Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

This dissertation examines changes in Earth's energetic-particle environment using measurements from the Calorimetric Electron Telescope (CALET) on the International Space Station. Although CALET was designed primarily for high-energy cosmic-ray and gamma-ray observations, its count rates provide nearly continuous measurements of particles of ~1 MeV and above. The analysis uses the CHDX, CHDY, and IMC4X channels, which have approximate electron thresholds of $>\!1.5$, $\!>3.4$, and $\!>8.2$~MeV and proton thresholds of $>\!17$, $\!>37$, and $\!>52$~MeV, respectively. Because these measurements do not identify particle species, the observations are interpreted using their geographic and geomagnetic distributions, the relative responses of the three channels, GOES proton measurements, and solar-wind and geomagnetic data from NASA OMNI.

A geomagnetic superstorm in May 2024 produced the largest radiation-belt reconfiguration examined in this work. CALET observed a persistent relativistic-electron population spanning approximately $L=2.2$--3.2, including much of the nominal slot region, as well as changes in the proton-dominated South Atlantic Anomaly. During a comparatively quiet interval from May 26 until the June 28 storm, exponential fits over $2.8

Similar analysis was applied to storms in September 2017, October 2024, November 2025, and January 2026. September 2017, October 2024, and January 2026 produced persistent electron enhancements near the outer slot region and inner edge of the outer belt, while the November 2025 event failed to produce a significant injection despite its comparable size. A survey of more than ten years of CALET data identified 22 distinct slot-region filling events. Fourteen followed geomagnetic storms with Dst below $-100$~nT, while the remaining events were associated with high-speed solar-wind streams, abrupt changes in the interplanetary magnetic field, or weaker geomagnetic storms. These observations show that the occurrence and magnitude of slot region filling by relativistic electrons depends on the timing and duration of magnetospheric compression, southward IMF/convection, particle injections, wave activity, and the pre-event particle population, rather than on storm strength alone.

Date

8-21-2026

Committee Chair

Michela Negro

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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