Previously a mode conversion model was presented which allowed for both the vertical inhomogeneity and anisotropy of the ionosphere. Horizontal inhomogeneity along the direction of propagation was modeled by a slab approximation. The required height gain functions were determined by full‐wave solutions and their associated integrals evaluated numerically. In this paper results of a modified mode conversion model are compared with those of the original. In the modified model, height gain functions are discarded above some height h in the guide and are approximated below height h by Airy functions. Merits of the modified version are that a full‐wave program for height gains is not required, and that the associated integrals may be performed analytically. An obvious disadvantage is the free parameter h. The modified mode conversion model can be implemented with about the same ease as a WKB method and has one distinct advantage over the latter, namely, that mode numbering in any given slab is immaterial. Results of both mode conversion models are compared at several frequencies in the VLF band with experimental sunrise results obtained with a multifrequency oblique sounder system located on the island of Hawaii. In particular, comparisons are made with vertical field measurements in southern California. Approximate agreement is found. Discrepancies between modelling results and data are generally larger than the differences between the two mode conversion models.
Prediction of LF signal strengths is usually done by using a single “average” model of the ionosphere. However, this approach suffers from the fact that signal strength is a nonlinear function of ionospheric parameters. Hence the “average” ionosphere does not always give the correct average signal strength. In addition, the standard deviation of signal strength varies considerably with position, in general being larger in the vicinity of modal interference nulls. A model where the daytime ionosphere is treated statistically is presented. The propagation model used is an earth ionosphere wave‐guide. The ionospheric electron density profile is specified by a reference height h′ and slope β. These parameters are assumed to be distributed in a jointly normal distribution. Best fit values for this model are obtained using measurements made at two points on either side of a deep modal interference null located about 1000 km from a 59‐kHz transmitter. The measured signal strength mean, and standard deviation were used to obtain a best fit mean, standard deviation and correlation coefficient for the β and h′ distribution. The resulting ionospheric parameters give signal statistics that fit the measured mean and standard deviation at both monitoring points within 1 dB.
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