Relation between changes in foF2 and hmF2 within various time intervals

The relation between the critical frequency foF2 and F2-layer height hmF2 is considered for ten ionospheric stations in the periods before and after 1980. It is shown that in the earlier period the relation between foF2 and hmF2 is well pronounced. In the later period, a distortion of this relation is observed. The statistical characteristics of the foF2 dependence on hmF2 are spoiled. That shows that due to the cooling and contraction of the upper atmosphere the height distribution of the photochemical parameters governing the equilibrium concentration in the layer maximum changes. A larger contribution to this effect is evidently provided by changes in the atom-to-molecule concentrations ratio.