Impact study of four Sudden Stratospheric Warming / Elevated Stratopause events on the VLF signal in high latitudes

Abstract

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are mid-to-high latitudinal, atmospheric wave-driven winter phenomena leading to significant changes in wind, temperatures, and vertical mass transport, especially in stratospheric and mesospheric altitudes. Likely SSW and ES-induced changes also cause modifications in the sensitive D-region ionization (~60-90 km). This bottom side of the Ionosphere behaves together with the Earth-surface as a reflection boundary for Very Low Frequency (VLF) transmission, used for long-distance communication. Since perturbations of the D-region ionization are also notable in the VLF signal, it serves as a useful 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 Tx-Rx link NRK-NyÅlesund, to gain further knowledge about interactions between the ionospheric D-region and the atmosphere during these significant atmospheric phenomena. For three of four SSW/ES events occurring during the winter periods of 2009, 2010, and 2018, a similar VLF signal amplitude variation is observed, characterized by a significant increase during the wind reversal in the signal amplitude followed by a decrease during the ES. This study aims to reveal a possible mechanism driving these similar VLF signal amplitude variations, involving an unexpectedly strong correlation with the mesospheric water vapor concentration. Furthermore, causes for the different VLF signal amplitude variations for the SSW/ES event in winter 2019 are discussed.