Lower Ionosphere Effects on Narrowband Very Low Frequency Transmission Propagation: Fast Variabilities and Frequency Dependence

The man-made narrowband very low frequency transmission from Rhauderfehn, Germany, is studied using high accuracy, microsecond time resolution measurements in Bath, UK. The high time resolution enables a novel comparison of the measurements with a detailed simulation of the transmitted signal. It is found that the wave propagation frequency response exhibits nonlinear amplitude and phase changes with frequency over the narrow transmission bandwidth during the time of the measurements (13 May 2011 15:00:03–15:00:09 UTC). The high time resolution also enables measurements of fast variabilities in the wave propagation <5 ms. The fast propagation variabilities are likely to originate from integrated ionospheric variability over the propagation path or fast ionospheric processes. The wave propagation frequency response measurement has potential benefits in the study of the lower ionosphere, in particular during highly variable perturbations such as those caused by lightning.