Numerical modelling of sub-ionospheric Very Low Frequency radio signal anomalies during the Samos (Greece) earthquake (M = 6.9) on October 30, 2020

Lower ionospheric perturbation during earthquake activity is well recognized by the anomalous behavior of Very Low or Low Frequency (VLF/LF) radio signal. In this study, we present the sub-ionospheric VLF signal modulation during the Samos earthquake that took place on October 30, 2020 (M = 6.9). We use the conventional terminator time method to check the seismogenic impression in the VLF signal during the local sunrise and sunset times. VLF signals from two transmitters ISR (Israel) and TBB (Turkey) are used for this analysis and those are recorded in a receiving station UWA in Greece. Both signals show a strong shift in the sunrise and sunset terminator times. The signal profiles are numerically simulated using the Long Wavelength Propagation Capability (LWPC) program by incorporating Wait’s two-component ionospheric parameters (β and h′). The simulation gives highly satisfactory outcomes. The outputs of LWPC are used to compute the electron density profile of the lower ionosphere. It is found that the electron density gets significantly modulated during the maximum shift in the terminator times. The electron density is found to have both increased and decreased from its normal value before the earthquake. The VLF path dependency is strongly observed in the change in the electron density profile. For ISR-UWA, nighttime reflection height has been computed using the modal conversion technique, and it is found to be similar to the LWPC outcomes. The overall condition was geomagnetically quiet, and the results have no contamination due to solar-terrestrial interaction.