LOWER HYBRID-RESONANCE ACCELERATION OF ELECTRONS AND IONS IN SOLAR-FLARES AND THE ASSOCIATED MICROWAVE EMISSION

Nonlinear wave processes at the lower hybrid resonance frequency are e xtremely important in transferring energy between different particle p opulations and fields in both astrophysical and laboratory plasmas. In this paper we investigate particle acceleration resulting from the re laxation of unstable ion ring distributions, producing strong wave act ivity at the lower hybrid resonance frequency, which collapses to prod uce energetic electron and ion tails. We apply the results to the prob lem of explaining energetic particle production in solar flares. There are several mechanisms whereby unstable ion distributions could be fo rmed in the solar atmosphere, including reflection at perpendicular sh ocks, tearing mode reconnection, and loss cone depletion. Numerical si mulations of ion ring relaxation processes, obtained using a 2 1/2-dim ensional fully electromagnetic, relativistic particle in cell code are presented. The results show the simultaneous acceleration of electron s to energies in the range 10-500 keV, and ions to energies of the ord er of 1 MeV. The electron flux is sufficiently high to account for fla re hard X-ray emission, on the basis of the thick-target model. The Me V 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 hyb rid or MHD wave turbulence. Particle energization is observed to occur inside elongated nonlinear wavepackets, indicating that the accelerat ion process is highly filamented. Our simulations also show wave gener ation at the electron cyclotron frequency. We suggest that this proces s could play a role in the production of solar millisecond radio spike s, which are normally attributed to the cyclotron maser instability.