Simulation of mode conversion from UHR-mode wave to LO-mode wave in an inhomogeneous plasma with different wave normal angles

We have investigated a linear mode conversion process among UHR-mode, Z-mode, and LO-mode waves by a computer simulation solving Maxwell's equations and the motion of a cold electron fluid. The characteristics of the wave coupling process occurring in the cold magnetized plasma were examined in detail for the case of an inhomogeneity of plasma density lying perpendicular to the ambient magnetic field. The dependence of the conversion efficiency on the incident wave normal angle, wave frequency, background plasma frequency, and steepness of density gradient was studied. The results show that an efficient mode conversion occurred in the conversion process from Z-mode to LO-mode waves rather than from the coupling between UHR-mode to LOmode waves. They also show that the highest conversion efficiency was obtained under the specific condition of the wave normal angle for the incident waves. In the specific case of such critical wave normal angles, we found that perpendicular components of refractive indexes became zero at the site of mode conversion, which is consistent with previously published results. We also show that the range of the critical normal angle varied depending on both the plasma frequency and the wave frequency. The simulation results also reveal that, when the steepness of the density gradient was taken into consideration, efficient mode conversion could be expected even in the case of the mismatch of the refractive indexes preventing the close coupling of plasma waves.