We present the first multi‐instrumental results on the ionospheric response to the geomagnetic storm of 17–18 March 2015 (the St. Patrick's Day storm) that was up to now the strongest in the 24th solar cycle (minimum SYM‐H value of −233 nT). The storm caused complex effects around the globe. The most dramatic positive ionospheric storm occurred at low latitudes in the morning (~100–150% enhancement) and postsunset (~80–100% enhancement) sectors. These significant vertical total electron content increases were observed in different local time sectors and at different universal time, but around the same area of the Eastern Pacific region, which indicates a regional impact of storm drivers. Our analysis revealed that this particular region was most concerned by the increase in the thermospheric O/N₂ ratio. At midlatitudes, we observe inverse hemispheric asymmetries that occurred, despite the equinoctial period, in different longitudinal regions. In the European‐African sector, positive storm signatures were observed in the Northern Hemisphere (NH), whereas in the American sector, a large positive storm occurred in the Southern Hemisphere, while the NH experienced a negative storm. The observed asymmetries can be partly explained by the thermospheric composition changes and partly by the hemispherically different nondipolar portions of the geomagnetic field as well as by the IMF By component variations. At high latitudes, negative ionospheric storm effects were recorded in all longitudinal regions, especially the NH of the Asian sector was concerned. The negative storm phase developed globally on 18 March at the beginning of the recovery phase.

The 15 January 2022 climactic eruption of Hunga volcano, Tonga, produced an explosion in the atmosphere of a size that has not been documented in the modern geophysical record. The event generated a broad range of atmospheric waves observed globally by various ground-based and spaceborne instrumentation networks. Most prominent was the surface-guided Lamb wave (≲0.01 hertz), which we observed propagating for four (plus three antipodal) passages around Earth over 6 days. As measured by the Lamb wave amplitudes, the climactic Hunga explosion was comparable in size to that of the 1883 Krakatau eruption. The Hunga eruption produced remarkable globally detected infrasound (0.01 to 20 hertz), long-range (~10,000 kilometers) audible sound, and ionospheric perturbations. Seismometers worldwide recorded pure seismic and air-to-ground coupled waves. Air-to-sea coupling likely contributed to fast-arriving tsunamis. Here, we highlight exceptional observations of the atmospheric waves.

Natural hazards (NH), such as earthquakes, tsunamis, volcanic eruptions, and severe tropospheric weather events, generate acoustic and gravity waves that propagate upward and cause perturbations in the atmosphere and ionosphere. The first NH‐related ionospheric disturbances were detected after the great 1964 Alaskan earthquake by ionosondes and Doppler sounders. Since then, many other observations confirmed the responsiveness of the ionosphere to NH. Within the last two decades, outstanding progress has been made in this area owing to the development of networks of ground‐based dual‐frequency Global Navigation Satellite Systems (GNSS) receivers. The use of GNSS‐sounding has substantially enlarged our knowledge about the solid earth/ocean/atmosphere/ionosphere coupling and NH‐related ionospheric disturbances and their main features. Moreover, recent results have demonstrated that it is possible to localize NH from their ionospheric signatures and also, if/when applicable, to obtain the information about the NH source (i.e., the source location and extension and the source onset time). Although all these results were obtained in retrospective studies, they have opened an exciting possibility for future ionosphere‐based detection and monitoring of NH in near‐real time. This article reviews the recent developments in the area of ionospheric detection of earthquakes, tsunamis, and volcanic eruptions, and it discusses the future perspectives for this novel discipline.

International audienceWe provide here a preliminary analysis of the ionospheric perturbations observed after the 11 March 2011 Tohoku Earthquake using a GPS-derived Total Electron Content (TEC) technique. Such anomalies are routinely observed after seismic events of magnitude M w = 6 and more. Here, we use the high density and the wide coverage of the Japanese Global Positioning System (GPS) network GEONET to image the ionosphere just after the main shock. We describe ionospheric perturbations with exceptional extension in amplitude and duration. As already seen in earlier events, a first intense signal is observed about 10 minutes after the seismic rupture; the first response consists in two modes: one propagating beyond 3 km/s and the other at nearly 1 km/s. A further analysis of TEC time series of the latter mode near the source shows the typical frequencies of acoustic resonance. Beyond 400 km from the source, both the tsunami induced gravity wave and a third mode are imaged, the latter for the first time. We show that the pattern of this slow (225 m/s ± 10 m/s) and long period gravity wave (1.8 ± 0.2 mHz) is most visible in the NorthWest of the epicentral area. This description is corroborated by a computation of the normal modes of the solid Earth-atmosphere system

On 6 September 2017, the Sun emitted two significant solar flares (SFs). The first SF, classified X2.2, peaked at 09:10 UT. The second one, X9.3, which is the most intensive SF in the current solar cycle, peaked at 12:02 UT and was accompanied by solar radio emission. In this work, we study ionospheric response to the two X‐class SFs and their impact on the Global Navigation Satellite Systems and high‐frequency (HF) propagation. In the ionospheric absolute vertical total electron content (TEC), the X2.2 SF caused an overall increase of 2–4 TECU on the dayside. The X9.3 SF produced a sudden increase of ~8–10 TECU at midlatitudes and of ~15–16 TECU enhancement at low latitudes. These vertical TEC enhancements lasted longer than the duration of the EUV emission. In TEC variations within 2–20 min range, the two SFs provoked sudden increases of ~0.2 TECU and 1.3 TECU. Variations in TEC from geostationary and GPS/GLONASS satellites show similar results with TEC derivative of ~1.3–1.7 TECU/min for X9.3 and 0.18–0.24 TECU/min for X2.2 in the subsolar region. Further, analysis of the impact of the two SFs on the Global Navigation Satellite Systems‐based navigation showed that the SF did not cause losses‐of‐lock in the GPS, GLONASS, or Galileo systems, while the positioning error increased by ~3 times in GPS precise point positioning solution. The two X‐class SFs had an impact on HF radio wave propagation causing blackouts at <30 MHz in the subsolar region and <15 MHz in the postmidday sector.