Sir Edward Appleton and early ionosphere research

Abstract

The earliest suggestion that there might be a conducting layer in the earth's atmosphere was made in 1839 by Gauss, who was concerned to explain the diurnal geomagnetic phenomena, and who pointed out that a possible hypothesis was that they arose from electric currents encircling the earth somewhere in the atmosphere. However, at that time, no mechanism to account for the existence of a conducting layer could be suggested, nor was there any knowledge of how air could become conducting; indeed, until ionizing radiation began to be studied, it was even suggested that air naturally became conducting at low pressure. It was soon apparent that the propagation of radio waves beyond the horizon could not be explained as the result of diffraction. Heaviside and Kennelly, at about the same time, both proposed that a conducting layer in the atmosphere was responsible. It was Appleton who first showed, by an elegant and well planned series of experiments, that this was indeed the case; or to put it more precisely, that at a point distant from the transmitter there was, in addition to a wave received along the surface of the earth, a second wave descending from the sky.