Energy‐Dependent Inward Transport of Storm‐Time Radiation Belt Electrons Observed By CIRBE/REPTile‐2

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Abstract
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.

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