Parametric Study of Asymmetric Propagation of Guided and Unguided EMIC Waves Near the Local Characteristic Frequencies

Previous studies demonstrate that the propagation of the guided and unguided electromagnetic ion cyclotron (EMIC) waves near the local characteristic frequencies in a dipole field shows prominent asymmetry with the wave vector $k$ pointing toward lower L shells (wave normal angle ) and higher L shells ($\psi >0^{\circ }$) at low magnetic latitudes () using representative cases. The $\psi$ and ${\theta }_M$ dependence of this asymmetric behavior and the role of magnetic gradients in this process are not clear. Using full-wave simulations and ray theories, a parametric study is conducted to address these questions. We find that the refraction due to the magnetic gradient term can be quantified by $\Delta \Psi$ (the variation of $\psi$ during the wavelength) primarily contributed by the product of the normalized derivative of the refractive index, the magnetic gradient and the unit vector perpendicular to $k$. At low ${\theta }_M$, the sign of $\Delta \Psi$ generally remains constant and $\left|\Delta \Psi \right|$ is large so that $k$ with $\psi >0^{\circ }$ keeps deviating from the field line while $k$ with has the potential to be close to the field line. This explains that the guided left-handedly polarized (LHP) EMIC wave with may reverse its polarization and propagates to much lower altitudes if $\psi$ is within certain angular window. This also explains the $k$ for the maximum coupling efficiency from the unguided to guided LHP EMIC mode can be tens of degrees away from the field-aligned direction. As ${\theta }_M$ increases, the asymmetry of $\Delta \Psi$ and the propagation of EMIC waves near the local characteristic frequencies becomes smaller.