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Earlier studies of the scattering of radio wave by Ionospheric irregularities are almost exclusively concerned with random or turbulent irregularities with averaged smooth spectra. However, the scattering by wavelike irregularities, whose wavelengths are of the order of the radio wavelength, is another important subject which has escaped the attention of most Ionospheric radio workers. This is especially important when the direct (not statistical) property of the scattering is concerned, such as the individual sporadic-E traces on sonograms. The purpose of the present paper is to study the physical mechanism of resonant scattering by wavelike irregularities; the practical importance of the scattering by such wavelike irregularities will be discussed in the following papers in connection with the partial transparency of the radio echoes scattered from the ionosphere including sporadic-E irregularities.

HF Doppler variations caused by Ionospheric disturbancesare studied using an ionosphere model containing sinusoidal traveling electron density fluctuations. The present study is different from earlier works in that we use a more realistic ionosphere model and a more accurate numerical method than previous works using corrugated specular reflector models. Our study gives a clue to estimate the TID-associated fluctuations of Ionospheric electron density by means of HF Doppler measurements. It is shown that some kinds of characteristic HF Doppler traces result depending on the wavelength of the disturbance and its traveling direction. Numerical results suggest that more or less 5% of the background electron density can explain most of the quasi-periodic variations on the observed HF Doppler records.

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