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We review the mechanism of multi-step vertical coupling (MSVC) via secondary and higher-order gravity waves (GWs), and its relevance for observed GW perturbations and the circulation in the upper mesosphere and thermosphere. Since the momentum deposition following the breaking or dissipation of a GW packet is localized in space and time, it leads to an imbalance in the ambient flow which in turn results in the generation of secondary or higher-order GWs. This local “body force” (LBF) mechanism is essential for MSVC. We argue that small-scale secondary GWs resulting directly from GW instability form a macro-turbulent cascade that leads to the LBF. We present a simple scale analysis supporting this interpretation with respect to observed GW spectra. Several examples of MSVC are reviewed. These include 1) an explanation of the observed persistent GWs and prevailing eastward winds in the winter mesopause region at middle to high latitudes via secondary GWs, 2) evidence that many of the daytime traveling ionospheric disturbances in the F region during winter and low geomagnetic activity are driven by higher-order GWs from MSVC, 3) the dependence of MSVC during wintertime on the strength of the polar vortex, and 4) the secondary GW disturbances in the thermosphere and ionospheric that were triggered by the Tonga volcanic eruption on January 15, 2022. Furthermore, we describe the GW-resolving whole-atmosphere model that was primarily used in corresponding studies of MSVC, and we discuss some open questions. 

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. The Hunga Tonga–Hunga Ha′apai volcano erupted on 15 January 2022, launching Lamb waves and gravity waves into the atmosphere. In this study, we present results using 13 globally distributed meteor radars and identify the volcanogenic gravity waves in the mesospheric/lower thermospheric winds. Leveraging the High-Altitude Mechanistic general Circulation Model (HIAMCM), we compare the global propagation of these gravity waves. We observed an eastward-propagating gravity wave packet with an observed phase speed of 240 ± 5.7 m s−1 and a westward-propagating gravity wave with an observed phase speed of 166.5 ± 6.4 m s−1. We identified these waves in HIAMCM and obtained very good agreement of the observed phase speeds of 239.5 ± 4.3 and 162.2 ± 6.1 m s−1 for the eastward the westward waves, respectively. Considering that HIAMCM perturbations in the mesosphere/lower thermosphere were the result of the secondary waves generated by the dissipation of the primary gravity waves from the volcanic eruption, this affirms the importance of higher-order wave generation. Furthermore, based on meteor radar observations of the gravity wave propagation around the globe, we estimate the eruption time to be within 6 min of the nominal value of 15 January 2022 04:15 UTC, and we localized the volcanic eruption to be within 78 km relative to the World Geodetic System 84 coordinates of the volcano, confirming our estimates to be realistic. 

Abstract. Atmospheric gravity waves and traveling ionospheric disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward-propagating atmospheric gravity waves can be simultaneously observed with the EISCAT very high frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of traveling ionospheric disturbances are notably impacted by the fall transition of the mesosphere and lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere. 

Abstract In Vadas et al. (2024,https://doi.org/10.1029/2024ja032521), we modeled the atmospheric gravity waves (GWs) during 11–14 January 2016 using the HIAMCM, and found that the polar vortex jet generates medium to large‐scale, higher‐order GWs in the thermosphere. In this paper, we model the traveling ionospheric disturbances (TIDs) generated by these GWs using the HIAMCM‐SAMI3 and compare with ionospheric observations from ground‐based Global Navigation Satellite System (GNSS) receivers, Incoherent Scatter Radars (ISR) and the Super Dual Auroral Radar Network (SuperDARN). We find that medium to large‐scale TIDs are generated worldwide by the higher‐order GWs from this event. Many of the TIDs over Europe and Asia have concentric ring/arc‐like structure, and most of those over North/South America have planar wave structure and occur during the daytime. Those over North/South America propagate southward and are generated by higher‐order GWs from Europe/Asia which propagate over the Arctic. These latter TIDs can be misidentified as arising from geomagnetic forcing. We find that the higher‐order GWs that propagate to Africa and Brazil from Europe may aid in the formation of equatorial plasma bubbles (EPBs) there. We find that the simulated GWs, TIDs and EPBs agree with EISCAT, PFISR, GNSS, and SuperDARN measurements. We find that the higher‐order GWs are concentrated at N at 200 km, in agreement with GOCE and CHAMP data. Thus the polar vortex jet is important for generating TIDs in the northern winter ionosphere via multi‐step vertical coupling through GWs.