The F1 Region

The F1 region is not unlike the E region in the sense that it obeys many of the features of Chapman theory. We look for a relation, which is formally similar to Equation 3.13. A recipe for foFl patterned after Chapman principles may be expressed as:

Like the E region, the F1 region exhibits more complicated behavior than that expressed by such a simple recipe. Specifically, it has been found that the geomagnetic latitude tends to exhibit some control over the F1 region electron densities. The term/ in Equation 3.15 depends upon sunspot number and magnetic latitude. It is also observed that the F1 region disappears (i.e., merges with the F2 region) at values of the solar zenith angle exceeding a certain value. This value itself depends upon both the sunspot number and the geomagnetic latitude. The Radio Sector of the International telecommunications Union (ITU-R, previously the CCIR) has developed a method for computing foFl taking all these factors into account [Muggleton, 1975]. The internationally adopted monthly median foFl formulation is based on the work of Ducharme et al. [1973]. The relation due to Davies [1990] gives a convenient but approximate expression for the F1-layer critical frequency:

Figure 3-9 shows the solar zenith angle control of foFl under sunspot maximum and minimum conditions.

The height of the F1 ledge, hFl, is taken to be range between 180 and 210 Km. From Chapman theory we anticipate that hFl will be lower in summer than in winter, and will be higher at midlatitudes than at low latitudes. Unfortunately the reverse is true. Explanations for this behavior may be found in a detailed study of scale height gradients, a nonvanishing movement term (as expressed in the continuity equation), or gradients in upper atmospheric chemistry.

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