Modelling the conversion of compressional waves into shear Alfven waves in Earth magnetosphere

Simulation results are presented for the linear conversion of compressional waves into shear Alfven waves in Earth magnetosphere. The simulations use a finite element discretization of the MHD wave equations on an unstructured tetrahedral grid. The grid was constructed so as to conform to a magnetospheric field configuration as calculated by the University of Michigan BATS-R-US code. The unstructured mesh is everywhere aligned along the magnetic field, so as to accurately preserve the strong anisotropy that characterizes the propagation of shear Alfven waves along magnetic field lines. Different conversion mechanisms are considered, including shear waves resulting from a single compressional pulse at different locations in the magnetosphere, as well as waves driven resonantly by monochromatic compressional pump wave. Our model agrees with earlier calculations made in the 1D or 2D approximation, or in an idealized dipole geometry. To our knowledge, however, this is the first calculation of this type of mode conversion reported in a realistic (computed) magnetospheric configuration.