Concerning the Nature of Radio Fade-Out

Radio fade-outs in the Earth's sunlit hemisphere coincident with bright solar chromospheric eruptions are examined by means of vertical-incidence recording of ionospheric echoes with a wide range multifrequency technique. It is concluded from the series of observations examined that a fade-out is best described as an upward projection of the lower or absorption limit of echo-return, and that the fade-out is caused by intense absorption of ionizing radiation, emanating from bright chromospheric eruptions, at the base of, or below, the E region, that is, below the 110-km level. It is shown that no appreciable absorption of chromospheric-eruption radiation takes place at levels higher than that of the E region; therefore, a process of selective absorption must operate. Since the principal geomagnetic effect associated with fade-outs is an augmentation of the diurnal variation of terrestrial magnetism, it appears that only the Stewart-Schuster "atmospheric-dynamo" theory of diurnal variation is possible under the restrictions imposed by these observations.