Impact of Sudden Stratospheric Warmings and Elevated Stratopause events on the VLF signal in high latitudes

Abstract

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in wind, temperatures and vertical mass transport, especially in stratospheric and mesospheric altitudes. Presumably, 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. Here we want to study the impact of SSW/ES events on the VLF signal amplitude in high latitudes. For the identification of SSW/ES induced perturbations of the VLF signal, the typical seasonal variation and outlier caused by noise, technical adjustments or solar events need to be removed. A quiet time curve, representing the seasonal VLF signal variation under undisturbed conditions, was developed with a polynomial fit of the composite. In preparation for the composite, the VLF data needed to be leveled due to artificial amplitude steps with technical origin in the timeseries. The leveling was done with help of the Pruned Exact Linear Time method. Additionally, outlier have been removed using the Median Absolute Deviation, a method from robust statistics. With help of the developed quiet time curve, VLF signal perturbations could be identified, caused by the SSW and ES events. Here we want to describe and discuss those VLF signal perturbations for multiple links in high latitudes, considering the different pathways between transmitter and receiver as the ES events vary strongly with longitude.