A new experimental method for study of the upper atmosphere

Abstract

The idea of utilizing the light scattered from a powerful searchlight‐beam directed to high altitudes has frequently been suggested for exploring the upper atmosphere. Examination of this possibility was made here some years ago, and no doubt elsewhere at earlier dates, but, in addition to technical difficulties connected with instrumental sensitivity, the background level of the light of the night sky constitutes a fundamental limitation. The light scattered from a powerful searchlight‐beam in the absence of clouds or dust (molecular scattering only) would be submerged in this background luminosity even for altitudes which are reached by instrument‐carrying balloons. However, an audacious suggestion was made in 1930 by E. H. Synge, 1 who proposed to concentrate the beams from an assemblage of several hundred searchlights (for example, with the help of the military authorities) on a common volume of the upper air. Utilizing a 36‐inch mirror in conjunction with the photo‐cell at the receiver, Synge calculated that with one hundred large searchlights it should be possible to measure the light scattered by the atmosphere (cloud‐free) at a height of 30 km with an accuracy of possibly one per cent. The stimulus of this large‐scale proposal led one of us (M.A.T.) shortly thereafter to discuss the problem at a meeting of the Washington Atomic‐Physics Colloquium and to point out that by using a modulated searchlight‐beam , thereby placing an “identification‐tag” on the scattered light it is desired to observe, a method of wholly new possibilities emerged from the older idea. Not only does this make available the convenience of resonance‐instruments for high‐sensitivity observations (alternating‐current amplification), but the background limit is no longer the total light of the night sky entering the receiver but only that Fourier component of the (variable) sky‐light which has the same frequency as the searchlight‐modulation. On this basis it was suggested that the use of a single modulated searchlight, with a 36‐inch receiving mirror, gave promise of making accessible to examination the unexplored levels of the atmosphere at heights above 30 km.