Mode conversion and auroral effects observed on a polar VLF propagation path

Abstract

Mode conversion effects on the VLF signal NPM 23.4 kHz received in Kiruna are discussed. The observations are interpreted in terms of the isotropic propagation model of Wait and Spies (1964). The propagation is well described by this model for daytime with β = 0.5 km −1 and h = 70 km and for nighttime with β = 0.5 km −1 and h slightly larger than 90 km. The difference between the sunrise and sunset mode interference is explained by a lower reflection height after sunset than before sunrise. The scatter in the data is referred to variations in the times of effective sunrise and sunset. Seasonal and solar cycle variations are observed. Auroral activity affects the average propagation conditions in such a way that the effective reflection height decreases with increasing Kp. Particle precipitation in the auroral zone can cause strong mode conversion.