VLF measurements and their use for investigating the ionospheric D-layer with the example of the October effect

Abstract

Very Low Frequencies (VLF) radio signals are constantly used for communication by navies because they can be received over very long distances. This is due to their reflections between the surface of the earth and the ionospheric D-layer, which acts as a waveguide. As their propagation is determined by the state of the D-layer, they are particular useful for the investigation of the D-layer itself. The daytime VLF radio signals observed over a year would be expected to follow the yearly variation of the solar zenith angle, as the solar radiation is driving the ionization in the ionospheric D-layer. On the contrary, various amplitude measurements show an asymmetry around fall by deviating from the expected variation defined by the solar zenith angle. Furthermore, the VLF signal amplitude shows a persistent sharp decrease during October at mid and high latitudes. This drastic change in the amplitude level is the so-called “October effect”. As the VLF signal amplitude follows the state of the D-layer, the D-layer in mid and high latitudes must undergo a seasonal variation. The influence of these seasonal ionospheric variations on the VLF signal propagation is investigated by the joint project “Analysis of the MEsosphere and Lower Ionosphere fall Effect” (AMELIE) between the Leibniz-Institute of Atmospheric Physics (IAP) and DLR.