed exponential characterization of the post-storm slot-region electron population decay, revealing complex energy- and $L$-dependent trends in effective lifetime. After the storm and prior to the next major disturbance, lifetimes were as follows for $L=2.8$--3.2: $41.3 \pm 4.2$ days for $>1.5$~MeV, $25.4 \pm 2.5$ days for $>3.4$~MeV, and $19.0 \pm 1.6$ days for $>8.2$~MeV electrons. Lower $L$ exhibited longer decay times for higher energies, e.g., the $>8.2$~MeV electron population at $L=2.3$ showed a fitted lifetime of $\sim$72 days.

Contrastingly, for $L\gtrsim2.7$, the $>1.5$~MeV electron lifetimes plateaued at $\sim$38--40 days, while the higher-energy populations demonstrated a minimum around $L=3$. These detailed decay profiles support the coexistence of collisional losses dominating at lower $L$ and energy-dependent wave-particle (notably hiss and EMIC) interactions controlling higher $L$ electron lifetime dynamics.

Furthermore, the persistence of the slot region population was modulated by subsequent geomagnetic storms; major events (e.g., June 28, August 4/12, September 12/17) produced abrupt dropouts and recoveries, with lower $L$ shells exhibiting more gradual and less pronounced variability compared to the higher $L$.

SAA Proton Dynamics

CALET also resolved notable changes in the South Atlantic Anomaly (SAA), detecting morphology and intensity shifts—particularly on the eastern boundary—attributable to storm-induced deep SEP proton injection. These observations, corroborated by EPT and REPTile-2, highlight the coupled nature of electron and proton belt responses under extreme space weather conditions.

Theoretical and Practical Implications

The extended durability of the relativistic electron storage ring challenges certain theoretical lifetimes derived from quasilinear diffusion models, which often predict shorter electron persistence, especially below $L=3$. The observed energy and $L$ dependencies constrain diffusion coefficients, elucidate the operative balance between hiss/EMIC scattering and Coulombic interactions, and suggest potential gaps in current inner magnetospheric wave models. The contrast with historical events (2003, 1989, etc.) implies critical sensitivity to storm strength, plasmaspheric erosion, and subsequent wave environments.

From a practical perspective, the findings have significant implications for spacecraft mission risk assessment, since the previously quiescent slot region may become a high-radiation environment for extended periods after superstorms.

Outlook

Continued operation of CALET through at least 2030 ensures the potential for further capturing rare events and developing predictive models integrating microphysical (wave-particle/transport) and macrophysical (global convection, magnetopause dynamics) processes. Enhanced multi-point, multi-energy mapping remains essential for further constraining wave power distributions, cross-species precipitation rates, and finally, for improving inner zone and slot region energetic electron forecasts for space weather mitigation.

Conclusion

This study demonstrates, using CALET/ISS, that the May 2024 superstorm created an unprecedented, long-lived multi-MeV electron storage ring in the slot region ($L = 2.2$--$3.2$), invalidating the classical notion of an impenetrable barrier below $L\sim2.8$ under extreme space weather. The persistence and decay of this population provide critical constraints on electron loss timescales, the operative role of hiss and EMIC waves, and the conditions leading to enhanced proton penetration into the SAA. These results necessitate both refinement of theoretical models and reassessment of radiation belt hazard protocols for LEO and MEO satellite operations (2602.03990).