[en] For the evaluation of the velocity diffusion, a set of difference equations is derived which describes the motion of an ion interacting with an externally excited lower hybrid wave in a nonuniform magnetic field. In addition to the well-known cyclotron harmonic resonance, the formula includes a phase shift of a wave-particle interaction. The latter, which has been missed in all previous papers, enhances the diffusion of ions in velocity space. The frequency shift due to the guiding-center drift is also included. It can govern the velocity diffusion when the particle energy is high. A numerical calculation for an example is presented to show how the velocity diffusion is enhanced. (author)