Figure 3-16: Long-term variation in Rn, of foF2, foFl and foE at noontime. The x-axis is labeled "critical frequency" (in MHz). The critical frequency is the highest frequency for which skywave propagation can be supported at vertical incidence. The ordinary ray critical frequency can be shown to be equivalent to the electron plasma frequency. The plasma frequency, in turn, is proportional to the square root of the electron density. Because 12-month running averages are used in the plot, the seasonal effects seen in Figure 3-15 are smoothed out. While the critical frequencies for the three layers all increase with solar activity, the F2 layer shows the largest dependence on sunspot number. From Goodman ,
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