The Effect of the SSW/ES Event in 2018 on the VLF Signal of Three Transmitter-Receiver Links in High Latitudes

Kurzfassung

Man-made very low frequency (VLF, 3–30 kHz) radio signals travel from transmitter to receiver within the Earth-ionosphere waveguide, confined between the Earth’s surface and the lower ionosphere in approximately 60–90 km altitude, known as the D-region. While ground conditions do not change significantly over time, variations in the reflected VLF signal can be linked to changes in the upper reflection boundary, thus to changes in the ionization of the D-region. Ionization variability in this region can be driven by atmospheric processes such as Sudden Stratospheric Warmings (SSWs) and Elevated Stratopause (ES) events. These wave-driven SSW/ES events, typically occurring at mid- to high latitudes, cause significant disturbances in temperature, wind patterns, and vertical mass transport. In this study, we investigated how the SSW/ES event of the 2017–2018 winter season affected VLF signal amplitudes across three high-latitude transmitter-receiver (Tx–Rx) links. During the SSW period, all three links showed an increase in amplitude. However, in the subsequent ES period, amplitude variations differed among the links. Only one link showed a decrease in amplitude – a signature also observed in previous ES events and likely characteristic of such events under quiet solar and geomagnetic conditions. Our analysis indicates that the 2018 ES event displayed considerable spatial variability. Consequently, the processes responsible for amplitude decreases did not occur uniformly across all VLF propagation paths. We conclude that the observed differences in VLF amplitude behavior among the three Tx–Rx links are due to the spatial distribution of the 2018 ES event.