Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh11b1663k&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SH11B-1663
Physics
Plasma Physics
[2471] Ionosphere / Plasma Waves And Instabilities, [6984] Radio Science / Waves In Plasma, [7534] Solar Physics, Astrophysics, And Astronomy / Radio Emissions, [7847] Space Plasma Physics / Radiation Processes
Scientific paper
Radio emissions near the electron plasma or upper hybrid frequency region have been generated in various space plasmas, such as the solar corona and interplanetary medium (solar radio bursts), foreshock regions upstream of Earth’s bowshock, outer heliosphere, magnetospheres of Earth and other magnetized planets, and ionospheres. Because the radio emissions are a primary observable, understanding physical processes leading to wave emissions is fundamental to interpreting observations of the sun and the inner and outer heliosphere. At present, the role and importance of linear mode conversion (LMC) processes in producing such radio emissions is not well understood. Because early work on LMC suggested that its efficiency was inadequate to explain the emissions, subsequent research efforts primarily focused on nonlinear mechanisms due to the highly nonthermal levels of Langmuir waves. However, nonlinear processes could not explain the mixed polarizations of radio emissions. On the other hand, other studies of LMC suggested a much higher efficiency than previously expected and that LMC would dominate over the efficiency of nonlinear processes. More recently, our numerical wave simulations found that LMC is not only efficient, but also leads immediately to linearly and partially polarized radiations consistent with observations, thereby resolving the mixed polarization issue. Here we use numerical simulations of the warm plasma fluid equations to investigate the detailed characteristics of LMC as functions of the outgoing EM o and x modes, k, the angles φ between B0 and ▽N0, B0, and the length scale of the density irregularity L. These results extend the recent demonstration that LMC produces both o and x-mode radiation when φ = 0 [Kim et al., Phys. Rev. Lett., 99, 015003, 2007], contrary to previous expectations that only the o mode is produced. Our new results include: (1) Only one radio window exists, smoothly linking the previous two windows as φ, f and k change. (2) Both o and x-mode radiation are produced in the unmagnetized to moderately magnetized regimes, as found for the φ = 0 case. (3) Less than 10% of the incident ES energy is transformed into EM radiation. (4) In weakly magnetized situations almost equal amounts of o and x-mode radiation are produced, but only the o mode is produced for large enough B0. Applications of these results to natural solar and planetary emissions and to ionospheric modification experiments are also explored.
Cairns Iver H.
Kim Erik
Robinson Adam P.
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