Linear mode conversion of Langmuir/z-mode waves to radiation: Scalings of conversion efficiencies and propagation angles with temperature and magnetic field orientation

Abstract

Linear mode conversion (LMC) is the linear transfer of energy from one wave mode to another in an inhomogeneous plasma. It is relevant to laboratory plasmas and multiple solar system radio emissions, such as continuum radiation from planetary magnetospheres and type II and III radio bursts from the solar corona and solar wind. This paper simulates LMC of waves defined by warm, magnetized fluid theory, specifically the conversion of Langmuir/z-mode waves to electromagnetic (EM) radiation. The primary focus is the calculation of the energy and power conversion efficiencies for LMC as functions of the angle of incidence θ of the Langmuir/z-mode wave, temperature β=Te/mec2, adiabatic index γ, and orientation angle ϕ between the ambient density gradient ∇N0 and ambient magnetic field B0 in a warm, unmagnetized plasma. The ratio of these efficiencies is found to agree well as a function of θ, γ, and β with an analytical relation that depends on the group speeds of the Langmuir/z and EM wave modes. The results demonstrate that the energy conversion efficiency ϵ is strongly dependent on γβ, ϕ and θ, with ϵ∝(γβ)1/2 and θ∝(γβ)1/2. The power conversion efficiency ϵp, on the other hand, is independent of γβ but does vary significantly with θ and ϕ. The efficiencies are shown to be maximum for approximately perpendicular density gradients (ϕ≈90°) and minimal for parallel orientation (ϕ=0°) and both the energy and power conversion efficiencies peak at the same θ.