FDTD modeling of ULF waves in the magnetosphere and ionosphere

The magnetised plasma of near-Earth space (magnetosphere) supports two ultra-low frequency (ULF; 1-100 mHz), magnetohydrodynamic (MHD) oscillations known as the shear and fast Alfvén wave modes. The fast mode propagates across the ambient magnetic field, spreading ULF wave energy throughout the magnetosphere. For sufficiently large ionosphere conductance, the shear Alfvén mode forms field line resonances (FLRs) between the northern and southern ionospheres. Developing applications for remote sensing the magnetosphere using ULF waves involves an understanding of these resonance modes. While modeling the magnetosphere part of the solution is relatively straightforward, adding the boundary conditions imposed by the ionosphere and at the magnetopause is more challenging. The ionosphere boundary formulation is described in addition to the implementation of an absorbing layer at the outer boundary. This avoids previous unrealistic restrictions at both the inner and outer boundaries of MHD wave models.