Computational results of the effect of localized ionospheric perturbations on subionospheric VLF propagation

The purpose of this paper is to investigate the effect of localized perturbations in the lower ionosphere on subionospheric VLF propagation by means of the finite element methos. Owing to the complexity of the method, we have analyzed only a two-dimensional case where a localized perturbation lies on the great circle path between the transmitter and receiver, The first-order mode is assumed to be incident into the waveguide, and we have found significant oscillations in the changes of amplitude and phase in the vicinity of the perturbation (distance less than 600 km), which are due to mode conversion (i.e., conversion to higher-order modes) and subsequent multiple-mode propagation, In this paper, we present many computational results of the spatial distribution of scattered fields, frequency dependence, effects of enhancement factor, horizontal and vertical scale, and altitude of the perturbation, etc, Some important findings are summarized as follows: (1) The study of scattered fields has suggested that a single-mode propagation model does not hold and that the geometrical structure of the perturbation, especially in the horizontal direction, plays an important role in the scattering. (2) The magnitude of the perturbation, that is, the enhancement factor, effects mainly the magnitude of the scattered field.