Characteristic Variation of Region F₂ Ionization Throughout the Year

Abstract

IN a letter from W. M. Goodall, which appeared in NATURE of June 10, p. 977, there is made one further step in the elucidation of the complex behaviour of Region F₂ of the ionosphere, in that it is shown that there is a better correspondence between ionization and calcium flocculi figures for the central solar disk than there is between ionization and the usual sunspot numbers. From the determined relation between ionization and solar activity (as expressed by the calcium flocculi figures), it is possible to estimate the average seasonal variation of ionization over a number of years, the varying influence of solar activity being removed. Assuming, for example, that the solar activity had remained constant at a level expressed by the calcium flocculi character figure 3·0 throughout the period 1935–38 (actually, of course, it was gradually altering), the seasonal variation of the Region F₂ critical frequency at Washington (lat. 39° N.) is as shown in curve A in the accompanying figure. The trend of such a curve confirms the observations of Appleton and Naismith^1,2, who earlier found a similar variation in the northern hemisphere during sunspot minimum at both Slough and Tromso during the Polar Year 1932–33, and at Slough in succeeding years. The continuance of the radio work at Tromso by Harang³ has provided further confirmatory evidence, and we may now feel confident that a curve of this type represents fairly accurately the seasonal variation of noon critical frequency (and thus ionization) in the absence of marked ionospheric storms.