Linear Mode Conversion of Langmuir/z-mode Waves To Radiation In Plasmas With Various Magnetic Field Strength
Authors: Eun-Hwa Kim, Iver H. Cairns and Jay R. Johnson
Abstract:
Linear mode conversion of Langmuir/z waves to electromagnetic radiation near the plasma and upper
hybrid frequency in the presence of density gradients is potentially relevant to type II and III solar radio bursts,
ionospheric radar experiments, pulsars, and continuum radiation for planetary magnetospheres. Here, we
study mode conversion in warm, magnetized plasmas using a numerical electron fluid simulation code when the density
gradient has a wide range of angle, ω, to the ambient magnetic field, B0, for a range of incident Langmuir/z wavevectors.
Our results include: (1) Left- handed polarized ordinary (oL) and right-handed polarized extraordinary (xR) mode
waves are produced in various ranges of δ for Ω0 = (ωL/c)1/3(ωce/ω) < 1.5, where ωce is the (angular) electron cyclotron
frequency, ω is the angular wave frequency, L is the length scale of the (linear) density gradient, and c is the speed of light;
(2) the xR mode is produced most strongly in the range, 40o < δ < 60o, for intermediately magnetized
plasmas with Ω0 = 1.0 and 1.5, while it is produced over a wider range, 0o < δ < 90o , for weakly magnetized
plasmas with Ω0 = 0.1 and 0.7; (3) the maximum total conversion efficiencies for wave power from the Langmuir/z mode to radiation
are of order 50%–99% and the corresponding energy conversion efficiencies are 5%–14% (depending on the adiabatic index
γ and β = Te/mec2, where Te is the electron temperature and me is the electron) for various Ω0;
(4) the mode conversion window becomes wider as Ω0 and δ increase. Hence, the results in this paper confirm that linear mode conversion under
these conditions can explain the weak total circular polarization of interplanetary type II and III solar radio bursts
because a strong xR mode can be generated via linear mode conversion near δ ~ 45o
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Submitted to: Physics of Plasmas (July 2013)
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Download PPPL-4988 (pdf KB pp)
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