Transient Distortions of the South Atlantic Anomaly Radiation Environments Driven by Large-Scale Electric Fields

Energetic electrons in Earth's inner radiation belt pose significant hazards to spacecraft systems, with the strongest radiation in low-Earth orbit (LEO) mostly confined to the South Atlantic Anomaly (SAA) region. Once considered stable, the inner belt is now understood to exhibit significant variability. Using data from the low-Earth-orbit Macau Science Satellite-1 mission, we report transient distortions of the SAA radiation environments, observationally characterized by enhanced fluxes of energetic electrons either attached to or detached from the traditional SAA boundary. We show that these distortions are induced by large-scale electric-field perturbations that adiabatically alter the electron mirror heights, which can be further modulated by ultra-low-frequency waves. Test-particle simulations successfully reproduce the observational features and provide new constraints on the properties of the associated electric fields. These findings reveal a less recognized variability of the inner belt, extending the electron radiation risks beyond the expected SAA boundaries.