Effect of Field Line Torsion on the Polarization of ULF Waves

Abstract In this paper we suggest a simple modification of the dipole magnetic field which introduces field‐aligned currents and torsion to the field lines. The resulting field lines are not contained in the meridional planes and have resemblance to the geomagnetic field lines in the dawn and dusk flanks of the magnetosphere. We analyze polarization of the guided standing Shear Alfvén Waves (SAWs) in this background field. The resulting polarization is mixed, and pure toroidal and poloidal waves appear only in the limit of exactly dipole background magnetic field. We show that even a small amount of torsion of the background field lines leads to significant deviations from the pure toroidal and poloidal polarization. In contrast, the frequency of the standing waves is much less sensitive to small deviations from the dipole field. In our study the deviation of the wave polarization from either toroidal or poloidal is attributable entirely to the topology of the background magnetic field, as both the magnetic field strength and the plasma density are constrained to be axisymmetric.