An analysis of estimated values of maximum usable frequency throughout a sunspot cycle and associated electron density trends, at Christchurch, New Zealand

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Abstract
A method of deriving tables of sunspot cycle changes in hourly values of maximum usable frequencies from basic vertical sounding ionosphere data is described, and the technique developed for construction of virtual height curves, representative of median conditions, for a rapid computation of M factors for distances shorter than the standard 3000 kilometres is outlined. Some of the factors affecting accuracy of tables are critically examined. The linear relationship assumed between increase in foF2 and increase in running average sunspot number is observed to degenerate markedly as the high sunspot numbers of the 1957–58 maximum are reached. As this effect is particularly significant during daylight hours, ion density changes at noon L.M.T. are investigated and associated trends in true heights are also considered in an endeavour to determine possible causes of ion density loss. The evidence points to a temperature effect. Seasonal variation in noon maxima of foF2 throughout the sunspot cycle examined is found to oscillate from summer maxima at sunspot minimum through a period of equinoctial maxima with the rising cycle to winter maxima (the winter anomaly expected at temperate latitudes) near sunspot maximum. Approximations to ion densities and true heights for different seasons at these varying stages of the cycle are examined and reference made to the possible contribution of physical processes such as mixing of atmospheric gases and changes in temperature. Ratios of disturbance to calm day ion densities, examined seasonally and at different stages of the sunspot cycle, show pronounced seasonal and sunspot cycle variations and reveal large equinoctial and summer daytime departures. It is suggested that these are due to atmospheric mixing of gases resulting in ion density loss through an attachment-like process. The significance of increased F-layer stratification during disturbance is discussed.