[M-TT37] New Frontier of Earth Science pioneered by Dense GNSS Observation Networks

[MTT37-P09] Characteristics of ionospheric disturbances after the 2024 Noto earthquake measured with two dense GNSS observation networks in Japan

*Atsuki Shinbori¹, Yuichi Otsuka¹, Takuya Sori¹, Michi Nishioka², Septi Perwitasari² (1.Institute for Space-Earth Environment Research (ISEE), Nagoya University, 2.National Institute of Information and Communications Technology)

Keywords:Dense GNSS observation network, 2024 Noto earthquake, Traveling ionospheric disturbances, Atmospheric waves, Magnetic field line, Total electron content

The Earth’s ionosphere is formed through the partial ionization of the upper atmosphere by solar extreme ultraviolet (EUV) radiation and precipitation of energetic particles from the magnetosphere. The temporal and spatial variation of the electron density in the ionosphere is caused by both solar activity such as solar flare and lower atmospheric disturbances associated with metrological phenomena, earthquakes, tsunamis, and volcanic eruptions. Because such electron density variations in the ionosphere influence the propagation of electromagnetic radio waves used for satellite positioning and navigation, it is important to monitor and predict the occurrence of ionospheric disturbances as space weather research. It has been well-known that traveling ionospheric disturbances (TIDs) with a concentric wave structure appear 8 minutes after the occurrence of large earthquakes and volcanic eruptions [e.g., Hebert et al., 2020; Shinbori et al., 2022]. The generation mechanism of TIDs is the neutral oscillation in the thermosphere driven by acoustic waves which is generated by the vertical motion of lower atmosphere related to earthquakes and volcanic eruptions. In a case of the 2024 Noto earthquake with a magnitude of 7.6, we found an appearance of such TIDs with a concentric wave structure over Japan in the two-dimensional maps of detrended total electron content (TEC) obtained from two dense global navigation satellite system (GNSS) observation networks (GEONET and SoftBank) in Japan. In this study, we elucidate the characteristics of TIDs observed after the 2024 Noto earthquake by analyzing GNSS-TEC data with high spatial resolution of 0.02 degrees x 0.02 degrees in geographical latitude and longitude. To investigate solar wind and geomagnetic conditions on the day of the Noto earthquake, we used OMNI data and geomagnetic indices (AE and Dst) provided by NASA CDAweb and Kyoto University. In the present analysis, we used the 15-min high-pass filtered (detrended) TEC data and subtracted the 3-min running median value from the detrended TEC to eliminate the background TIDs with a much slower propagation velocity than the TIDs related to the earthquake. As a result, the TIDs with a concentric wave structure appeared around the epicenter of the earthquake approximately 8.5 minutes after the onset of the earthquake and the wave structure expanded radially with time. The amplitude of the southward propagating TIDs was much larger than that of the northward propagating TIDs. Considering an inclination of magnetic field lines over Japan, an angle between the wave vector of the acoustic wave propagating southward and magnetic field line becomes small. In this case, charged particles in the ionosphere can more easily move along the magnetic field line on the south side. Therefore, the amplitude of the TEC variations becomes larger on the south side of the epicenter. The initial excursion of the detrended TEC variations occurred almost along a spatial distribution of the epicenters of aftershocks around Noto Peninsula. This result suggests that the TEC variations are due to the acoustic waves generated by the ground and sea level rise associated with the earthquake. From the distance-time plot of the detrended TEC, the propagation speed of the TIDs was estimated as approximately 1,000 m/s. This propagation speed is almost consistent with that of the acoustic wave propagating in the thermosphere at an altitude of 550 km.

Acknowledgments
The SoftBank's GNSS observation data used in this study was provided by SoftBank Corp. and ALES Corp. through the framework of the "Consortium to utilize the SoftBank original reference sites for Earth and Space Science".