Field Line Resonances in the Non-axisymmetric Plasmasphere

Abstract:

Field line resonances (FLR) often occur in the magnetosphere owing to the coupling of compressional and transverse waves in inhomogeneous space plasmas. It has been well known that the wave energy is transferred from the fast mode to standing shear Alfven mode or FLR, which have been extensively studied in the models of axisymmetric geometry. However, the realistic plasmasphere often shows serious asymmetry between the dawn- and dusk-side or around the different local times. In such fully three-dimensional nonuniform space, it has not been investigated until now how much the previous ansatz works well. In this study, we have developed a new 3-D model, which allows an arbitrarily non-axisymmetric plasmasphere in dipole geometry. We have performed three-dimensional MHD simulations to investigate how FLR are affected by the inhomogeneity along each direction. Our numerical results show that the FLR persist regardless of field-aligned inhomogeneity, but their nature of shear mode and its polarization is disturbed seriously by the non-axisymmetric plasmasphere. Its polarization pattern is found to vary with the local angle between the magnetic surface and the constant Alfven speed surface. It is shown how the FLR signals appear in each LT location when reasonable non-axisymmetric plasmaspheres are assumed, where both ground and space signals are compared and discussed compared to the previous axisymmetric cases.