the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Apr 2026
Co-seismic infrasound in the ionosphere over Central Europe from the M8.8 Kamchatka 2025 earthquake observed by Doppler sounding at record heights
Abstract. 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 July 29, 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 in Czechia, more than 8000 km away 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.
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Status: open (until 19 Jun 2026)
- RC1: 'Comment on egusphere-2026-1206', Anonymous Referee #1, 30 May 2026 reply
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RC2: 'Comment on egusphere-2026-1206', Anonymous Referee #2, 11 Jun 2026
reply
Comment on the revised manuscript entitled "Co-seismic infrasound in the ionosphere over Central Europe from the M8.8 Kamchatka 2025 earthquake observed by Doppler sounding at record heights" by Chum et al.
This manuscript presents observations of co-seismic ionospheric disturbances associated with the 29 July 2025 M8.8 Kamchatka earthquake using continuous Doppler sounding systems in Czechia and Slovakia. The authors report the detection of ionospheric signatures at reflection heights of approximately 340 km, which they claim to represent the highest-altitude observation of co-seismic infrasound by HF Doppler sounding to date. The study combines Doppler observations, digisonde measurements, ray-tracing calculations, and full-wave numerical simulations to investigate the propagation and attenuation of earthquake-generated infrasound waves.
The topic is timely and relevant to the communities studying lithosphere--atmosphere--ionosphere coupling and coseismic ionospheric disturbances. The dataset is valuable, and the reported observations are potentially significant. In particular, the apparent detection of co-seismic infrasound at unusually high altitudes and the discussion of frequency-dependent attenuation during upward propagation are interesting and worthy of publication.
However, I would ask the authors to carefully consider the following points before the manuscript is accepted for publication.
1. Discussion of the OH airglow observations
In Lines 353--358, the authors state that "The time resolution of OH temperature measurement by GRIPS is 15 s. This sampling frequency … using the continuous Wavelet Transform (not shown)." This sentences showed that the temporal resolution of the OH airglow observations is insufficient to resolve the higher-frequency perturbations expected in association with the earthquake. If so, the meanings for discussing the OH airglow results is not clear.
OH airglow observations would be highly valuable if they were capable of detecting temporal variations similar to those reported by Snively et al. (2013). However, if the time resolution used in this study is unable to resolve such variations, the scientific value of presenting and discussing these observations becomes questionable. The authors should therefore provide a stronger justification for including the OH airglow results or consider removing this section from the manuscript.
2. Discussion of the Swarm satellite data
The inclusion of the Swarm satellite analysis also appears unnecessary. It should have been evident from a preliminary inspection of the satellite trajectories that no Swarm spacecraft passed sufficiently close to the region of interest during the period investigated in this study. Consequently, the absence of relevant observations does not provide meaningful information regarding the phenomenon under investigation.
Therefore, there appears to be little justification for describing the Swarm data in either the Data section or the Results section. Unless these observations contribute directly to the scientific objectives of the study, I recommend removing them in order to improve the focus and conciseness of the manuscript.
If the authors wish to incorporate additional datasets in addition to the Doppler sounding observations, GNSS-TEC measurements would likely provide more useful information. In fact, TEC observations are already discussed in the Introduction as a powerful tool for investigating ionospheric disturbances associated with seismic events. Compared with the OH airglow and Swarm datasets presented here, TEC measurements would be expected to provide more direct information on the spatial extent, propagation characteristics, and amplitude of the ionospheric response. Therefore, I encourage the authors to consider incorporating TEC observations, if available, rather than presenting datasets that do not substantially constrain the interpretation of the event.
3. Interpretation of Figure 2
According to Equation (2), the plasma vertical velocity, w_p, should be smaller than the neutral vertical velocity, w. However, Figure 2 appears to show the opposite relationship, with w being smaller than w_p. The reason for this apparent discrepancy is unclear. The authors should explain why the relationship shown in Figure 2 differs from that expected from Equation (2), or clarify whether there is a misunderstanding regarding the definition, derivation, or presentation of these quantities.
In addition, Figure 2(c) contains several different quantities, including the observed plasma vertical velocity (w_p), the neutral velocity (w) derived from the observations, the numerically simulated neutral velocity, and the result obtained over Slovakia, which is shown as a green dashed line. As a result, the figure is rather crowded and difficult to interpret.
In particular, the green dashed line is difficult to distinguish from the other curves. I recommend improving the figure by using more clearly distinguishable colours, line styles, and/or line widths, and by enhancing the overall readability of the plot. Since the scientific interpretation relies heavily on comparisons among these datasets, the figure should be redesigned so that all curves can be readily distinguished by the reader.
Citation: https://doi.org/10.5194/egusphere-2026-1206-RC2
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Review of manuscript entitled “Co-seismic infrasound in the ionosphere over Central Europe from the M8.8 Kamchatka 2025 earthquake observed by Doppler sounding at record heights” by Chum et al.
This study reported a unique observation of co-seismic infrasound wave signature at an unprecedented high altitude of the ionosphere by the continuous Doppler sounding systems (CDSS) in Czechia and Slovakia. This paper is novel in two aspects: (1) Co-seismic infrasound waves can propagate upward to the altitude of ionospheric F layer. (2) The ionospheric perturbations observed by Doppler sounder is an effective characterization of the response to earthquakes. The authors confirmed the presence of infrasound wave using ray tracing simulation, and also revealed the characteristics and underlying mechanisms of the upward propagation of infrasound waves generated by the earthquake. This paper should be published after considering the follow comments.