Experimental investigations of the ionospheric E-layer

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Abstract
This paper discusses the recent information gained about the E-layer of the ionosphere by various experimental techniques. The basic information is provided by penetration-frequency measurements made from routine pulse soundings, giving a general picture of the variation of the ionization during the day, with season and with latitude, as well as the effect of changing solar activity and of the magnetic current system. Special radio-reflection experiments to measure height changes have yielded important results, particularly concerning tidal movements. The detailed structure of the layer has also been deduced from high-resolution pulse soundings. Independent information about the structure of the layer has been determined directly using rockets, and shows marked differences from the structure deduced from ground observations. Observations with high-resolution ionosondes reveal that the reflections from the E-region are often very complex, and this introduces uncertainties in the identification of the characteristics of the normal E-layer. A brief discussion is given of observations of fine-structure in the layer. Rocket measurements of solar radiations and of the atmospheric composition have made it possible to predict the distribution of ionization in the E-layer; this prediction is in quite good agreement with the distribution determined from accurate soundings. Observations of changes in the ionization during solar flares and radio fade-outs are described.

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