Some measurements of upper-atmospheric ionisation

Abstract

Abstract In the wireless exploration of the electrical structure of the upper atmosphere a considerable simplification in the interpretation of the experimental results is effected if the emitting and receiving stations are placed very close together. In such a case it can be assumed that the waves have travelled to and from the reflecting region approximately along the same track, and, if care is taken that the emitting station radiates appreciably in an upward direction, we may further assume that reflection from the upper-atmospheric layer takes place at normal incidence. The advantages of such a simplification arise as follows. The two quantities which can, in general, be measured in the type of wireless experiments we are considering are (a) the group-time for a signal to traverse the atmospheric ray track, and (b) the angle of incidence θ0 of the waves at the lower boundary of the ionised region. From (a) we can deduce the equivalent path of the waves in their overhead journey. Also since sin θ0 is equal to μ, the refractive index at the highest point of the wave track, and since the relation between refractive index and ionisation is known, we can find from (b) the maximum ionisation content experienced by the waves. It is therefore clear that, in the case of experiments at normal incidence, it is not necessary to supplement the measurements of group-time by measurements of angle of incidence, since it may be assumed that the waves have been reflected at a region where the refractive index has been reduced to zero. Another advantage arises from the fact that the very complicated expressions for the refractive indices of the two component waves which travel in an ionised medium which is subject to the influence of the earth’s magnetic field, yield easily calculable values of the ionisation content in terms of the wave frequency for the particular conditions μ2 = 0. As a result of the investigations carried out within the last few years it has been shown that there exist in the upper atmosphere two ionised regions from which wireless waves are reflected. The lower of these two regions is the Kennelly-Heaviside layer, the existence of which was inferred by Kennelly and Heaviside more than a quarter of a century ago to account for successful communication by long waves over long distances. The upper region has been shown to be richer in ionisation than the lower, so that when waves penetrate the Kennelly-Heaviside layer they may still be reflected by the upper region. If, therefore, we project vertically upwards waves of gradually increasing frequency (and therefore gradually shorter wave-length) we find that, at a certain critical frequency, the Kennelly-Heaviside layer is just penetrated and reflection begins to take place from the upper region. Since, as has been stated above, we assume that reflection at vertical incidence takes place at a place where the refractive index tends to zero and since the expression for the refractive index, in terms of N the ionisation content and f the frequency of the waves, is known, the maximum value of N is evidently determined by the critical penetration frequency of the waves.