ABSORPTION AT THE ION-ION HYBRID-RESONANCE IN THE PRESENCE OF INCOHERENT DENSITY-FLUCTUATIONS

A numerical study is made of the effects caused by incoherent fluctuations in the minority density concentration on the absorption of fast Alfven waves at the ion-ion hybrid resonance. This work extends to the incoherent regime of a previous study by Romero and Morales [Phys. Fluids B 2, 2007 (1990)], which surveyed in detail the effects of single-mode (coherent) fluctuations. The system is represented by a Budden equation that properly describes the resonance and cutoff structure arising from spatial variations of the equilibrium magnetic field. For wave incidence such that the cutoff is encountered before the resonance layer, the absorption coefficient increases monotonically as the fluctuation amplitude is increased. A specific case is presented in which the absorption changes from approximately 0% to approximately 70% for minority density fluctuations delta-n/n0 approximately 60% that correspond to electron density variations of less than 5%. For wave incidence such that the resonance is encountered before the cutoff, the reflection coefficient increases for minority density fluctuations below 5% of the ambient value. For larger density fluctuations, the reflected energy saturates at approximately 20% of the incident wave energy.