<p>Natural hazards, that occur in the lower part of the Earth's atmosphere, the troposphere, are affecting higher atmosphere layers. Many of these phenomena, such as thunderstorms, heavy rainfalls, tsunamis, or volcanic eruptions create acoustic waves (AWs), and internal gravity waves (IGWs) in the atmosphere. These waves considerably affect the propagation of radio waves traveling through the ionosphere via transferring energy to this layer, causing significant variations in its parameters.</p> <p>In this paper, IGWs generated by thunderstorms and tsunamis are investigated. For this task, we used double-frequency measurements from the Global Navigation Satellite Systems (GNSS) along with radio occultation (RO) measurements from Low Earth Orbiting (LEO) satellites, such as Formosat3/ COSMIC (F3/C) and SWARM.</p> <p>The average increase of ionospheric irregularity amplitudes under severe thunderstorm conditions was 30% compared to calm conditions (non-lightning days). The Rate of TEC Index (ROTI) on high thunderstorm days showed an average increase of 25% within the lower ionosphere. In addition, a significant increase in amplitude and activity of IGWs and IAWs during the thunderstorms was observed.</p> <p>In the case of tsunamis, the critical frequency of the F2 layer (foF2) showed clear disturbances at the GNSS ground stations due to the tsunami-generated IGWs. During the tsunami, the Ionospheric Electron Density (IED) decreased in altitudes below approximately 300km by 27%. However, above this height, the IEDs increased by 64% up to about 750 km altitude, with a maximum amount of 3.77 × 105 elec/cm3 at 355 km altitude. The average increase of ROTI during the arrival of the tsunami to the stations was 8%.</p> <p><strong>Keywords: </strong>Natural hazards, ionospheric parameters, thunderstorm, tsunami, internal gravity waves, troposphere-ionosphere vertical coupling.</p>

The rapid displacement of the ocean floor during large ocean earthquakes or volcanic eruptions causes the propagation of tsunami waves on the surface of the ocean, and consequently internal gravity waves (IGWs) in the atmosphere. IGWs pierce through the troposphere and into the ionospheric layer. In addition to transferring energy to the ionosphere, they cause significant variations in ionospheric parameters, so they have considerable effects on the propagation of radio waves through this dispersive medium. In this study, double-frequency measurements of the Global Positioning System (GPS) and ionosonde data were used to determine the ionospheric disturbances and irregularities in response to the tsunami induced by the 2011 Tohoku earthquake. The critical frequency of the F2 layer (foF2) data obtained from the ionosonde data also showed clear disturbances that were consistent with the GPS observations. IGWs and tsunami waves have similar propagation properties, and IGWs were detected about 25 min faster than tsunami waves in GPS ground stations at the United States west coast, located about 7900 km away from the tsunami’s epicenter. As IGWs have a high vertical propagation velocity, and propagate obliquely into the atmosphere, IGWs can also be used for tsunami early warning. To further investigate the spatial variation in ionospheric electron density (IED), ionospheric profiles from FORMOSAT-3/COSMIC (F3/C) satellites were investigated for both reference and observation periods. During the tsunami, the reduction in IED started from 200 km and continued up to 272 km altitude. The minimum observed reduction was 2.68 × 105 el/cm3, which has happened at 222 km altitude. The IED increased up to 767 km altitude continuously, such that the maximum increase was 3.77 × 105 el/cm3 at 355 km altitude.

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