Resonant absorption of downward propagating electromagnetic hiss

Electromagnetic hiss is regularly detected by electric and magnetic wave receiver on board low-altitude satellites at high magnetic latitudes. We solve the full wave equation for electromagnetic magnetosonic waves (MSWs) initially launched earthward at high altitudes above the satellite. Owing to both the inhomogeneity of the medium (variation along the geomagnetic field) and the bi-ionic composition of the plasma (H+, O+), the wave field undergoes a singularity at the altitude where its frequency matches the local gyrofrequency of H+. This phenomenon is accompanied by strong modifications of the wave properties (for example, a polarization reversal) and by a resonant absorption that contributes to the heating of the protons. It is shown that a WKB point of view still provides a qualitative description of the full wave propagation, The resonant absorption can then be viewed as a three-step process as follows: (1) downward propagation of MSWs followed by (2) a reflection and by (3) a mode conversion into electromagnetic ion cyclotron waves, which are appropriate to heat the protons. During this process wave polarization reverses near the crossover point, where the mode conversion mainly occurs. The amplitude of the component B-perpendicular to is found to undergo a moderate increase at the point where the wave frequency matches the proton gyrofrequency. The analysis is extended to include kinetic effects resulting from the longitudinal motions of the electrons T-e. This aims at explaining the strong enhancement of the parallel electric field recorded onboard AUREOL 3. If T-e >> T-i, it is shown that this E(parallel to) can be interpreted as being due to ion acoustic waves excited by upward propagating electromagnetic ion cyclotron waves. The estimate of the size of E(parallel to) (and of B-perpendicular to) allows a precise diagnosis of the mode conversion of the incident electromagnetic hiss and therefore helps understanding how its energy is used to heat the protons.