Stochastic Acceleration by an Electrostatic Wave near Ion Cyclotron Harmonics

We study a nonlinear effect on ion motion of an electrostatic wave propagating perpendicularly to a uniform magnetic field. When the wave near the ion cyclotron harmonics has sufficiently large amplitude, the trapped motions of hot ions ($v_{\perp }>\frac{\omega }{k_{\perp }}$) become stochastic in the region of appreciable width near the separatrix. Numerical calculations show the tail formation in a $v_1$ distribution and theoretically predicted upper bound conditioned by the amplitude.