Evolution of High-m Poloidal Alfven Waves in a Dipole Magnetic Field

Abstract:

We investigate how initially high-m, poloidal Alfven waves evolve using a numerical model solving the ideal, cold, linear magnetohydrodynamic (MHD) equations in a 2D dipole coordinate system. The curved magnetic geometry provides a key difference between the poloidal and toroidal Alfven frequencies of any one field line. A polarisation rotation from poloidal towards toroidal predicted from the Cartesian box model theory still occurs, but now with the waves following contours of Alfven frequency, which moves the Alfven wave across field lines. The structure of these contours depends on the harmonic mode along the field line and the equilibrium. We find that the amplitude peak of the poloidal mode moves significantly radially outward in time. When the typically observed azimuthal phase motion of such waves is included, hodograms show a polarisation rotation from purely poloidal to a mixed poloidal/toroidal polarisation at all locations. Such features could be used to help interpret satellite observations of Pc4-5 poloidal ultralow frequency (ULF) waves in Earth's magnetosphere.