A new approach to mode conversion effects observed in a mid-latitude VLF transmission

Abstract

The seasonal variation of the mode conversion effect has been investigated as observed for more than three years on the VLF signal from NLK 18.6 kHz in Nishinomiya, Japan. A new analytical method is proposed for the further understanding mode conversion effects on long-distance VLF transmission. Using this method for our measurements during the sunrise transition in winter, the modal interference spacing distance D 21 N = 2.30 Mm has been derived uniquely consistent with VLF waveguide mode theory and suggests that the height of the earth-ionosphere waveguide is somewhat higher than 90 km at night during winter. Moreover, the quantity arg ( S 21 DN S 11 DN ), the argument of the ratio between complex two major mode conversion coefficients for day-night propagation, was found to be dependent on the length of the transition region in an essentially linear manner. This linear relationship indicates that a reference value of solar zenith angle corresponding to the effective sunrise in the ionosphere can be derived uniquely from the requirement of the VLF mode theory. It is noted that the nighttime height of the waveguide is not always constant owing to the geographical configuration of the propagation path. This leads to an effective extension of the transition region during summer.