Longterm Solar Activity Dependence

There is a clear tendency for the ionospheric critical frequencies to increase with sunspot number. Figure 3-15 shows the long-term variation of Ri2, 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 Section 3.4.2). A device for monitoring the D region absorption is the Riometer, where the magnitude of the absorption is proportional to the product of D region electron concentration and the electron collision frequency. From Figure 3-15, a slow 11-year modulation in the ionospheric parameters is evident.

Year

Figure 3-15: Variation in R12, foF2, foE, and 4 MHz absorption at noontime. The seasonal effects are clearly evident, the foE and D layer variations being out of phase with the foF2 variations (i.e., the seasonal anomaly). From Goodman [1991],

Year

Figure 3-15: Variation in R12, foF2, foE, and 4 MHz absorption at noontime. The seasonal effects are clearly evident, the foE and D layer variations being out of phase with the foF2 variations (i.e., the seasonal anomaly). From Goodman [1991],

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 Figure 3-16. This plot is rather unusual since it represents the running 12-month averages of the specified ionospheric parameters as well as the sunspot number. This disguises the seasonal effects observed in Figure 315.

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