Wesley Martin

Radial transport plays a fundamental role in controlling the dynamics of Earth's radiation belt electrons during geomagnetic storms. As a major energization mechanism, inward radial transport contributes to the refilling of the slot region, with an efficiency that depends on electron energy but remains insufficiently characterized. We present electron observations from the relativistic electron and proton telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE). Operating in low‐Earth‐orbit (LEO), CIRBE enabled repeated sampling of the trapped region approximately every 1.5 hr, though observations are primarily limited to electrons with small equatorial pitch angles. These observations reveal the inward transport of outer belt electrons to the slot region and inner belt and their energy dependence during storms of varying intensities in April, May, and November of 2023. Notably, the April 2023 storm produced a five‐order‐of‐magnitude enhancement of 0.4–1 MeV electron flux at L ≤ 3. In the phase space density (PSD) radial profile, this is reflected in the rapid inward transport of the inner edge of the outer belt. The PSD increased by around three orders of magnitude between L* ∼ 3.3 and L* ∼ 2.5 over ∼2–3 days for μ = 2 MeV/G, by around two orders of magnitude from L* ∼ 3.5 to L* ∼ 3.0 at μ = 20 MeV/G, and by less than two orders of magnitude and largely constrained above L* ∼ 3 at μ = 30 MeV/G. These observations provide an energy‐resolved LEO characterization of storm‐time deep penetration, showing that the magnitude and radial extent of the response vary strongly with energy and among storms with different intensities and prestorm states.

Following an intense geomagnetic storm (Dst < −150 nT) on 5 November 2023, electron fluxes in the outer radiation belt were reduced by more than three orders of magnitude at energies below ∼700 keV while they were enhanced at ∼1–3 MeV, leading to reversed energy spectra. Using high‐energy resolution measurements from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat, we track the evolution of electron spectra across L ≈ 2.5–4.5 over ∼12 days, clearly presenting the spatial extents and structures of reversed energy spectra. We also perform one‐dimensional pitch angle diffusion simulations induced by hiss and magnetosonic (MS) waves to reproduce the key features of the observed reversed spectra. These results highlight the unique observations of CIRBE and the important roles of hiss and MS waves in controlling electron dynamics in radiation belts after geomagnetic storms.

We investigate how the wave normal angle (WNA) and polarization of proton‐band electromagnetic ion cyclotron (EMIC) waves change as they travel from their source to Earth. This paper marks a significant milestone as the first full‐wave simulation of proton‐band EMIC waves reflecting from the ionosphere. Our findings show that the WNA can change rapidly during propagation, primarily due to plasma inhomogeneities, such as variations in the Alfvén speed. The wave polarization is strongly related to the WNA, consistent with theory. Newly generated EMIC waves near the equator propagate with a WNA of 0, then the WNA gradually shifts to 90 as they move toward Earth. In contrast, reflecting waves having 90 of WNA at Earth maintain a relatively larger WNA even near the magnetic equator. As a result, only the newly generated waves close to the source, where the magnetic latitude is less than approximately 20, show left‐handed polarization, while linear polarization remains dominant throughout the rest of the propagation.