Search

Despite routine detection of coseismic acoustic‐gravity waves (AGWs) in Global Navigation Satellite System (GNSS) total electron content (TEC) observations, models of the earthquake‐atmosphere‐ionosphere dynamics, essential for validating data‐driven studies, remain limited. We present the results of three‐dimensional numerical simulations encompassing the entire coupling from Earth's interior to the ionosphere during the Mw ${M}_{w}$ 7.8 2016 Kaikoura earthquake. Incorporating the impact of data/model uncertainties in estimating the ionospheric state, the results show a good agreement between observed and simulated slant TEC (sTEC) signals, assessed through a set of metrics. The signals exhibit intricate waveforms, resulting from the integrated nature of TEC and phase cancellation effects, emphasizing the significance of direct signal comparisons along realistic line‐of‐sight paths. By comparing simulation results initialized with kinematic and dynamic source models, the study demonstrates the quantifiable sensitivity of sTEC to AGW source specifications, pointing to their utility in the analysis of coupled dynamics. Plain Language Summary: Earthquakes launch acoustic‐gravity waves (AGWs) into the atmosphere, spanning periods from seconds to minutes, that can reach the ionosphere at ∼ ${\sim} $100–400 km altitude. The majority of AGW detections in the ionosphere are performed with the use of GNSS signals collected with ground‐based receivers that nowadays comprehensively cover seismically active regions. However, the modeling of earthquake‐atmosphere‐ionosphere processes together, essential for validating and supporting data‐driven studies, remains rare. We present the outcomes of three‐dimensional numerical modeling of interconnected processes, spanning from Earth's interior to the ionosphere. We conducted a case study focused on the 2016 Mw ${M}_{w}$ 7.8 earthquake in New Zealand, renowned for its complexity and comprehensive observations of coseismic AGWs recorded with GNSS signals. Our results demonstrate a high level of accuracy of simulated GNSS signals, also revealing the high sensitivity to the chosen earthquake model and the complexity of resulting ionospheric signals, highlighting the necessity of attributing realistic geometries of GNSS TEC observations. The findings highlight the potential for using GNSS signals to investigate coseismic AGWs to infer characteristics of earthquakes. Key Points: Acoustic‐gravity wave‐driven slant total electron content (sTEC) signals are analyzed in terms of amplitude, waveform, and onset timeIntricate sTEC signal waveforms result from ionospheric fluctuations measured along lines‐of‐sight between satellites and receiversHigh sensitivity of sTEC signals to acoustic‐gravity wave source specification provides additional basis for earthquake characterization [ABSTRACT FROM AUTHOR]

Numerical simulations of mesopause airglow (MA) fluctuations induced by tsunami‐generated acoustic and gravity waves (TAGWs) are performed. Simulated tsunamis over realistic bathymetry are used to excite atmospheric waves at the surface level of a three‐dimensional nonlinear and compressible neutral atmospheric model. The model incorporates the dynamics and chemistry of hydroxyl OH(3,1) MA under nighttime assumptions. We report case study results of eight recent large tsunami events and demonstrate that TAGW‐induced MA fluctuations are readily detectable with modern ground‐ and space‐based imagers, and may provide quantitative insight. The amplitudes of MA fluctuations reflect the evolution of ocean surface displacements, enhancing or decreasing accordingly, and revealing the tsunami's lobes and local wave focusing. The results suggest that MA observations have potential to supplement early‐warning systems, providing information on spatial and temporal evolution of tsunami waves of ∼10 cm and higher for the cases shown. They may find applications in tsunami tracking over large open ocean areas, as well as in the investigation or reconstruction of tsunami source characteristics. Key Points: Three‐dimensional numerical simulations of mesopause airglow (MA) responses to realistic tsunami‐generated acoustic‐gravity waves (TAGWs) are performedLarge tsunamis can generate readily detectable and quantifiable fluctuations in MA via TAGWsMA observations may supplement tsunami tracking, early‐warning systems and source process investigations [ABSTRACT FROM AUTHOR]

Simulations of hypothesized but unobserved mesopause airglow (MA) disturbances generated by infrasonic acoustic waves (IAWs) during the 2016 M7.8 Kaikoura earthquake are performed. Realistic surface displacements are calculated in a forward seismic wave propagation model and incorporated into a 3‐D nonlinear compressible neutral atmosphere model as a source of IAWs at the surface‐air interface. Inchin et al. (2021), https://doi.org/10.1029/2020av000260 previously showed that Global Positioning System‐based total electron content (TEC) observations can be used to constrain the finite‐fault kinematics of the Kaikoura earthquake. However, due to limitations of Global Navigation Satellite System network coverage and coalescence of nonlinear IAW fronts, they pointed to the relative insensitivity of the observed near‐zenith TEC perturbations to the rupture evolution on the Papatea fault (PF). Here, we demonstrate that MA observations may have been able to supplement the investigation of the PF, providing information on both the timing of rupture initiation and its direction of propagation. The amplitudes of perturbations of vertically integrated volume emission rates for the simulated hydroxyl (OH)(3,1) and atomic oxygen O(1S) 557.7 nm reach ∼18% peak‐to‐peak, and ∼3.2% (5.8 K) peak‐to‐peak perturbations in OH(3,1) temperature. Our results suggest that observations of nighttime MA imprints of coseismic IAWs are feasible with ground‐based imagers, and may supplement the study of finite‐fault kinematics of large crustal earthquakes. Key Points: Simulations of mesopause hydroxyl and OI layer responses to infrasonic acoustic waves driven by the 2016 M7.8 Kaikoura earthquake are performedAirglow perturbations from coseismic infrasonic acoustic waves replicate the progression of ruptureAirglow observations may supplement traditional seismic data sets for future investigations of large earthquake finite‐fault kinematics [ABSTRACT FROM AUTHOR]

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 at z = 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. [ABSTRACT FROM AUTHOR]

The investigation of atmospheric tsunamigenic acoustic and gravity wave (TAGW) dynamics, from the ocean surface to the thermosphere, is performed through the numerical computations of the 3D compressible nonlinear Navier‐Stokes equations. Tsunami propagation is first simulated using a nonlinear shallow water model, which incorporates instantaneous or temporal evolutions of initial tsunami distributions (ITD). Ocean surface dynamics are then imposed as a boundary condition to excite TAGWs into the atmosphere from the ground level. We perform a case study of a large tsunami associated with the 2011 M9.1 Tohuku‐Oki earthquake and parametric studies with simplified and demonstrative bathymetry and ITD. Our results demonstrate that TAGW propagation, controlled by the atmospheric state, can evolve nonlinearly and lead to wave self‐acceleration effects and instabilities, followed by the excitation of secondary acoustic and gravity waves (SAGWs), spanning a broad frequency range. The variations of the ocean depth result in a change of tsunami characteristics and subsequent tilt of the TAGW packet, as the wave's intrinsic frequency spectrum is varied. In addition, focusing of tsunamis and their interactions with seamounts and islands may result in localized enhancements of TAGWs, which further indicates the crucial role of bathymetry variations. Along with SAGWs, leading long‐period phases of the TAGW packet propagate ahead of the tsunami wavefront and thus can be observed prior to the tsunami arrival. Our modeling results suggest that TAGWs from large tsunamis can drive detectable and quantifiable perturbations in the upper atmosphere under a wide range of scenarios and uncover new challenges and opportunities for their observations. Key Points: Numerical simulations of acoustic‐gravity waves generated by tsunamis are performed with 3D compressible nonlinear neutral atmosphere modelBathymetry variations markedly impact the propagation of tsunamis and tsunamigenic AGWs and may lead to their nonlinear evolutionPhase fronts of tsunamigenic AGWs and thermospherically generated secondary AGWs arrive prior to the tsunami and may explain observations [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Large earthquakes can produce substantial Earth's surface deformation. For earthquakes that occur offshore, sea floor deformation may also result in ocean surface displacements that generate tsunamis. At the interface with the atmosphere, these displacements can serve as a source of very low frequency acoustic waves. Reaching the upper atmosphere (∼70 km and higher), these waves can be strong enough to form shock waves and generate observable disturbances. This paper reports the results of numerical simulations of ∼80–90‐km altitude airglow disturbances driven by strong acoustic waves excited during a hypothetical nighttime equivalent of the 2011 Tohoku‐Oki earthquake (magnitude 9.1). Modeling results show that, for such large earthquakes if occurring at nighttime, airglow disturbances could be readily detected with specialized ground‐ and space‐based imagers. The possibility to detect airglow disturbances earlier than the arrival of the tsunami at the shore points to the potential applicability of such observations for tsunami early‐warning systems. Also, the results suggest that such observations can be a useful tool for the characterization of earthquake processes and the propagation of seismic waves. Key Points: Mesopause airglow responses to infrasonic acoustic waves generated by near‐epicentral seismic motions are investigated numerically in 3DLarge earthquakes can drive substantial and detectable perturbations in mesopause airglow via acoustic wavesAirglow observations may help to investigate earthquake dynamics and supplement tsunami early‐warning systems [ABSTRACT FROM AUTHOR]

Near‐ and far‐field ionospheric responses to atmospheric acoustic and gravity waves (AGWs) generated by surface displacements during the 2015 Nepal Mw7.8 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 and ∼6‐10 min for acoustic and gravity waves, respectively), propagation velocities (∼0.92 km/s for acoustic waves) and amplitudes (up to ∼2 TECu). Simulated far‐field CIDs correspond to long‐period (∼4 mHz) Rayleigh waves (RWs), propagating with the same phase velocity of ∼4 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. Key Points: Seismically generated AGW dynamics from ground to exobase are simulated based on realistic spatial and temporal surface displacementsSimulation results of near‐epicentral ionospheric responses to AGWs are consistent with observations in amplitudes, periods, and speedsResults identify challenges and opportunities for faulting mechanism characterization based on ionospheric observations [ABSTRACT FROM AUTHOR]

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 is ∼2 × that 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. Key Points: Mountain wave breaking in the MLT couples directly to secondary wavesTidal winds act like a filter for secondary wave spectra reaching the thermosphereNonprimary wave forcing is significant and distribution is governed by thermospheric winds [ABSTRACT FROM AUTHOR]

We present multilayer observations and numerical simulations of gravity waves (GWs) generated by a series of Mesoscale Convective Systems over the midwestern United States. Strong semiconcentric GWs were observed and modeled, which couple from their tropospheric sources to the thermosphere, displaying strong nonlinearity indicated by instability, breaking, and formation of turbulent vortices. GWs in the stratosphere display a large range of horizontal scales from 34–400 km; however, the smaller wavelength waves break rapidly in the mesosphere and lower thermosphere. Larger‐scale (≥150 km) waves dominate in the thermosphere and display northwestward propagation at 200–300 km altitude, opposing the mean winds. Despite strong molecular viscosity and thermal conductivity in the thermosphere, steepened wave fronts, which may indicate nonlinearity, is identified in 630 nm airglow imagers. The agreement between model and data suggests new opportunities for data‐constrained simulations that span multilayer observables, including mesosphere and lower thermosphere‐region airglow not captured for this event. Key Points: Coherent gravity waves were observed above thunderstorms, in the stratosphere by AIRS and thermosphere by MANGOSimulations of waves generated by convective sources defined by radar data closely match the observations in scaleResults indicate that gravity waves may evolve nonlinearly well into the thermosphere and ionosphere [ABSTRACT FROM AUTHOR]

The nonlinear propagation of low‐frequency acoustic waves through the turbulent fluctuations induced by breaking mountain gravity waves is investigated via 2‐D numerical simulations of the Navier‐Stokes equations, to understand the effects of atmospheric dynamics on ground‐based infrasound measurements. Emphasis is placed on acoustic signals of frequency around 0.1 Hz, traveling through tens‐of‐kilometers‐scale gravity waves and subkilometer‐scale turbulence. The sensitivity of the infrasonic phases to small‐scale fluctuations is found to depend on the altitudes through which they are refracted toward the Earth. For the considered cases, the dynamics in the stratosphere impact the refracting acoustic waves to a greater extent than those in the thermosphere. This work clearly demonstrates the need for accurate descriptions of the effects of atmospheric dynamics on acoustic propagation, such as here captured by the full set of fluid dynamic equations, as well as of the subsequent effects on measured signals. Plain Language Summary: Infrasound is the low‐frequency part of the acoustic spectrum, with periods ranging from around 0.05 s to about 300 s. Infrasonic waves are generated by a large variety of natural and artificial sources (such as earthquakes, volcanic eruptions, auroras, thunderstorms, explosions, rocket launches, and sonic booms) and can propagate up to the thermosphere and over thousands of kilometers on the ground. The ground recordings are currently employed in numerous applications, including the detection and localization of specific sources (such as clandestine nuclear tests) as well as the research in the domain of the atmospheric dynamics. Mountain gravity waves are buoyant motions associated with the gravitational acceleration and are excited as air flows over mountain ranges. They can have horizontal wavelengths as small as 10 km and as high as 100 km and, if sufficiently strong, can break and lead to smaller‐scale turbulence. The subsequent turbulent fluctuations can severely influence the infrasonic waves. This paper reports the results of numerical simulations of the atmospheric propagation of infrasonic signals through the inhomogeneities induced by the breaking of mountain waves. The present analysis provides new insights into the understanding of the effects of the aforementioned turbulent perturbations on the infrasonic signals potentially recorded on the Earth's surface. Key Points: The propagation of infrasonic waves through the turbulence induced by the breaking of mountain waves is investigated numericallyThe numerical model is based on the 2‐D Navier‐Stokes equationsThe influence of small‐scale turbulent fluctuations on infrasonic ground recordings is analyzed [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: This paper reports new numerical model results on how the ionosphere—the ionized, conducting region of the upper‐atmosphere—responds to strong acoustic waves launched by earthquakes. The effects of these waves in the ionosphere can be observed from ground: They are measured as variations in Global Positioning System satellite signals received at the ground, which translate to variations in integrated electron content of the ionosphere. They can be detected also via small fluctuations in magnetic field, which are driven by ionospheric currents. The model results suggest that both measurement techniques can provide insight into the waves and their sources; new guidance for interpreting these measurements is provided. Further, results demonstrate a new capability for simulating these processes in three dimensions, which can be applied to more‐realistic case studies. Key Points: Earthquake‐driven TEC and magnetic perturbations are studied with a 3‐D modelMagnetic fluctuations driven by infrasonic acoustic waves are detectableLocal geomagnetic field geometry affects directionality of TEC responses [ABSTRACT FROM AUTHOR]

Abstract: Gravity waves not subject to breaking or filtering will dissipate due to viscosity and thermal conduction in the thermosphere. However, the evolutions of wave packets, and the altitudes they reach, are highly dependent upon the spectral content. In this paper, a 2‐D numerical model is used to investigate the effect of spatial localization (and thus spectral content) of a wave packet on its dissipation, dispersion, and spectral evolution. It is found that most wave packets launched below the thermosphere evolve to smaller central vertical wavelengths as the faster, longer vertical wavelength components reach the dissipative thermosphere and are removed first, leaving the shorter, slower components to become dominant at later times. This effect is greater for more spatially localized packets (spectrally broadband) as rapid dispersion leads to the rapid spreading of the wave over large altitude regions that could be interpreted as different waves (i.e., from different sources) by instruments observing different altitudes. Dispersion can also be accelerated by the refractive effects of the thermospheric temperature gradient. Initially, Gaussian broadband packets can evolve into asymmetric distributions which are not well described by standard assumptions (e.g., Gaussian packets), requiring instead numerical simulation to properly describe them. In the case that the vertical scale is smaller than the scale height, and dissipation acts immediately on the packet (i.e., it is generated in situ in the dissipative thermosphere), then the scale‐dependent nature of dissipation removes the shorter wavelengths components first, leading to the spectrum evolving toward larger vertical wavelengths. [ABSTRACT FROM AUTHOR]

This study examines how thermospheric motions due to gravity waves (GWs) drive ion upflow in the F region, modulating the topside ionosphere in a way that can contribute to ion outflow. We present incoherent scatter radar data from Sondrestrom, from 31 May 2003 which showed upflow/downflow motions, having a downward phase progression, in the field‐aligned velocity, indicating forcing by a thermospheric GW. The GW‐upflow coupling dynamics are investigated through the use of a coupled atmosphere‐ionosphere model to examine potential impacts on topside ionospheric upflow. Specifically, a sequence of simulations with varying wave amplitude is conducted to determine responses to a range of transient forcing reminiscent of the incoherent scatter radar data. Nonlinear wave effects, resulting from increases in amplitude of the modeled GW, are shown to critically impact the ionospheric response. GW breaking deposits energy into smaller scale wave modes, drives periods of large field‐aligned ion velocities, while also modulating ion densities. Complementary momentum transfer increases the mean flow and, through ion‐neutral drag, can increase ion densities above 300 km. Ionospheric collision frequency (cooling) and photoionization effects (heating), both dependent on ionospheric density, modify the electron temperature; these changes conduct quickly up geomagnetic field lines driving ion upflow at altitudes well above initial disturbances. This flow alters ion populations available for high‐altitude acceleration processes that may lead to outflow into the magnetosphere. We have included a representative source of transverse wave heating which, when supplemented by our GWs, illustrates strengthened upward fluxes in the topside ionosphere. Key Points: High‐latitude ionospheric upflow can be modulated by thermospheric gravity wavesF region density perturbations due to gravity wave modulations, through heating and cooling processes, modulate the electron temperatureIncreases in electron temperature quickly conduct up the magnetic field lines and drive upflow at altitudes above the initial heating region [ABSTRACT FROM AUTHOR]

Abstract: A 2‐D numerical model is used to compare interactions between small‐scale (SS) (25 km horizontal wavelength, 10 min period) and medium‐scale (MS, 250 km horizontal wavelength, 90 min period) gravity waves (GWs) in the Mesosphere and Lower Thermosphere within three different limits. First, the MS wave is specified as a static, horizontally homogeneous ambient atmospheric feature; second, a linear interaction is investigated between excited, time‐dependent SS and MS waves, and third, a fully nonlinear interaction at finite amplitudes is considered. It is found that the finite‐amplitude wave interactions can cause SS wave breaking aligned with the phase fronts of the MS waves, which induces a permanent mean flow and shear that is periodic in altitude. This impedes SS wave propagation into in the upper thermosphere and dissipation by molecular diffusion, when compared to linear amplitude simulations. Linear cases also omit self‐acceleration‐related instabilities of the SS wave and secondary wave generation, which modulate the MS wavefield. Neither the linear or nonlinear cases resemble the static approximation, which, by reducing a dynamic wave interaction to a static representation that is vertically varying, produces variable momentum flux distributions that depend strongly upon the amplitude and phase of the larger‐scale wave. This is an approximation made by GW parameterization schemes, and results suggest that including time‐dependent effects and feedback mechanisms for interactions between resolved and parameterized waves will be an important area for future investigations especially as general circulation models begin to resolve MS GWs explicitly. [ABSTRACT FROM AUTHOR]

Abstract: Since 2010, Utah State University has operated an infrared Advanced Mesospheric Temperature Mapper at the Amundsen–Scott South Pole station to investigate the upper atmosphere dynamics and temperature deep within the vortex. A surprising number of “frontal” gravity wave events (86) were recorded in the mesospheric OH(3,1) band intensity and rotational temperature images (typical altitude of ~87 km) during four austral winters (2012–2015). These events are gravity waves (GWs) characterized by a sharp leading wave front followed by a quasi‐monochromatic wave train that grows with time. A particular subset of frontal gravity wave events has been identified in the past (Dewan & Picard, 1998) as “bores.” These are usually associated with wave ducting within stable mesospheric inversion layers, which allow them to propagate over very large distances. They have been observed on numerous occasions from low‐latitude and midlatitude sites, but to date, very few have been reported at high latitudes. This study provides new analyses of the characteristics of frontal events at high latitudes and shows that most of them are likely ducted. The occurrence of these frontal GW events over this isolated region strongly supports the existence of horizontally extensive mesospheric thermal inversion layers over Antarctica, leading to regions of enhanced stability necessary for GW trapping and ducting. [ABSTRACT FROM AUTHOR]

Numerical simulations demonstrate theoretical predictions that gravity waves with short periods (∼4-8 min) in the mesosphere and lower thermosphere may force secondary acoustic waves, with harmonic periods (∼2-4 minutes), that can reach detectable amplitudes in the thermosphere and ionosphere. The mechanism is through their vertical fluxes of vertical momentum, which lead to forcing as they are disrupted by varying stratification or instability. This is shown likely to occur where horizontally or radially opposing gravity waves interact at large amplitudes, such as above large convective sources, and after overturning. Evanescence and reflection of the waves can lead to further enhancements of the vertical fluxes and the potential for forcing. Results thus identify one of likely several mechanisms for the nonlinear conversion from gravity waves to acoustic waves, to elucidate an unappreciated source of vertical coupling. [ABSTRACT FROM AUTHOR]