Impact of Sudden Stratospheric Warming and Elevated Stratopause Events on the Very Low Frequency Radio Signal

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are mid-to-high latitudinal, atmospheric wave-driven phenomena leading to significant changes in wind, temperatures, and vertical mass transport, especially at stratospheric and mesospheric altitudes. Presumably, SSW and ES-induced changes also cause modifications in the highly variable D-region ionization. This bottom side of the ionosphere behaves with the Earth's surface as a reflection boundary for Very Low Frequency (VLF) radio signal transmission used for long-distance communication. Since perturbations of the D-region ionization are also notable in the VLF signal, it is a valuable tool for continuous investigations of the D-region. Here, we study the impact of four SSW/ES events on the VLF signal amplitude between the high latitude transmitter-receiver link Keflavik, Iceland, to Ny-Ålesund, Svalbard, to gain further knowledge about interactions between the D-region and the atmosphere during these atmospheric phenomena. For three of four SSW/ES events, a very similar VLF signal amplitude signature is observed, characterized by a significant increase during the SSW period in the signal amplitude followed by a decrease during the ES period. This study aims to reveal a possible mechanism driving these similar VLF signal amplitude variations, involving modified electron neutral collision frequencies and electron densities due to changed temperatures and minor constituent concentrations according to the SSW/ES events. However, the VLF signal amplitude for one event increased 2 weeks later than during the other three events and did not show a decrease during the ES period. Possible causes for the different VLF signal amplitude variations are discussed.