Non‐seasonal change of F₂‐region ion‐density

Observation of noon F 2 ‐region ion‐density at stations in both the Northern and Southern hemispheres shows that variations in one are not predicted at the other with the hypotheses which have been advanced to explain them. Observations made at Washington by the National Bureau of Standards, and at Watheroo by the Department of Terrestrial Magnetism, Carnegie Institution of Washington, are analyzed. It is found that a variation‐component exists in the data from 1935–37 which is in the same phase at both stations. This term has a principal period which, over the period studied, is indistinguishable from one year. Its amplitude is about equal to that of the seasonal variation, maintaining a nearly constant ratio to the changing background ion‐density. This non‐seasonal variation is, therefore, of sufficient amplitude, if neglected, to vitiate quantitative calculations of seasonal effects based upon the observations from one hemisphere alone. While the average magnitude of the background ion‐density is closely related to annual averages of solar activity, departures of the non‐seasonal component from the mean show no sensible correlation with monthly sunspot‐averages. The data and methods are examined to determine whether inhomogeneity of data, or methods of analysis, could induce the effect artificially. It is concluded that the amplitude of such induced errors must be small compared to the amplitude of the non‐seasonal effect. Application of the heating hypothesis in its present form leads to a heating ratio more than 100 times as great at Washington as at Watheroo, which does not seem probable. Correction of the data for the non‐seasonal effect leads to a heating ratio for summer to winter of about 10 to 1 for both stations. Possible causes of the non‐seasonal term are considered. The paucity of data from widely separated locations through the 24 hours limits speculation. In reviewing the implications of the data, the writers are inclined to the view that the cause is associated with the Earth or its motion, though the arguments presented on the basis of existing data are not sufficient to establish this view unequivocally.