Nonlinear interaction of outer zone electrons with VLF waves

Analytical expressions have been obtained for three different types of cyclotron‐resonant interaction of a test particle with a monochromatic electromagnetic wave: diffusion, phase bunching, and phase trapping. All three lead to changes in particle energy and pitch angle. They are evaluated for VLF waves and electrons at L = 5.5, to try to account for observed electron acceleration following magnetic storms. The results are that diffusion may be too slow, while phase bunching leads to deceleration. A maximal estimate of phase trapping, considered in isolation, leads to the acceleration of at least some 0.1 MeV electrons to ∼1 MeV in about 1 minute.