Records are reproduced showing diffuse echoes from the F-region of the ionosphere received continuously at night in equatorial regions over a wide range of wave-frequency. Tney are interpreted as due to Rayleigh scattering by spatial irregularities in the distribution of electron-density at or above a definite level in the F-region. Because of the highly dispersive nature of the ionosphere, there is no marked dependence of Rayleigh scattering upon wave-frequency such as there is for a non-dispersive medium. According to this interpretation, variation of the maximum wave-frequency to which diffuse echoes can be followed has nothing to do with variation of the maximum electron-density of an ionospheric region, but merely indicates variation in the size of irregularities in electron-density. Scattering of this type may have some bearing upon the phenomenon of persistence of E-region echoes to wave-frequencies greater than the critical penetration-frequency of the E-region.
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