The effect of the SSW/ES event in 2018 on the VLF signal of three transmitter-receiver links in high latitudes

Abstract

Man-made radio signal transmissions with low frequencies (VLF, 3-30 kHz) travel from transmitter to receiver by being reflected between the Earth's surface and the bottom side of the ionosphere (D-region, 60-90 km). Since the D-region is the VLF's upper reflection boundary, its ionization perturbations are notable in the VLF signal amplitude. The highly variable ionization can be affected by atmospheric phenomena like Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events. The atmospheric wave-driven SSW/ES events occur in mid-to-high latitudes and cause significant changes in wind, temperatures, and vertical mass transport. We studied the impact of the SSW/ES event in the winter of 2017/2018 on the VLF signal amplitude of three high-latitude transmitter-receiver (Tx-Rx) links. An increase in the amplitude during the SSW period could be observed for all three links. However, during the ES period, the amplitude variations differ between the Tx-Rx links. Only one link shows an amplitude decrease during the ES period, which was also observed for other ES events and seems to be a typical amplitude anomaly during an ES event under quiet solar and geomagnetic conditions. We found that the ES event in 2018 shows a substantial spatial variability. Thus, processes leading to the amplitude decrease during the ES period do not occur along all pathways to the same extent. We conclude that the differences between the VLF signal amplitude variations of the three links under consideration are caused by the spatial variability of the ES event in 2018.