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Vidmar, R. J.

Publications and source records attributed to Vidmar, R. J..

4 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Long-delayed radio echoes - Mechanisms and observations

Experiments carried out in Alaska with the object of observing long-delayed radio echoes (LDE) are described. The results are subjected to critical sonogram analysis, together with data form previous experiments carried out at Stanford and elsewhere. It is concluded that, although strong supporting evidence for sporadic LDE phenomena has accumulated over the late 50 years, and a variety of plausible mechanisms have been proposed, stringent analysis of records such as those discussed in the paper almost invariably reveal alternative possible explanations for them. Further detailed experimentation is required on mechanisms, for which the phenomenon seems best established, e.g., round-the-world echoes and other forms of guided propagation.

Vidmar, R. J.↗
NASA NTRS1983

Delta function excitation of waves in the earth's ionosphere

Excitation of the earth's ionosphere by delta function current sheets is considered, and the temporal and spatial evolution of wave packets is analyzed for a two-component collisional F2 layer. Approximations of an inverse Fourier-Laplace transform via saddle point methods provide plots of typical wave packets. These illustrate cold plasma wave theory and may be used as a diagnostic tool since it is possible to relate specific features, e.g., the frequency of a modulation envelope, to plasma parameters such as the electron cyclotron frequency. It is also possible to deduce the propagation path length and orientation of a remote radio beacon.

Vidmar, R. J.↗
NASA NTRS1982

Observational dispersion relations

Pulse propagation in plasmas has been widely studied in relation to instabilities and their classification as convective or absolute. Such analyses emphasize the importance of considering the summation of a spectrum of modes, rather than simply making deductions from Brillouin dispersion relations plotted either for real frequency and complex wavenumber, or for complex frequency and real wavenumber. In particular, erroneous conclusions may be drawn about the magnitude and direction of propagation of a transient pulse. In this paper, the point of view is taken that an observer remote from the source would measure an 'observational dispersion relation'. The nature of this dispersion relation is examined for absorbing or convectively unstable plasmas by asymptotic analysis, and for different source waveforms. Illustrative results are presented for propagation in cold plasmas and for beam-plasma interaction.

Crawford, F. W.↗