Astronomy and Astrophysics – Astronomy
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusmsp21b..05k&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #SP21B-05
Astronomy and Astrophysics
Astronomy
6949 Radar Astronomy, 7509 Corona, 7514 Energetic Particles (2114), 7839 Nonlinear Phenomena, 7868 Wave/Wave Interactions
Scientific paper
We develop a theory for radar signal scattering by anisotropic Langmuir turbulence in the solar corona due to a t+l = t process. Langmuir turbulence is considered (thought) to be generated in a cone by a narrow field-aligned electron beam e.g. by an electron beam responsible for a type III solar burst. Expressions for the scattering altitudes, frequency shifts, cross-sections, efficiency of the process, and optical depth are obtained. We show that the absolute value of the radar echo frequency shift is equal to the local plasma frequency at the scattering spot. The frequency shift is positive for the coalescence case and negative for the decay case. The minimum echo frequency shift is determined by the minimal phase velocity of the Langmuir waves, the maximum shift is determined by the electron beam velocity, but in any case can not exceed -ωt/2 and ωtt, where ωt is the frequency of a radar signal. Angular characteristics of the scattered signal differ dramatically for the cases of coalescence and decay. The signal is scattered into a narrow cone high above the specular reflection point (ωp ≪ ωt), but in the vicinity of ωp ~ ωt/2 the red echo is scattered isotropically, while the blue echo is scattered into a even narrower cone. We show that the scattering efficiency increases with increase of the local plasma frequency as well as the radar frequency.
One should note, that at ωp = ωt/2 and ωp = ωt efficiency goes to infinity. Strong peaks of the echo at ωt/2 and 2ωt, may be observed only if the velocity of electron beam exceeds c/√{3}. The considered t+l = t process is essential and can be successfully used for radar studies of the spectral energy density of anisotropic Langmuir turbulence.
Khotyaintsev Mykola V.
Konovalenko O. O.
Mel'nik Valentin N.
Thide' B.
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