Figure 5. Variations of the F2-layer parameters: foF2 (circles), the vertical drift velocity wi (dashed curve), and layer peak altitude hmF2 (dashed-doted curve) on (a) January 24 (quiet day) and (b) January 25 (disturbed day), 1974 at the Boulder station according to Mikhailov et al. [1989]. The mean diurnal variation of foF2 observed on January 24 is shown by the solid line in the panel (b).