A theory of medium-energy (~ keV) electrons and heavy ions in Jupiter's magnetosphere is presented. Lower hybrid waves are generated by the combined effects of a ring instability of neutral wind pickup ions and the modified two-stream instability associated with transport of cool Iogenic plasma. The quasi-linear energy diffusion coefficient for lower hybrid wave-particle interactins is evaluated, and several solutions to the diffusion equation are given. Calculations based on measured wave properties show that the noise modifies substantially the particle distribution functions. The effects are to accelerate superthermal ions and electrons to keV energies and to thermalize the pickup ions on time scales comparable to the particle residence time. The [S2+>/[S+> ratio at medium energies is a measure of the relative contribution from Iogneic thermal plasma and neutral wind ions, and this important quantity should be determined from future measurement. The theory also predicts a p referential acceleration of heavy ions with an acceleration time that scales inversely with the root of the ion mass. Electrons accelerated by the process contribute to further reionization of the neutral wind by electron impact, thus providing a possible confirmation of Alfv¿n's critical velocity effect in the Jovian magnetosphere.