A numerical model to investigate the polarisation azimuth of ULF waves through an ionosphere with oblique magnetic fields

Abstract. A one dimensional, computational model for the propagation of ultra low frequency (ULF; 1-100 mHz) wave fields from the Earth's magnetosphere through the ionosphere, atmosphere and into the ground is presented. The model is formulated to include solutions for high latitudes where the Earth's magnetic field, (B₀), is near vertical and for oblique magnetic fields applicable at lower latitudes. The model is used to investigate the wave polarisation azimuth in the magnetosphere compared with the ground wave fields, as a function of the dip angle of B₀. We find that for typical ULF wave scale sizes, a 90° rotation of the wave polarisation azimuth from the magnetosphere to the ground occurs at high latitudes. However, this effect does not necessarily occur at lower latitudes in all cases. We show that the degree to which the wave polarisation azimuth rotates critically depends on the properties of the compressional ULF wave mode.