The modelling of VLF Trimpis using both finite element and 3D Born modelling

This paper investigates the numerical modelling of VLF Trimpis produced by a D region inhomogeneity on the Great Circle Path. Two different codes are used. The first is a 2D finite element method (FEM) code, whose solutions a re valid in the non-Born limit. The second is a 3D model that invokes the B orn approximation. The predicted Trimpis from these codes compare closely, thus confirming the validity of both models. The modal scattering matrices have a comparable structure, and indicate strong scattering between the dom inant TM modes. Analysis of the scattering matrix from the FEM code delinea tes the Born regime. For a LIE with a radius of 100kms, the Born approximat ion becomes invalid at an electron density perturbation of about 8 el/cc.