Synoptic view on sub-ionospheric VLF/LF amplitude and phase variations at the Graz seismo-electromagnetic facility

This presentation discusses physical processes related to potentially seismic and non-seismic disturbances along VLF/LF paths measured with two different receivers located side by side at the Space Research Institute (IWF) facility in Graz, Austria. At the same time both systems are embedded in international networks which gives the unique opportunity to probe the waveguide cavity over a large area.
In general, a variety of VLF/LF amplitude and phase variations are ubiquitous at wide scales throughout the cavity. We analyse such signals observed in the period 2018-2020 (solar minimum, i.e. less external forcing of the upper ionospheric boundary) in the time- and frequency-domain for several paths. In this attempt we aim to single out natural disturbances, characterise the source event, and figure out the lithosphere-atmosphere-ionosphere coupling mechanism. For known seismic events we consider the so-called Dobrovolsky-Bowman relationship [1,2] allowing to estimate the pre-seismic zone crossed by the VLF/LF paths.
The findings open up good prospects for an automated monitoring and characterisation of source phenomena who affect the electric field of VLF/LF sub-ionospheric links.

Ref:
[1] Dobrovolsky, I.P., Zubkov, S.I., and Miachkin, V.I., Estimation of the size of earthquake preparation zones, PAGEOPH 117, 1025–1044, 1979.
https://doi.org/10.1007/BF00876083
[2] Bowman, D.D., Ouillon, G., Sammis, C.G., Sornette, A., and Sornette, D., An observational test of the critical earthquake concept, JGR Solid
Earth, 103, B10, 24359-24372, 1998. https://doi.org/10.1029/98JB00792