Drift‐Resonance Driven by Field Line Resonance Gradients: A Mechanism for Non‐Dispersive Microinjection Events

Abstract Ultra‐low‐frequency waves in the magnetosphere often produce field line resonances with strong spatial gradients in wave amplitude. We analyze a form of drift resonance driven by these gradients near the resonant field line. This mechanism differs fundamentally from conventional gradient‐ and curvature‐driven drift resonance and predicts an energy‐independent, non‐dispersive signature in particle flux. Observations from the Magnetospheric Multiscale mission reveal a non‐dispersive electron microinjection event occurring near the resonant L ‐shell of a toroidal wave, providing the first direct evidence for this resonance type in the terrestrial magnetosphere. We present a classification of particle‐wave interactions that includes both trapped and passing drift‐resonant populations, demonstrating how field line resonance structures can generate rapid, coherent microinjections distinct from classical drift‐resonance behavior.