Nonlinear Acceleration of Ultrarelativistic Electrons in the Outer Radiation Belt Disrupted by Transverse Wave Modulations

Abstract Whistler‐mode waves scatter ultrarelativistic electrons in the radiation belts and accelerate them through resonant interactions. In simplified models, nonlinear phase trapping by high‐amplitude waves can increase electron energy by several MeV within seconds. However, the acceleration rate in realistic wave packets is slower due to small‐scale wave field structures reducing trapping efficiency. While previous studies focused on short field‐aligned amplitude modulations and phase jumps, we examine the effects of transverse modulations, which have been observed to reach scales comparable to ultrarelativistic electron gyroradii. Using test‐particle simulations, we demonstrate that these modulations disrupt the acceleration process. Our numerical results suggest that nonlinear trapping plays a negligible role in accelerating electrons above a certain energy limit, reinforcing the diffusive nature of wave‐driven electron transport at multi‐MeV energies. Unlike field‐aligned structures, transverse phase incoherence modifies the effective wave spectrum and allows for resonance, making amplitude modulations a necessary component for suppression of acceleration.