The Dominant Role of the Electron Isotropy Boundary in Controlling Earth's Outer Radiation Belt Electron Lifetimes

Abstract Field‐line curvature scattering (FLCS) is believed to be the primary mechanism forming electron isotropy boundaries (IB) and can rapidly scatter relativistic electrons from the outer radiation belt. However, its direct and quantitative impact on controlling outer belt electron lifetimes has never been directly assessed. Using simultaneous observations of IBs from low‐altitude satellites and in situ electron fluxes from equatorial satellites, we report IBs intruding into the outer belt (reaching L ∼ 4.5), closely synchronized with sharp flux radial gradients near IBs, caused by significant electron loss outside IBs during a 4‐day storm recovery period. By combining observations with simulations, we provide the first direct and quantitative evidence that FLCS‐induced electron loss outside the IB dominantly controls the outer belt electron lifetimes. Our findings reveal that this simple yet fundamental physical process, which has been historically neglected in global radiation belt models, can explain the outer electron belt configuration.