Developing an analytical model for VLF signal amplitudes with consideration of the fall effect

Abstract

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 D-layer of the ionosphere. On the contrary, various amplitude measurements show an asymmetry around fall by deviating from the expected variation defined by the solar zenith angle. This asymmetry is referred to as the fall effect. As this investigation is an ongoing effort, based on the DLR project "Analysis of the MEsosphere and Lower Ionosphere fall Effect (AMELIE)" joined together with the IAP, the current status of the investigation of the fall effect will be presented. Finding a suitable description of the size of the asymmetric behavior will help in further research on the main factors that affect the ionosphere. For understanding these seasonal influences, short-term disturbances must be eliminated, so we developed an analytical model, confined to exemplary links in the northern hemisphere, which gives an expected, quite-day value for a given day and signal path. Perturbations are caused by, e.g., solar flares. Comparing the derived unperturbed value of the model with the measurements will additionally allow to detect such impacts.