On the ionization of the upper atmosphere

The subject of the ionization of the upper atmosphere is one of extreme importance to students both of Terrestrial Magnetism and of Atmospheric Electricity, and from various points of view there are indications that this region of the atmosphere is to be treated as one of relatively high electrical conductivity. One of the first theories which took this hypothesis as one of its bases was that developed by Schuster 1 to account for the diurnal variation of the Earth's magnetism. At the time when Schuster's paper was written the most natural source in which to seek an explanation of the high conductivity was the ultra‐violet light from the Sun. The great increase in our knowledge of photo‐electric ionization within the last few years renders it now possible to make a closer scrutiny of the consequences to be anticipated from the ultra‐violet ionization alone. Any conclusions as to the insufficiency of this source for accounting for such a conductivity as that cited by Schuster must not, of course, be considered in any way as a criticism regarding the broad principles underlying Schuster's work, since the ultra‐violet light is by no means the only source of ionization of the atmosphere. Indeed, Schuster himself questioned the sufficiency of this source, and he took pains to point out that other sources were available. Thus, the rate of production of ions in a closed vessel increases at altitudes of 10 kilometers to many times its value at the Earth's surface, and this fact suggests a powerful ionization agent in the upper atmosphere other than the ultra‐violet light. In spite of the existence of various agencies which may be invoked to account for the ionization in question, it is not without interest to discuss in terms of modern views as to photo‐electric ionization the possibilities inherent in the ultra‐violet light alone.