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The Hunga‐Tonga Hunga‐Ha'apai volcano underwent a series of large‐magnitude eruptions that generated broad spectra of mechanical waves in the atmosphere. We investigate the spatial and temporal evolutions of fluctuations driven by atmospheric acoustic‐gravity waves (AGWs) and, in particular, the Lamb wave modes in high spatial resolution data sets measured over the Continental United States (CONUS), complemented with data over the Americas and the Pacific. Along with >800 barometer sites, tropospheric observations, and Total Electron Content data from >3,000 receivers, we report detections of volcano‐induced AGWs in mesopause and ionosphere‐thermosphere airglow imagery and Fabry‐Perot interferometry. We also report unique AGW signatures in the ionospheric D‐region, measured using Long‐Range Navigation pulsed low‐frequency transmitter signals. Although we observed fluctuations over a wide range of periods and speeds, we identify Lamb wave modes exhibiting 295–345 m s⁻¹phase front velocities with correlated spatial variability of their amplitudes from the Earth's surface to the ionosphere. Results suggest that the Lamb wave modes, tracked by our ray‐tracing modeling results, were accompanied by deep fluctuation fields coupled throughout the atmosphere, and were all largely consistent in arrival times with the sequence of eruptions over 8 hr. The ray results also highlight the importance of winds in reducing wave amplitudes at CONUS midlatitudes. The ability to identify and interpret Lamb wave modes and accompanying fluctuations on the basis of arrival times and speeds, despite complexity in their spectra and modulations by the inhomogeneous atmosphere, suggests opportunities for analysis and modeling to understand their signals to constrain features of hazardous events.

A 2D nonlinear, compressible model is used to simulate the acoustic‐gravity wave (AGW, i.e., encompassing the spectrum of acoustic and gravity waves) response to a thunderstorm squall‐line type source. We investigate the primary and secondary neutral AGW response in the thermosphere, consistent with waves that can couple to the F‐region ionospheric plasma, and manifest as Traveling Ionospheric Disturbances (TIDs). We find that primary waves atz = 240 km altitude have wavelengths and phase speeds in the range 170–270 km, and 180–320 m/s, respectively. The secondary waves generated have wavelengths ranging from ∼100 to 600 km, and phase speeds from 300 to 630 m/s. While there is overlap in the wave spectra, we find that the secondary waves (i.e., those that have been nonlinearly transformed or generated secondarily/subsequently from the primary wave) generally have faster phases than the primary waves. We also assess the notion that waves with fast phase speeds (that exceed proposed theoretical upper limits on passing from the mesosphere to thermosphere) observed at F‐region heights must be secondary waves, for example, those generated in situ by wave breaking in the lower thermosphere, rather than directly propagating primary waves from their sources. We find that primary waves with phase speeds greater than this proposed upper limit can tunnel through a deep portion of the lower/middle atmosphere and emerge as propagating waves in the thermosphere. Therefore, comparing a TID's/GWs phase speed with this upper limit is not a robust method of identifying whether an observed TID originates from a primary versus secondary AGW.

A numerical study of the effects of seismically generated acoustic waves in the ionosphere is conducted using a three‐dimensional (3‐D) ionospheric model driven by an axisymmetric neutral atmospheric model. A source consistent with the 2011 Tohoku earthquake initial ocean surface uplifting is applied to simulate the subsequent responses. Perturbations in electron density, ion drift, total electron content (TEC), and ground‐level magnetic fields are examined. Results reveal strong latitude and longitude dependence of ionospheric TEC, and of ground‐level magnetic field perturbations associated with acoustic wave‐driven ionospheric dynamo currents. Results also demonstrate that prior two‐dimensional models can capture dominant meridional responses of TEC over latitude, even though dynamics at other longitudes are not resolved. Conclusions support that TEC and magnetic signatures can arise from nonlinear acoustic waves generated by strong earthquakes; simulations elucidate the comprehensive physics of their 3‐D ionospheric responses.

Near‐ and far‐field ionospheric responses to atmospheric acoustic and gravity waves (AGWs) generated by surface displacements during the 2015 Nepal7.8 Gorkha earthquake are simulated. Realistic surface displacements driven by the earthquake are calculated in three‐dimensional forward seismic waves propagation simulation, based on kinematic slip model. They are used to excite AGWs at ground level in the direct numerical simulation of three‐dimensional nonlinear compressible Navier‐Stokes equations with neutral atmosphere model, which is coupled with a two‐dimensional nonlinear multifluid electrodynamic ionospheric model. The importance of incorporating earthquake rupture kinematics for the simulation of realistic coseismic ionospheric disturbances (CIDs) is demonstrated and the possibility of describing faulting mechanisms and surface deformations based on ionospheric observations is discussed in details. Simulation results at the near‐epicentral region are comparable with total electron content (TEC) observations in periods (3.3 and6‐10 min for acoustic and gravity waves, respectively), propagation velocities (0.92 km/s for acoustic waves) and amplitudes (up to2 TECu). Simulated far‐field CIDs correspond to long‐period (4 mHz) Rayleigh waves (RWs), propagating with the same phase velocity of4 km/s. The characteristics of modeled RW‐related ionospheric disturbances differ from previously‐reported observations based on TEC data; possible reasons for these differences are discussed.

Near‐epicentral mesopause airglow perturbations, driven by infrasonic acoustic waves (AWs) during a nighttime analog of the 2011 M9.1 Tohoku‐Oki earthquake, are simulated through the direct numerical computation of the 3D nonlinear Navier‐Stokes equations. Surface dynamics from a forward seismic wave propagation simulation, initialized with a kinematic slip model and performed with the SPECFEM3D_GLOBE model, are used to excite AWs into the atmosphere from ground level. Simulated mesopause airglow perturbations include steep oscillations and persistent nonlinear depletions up to 50%and 70%from the background state, respectively, for the hydroxyl OH(3,1) and oxygen O(¹S) 557.7‐nm emissions. Results suggest that AWs excited near a large earthquake's epicenter may be strong enough to drive fluctuations in mesopause airglow, some which may persist after the AWs have passed, that could be readily detectable with ground‐ and/or satellite‐based imagers. Synthetic data demonstrate that future airglow observations may be used for the characterization of earthquake mechanisms and surface seismic waves propagation, potentially complementing tsunami early‐warning systems based on total electron content (TEC) observations.

A strong mountain wave, observed over Central Europe on 12 January 2016, is simulated in 2D under two fixed background wind conditions representing opposite tidal phases. The aim of the simulation is to investigate the breaking of the mountain wave and subsequent generation of nonprimary waves in the upper atmosphere. The model results show that the mountain wave first breaks as it approaches a mesospheric critical level creating turbulence on horizontal scales of 8–30 km. These turbulence scales couple directly to horizontal secondary waves scales, but those scales are prevented from reaching the thermosphere by the tidal winds, which act like a filter. Initial secondary waves that can reach the thermosphere range from 60 to 120 km in horizontal scale and are influenced by the scales of the horizontal and vertical forcing associated with wave breaking at mountain wave zonal phase width, and horizontal wavelength scales. Large‐scale nonprimary waves dominate over the whole duration of the simulation with horizontal scales of 107–300 km and periods of 11–22 minutes. The thermosphere winds heavily influence the time‐averaged spatial distribution of wave forcing in the thermosphere, which peaks at 150 km altitude and occurs both westward and eastward of the source in the 2 UT background simulation and primarily eastward of the source in the 7 UT background simulation. The forcing amplitude is2that of the primary mountain wave breaking and dissipation. This suggests that nonprimary waves play a significant role in gravity waves dynamics and improved understanding of the thermospheric winds is crucial to understanding their forcing distribution.

The variations of the horizontal phase velocity of an internal gravity wave, generated by wave “blocking” or “reflection” due to an inhomogeneous wind field, have been predicted theoretically and numerically investigated but had yet to be captured experimentally. In this paper, through a collaborative observation campaign using a sodium (Na) Temperature/Wind lidar and a collocated Advanced Mesospheric Temperature Mapper (AMTM) at Utah State University (USU), we report the first potential evidence of such a unique gravity wave process. The study shows that a small‐scale wave, captured by the AMTM, with initial observed horizontal phase velocity of 37 ± 5 m/s toward the northwest direction, experienced a large and increasing headwind as it was propagating in the AMTM field of view. This resulted in significant deceleration along its initial traveling direction, and it became quasi‐stationary before it was “reflected” to the opposite direction at later time. The USU Na lidar measured the horizontal wind and temperature during the event, when the wave was found traveling within a temperature inversion layer and experiencing an increasing headwind relative to the wave. The wind agrees well with the expected value for wave blocking suggested by the wave tracing theory, implying the existence of a large horizontal wind gradient that night near the OH layer altitudes. The study indicates the critical role of horizontal winds and their horizontal gradients in determining propagation in vertical and horizontal directions.