Coupling Between Earth′s Magnetotail and the Outer Radiation Belt via Field-Line Curvature Scattering

The Earth's outer radiation belt is populated by relativistic ($\ge 500$ keV) electrons, which are typically confined by the strong dipole magnetic field but can precipitate into the atmosphere through scattering by electromagnetic waves. In contrast, the magnetotail primarily contains electrons with energies below 200 keV, which are predominantly scattered and precipitated due to magnetic field-line curvature scattering (FLCS). In this study, we demonstrate that FLCS can also scatter and precipitate relativistic electrons from the outer radiation belt. Using coordinated observations from the ERG/Arase satellite and low-altitude ELFIN CubeSats in the outer radiation belt, we compare electron fluxes across different $L$-shells and energy ranges. Our analysis reveals that the outer edge of the radiation belt exhibits isotropic electron populations above a minimum energy that increases with proximity to Earth. Such isotropization energy dependence on distance, or $L$-shell, agrees with that observed simultaneously at the ELFIN satellite, at low-Earth orbit, where it has been known as the electron isotropy boundary (IBe). This agreement between low-altitude and near-equatorial observations during satellite conjunctions suggests that the IBe pattern may extend to the outskirts of the traditional outer radiation belt. From that distance, the associated FLCS may facilitate precipitation of relativistic electrons up to several MeV. Therefore, FLCS—known to shape the IBe pattern —plays a key role in radiation belt dynamics.