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

Abstract

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.

Simulation of mode conversion process from upper-hybrid waves to LO-mode waves in the vicinity of the plasmapause

“…Based on the results of the recent simulation studies, the difference of the beaming angle could be explained by considering the condition that the parallel component of the refractive index is slightly different from the critical value in the mode conversion process (Kalaee et al, 2009). The results of present study focus on comparison between simulation and observation thereby suggest the importance of the spatial scale of the inhomogeneity in considering the generation process of LO-mode waves through the mode conversion process in actual space.…”

“…The relative wave amplitude in each angle is estimated from the conversion efficiency. Therefore, under the condition that efficient mode conversion is expected, the beaming angle of LO-mode waves could be different with the LMCT prediction Especially, under the condition that the spatial scale of the density gradient is of the order of the wavelength of incident UHR-mode waves, the efficient mode conversion could be expected and the radio window angle of generated LO-mode waves becomes different from the LMCT prediction (e.g., Kalaee et al, 2009). In the present study we have pointed out that the inhomogeneity with the spatial scale comparable with the wavelength is important in understanding the emission mechanism.…”

“…In a case that the parallel component of the refractive index of incident waves is slightly different from the critical value of the refractive index, two branches are disconnected and there is an evanescent layer even though the perpendicular component of the refractive index becomes zero for both LO-mode and Z-mode waves. Although the LMCT predicts that the mode conversion does not occur in such a case, the recent simulation results by Kalaee et al (2009) have shown that the effective mode conversion can be expected under the condition in which the spatial scale of the density gradient is of the order M. J. Kalaee et al: Conversion process from upper-hybrid waves to LO-mode waves of the wavelength of the incident UHR-mode waves.…”

“…There arises difficulty in theoretical treatments in a case that the spatial size of inhomogeneity is of the order of the wavelength of LO-mode waves, where the WKB approximation is invalid. The simulation studies have also pointed out that efficient LO-mode wave generation is expected in the case that the spatial scale of the density gradient is the order of wavelength (Katoh and Iizima, 2006;Kalaee et al, 2009). …”

“…We study the wave coupling process among UHR-mode, Z-mode and LO-mode waves by using a spatially twodimensional electron fluid model (Katoh, 2003;Katoh and Iizima, 2006;Kalaee et al, 2009). The basic equations are given as follows;…”