Effects of Oblique Wave Normal Angle and Noncircular Polarization of Electromagnetic Ion Cyclotron Waves on the Pitch Angle Scattering of Relativistic Electrons

Electromagnetic ion cyclotron (EMIC) waves have long been considered major plasma waves that can cause the atmospheric precipitation of electrons. In this work, we perform test particle calculations of the interactions between relativistic electrons and EMIC waves with oblique wave normal angle (WNA) and noncircular polarization. We demonstrate advective and diffusive changes in electron pitch angle (PA) under a uniform background magnetic field for broad ranges of kinetic energy (KE), PA, WNA, and ellipticity. First, we find that the direction of advective PA changes can be of either sign, being positive (negative) for a lower (higher) initial PA. Second, an oblique WNA can enhance the advection effects on PA changes, particularly for KE similar to 2.5-4 MeV and a broad range of initial PAs. In contrast, a more linear polarization can decrease advection for a broad range of KE and initial PAs. Third, phase space trajectories depend strongly on specific values of WNA, ellipticity, KE, and PA. Sometimes a phase-averaged PA change is a net result of a time-dependent mixture of the phase trapping, untrapping, and bunching effects. Lastly, the overall effect of finite WNA and ellipticity on diffusive PA changes is less systematic than on advection. Nevertheless, we identify a range of KE and PA where the diffusion tends to increase for a larger WNA and decrease for a more linear polarization. The results in this work emphasize a strong necessity for incorporating precise information on WNA and ellipticity into evaluations of electron scattering by EMIC waves.