Acoustic-gravity Waves Research Articles

Geothermal Anomalies and Coupling with the Ionosphere before the 2020 Jiashi Ms6.4 Earthquake

Rock temperature reflects the adjustment in crustal stress, and the fluctuation of ionospheric electron concentration is closely related to short-term disturbances of the stress field. Their coupling may reveal short-term effects before strong earthquakes. This study explores the rock temperature changes and mechanical-electrical coupling in the lithosphere–ionosphere before the Jiashi Ms6.4 earthquake on 19 January 2020. The observed data were detrended by general polynomial piecewise fitting; three observation points within 150 km of the epicenter were found to show significant temperature fluctuations in the 15 days before the earthquake. The peak occurred synchronously five days before the earthquake, and the variation range was approximately 10−3 orders of magnitude. Five days before the earthquake, the electromagnetic satellite Zhangheng-1 synchronously observed an anomalous electron concentration in orbit near the epicenter, with a maximum value of 2.01 × 1010 m−3. The loading/unloading response ratio (LURR) was calculated using small earthquakes within 100 km of the epicenter; it showed that the large changes in rock temperature and the ionosphere occurred at high LURR, indicating high-stress accumulation in the region. Various anomalies appeared simultaneously and may indicate fault rupture, which may be caused by an acoustic-gravity wave, indicating a synchronous coupling between the lithosphere atmosphere and the ionosphere.

The characteristics of the 2022 Tonga volcanic tsunami in the Pacific Ocean

Abstract. On 15 January 2022, an exceptional eruption of the Hunga Tonga–Hunga Ha'apai volcano generated atmospheric and tsunami waves that were widely observed in the oceans globally, gaining remarkable attention from scientists in related fields. The tsunamigenic mechanism of this rare event remains enigmatic due to its complexity and lack of direct underwater observations. Here, to explore the tsunamigenic mechanisms of this volcanic tsunami event and its hydrodynamic processes in the Pacific Ocean, we conduct statistical analysis and spectral analysis of the tsunami recordings at 116 coastal gauges and 38 deep-ocean buoys across the Pacific Ocean. Combined with the constraints of some representative barometers, we obtain the plausible tsunamigenic origins of the volcano activity. We identify four distinct tsunami wave components generated by air–sea coupling and seafloor crustal deformation. Those tsunami components are differentiated by their different propagating speeds or period bands. The first-arriving tsunami component with an ∼ 80–100 min period was from shock waves spreading at a velocity of ∼ 1000 m s−1 in the vicinity of the eruption. The second component with extraordinary tsunami amplitude in the deep ocean was from Lamb waves. The Lamb wave with a ∼ 30–40 min period radically propagated outward from the eruption site with spatially decreasing propagation velocities from ∼ 340 to ∼ 315m s−1. The third component with a ∼ 10–30 min period was probably from some atmospheric-gravity-wave modes propagating faster than 200 m s−1 but slower than Lamb waves. The last component with a ∼ 3–5 min period originated from partial caldera collapse with dimension of ∼ 0.8–1.8 km. Surprisingly, the 2022 Tonga volcanic tsunami produced long oscillation in the Pacific Ocean which is comparable with that of the 2011 Tohoku tsunami. We point out that the long oscillation is associated not only with the resonance effect with the atmospheric acoustic-gravity waves but more importantly with their interactions with local bathymetry. This rare event also calls for more attention to the tsunami hazards produced by an atypical tsunamigenic source, e.g. volcanic eruption.

Time-domain wave response of a compressible ocean due to an arbitrary ocean bottom motion

The time-domain motion of a finite depth ocean subject to an arbitrary (in both time and space) imposed displacement of the bottom is studied under the assumption of linear theory. This solution provides results for this limiting case which may be helpful for benchmarking. The focus is on the numerical simulation of the near-field waves with application to the simulation of tsunami waves. The fluid domain is assumed two-dimensional, and the effect of compressibility is included. The time-domain solution is built from the frequency domain solution taking a Fourier series expansion of the bottom motion. This expansion allows complex displacements to be simulated. The solution in the frequency domain is expressed as a sum over modes. The time-domain solution is calculated by numerical evaluation of the Fourier transform in time, allowing arbitrary time-dependent motion. This code is extremely efficient and highly accurate, and there is no time–stepping so that errors do not accumulate in time. The eigenfunction expansion method to obtain the velocity potential for a flat ocean bottom case is independently derived. A shallow water limit for all the above cases is provided, giving a method to check the correctness of the numerical solution. Separate treatment for all the situations under the compressible assumption is also performed. The horizontal and vertical particle velocities are graphically presented for the time-harmonic oscillation. Time-dependent surface wave propagation is computed to show the initiation of tsunami waves in the deep ocean and their subsequent propagation. The calculations presented here allow for the simulation of tsunami wave generation and to investigate various effects, including the role of acoustic gravity waves. It is shown that the compressibility is not always significant, but that when the water is either sufficiently deep or the rise sufficiently rapid, acoustic gravity waves are produced. It is shown that, in this case, the ocean surface undergoes a rapid oscillation and that this may be a method to detect tsunamis.

On the propagation of acoustic–gravity waves due to a slender rupture in an elastic seabed

The propagation of waves from a vertical uplift of a slender rectangular fault in a sea of constant depth is discussed, accounting for water compressibility, gravity and seabed elasticity. The compressed water column results in the generation of acoustic–gravity waves that travel at the speed of sound in water. Acoustic–gravity waves are found to terminate after a finite time, with the decay time most influenced by seabed rigidity, which is in contrast to the rigid stationary-phase model where signals persist indefinitely. At certain frequencies acoustic–gravity waves couple with the elastic seabed and travel at the shear velocity (speed of sound in an elastic solid). Improved estimates of the critical frequencies are derived. Moreover, besides the usual tsunami, a second – very small amplitude – surface wave mode travelling at the speed of sound arises under certain frequencies. We derive the cut-off frequency for this mode. The acoustic modes possess a frequency spectrum which depends on the time evolution and spatial properties of the rupture. We find that appropriate filtering of the acoustic–gravity wave signal can reveal characteristic peaks that encode information on the fault's geometry and dynamics.

Disturbances of Doppler Frequency Shift of Ionospheric Signal and of Telluric Current Caused by Atmospheric Waves from Explosive Eruption of Hunga Tonga Volcano on January 15, 2022

After the explosive eruption of the Hunga Tonga volcano on 15 January 2022, disturbances were observed at a distance of about 12,000 km in Northern Tien Shan and regarded variations in the atmospheric pressure, in telluric current, and in the Doppler frequency shift of ionospheric signal. At 16:00:55 UTC, a pulse of atmospheric pressure was detected there, with peak amplitude of 1.3 hPa and propagation speed of 0.3056 km/s, equal to the velocity of Lamb waves. In the variations in the Doppler frequency shift, disturbances of two types were registered on the 3212 km and 2969 km long inclined radio paths, one of which arose as a response to the passage of a Lamb wave (0.3059 km/s) through the reflection point of the radio wave and another as reaction to an acoustic-gravity wave (0.2602 km/s). Two successive perturbations were also detected in the records of telluric current at the arrival times of the Lamb and acoustic-gravity waves at the registration point. According to the parameters of the Lamb wave, the energy transfer into the atmosphere upon the explosion of the Hunga Tonga volcano was roughly estimated to be 2000 Mt of TNT equivalent.

Approximation of Small Amplitude Waves Short in Vertical in the Atmosphere Taking Into Account the Average Wind

Using the method of different scales, formulas for the hydrodynamic fields of acoustic-gravity waves (AGWs) with vertical wavelengths small compared to the scales of changes in the background temperature and wind fields are derived. These formulas are equivalent to the conventional WKB approximation, but explicitly include the vertical gradients of the background fields. The conditions for the applicability of the obtained formulas for describing the propagation of AGWs from the troposphere to the thermosphere are formulated and analyzed. The absence of singular points (critical levels) in the equations for wave modes in the analyzed height range is one of the conditions for the applicability of approximate formulas. For the wind from the empirical HWM model, singular points are often located below 200 km and are typical for internal gravity waves (IGWs), with lengths of the order of 10 km. As the wavelength increases, the number of singular points decreases. For IGWs with scales on the order of 300 km or more, there are usually no singular points. It is shown that IGWs with periods of less than 20 min propagating upward from tropospheric heights usually have one turning point in the altitude range from 100 to 130 km. The obtained formulas are useful, in particular, for parametrization of AGW effects in numerical models of atmospheric dynamics and energy.

Destructive Potential of Planetary Meteotsunami Waves beyond the Hunga Tonga–Hunga Ha‘apai Volcano Eruption

Abstract Worldwide tsunamis driven by atmospheric waves—or planetary meteotsunami waves—are extremely rare events. They mostly occur during supervolcano explosions or asteroid impacts capable to generate atmospheric acoustic-gravity waves including the Lamb waves that can circle the globe multiple times. Recently, such ocean waves have been globally recorded after the Hunga Tonga–Hunga Ha‘apai volcano eruption on 15 January 2022, but did not pose any serious danger to the coastal communities. However, this study highlights that the mostly ignored destructive potential of planetary meteotsunami waves can be compared to the well-studied tsunami hazards. In practice, several process-oriented numerical experiments are designed to force a global ocean model with the realistic atmospheric response to the Hunga Tonga–Hunga Ha‘apai event rescaled in speed and amplitude. These simulations demonstrate that the meteotsunami surges can be higher than 1 m (and up to 10 m) along more than 7% of the world coastlines. Planetary meteotsunami waves thus have the potential to cause serious coastal damages and even human casualties during volcanic explosions or asteroid impacts either releasing intense acoustic energy or producing internal atmospheric gravity waves triggering the deep-ocean Proudman resonance at a speed of ∼212 m s−1. Based on records of catastrophic events in Earth’s history, both scenarios are found to be realistic, and consequently, the global meteotsunami hazards should now be properly assessed to prepare for the next big volcanic eruption or asteroid impact even occurring inland.

Detecting the January 15, 2022 Hunga Tonga-Hunga-Ha'apai volcanic eruption (SW Pacific) high up in the sky through “ionospheric resonance”

Summary We report a mode of ionospheric resonance induced by the January 15, 2022, Hunga Tonga-Hunga-Ha'apai volcanic eruption, captured globally through ground-based GNSS networks. In the near-field region, we observe a strong ionospheric disturbance in the total electron content, including contributions from acoustic waves and gravity waves. In contrast, propagation of gravity waves, seismic airwaves diffracted by topographic barriers, and/or atmospheric Lamb waves can be documented for far-field ionospheric disturbances. Specifically, the atmospheric wave propagation towards the Hawaii Islands involved three distinct modes of ionospheric perturbation through Rayleigh wave, gravity waves and atmospheric Lamb waves. Further, we also noticed that the peak-to-peak amplitude of the disturbance reached ∼ 8.7 per cent of the background electrons, which is significantly higher than the recent volcanic explosions in the Japanese Islands and the 2020 chemical explosion in Beirut. The ionospheric total electron content perturbations can provide important information about the volcanic source, hence they could be useful as a measure of eruption intensity and for real-time hazard monitoring.

Wave disturbances of the atmosphere in a spatially inhomogeneous flow

Analysis of measurements by the Dynamics Explorer 2 satellite indicates a close connection between atmospheric wave disturbances and wind circulation in the polar thermosphere. According to satellite observations, large-amplitude acoustic-gravity waves are systematically observed in the regions of formation of powerful wind systems. At the same time, the azimuths of AGW propagation are spatially consistent with the directions of wind circulation. Waves propagate mainly against the wind, and their amplitude is approximately proportional to the wind speed. In order to explain the experimental data, the paper theoretically investigated the changes in the amplitudes of AGW in a horizontally inhomogeneous wind flow. The dispersion equation of the AGW in the reference frame of the medium, which moves with a non-uniform speed, was obtained. When obtaining it, inertial forces were taken into account, as well as the change in the background density of the atmosphere in an inhomogeneous flow. It is shown that under the slow change in wind speed, the real part of this equation coincides with the dispersion equation of AGW for a stationary medium. An expression for the change in the amplitude of waves in a moving medium was obtained, according to which, in an oncoming inhomogeneous wind, the amplitude of the wave increases approximately according to a linear law, which is consistent with the data of satellite observations.

Properties of Acoustic-Gravity Waves at the Boundary of Two Isothermal Media

Properties of Acoustic-Gravity Waves at the Boundary of Two Isothermal Media

Global Electromagnetic Disturbances Caused by the Eruption of the Tonga Volcano on 15 January 2022

AbstractThe study is devoted to the analysis of geomagnetic field disturbances and the response of the Schumann resonance (SR) during the eruption of the Tonga volcano in 2022. The data on geomagnetic field variations at distances from 800 to 15,000 km from the volcano according to the INTERMAGNET network and parameters of SR signals recorded at Mikhnevo Observatory in Russia were used. The source of global geomagnetic disturbances are acoustic–gravity waves (AGWs), which caused changes in ionospheric conductivity, values of ionospheric currents, and the geomagnetic field. The propagation velocity of magnetic disturbances 263 ± 5 m/s, corresponding to the AGWs velocity, was determined and an independent estimate of the time of the eruption phase that caused the generation of the atmospheric wave (4:14 ± 10 UT) was obtained. A new method of processing the results of measurements of SR disturbance with a time resolution of 5 min instead of the usual 10–15 min allowed not only to detect but also to study this phenomenon in detail. The peculiarities of signals related to the number and energy of lightning discharges were revealed. Synchronous measurements of SR signals and geomagnetic field variations in a single observatory for the first time allowed to obtain an independent estimate of the eruption time and the electromagnetic disturbance propagation rate.

Acoustic-gravity waves generated by surface disturbances

The sudden generation of wave disturbances in the ocean is associated with a change in the pressure field in the liquid layer. Consequently, compression-type of waves, known as acoustic-gravity waves, form and radiate at the speed of sound, carrying information on the event source, namely its magnitude and location. This information can be recorded by hydrophones hundreds of kilometres away from the event, and if analyzed at near-real time using an inverse model it could allow for an early warning system. The near-real time analysis requires an explicit form of the pressure field induced by the acoustic-gravity waves. In this work, we derive an explicit solution for the bottom pressure field using stationary phase approximation. Then, to demonstrate the effectiveness of the warning system, we apply an inverse model to calculate the generation time and location of the event.

Properties of acoustic-gravity waves at the boundary of two isothermal media

Properties of acoustic-gravity waves at the boundary of two isothermal media

Decay times of atmospheric acoustic–gravity waves after deactivation of wave forcing

Abstract. High-resolution numerical simulations of non-stationary, nonlinear acoustic–gravity waves (AGWs) propagating upwards from surface wave sources are performed for different temporal intervals relative to activation and deactivation times of the wave forcing. After activating surface wave sources, amplitudes of AGW spectral components reach a quasi-stationary state. Then the surface wave forcing is deactivated in the numerical model, and amplitudes of vertically traveling AGW modes quickly decrease at all altitudes due to discontinuations of the upward propagation of wave energy from the wave sources. However, later the standard deviation of residual and secondary wave perturbations experiences a slower quasi-exponential decrease. High-resolution simulations allowed, for the first time, for the estimation of the decay times of this wave noise produced by slow residual, quasi-standing and secondary AGW spectral components, which vary between 20 and 100 h depending on altitude and the rate of wave source activation and deactivation. The standard deviations of the wave noise are larger for the case of sharp activation and deactivation of the wave forcing compared to the steep processes. These results show that transient wave sources may create long-lived wave perturbations, which can form a background level of wave noise in the atmosphere. This should be taken into account in parameterizations of atmospheric AGW impacts.

The Seismo-Ionospheric Disturbances before the 9 June 2022 Maerkang Ms6.0 Earthquake Swarm

Based on the multi-data of the global ionospheric map (GIM), ionospheric total electron content (TEC) inversed from GPS observations, the critical frequency of the F2 layer (fOF2) from the ionosonde, electron density (Ne), electron temperature (Te), and He+ and O+ densities detected by the China Seismo-Electromagnetic Satellite (CSES), the temporal and spatial characteristics of ionospheric multi-parameter perturbations were analyzed around the Maerkang Ms6.0 earthquake swarm on 9 June 2022. The results showed that the seismo-ionospheric disturbances were observed during 2–4 June around the epicenter under quiet solar-geomagnetic conditions. All parameters we studied were characterized by synchronous changes and negative anomalies, with a better consistency between ionospheric ground-based and satellite observations. The negative ionospheric anomalies for all parameters appeared 5–7 days before the Maerkang Ms6.0 earthquake swarm can be considered as significant signals of upcoming main shock. The seismo-ionospheric coupling mechanism may be a combination of two coupling channels: an overlapped DC electric field and an acoustic gravity wave, as described by the lithosphere–atmosphere–ionosphere coupling (LAIC). In addition, in order to make the investigations still more convincing, we completed a statistical analysis for the ionospheric anomalies of earthquakes over Ms6.0 in the study area (20°~40° N, 92°~112° E) from 1 January 2019 to 1 July 2022. The nine seismic events reveal that most strong earthquakes are preceded by obvious synchronous anomalies from ground-based and satellite ionospheric observations. The anomalous disturbances generally appear 1–15 days before the earthquakes, and the continuity and reliability of ground-based ionospheric anomaly detection are relatively high. Based on the integrated ionospheric satellite–ground observations, a cross-validation analysis can effectively improve the confidence level of anomaly identification and reduce the frequency of false anomalies.

Global Ionospheric Disturbance Propagation and Vertical Ionospheric Oscillation Triggered by the 2022 Tonga Volcanic Eruption

The Tonga volcano erupted on 15 January 2022, at 04:15:45 UTC, which significantly influenced the atmosphere and space environment, at the same time, an unprecedented opportunity to monitor ionospheric anomalies is provided by its powerful eruption. In current studies of traveling ionospheric disturbance (TID) triggered by the 2022 Tonga volcanic eruption, the particular phenomenon of ionospheric disturbances in various parts of the world has not been reasonably explained, and the vertical ionospheric disturbances are still not effectively detected. In this paper, we calculate the high-precision slant total electron content (STEC) from more than 3000 ground-based GPS stations distributed around the world, then we obtain the radio occultation (RO) data from near-field COSMIC-2 profiles and investigate the horizontal TID and the vertical ionospheric disturbances by the singular spectrum analysis (SSA). Horizontal TID propagation captured by GPS STEC results indicates that acoustic-gravity waves dominate the energy input at the beginning of the ionospheric disturbance with an approximate speed of 1050 m/s initially. With the dissipation of the shock energy, lamb waves become a dominant mode of ionospheric disturbances, moving at a more stable speed of about 326 m/s to a range of 16,000 km beyond the far-field. Local characteristics are evident during the disturbance, such as the ionospheric conjugation in Australia and the rapid decay of TID in Europe. The shock-Lamb-tsunami waves’ multi-fluctuation coupling is recorded successively from the COSMIC-2 RO observation data. The shock and Lamb waves can perturb the whole ionospheric altitude. In contrast, the disturbance caused by tsunami waves is much smaller than that of acoustic-gravity waves and Lamb waves. In addition, influenced by the magnetic field, the propagation speed of TID induced by Lamb waves is higher towards the northern hemisphere than towards the southern hemisphere.

Ionospheric Disturbances after the 2022 Hunga Tonga-Hunga Ha’apai Eruption above Indonesia from GNSS-TEC Observations

On 15 January 2022, a VEI 5 eruption occurred at the submarine Hunga Tonga-Hunga Ha’apai (HTHH) Volcano in the Southwest Pacific, causing an ash plume reaching a height of 50–55 km. The eruption generated strong acoustic-gravity waves in the near-field and stations all over the world recorded Lamb waves (LW) that travelled around the earth multiple times at a speed of ~0.3 km/s. Here we report ionospheric anomalies due to the LW over Indonesian islands, 5000–10,000 km away from the volcano, in terms of changes in total electron contents (TEC) using the nationwide network of GNSS stations. We detected ionospheric anomalies travelling above Indonesia several times both westward and eastward. The first passage of LW over Java caused strong TEC increases of >12 TECU. The wave circled the earth and returned to Java on subsequent days. The second passage was recorded early 1/17, the anomaly decayed to 6 TECU. We also detected the passage of long-path waves propagating from west to east. In addition to such anomalies, we examined the existence of ionospheric disturbances apparently propagating from the geomagnetic conjugate point of the volcano that could possibly emerge in Indonesia. However, their signatures in Indonesia were not clear.

Modeling of infrasonic and acoustic-gravity wave propagation, nonlinear evolution, and observable effects

Mechanical disturbances associated with hazardous events—e.g., earthquakes, explosions (volcanic or man-made)—and severe weather – generate broad spectra of infrasound and acoustic-gravity waves (AGWs). These wave signals may provide diagnostic insight into the processes that generated them. They are routinely detected as fluctuations in atmospheric pressure, measured at ground or from balloon-borne platforms; at lower frequencies (<1 Hz), and where they may attain sufficient amplitudes at high altitudes, they may also be measured via the fluctuations that they impose in densities of layered species throughout the atmosphere and ionosphere. Thus, they provide complementary remote sensing opportunities, where waves and their effects, especially above and surrounding larger sources, may be diagnosed as they propagate. We review recent progress and techniques for high-fidelity modeling and simulation, to capture the propagation and evolution of low-frequency infrasound and AGWs throughout the atmosphere, from 0–500 km altitude (from surface to exobase). Strategies to (1) efficiently extend model simulation domains well into the diffusive thermosphere, to (2) connect models of atmospheric dynamics to those for other measurable processes (e.g., the ionosphere), and to (3) construct simulations that extend from source processes to specific remote sensing methodologies are discussed.

Geomagnetic Field Disturbance by an Acoustic-Gravity Wave Generated by Ionizing Radiation of Solar Flares

Geomagnetic Field Disturbance by an Acoustic-Gravity Wave Generated by Ionizing Radiation of Solar Flares

Acoustic-gravity waves generated by a point source on the ground in a stratified atmosphere-Earth structure

SUMMARY An analytically based method is proposed to simulate the acoustic-gravity waves in the horizontally stratified atmosphere-Earth structure generated by a point force on the Earth's surface. The method solves the linear momentum, continuity and adiabaticity equations in the atmosphere and elastodynamic equations in the solid Earth in the frequency–wavenumber domain. The time-domain waveforms are obtained by wavenumber integration and fast Fourier transform with respect to the frequency. Numerical simulations are conducted to investigate the properties of the acoustic-gravity waves, including both the high-frequency acoustic-mode waves and low-frequency gravity-mode waves. Simulations of the high-frequency responses show that disturbances in the atmosphere with three apparent horizontal velocities can be identified. They are, namely, the direct acoustic-mode wave generated by source travelling with the sound speed, the head wave generated by the seismic P-wave travelling with apparent horizontal speed identical to the P velocity, and the head wave generated by the Rayleigh wave with a horizontal speed same to the Rayleigh wave velocity. Simulations of the low-frequency responses show that the gravity-mode wave and Lamb wave can be identified. The gravity-mode wave travels with a speed lower than the sound seed and does not reach everywhere, especially the area directly above the source. The Lamb wave travels along the Earth surface with a speed of about 310 m s–1 and its energy decays with the altitude. We also apply our method to explaining the Doppler sounding data observed in Taiwan area during the 2011 Tohoku M 9 earthquake, and find good agreement between the predicted signals and observed data in the arrival time and wave envelope associated with the Rayleigh wave.