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Abstract

Large earthquakes and volcanic eruptions disturb ionosphere in various ways, and we can observe them with global navigation satellite system receivers as changes in total electron content (TEC), number of electrons along the line-of-sights connecting the receivers and the satellites. I first review ionospheric disturbances related to earthquakes. They appear ~10 minutes or later after earthquakes as sudden TEC changes propagating with three different speeds, ~4 km/s (Rayleigh wave), ~0.8 km/s (direct acoustic wave), and ~0.25 km/s (internal gravity wave). The disturbances caused by direct acoustic waves from the epicenters are useful to estimate moment magnitudes of earthquakes, although we should take care of the influences of geomagnetic fields and line-of-sight geometries on the observed amplitudes. Slow fault slip signatures were recently found to emerge as the large amplitudes of internal gravity wave signals relative to those by acoustic waves. Secondly, I discuss ionospheric TEC changes by large volcanic eruptions. They emerge in two distinct forms, long-lasting harmonic oscillations or short impulsive changes, often associated with Plinian continuous eruptions and Vulcanian explosive eruptions, respectively. In both types, disturbance amplitudes normalized by background vertical TEC provide useful measures of the eruption intensities. I also review observations of ionospheric responses to the 2015 January eruption in Tonga, where several brand-new TEC changes have been observed, e.g., the Lamb waves circulating the Earth multiple times, long-lasting harmonic oscillations caused by atmospheric modes, and sudden electron depletion possibly by the injection of water vapor molecules into the upper atmosphere.