Spring?Fall Asymmetry in VLF Amplitudes Recorded in the North Atlantic Region: The Fall?Effect

A spring-fall asymmetry is observed in daytime amplitude values of very low frequency (VLF) radio wave signals propagating over the North Atlantic during 2011–2019. We explore processes behind this by comparing against mesospheric mean temperatures and semidiurnal solar tide (S2) winds. The radiation influence on VLF subionospheric propagation was removed from values, isolating fall-effect. Similarly, symmetric background level undertaking comparable analysis. During fall, all three analyzed parameters experience significant deviation their levels. variation spring explained seasonal illumination conditions, while fall-effect can be interpreted as a zonal wind reversal associated with both S2 enhancement, temperature reductions. Decreases produce decreases collision frequency, reducing signal absorption, driving asymmetry.