Stochastic acceleration by an electrostatic wave near ion cyclotron harmonics

A nonlinear effect of a large amplitude electrostatic wave propagating perpendicularly to a static magnetic field on the motion of an ion is studied. The case where the frequency of the wave w is sufficiently close to a multiple of the ion cyclotron frequency is considered. It is found that the trapping motion is influenced by periodic forces. These forces lead to randomization of the trapping motion near the separatrix, and ions are expected to be stochastically accelerated. In order to confirm the results, numerical calculations are carried out.