Annual Variations in E-Layer Ionization

Abstract

IN the E-layer of the ionosphere, the variations in the critical frequency (ƒ) depend mainly on the solar zenith angle (χ) and the phase of the cycle of solar activity. If solar activity is assumed to remain constant, the annual variation in ƒ at local noon can be represented by : and the hour-to-hour variation during the day by : In these equations, ƒ_a and ƒ_d are assumed to be constant; the suffixes tand 12 denote the values of ƒ and χ at time t and at local noon, respectively. For a simple Chapman layer, m = n = 0.25, but Nicolet and Bossy¹ have shown that, under certain conditions, m and n may differ from this value. Since the parameters ƒ_a and ƒ_d both correspond to χ = 0, they are sometimes referred to as the critical frequency at the ‘sub-solar point’. This terminology is misleading because it suggests that they should be equal to each other and also to the value of the actual critical frequency (ƒ₀) at the geographical location where χ = 0. In fact, each of the parameters ƒ_a and ƒ_d represents the value of ƒ which results from the extrapolation of a series of measured values to a hypothetical condition χ = 0 ; they are not equal because different sets of data must necessarily be used in performing the two extrapolations and neither ƒ_a and ƒ_d is equal to ƒ₀ except where the Sun is overhead at noon. By combining equations (1) and (2), it is easily shown that the relation between ƒ_a and ƒ_d is given by : This equation implies that ƒ_d and ƒ_a cannot both be constant as was previously assumed. It appears that no evidence has yet been published to prove that the observed values of the E-layer critical frequency vary in conformity with equation (3); but this can easily be verified by comparing E-layer data which have been published by Minnis and Bazzard² and by Eyfrig³. It will be shown that, near the minimum of the solar cyele, ƒ_a is constant and ƒ_d varies with χ₁₂.