VLF Trimpi modelling on the path NWC-Dunedin using both finite element and3D 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 to model Trimpis on the path NWC-Dunedin. The first is a 2D Finite El ement Method Code (FEM), whose solutions are rigorous and valid in the stro ng scattering or non-Born limit. The second code is a 3D model that invokes the Born approximation. The predicted Trimpis from these codes compare ver y closely, thus confirming the validity of both models. The modal scatterin g matrices for both codes are analysed in some detail and are found to have a comparable structure. They indicate strong scattering between the domina nt TM modes. Analysis of the scattering matrix from the FEM code shows that departure from linear Born behaviour occurs when the inhomogeneity has a h orizontal scale size of about 100 km and a maximum electron density enhance ment at 75 km altitude of about 6 electrons. (C) 1998 Elsevier Science Ltd. All rights reserved.