Dependence of the Critical Frequency of the Ionospheric E-layer on Solar Zenith Angle and the Annual Variation in E-layer Ionization

Abstract

IT is generally known that ƒ₀E, the critical frequency of the ionospheric E layer, varies in a fairly regular manner with a law of the form: where χ is the solar zenith angle. This form can be applied to both diurnal and annual variations, and generally both ƒ₀ and n are different for these two variations. To distinguish these two cases we use ƒ_d and m for diurnal variations, and ƒ_a and n for annual variations. It is known that m is greater than n except at stations in equatorial regions and at very high latitudes (see Fig. 1), while ƒ_d is greater than ƒ_a everywhere¹.