Abstract
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 LO-mode 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.
Highlights
A large variety of plasma waves exist in space plasmas, depending on the conditions of the medium
3.2.4 Summary and discussion of Case 2 Based on the simulation results of Case 2, we found that the conversion efficiency of the double-mode conversion process is strongly dependent on the wave normal angle and the wave frequency of the incident waves
Our simulation results confirm that the wave energy of the UHR-mode waves tend to be converted into the LO-mode waves through the mode conversion process
Summary
A large variety of plasma waves exist in space plasmas, depending on the conditions of the medium. The refractive index is determined by the characteristics of waves, such as the wave frequency and the wave normal angle, and by plasma parameters, such as the number density of the plasma and the background magnetic field strength. In an inhomogeneous plasma where these parameters vary as a function of position, the local refractive index is a function of position (Stix, 1992). The mode conversion, identified as a change in the propagation modes of plasma waves, is one of the generation mechanisms of radio emissions occurring in an inhomogeneous plasma. Mj lhus (1983, 1990) discussed the linear mode conversion of an ordinary polarized electromagnetic wave in a magnetized plasma with a density gradient parallel to the magnetic field and developed an expression for the conversion coefficient as a function of the incident wave normal angle. The magnetic field leads to a narrowing of the range of angles of incidence, leading to linear conversion when compared with the case of the unmagnetized field
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