THE experimental investigation of the electrical structure of the upper atmosphere by means of wireless waves as the exploring agency has shown that there exist two main ionised regions from which such waves may be reflected. The lower of these regions is the Kennelly-Heaviside layer, which reflects long waves in long-distance transmission. The upper region is found to be much richer in ionisation than the lower, so that if we project vertically upwards waves of gradually increasing frequency (and therefore gradually shorter wave-length) we find that, at a certain critical frequency, the lower region is just penetrated and reflection takes place from the upper region.
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Journal of Atmospheric and Terrestrial Physics (1974)
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