Study of Ionospheric Plasma Turbulence Caused by Powerful Ground-Based HF Radio Transmissions

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It is proposed to continue the observations of ionospheric plasma turbulence by radar back-scatter. Observations in 1990 and 1991 greatly increased the understanding of ionospheric Langmuir turbulence generated by a radio wave reflected in the F-region. In the proposed observations spectra of Langmuir turbulence will be obtained at 150 m height intervals every ms. Preliminary results of this type obtained in August 1991 show the temporal development and decay of the spectrum of Langmuir turbulence for 10 heights 150 m apart, for pulsed HF transmissions of 10 ms duration at 1 ms time intervals. It is also proposed to conduct theoretical investigations using an analytical model of Langmuir turbulence. In that model the free Langmuir mode is considered as the untrapped part of long wavelength plasma turbulence excited by the modulational instability. A generalization of the existing analytical model for the case of a finite magnetic field appropriate for the ionospheric experiments is also proposed.