Signatures of Resonant Mode Conversion at the Magnetopause

At low frequency, the MHD equations describe coupling between the compressional and Alfvén wave near the Alfvén resonance. Coupling between the modes occurs when there is a gradient in the Alfvén velocity at the location where the wave frequency matches the Alfvén resonant frequency (kallel VA). However, the modes are coupled only in the case where there is a wave perturbation (ky) perpendicular to the magnetic field (z) and to the direction of the Alfven velocity gradient (x). At higher frequency, f > 0.1 fci) coupling may also occur due to the polarization drift. We solve the full fluid wave equations for a 1D magnetopause equilibrium model to determine the efficiency of mode conversion including coupling due to (f/fci) effects and ky. Wave absorption is quantified by comparing the incoming and reflected compressional wave Poynting fluxes for numerical solutions obtained by solving the corresponding difference equation on a grid with appropriate boundary conditions. For the magnetopause equilibrium model, we find that mode conversion efficient peaks where (ky VA/ ω) ~ 0.4 and f/fci ~ 0.3 with nearly total absorption. The efficiency is significantly weaker (10%) in the limit f \ll fci as in the MHD approach. We also discuss the effect of heavy ions on the mode conversion process. We compare the mode conversion efficiency with hybrid simulations and discuss kinetic effects on the mode conversion process. Finally, we consider these results in the context of magnetopause wave measurements and discuss implications for particle transport.