Abstract
The mode conversion process responsible for radio wave generation has been studied for several decades; however, the properties of the condition required for an efficient conversion process are still unknown. The aim of this study is to determine the value of plasma frequency required for an efficient mode conversion process from slow Z-mode to left-hand ordinary (LO)-mode waves in the matching cases, where the two branches of the dispersion relation of the two modes are perfectly connected. We derive the dispersion relations for electromagnetic wave propagation in an inhomogeneous plasma considering Snell’s law and investigated them in detail. We quantify the minimum variation of plasma frequency required for the efficient mode conversion process, which we call “the plasma frequency demand.” We show that the condition required for the efficient mode conversion can be satisfied by waves propagating first toward the high-density region and then returning toward the low-density region before reaching the region where the wave frequency matches the cutoff frequency; therefore, a large inhomogeneity is not always required. We show that the angle between the background magnetic field and the density gradient has a significant effect on the plasma frequency demand.
Highlights
The mode conversion responsible for radio wave generation has been studied for several decades (e.g., Oya 1971, 1974; LaBelle 2018) and owing to the importance of this subject, it is still an active topic in a lot of research
Fast Z-mode waves of frequency ω can exist in the higher density region as long as the wave frequency is higher than the Z-mode wave cutoff frequency ωZ−cutoff, the waves change their propagation direction toward the lower density region before reaching the region where ω = ωZ−cutoff and propagate further into the lower density region passing through the layer with ω = ωp, where the waves have the possibility to convert into left-hand ordinary (LO)-mode waves
Theory and assumptions First, we review the coupling properties of the Z-mode and LO-mode waves based on the cold plasma theory, and we extend our discussion to formulate the plasma frequency for the mode conversion processes
Summary
Introduction The mode conversion responsible for radio wave generation has been studied for several decades (e.g., Oya 1971, 1974; LaBelle 2018) and owing to the importance of this subject, it is still an active topic in a lot of research. Jones (1980) considered the condition for the mode conversion under the assumption that the magnetic field direction is perpendicular to the density. Kalaee and Katoh Earth, Planets and Space (2020) 72:95 gradient and derived a formula for the beaming angle of LO-mode waves. Kalaee and Katoh (2016b) considered the magnetic field direction arbitrary to the density gradient to generalize the condition for the mode conversion and presented the formula for the beaming angle of LO-mode waves. Let us consider a situation where slow Z-mode waves of frequency ω propagate toward a region of higher plasma density. These waves become fast Z-mode waves after passing through the layer with ω = ωp.
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