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Abstract The biggest volcanic eruption since 1991 happened on 15 January 2022 on the island of Hunga Tonga‐Hunga Haʻapai (20.6°S; 175.4°W) in the South Pacific between 4:00 and 4:16 UT. The updrafts from the eruption reached 58 km height. In order to observe its ionospheric effects, approximately 750 GNSS receivers in New Zealand and Australia were used to calculate the detrended total electron content (dTEC). Traveling ionospheric disturbances (TIDs) were observed over New Zealand 1.0–1.5 hr after the volcano eruption, with a horizontal wavelength () of 1,525 km, horizontal phase velocity () of 635 m/s, period (τ) of 40 min, and azimuth (α) of 214°. On the other hand, TIDs were observed 2–3 hr after the eruption in Australia with,,τ, andαof 922 km, 375 m/s, 41 min, and 266°, respectively. Using reverse ray tracing, we found that these GWs originated atz > 100 km at a location ∼500 km south of Tonga, in agreement with model results for the location of a large amplitude body force created from the breaking of primary GWs from the eruption. Thus, we found that these fast GWs were secondary, not primary GWs from the Tonga eruption. 

Abstract We analyze the gravity waves (GWs) from the ground to the thermosphere during 11–14 January 2016 using the nudged HI Altitude Mechanistic general Circulation Model. We find that the entrance, core and exit regions of the polar vortex jet are important for generating primary GWs and amplifying GWs from below. These primary GWs dissipate in the upper stratosphere/lower mesosphere and deposit momentum there; the atmosphere responds by generating secondary GWs. This process is repeated, resulting in medium to large‐scale higher‐order, thermospheric GWs. We find that the amplitudes of the secondary/higher‐order GWs from sources below the polar vortex jet are exponentially magnified. The higher‐order, thermospheric GWs have concentric ring, arc‐like and planar structures, and spread out latitudinally to 10 − 90°N. Those GWs with the largest amplitudes propagate against the background wind. Some of the higher‐order GWs generated over Europe propagate over the Arctic region then southward over the US to ∼15–20°N daily at ∼14 − 24 UT (∼9 − 16 LT) due to the favorable background wind. These GWs have horizontal wavelengthsλ_H ∼ 200 − 2,200 km, horizontal phase speedsc_H ∼ 165 − 260 m/s, and periodsτ_r ∼ 0.3 − 2.4 hr. Such GWs could be misidentified as being generated by auroral activity. The large‐scale, higher‐order GWs are generated in the lower thermosphere and propagate southwestward daily across the northern mid‐thermosphere at ∼8–16 LT withλ_H ∼ 3,000 km andc_H ∼ 650 m/s. We compare the simulated GWs with those observed by AIRS, VIIRS/DNB, lidar and meteor radars and find reasonable to good agreement. Thus the polar vortex jet is important for facilitating the global generation of medium to large‐scale, higher‐order thermospheric GWs via multi‐step vertical coupling. 

Abstract The moving solar terminator (ST) generates atmospheric disturbances, broadly termed solar terminator waves (STWs). Despite theoretically recurring daily, STWs remain poorly understood, partially due to measurement challenges near the ST. Analyzing Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) data from NASA's Ionospheric Connection Explorer (ICON) observatory, we present observations of STW signatures in thermospheric neutral winds, including the first reported meridional wind signatures. Seasonal analysis reveals STWs are most prominent during solstices, when they intersect the ST about ∼20° latitude from the equator in the winter hemisphere and have phase fronts inclined at a ∼40° angle to the ST. We also provide the first observed STW altitude profiles, revealing large vertical wavelengths above 200 km. Comparing these observations to four different models suggests the STWs likely originate directly or indirectly from waves from below 97 km. STWs may play an under‐recognized role in the daily variability of the thermosphere‐ionosphere system, warranting further study. 

Abstract We analyze the gravity waves (GWs) from the ground to the thermosphere during 11–14 January 2016 using the nudged HI Altitude Mechanistic general Circulation Model. We find that the entrance, core and exit regions of the polar vortex jet are important for generating primary GWs and amplifying GWs from below. These primary GWs dissipate in the upper stratosphere/lower mesosphere and deposit momentum there; the atmosphere responds by generating secondary GWs. This process is repeated, resulting in medium to large‐scale higher‐order, thermospheric GWs. We find that the amplitudes of the secondary/higher‐order GWs from sources below the polar vortex jet are exponentially magnified. The higher‐order, thermospheric GWs have concentric ring, arc‐like and planar structures, and spread out latitudinally to 10 − 90°N. Those GWs with the largest amplitudes propagate against the background wind. Some of the higher‐order GWs generated over Europe propagate over the Arctic region then southward over the US to ∼15–20°N daily at ∼14 − 24 UT (∼9 − 16 LT) due to the favorable background wind. These GWs have horizontal wavelengthsλ_H ∼ 200 − 2,200 km, horizontal phase speedsc_H ∼ 165 − 260 m/s, and periodsτ_r ∼ 0.3 − 2.4 hr. Such GWs could be misidentified as being generated by auroral activity. The large‐scale, higher‐order GWs are generated in the lower thermosphere and propagate southwestward daily across the northern mid‐thermosphere at ∼8–16 LT withλ_H ∼ 3,000 km andc_H ∼ 650 m/s. We compare the simulated GWs with those observed by AIRS, VIIRS/DNB, lidar and meteor radars and find reasonable to good agreement. Thus the polar vortex jet is important for facilitating the global generation of medium to large‐scale, higher‐order thermospheric GWs via multi‐step vertical coupling. 

Abstract We use high temporal‐resolution mesoscale imagery from the Geostationary Operational Environmental Satellite‐R (GOES‐R) series to track the Lamb and gravity waves generated by the 15 January 2022 Hunga Tonga‐Hunga Ha'apai eruption. The 1‐min cadence of these limited area (∼1,000×1,000 km²) brightness temperatures ensures an order of magnitude better temporal sampling than full‐disk imagery available at 10‐min or 15‐min cadence. The wave patterns are visualized in brightness temperature image differences, which represent the time derivative of the full waveform with the level of temporal aliasing being determined by the imaging cadence. Consequently, the mesoscale data highlight short‐period variations, while the full‐disk data capture the long‐period wave packet envelope. The full temperature anomaly waveform, however, can be reconstructed reasonably well from the mesoscale waveform derivatives. The reconstructed temperature anomaly waveform essentially traces the surface pressure anomaly waveform. The 1‐min imagery reveals waves with ∼40–80 km wavelengths, which trail the primary Lamb pulse emitted at ∼04:29 UTC. Their estimated propagation speed is ∼315 ± 15 m s⁻¹, resulting in typical periods of 2.1–4.2 min. Weaker Lamb waves were also generated by the last major eruption at ∼08:40–08:45 UTC, which were, however, only identified in the near field but not in the far field. We also noted wind effects such as mean flow advection in the propagation of concentric gravity wave rings and observed gravity waves traveling near their theoretical maximum speed. 

Abstract The Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) and CHAllenging Minisatellite Payload (CHAMP) satellites measure in‐situ thermospheric density and cross‐track wind. When propagating obliquely to the satellite track in a horizontal plane (i.e., not purely along‐track or cross‐track), gravity waves (GWs) can be observed both in the density and cross‐track wind perturbations. We employ the Wavelet Analysis, red noise model, dissipative dispersion and polarization relations for thermospheric GWs, and specific criteria to determine whether a quiet‐time (Kp < 3) thermospheric traveling atmospheric disturbances (TADs) event is a GW or not. The first global morphology of thermospheric GWs instead of TADs is reported. The fast intrinsic horizontal phase speed (c_IH> 600 m/s) of most GWs suggests that they are not generated in the lower/middle atmosphere (wherec_IH < 300 m/s). A second population of GWs with slower speeds (c_IH = 50–250 m/s) in GOCE are likely from the lower/middle atmosphere, but they occur much less frequently in CHAMP. GW hotspots occur during the high‐latitude and the winter midlatitude regions. GW amplitudes exhibit semi‐annual and annual variations. These findings suggest that most GOCE and CHAMP GWs are higher‐order GWs from primary GW sources in the lower/middle atmosphere. Finally, the average propagation direction of the CHAMP GWs exhibits a clear diurnal cycle, with clockwise (counterclockwise) occurring in the northern (southern) hemisphere and equatorward propagation occurring at ∼13 LST. This suggests that the predominant GW propagation direction is opposite to the background wind direction. 

Abstract We analyze daytime quiet‐time MSTIDs between 2013 and 2015 at the geomagnetic equatorial and low latitude regions of the Chilean and Argentinian Andes using keograms of detrended total electron content (dTEC). The MSTIDs had a higher occurrence rate at geomagnetic equatorial latitudes in the June solstice (winter) and spring (SON). The propagation directions changed with the season: summer (DJF) [southeast, south, southwest, and west], winter (JJA) [north and northeast], and equinoxes [north, northeast, south, southwest, and west]. In addition, the MSTIDs at low latitudes observed between 8:00 and 12:00 UT occur more often during the December solstice and propagate northwestward and northeastward. After 12:00 UT, they are mostly observed in the equinoxes and June solstice. Their predominant propagation directions depend on the season: summer (all directions with a preference for northeastward), autumn (MAM) [north and northeast], winter (north and northeast), and spring (north, northeast, and southwest). The MSTID propagation direction at different latitudes was explained by the location of the possible sources. Besides, we calculated MSTIDs parameters at geomagnetic low latitudes over the Andes Mountains and compared them with those estimated at the geomagnetic equatorial latitudes. We found that the former is smaller on average than the latter. Also, our observations validate recent model results obtained during geomagnetically quiet‐time as well as daytime MSTIDs during winter over the south of South America. These results suggest that secondary or high‐order gravity waves (GWs) from orographic forcing are the most likely source of these MSTIDs.