Particle orbits in the presence of the fast wave fields are calculated. It is found that the resonant interaction of ions with the wave can be neglected at ω0 = 2ωci, provided that the wave electric field strength is less than a critical value Ec( = 4k ∥Tle). The dominant effect on the dispersion relation of the drift-cyclotron loss-cone mode is thus found to be due to the nonresonant effects of the wave on electrons and ions. It is found that a fast wave with ω0 = 2ωci destabilizes the mode. For ω0<2ωci, fast waves can stabilize the mode for certain values of the plasma β which is calculated numerically.
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