On lunar tides in the upper atmosphere

Abstract
Although the earliest radio investigations of the ionosphere consisted of systematic measurements of the equivalent heights of reflexion, the tendency in recent years has been to concentrate on measurements of equivalent height as a function of electric wave frequency, since, from such information, maximum ionization concentrations may be estimated. Thus we may say that, in the early measurements, the relation between equivalent height h' and time t was sought, while, more recently, attention has been chiefly focused on the relation between h' and the frequency f . Now it happens that the determination of the critical penetration frequency f c of any ionospheric region, which proceeds by way of ( h', f ) data, does not demand great accuracy in the determination of h' , so that it is easy to understand why little attention has been given to improving the technique of equivalent height measurements. There are, however, a number of interesting ionospheric problems which may be attacked by way of accurate ( h', t ) determinations. It is, for example, clear from simple theory that the actual height at which any ionospheric region is produced should depend on solar altitude, the region being the lower the higher the sun. The variation of the height with the solar zenith distance X depends, moreover, on the local scale height H , so that the study of the variation of region height with the diurnal and seasonal variations of X permits a comparison with theory, and thus a determination of H .

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