Nonlinear wave processes at the lower hybrid resonance frequency are extremely important in transferring energy between different particle populations and fields in both astrophysical and laboratory plasmas. In this paper we investigate particle acceleration resulting from the relaxation of unstable ion ring distributions, producing strong wave activity at the lower hybrid resonance frequency, which collapses to produce energetic electron and ion tails. We apply the results to the problem of explaining energetic particle production in solar flares. There are several mechanisms whereby unstable ion distributions could be formed in the solar atmosphere, including reflection at perpendicular shocks, tearing mode reconnection, and loss cone depletion. Numerical simulations of ion ring relaxation processes, obtained using a 2 1/2-dimensional fully electromagnetic, relativistic particle in cell code are presented. The results show the simultaneous acceleration of electrons to energies in the range 10-500 keV, and ions to energies of the order of 1 MeV. The electron flux is sufficiently high to account for flare hard X-ray emission, on the basis of the thick-target model. The MeV ions have insufficient energy to account for gamma-ray line emission in the 4-6 MeV range, but they provide a seed population for further acceleration, which could result from the presence of either lower hybrid or MHD wave turbulence. Particle energization is observed to occur inside elongated nonlinear wavepackets, indicating that the acceleration process is highly filamented. Our simulations also show wave generation at the electron cyclotron frequency. We suggest that this process could play a role in the production of solar millisecond radio spikes, which are normally attributed to the cyclotron maser instability.
