A Study on the VLF/LF Long Term Amplitude Oscillations Associated with Frequencies 37.5 kHz and 45.9 kHz Received at Keil Longwave Monitor, Keil, Germany

The middle and upper atmosphere of Earth displays many large-scale oscillationsin several parameters. Of these oscillations, Atmospheric Oscillation (AO)and Semi-annual Oscillation (SAO) are prominent ones. In this work, wehave analyzed the Very Low Frequency/Low Frequency (VLF/LF) data fromtwo of the transmitters of frequency 45.9 kHz at NSY, Sicily, Italy and 37.5kHz at Grindavik, Iceland. The selected Trasmitter-Receiver Great Circle Path(TRGCP) is middle latitude which is marine in case of Grindavik TRGCP andterrestrial in case of NSY TRGCP. The VLF/LF signals are bounced back andforth from D-layer of ionosphere (altitude at ~65 km during day and ～95 kmduring night) in Earth-ionospheric waveguide. This infers the presence of atmosphericoscillations as a consequence of change in ionization and recombinationrates. Many works related to AO and SAO are mostly done only for equatorialregion of the ionosphere and authors have reported the elegant dominancyof AO and SAO in the VLF/LF amplitude pattern over years. To our surprise,in our work it is seen that not only AO and SAO but also other oscillations arenecessary to model the oscillation pattern of middle latitude ionosphere.