Linear mode conversion of Langmuir/z-mode waves to radiation in plasmas with various magnetic field strength

posted Dec 16, 2013, 11:15 AM by Eun-Hwa Kim
Eun-Hwa Kim, Iver. H. Cairns and Jay R. Johnson

Phys. Plasmas 20, 122103 (2013)

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 plasmasusing a numerical electron fluid simulation code when the density gradient has a wide range of angle, δ, to the ambient magnetic field, B 0, 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(ωc e) < 1.5, whereωc e 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, 40° < δ < 60°, for intermediately magnetized plasmas with Ω0 = 1.0 and 1.5, while it is produced over a wider range, 0° ≤ δ ≤ 90°, 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 energyconversion efficiencies are 5%14% (depending on the adiabatic index γ and β = T e/m e c 2, where T e is the electron temperature and m e 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 circularpolarization of interplanetary type II and III solar radio bursts because a strong xR mode can be generated via linear mode conversion near δ ∼ 45°.