The absorption of short radio waves in the ionospheric D and E regions

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The difficulties of separating the measured absorption of short radio waves into separate contributions by the D and E regions are discussed. Such a separation is only possible on the basis of certain theoretical models of the two regions. A better knowledge of the electron density and the collision frequency in the two regions is necessary before the critical examination of such models. Recent experimental evidence suggests that most of the absorption of waves reflected from the E region on a normal day occurs within a few kilometers of the level of reflection. Difficulties of obtaining true electron density versus height profiles for the E region from routine ionograms are discussed. Other more direct ground-based methods of measuring the electron density as a function of height exist. Some of these methods have been used in the past to measure the electron density in the lower part of the D region. It is suggested that one of these methods, the observation of backscattered echoes from irregularities, would be a powerful tool for the determination of electron densities in both the E and the D region. Certain modifications to the technique previously employed in such observations are proposed. With these modifications the observations would allow the independent determination of the electron density and the collision frequency as functions of the height. The possibility of using this modified technique for the study of E and D region electron densities and collision frequencies both during normal days and days of abnormal absorption is examined.