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

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 increase during an ES is accompanied by strongly enhanced positive zonal winds and a downward directed mass transport, which leads to changes in neutral chemistry.
Very low frequency (VLF) signals transmission, which is used for long distance communication, is generally conducted from a transmitter station to a receiver station within the so-called wave guide. This is the region between the Earth surface and the bottom side of the ionosphere (~60-90 km), which is behaving as a reflection boundary. Any changes in D-region ionization are able to modify the propagation of the VLF signal.
The above described significant changes in wind, temperature and neutral composition during SSW/ES events occur within the VLF reflection heights and likely influence the VLF propagation.

For the identification of SSW/ES induced perturbations of the VLF signal we need to remove the typical seasonal variation and outliers caused by noise, technical adjustments or solar events.  For this purpose, a quiet time curve is required, which represents the seasonal VLF signal variation under undisturbed conditions, for each link respectively. We developed the quiet time winter curve with a polynomial fit of the wintertime 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, outliers have been removed using the Median Absolute Deviation, a method from robust statistics.

The developed quiet time winter curve allows us to determine VLF signal perturbations, which we analyze to examine the impact of SSW/ES events on the VLF signal. Furthermore, by studying different links in high latitudes, we want to investigate if there occur longitudinally differences in the VLF signal perturbation as the ES events vary strongly with longitude.