Co-seismic infrasound in the ionosphere over Central Europe from the M 8.8 Kamchatka 2025 earthquake observed by Doppler sounding at record heights

Unique observations of co-seismic infrasound waves and disturbances in the ionosphere recorded by continuous Doppler sounding systems (CDSS) in Czechia and Slovakia during geomagnetically quiet period and associated with the Kamchatka M8.8 earthquake on 29 July 2025, are analysed and discussed. It is shown by simultaneous ionospheric sounding by a digisonde that the co-seismic infrasound waves were detected by the CDSS at a record height of about 340 km over Czechia, which is much higher than in previous observations, at distances greater than 8000 km from the epicentre. The Doppler shift oscillations caused by ionospheric plasma quasiperiodic movement induced by the infrasound waves had a frequency around 0.005 Hz and were observed approximately 12 min after the arrival of causative Long period surface seismic waves in Czechia. The frequency spectrum of the vertical ground surface motion that generated the infrasound waves was much broader, including more intense fluctuations with frequencies around 0.05 Hz. However, the higher frequency infrasound waves were attenuated during their propagation upward and did not reach the observation altitude, which is confirmed by numerical simulation that is in a good agreement with the CDSS observation. The numerical simulation also proves that it is necessary to consider air/plasma compression when calculating air particle velocities from the measured Doppler shift values. Combination of the numerical simulation with measurements using the CDSS and ionosonde is therefore a useful tool for better understanding and monitoring of co-seismic infrasound that propagates up to the altitudes of ionospheric F2 layer.