Long-Term Solar-Activity Dependence of the Ionospheric Layers

There is a clear tendency for the ionospheric critical frequencies to increase with sunspot number. Figure 14 shows the long-term variation of R12, foF2, and foE, and the D-layer absorption level (at 4 MHz), for noontime conditions. The D region is best characterized by the amount of absorption it introduces (see the subsection “Ionospheric Layering” above). A device for monitoring the D-region absorption is the riometer, which evaluates it as the product of D-region electron concentration and the electron collision frequency. From Figure 14, a slow 11-year modulation in the ionospheric parameters is evident. After smoothing, the results correlate well with sunspot number. Superimposed on this solar epochal variation is an annual variation, with D-region absorption and foE exhibiting summertime maxima, while foF2 exhibits a wintertime maximum (i. e., seasonal anomaly).

The slow but definite dependence upon mean sunspot number is illustrated in Fig. 15. This plot is unusual in that it presents running 12-month averages of the specified ionospheric parameters as well as of the sunspot number. This obscures the seasonal effects observed in Fig. 14.

Fig. 11. Variations in the hourly values of foF2 as a function of the time of day, for January solar maximum conditions at a northern-hemisphere midlatitude site. The range of day-to-day variability in foF2 is ~ ±10%, suggesting a variation in NmaxF2 of ~ ±5%. [From Davies (1).]

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