Sub-ionospheric VLF/LF waveguide variations related to natural hazards events over Southern Europe
Abstract
This study outlines a range of sub-ionospheric variations measured along the paths between narrowband VLF/LF transmitters and the mid-latitude receiver knot in Graz, Austria. The facility is part of an extensive receiver network with the ability to sample natural hazards prone areas based on various radio link combinations in the earth-ionosphere waveguide. In particular we investigate ionospheric disturbances related to magnitude M > 5.5 earthquakes over Southern Europe and volcanic eruptions of Mount Etna, Italy, in the year 2021. For both groups we consider a lithosphere-atmosphere-ionosphere coupling (LAIC) mechanism by atmospheric waves and show characteristic VLF/LF signal behavior before and after the events. A third domain is sporadic external waveguide modulations due to x-ray solar flares of the current solar cycle 25. Comparisons between VLF/LF electric field fluctuations and x-ray flux measurements from GOES satellite data are presented. In general, the ground-based VLF/LF receiver network demonstrate the potential for applications in continuous natural hazards monitoring over larger spatial areas. The measurements can be augmented with complementary ground- and space-based observation capabilities to provide a more complete picture of the events.