Bakerian Lecture.—Some phenomena of the upper atmosphere

Abstract 1. Known facts relating to the upper atmosphere have increased rapidly in recent years, through observations of many kinds—concerning meteors, the absorption and emission of light by the atmosphere, the propagation of radio and sound waves, and the daily and irregular changes in the earth’s magnetism. Among other things they indicate, more or less definitely, the existence, and some of the properties, of a layer containing a small but abnormal and important amount of ozone, the absolute density of which is greatest at about 50 km. height; this ozone absorbs 4 or 5 per cent, of the incident solar radiation, and the air is thereby heated to a temperature exceeding that of the air near the ground. The observations also indicate the presence of two strongly ionised layers, at altitudes of about 100 km. and 220 km.; and of a region, of unknown height, where oxygen exists in a metastable atomic state, from which it passes to a state of less energy, with the emission of the green auroral light. It is desirable to co-ordinate these facts, with the help of laboratory data, general physical theory, mathematical analysis, and intuition or speculation. The large gaps in our knowledge unfortunately leave much room for individual differences of opinion, but nevertheless real progress in co-ordination has been made, and the range of possible hypotheses is being steadily narrowed down. In this lecture I present a combined picture of many of the phenomena, in the form that now seems to me most probable, though without concealing the uncertainties affecting many points.