VLF measurements and their use for investigating the October effect in the ionospheric D-Region

Abstract

Very low frequency (VLF) radio signals, used by the Navies for submarine communications, can be received over very long distances, as the Earth's surface and the ionospheric D region act as a waveguide. Since VLF propagation is determined by the state of the lower ionosphere, VLF signals are suitable for the continuous monitoring of the D region. As solar radiation is the main source of ionization of the ionosphere, a symmetric annual variation as a function of the solar zenith angle is expected. However, various amplitude measurements show a deviation in autumn. More specifically, a reoccurring significant decrease in October is observed at mid and high latitudes. This drastic change in amplitude level is referred to as the October effect. In this study, we investigate the extent of the October effect and its dependencies on latitude and longitude. Here, a steeper decrease is found at higher latitudes. Furthermore, the October effect first occurs over the American sector and seems to move eastward towards Europe. Since it occurs with the zonal wind reversal, a strong connection with the dynamics of the neutral atmosphere is suggested. Finally, we compare our results with the FIRI model and well-established propagation simulations using LWPC (Long Wavelength Propagation Capability code). Both of the existing approaches do not reflect the October effect so far and need revision.