Annual Variation of the Critical Frequencies of the E- and F₂-Layers

Abstract

THE successive accumulation of data on the critical frequencies, that is, the limiting penetrating frequencies, of the E- and F-layers determined from noon observations show the following main features¹: the critical frequencies of the E-layer show a regular annual variation depending on the height of the sun, whereas the critical frequencies of the F₂-layer show an inverse annual variation, with high values during winter and lower values in summer. Appleton² has explained this as a temperature effect, the density of the air at 200–400 km. in winter being higher on account of the lower temperature. According to the simple theory of photoionization, the maximum electron density, that is, the critical frequency, will depend on the pressure.