Kinetic model of Alfvén wave light-ion gyroresonance heating

The absorption of Alfvén waves by gyroresonant interaction with the light-ions in an ionospheric oxygen-hydrogen plasma with a parallel magnetic field gradient is reconsidered, taking into account simultaneously the plasma inhomogeneity and the finite temperature of the resonant ions (T_i ≠ 0). A kinetic full wave equation is derived, that is valid in the vicinity where the wave frequency matches the local proton gyrofrequency. It is analytically solved for the case of a Lorentzian distribution function. The energy transmission, reflection and absorption coefficients, for waves incident from the high magnetic field side onto the gyroresonant interaction region, are found to be the same as for the cold plasma case (T_i = 0). Conversely, the absorption of waves incident from the low magnetic field side is found to be enhanced and strongly depends on the ionic temperature, whereas their transmission to the high magnetic field side occurs still in the same proportion as for the cold plasma case. Elaborating local dispersion curves and evaluating the full wave solutions enable us to interpret qualitatively these results and to extrapolate them for the case of a Maxwellian distribution function. For realistic ionospheric plasma conditions a sensible increase of the gyroresonant absorption of the waves is thus found.