The Impact of Sudden Stratospheric Warmings and Elevated Stratopause Events on the VLF signal

Abstract

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in atmospheric dynamics and temperatures. SSWs are characterized by a sudden warming in the stratosphere by up to 90K and a mesospheric cooling by up to 30K. At the same time the background wind decelerates and can even revers which modifies the vertical mass transport. Occasionally SSW are followed by an ES where the stratopause at 50-60 km vanishes and subsequently reforms in elevated altitude ranges of 70-85 km. This leads to a temperature increase of up to 50 K in mesospheric heights. The temperature changes during a SSW and ES event can have an impact on the VLF signal absorption. In theory a decrease/increase in temperature can lead to a decreased/enhanced VLF signal absorption due to a decreased/increased collision frequency. Here we want to investigate the impact of a SSW and ES on VLF signals using the VLF signals of three transmitter and receiver combinations, with reflections points located in high latitudes. For the winter 2008/2009 we found an anticorrelation between VLF amplitude perturbations and the temperature at 70 km at midpoint location confirming the theory. We aim to discuss possible mechanisms leading to those observed perturbations in the VLF signals during SSW and ES event.