Use of scintillation theory to explain frequency-spread on F-region ionograms

Frequency-spread on F -region ionograms is to be explained primarily with the aid of large-scale irregularities of ionization density in the F -region of the ionosphere. Large-scale irregularities are ones whose sizes extend from the Fresnel scale up to the outer scale. This means scales ranging from about two kilometres up to some tens of kilometres, or wavelengths ranging from about ten kilometres up to several hundred kilometres. Irregularities of scale in excess of the Fresnel scale cause refractive scattering of a vertically incident wave on its upward journey through the F -region and on its return journey. Irregularities also cause the critical surface separating underdense ionization from overdense ionization to be rough. Irregularity scales from the Fresnel scale up to the outer scale cause reflection from this surface to take the form of glints. However, this mechanism by itself does not produce enough frequency-spread to explain satisfactorily the more extensive forms of spread- F . In these circumstances refractive scattering of a wave on its upward journey into the F -region ceases to be small-angle scattering before the reflecting stratum is reached. Return of energy to the ionosonde becomes possible by multiple refractive scattering alone. This is diffuse scattering by irregularities ofionization density with scales ranging from the Fresnel scale up to the outer scale. The mathematical theory of scintillation needs to be extended so as to cover development of large-angle multiple refractive scattering.