Theoretical and experimental sunrise mode conversion results at VLF

Abstract

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.