Effect of the field line torsion on the polarization of ULF waves

Polarization of shear Alfven waves observed in the terrestrial magnetosphere is almost never exactly poloidal or toroidal. Ultra Low Frequency (ULF) waves of mixed polarization naturally appear if we assume that the background magnetic field lines are not contained in the meridional planes, i.e. if they have non-zero torsion. To illustrate the effect of torsion on the polarization of ULF waves, we developed a simple analytical model of the magnetic field with non-planar field lines which are similar to the magnetic field lines in the dawn or dusk flanks of the magnetosphere. This field is explicitly characterized by a control parameter describing the degree of deviation from the dipole magnetic field. Shear Alfven waves in this background field are described using covariant-contravariant formalism which allows a self-consistent calculation of the wave polarization. Our calculations show that even small torsion of the background magnetic field lines leads to significant deviations of the wave polarization from pure poloidal or toroidal direction. In contrast, the frequencies of the ULF remain practically unaffected by the torsion of the background magnetic field. The results of our model calculations show that the electric field of the commonly observed quasi-toroidal mode has an azimuthal component and, therefore, can effectively contribute the energization of the charged particles which undergo gradient-curvature drift in the inner magnetosphere.