Spatial and temporal variations in ionosphereric radio wave absorption

The D and E Regions of the Ionosphere are reviewed historically; the methods of measurement of both the electron density and the gross properties of the neutral atmosphere are discussed. The various reactions of electron production and loss are listed, and their relative importance assessed. The mechanism of ionospheric radio wave absorption is described; the Sen and Wyller generalisation of the Appleton-Hartree equation yields an expression for the absorption coefficient, in which are implied certain temporal variations. The results of previous experimental work are considered. An analysis of long term A1 measurements at three stations, of approximate latitude 50° N, suggests that absorption does not follow a direct Cos n x law, the departure from such a law being a function of wave frequency, location, and sunspot number. Comparable effects are shown to exist at 7° N, 35° N, and 51° S. A theoretical absorption model is constructed, for which a revised profile for the electron loss coefficient, and a seasonally varying nitric oxide profile are proposed. It is suggested that the effect may be related to the ledge in electron density, occurring at around 87 km, which in turn may be caused by the steep gradient in the atomic oxygen concentration. The asymmetric seasonal variations in other relevant parameters are discussed.