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Hedlin, MAH, De Groot-Hedlin CD, Forbes JM, Drob DP.  2018.  Solar terminator waves in surface pressure observations. Geophysical Research Letters. 45:5213-5219.   10.1029/2018gl078528   AbstractWebsite

We report observations of waveforms in surface pressure made over several years by a network of ground-level barometers in the eastern United States. The waveforms can be reconstructed by superimposing the 4th through 10th subharmonics of the solar day. Some of these solar harmonics are likely generated by the temperature and pressure gradients across the solar terminators. The measurements presented here enable a wave detection analysis which indicates that some waveforms are coherent between stations with a median speed of 49.7 m/s to the southeast. We interpret these propagating signals, which are interference patterns created by internal gravity waves with periods that are subharmonics of a solar day, as a previously undiscovered type of terminator wave. The waveforms appear predominantly postsunrise during winter and postsunset in summer. Their quasi-eastward propagation direction suggests an analogy with "stern" waves left behind by the faster, westward-moving terminator.

De Groot-Hedlin, CD.  2016.  Long-range propagation of nonlinear infrasound waves through an absorbing atmosphere. Journal of the Acoustical Society of America. 139:1565-1577.   10.1121/1.4944759   AbstractWebsite

The Navier-Stokes equations are solved using a finite-difference, time-domain (FDTD) approach for axi-symmetric environmental models, allowing three-dimensional acoustic propagation to be simulated using a two-dimensional Cylindrical coordinate system. A method to stabilize the FDTD algorithm in a viscous medium at atmospheric densities characteristic of the lower thermosphere is described. The stabilization scheme slightly alters the governing equations but results in quantifiable dispersion characteristics. It is shown that this method leaves sound speeds and attenuation unchanged at frequencies that are well resolved by the temporal sampling rate but strongly attenuates higher frequencies. Numerical experiments are performed to assess the effect of source strength on the amplitudes and spectral content of signals recorded at ground level at a range of distances from the source. It is shown that the source amplitudes have a stronger effect on a signal's dominant frequency than on its amplitude. Applying the stabilized code to infrasound propagation through realistic atmospheric profiles shows that nonlinear propagation alters the spectral content of low amplitude thermospheric signals, demonstrating that nonlinear effects are significant for all detectable thermospheric returns. (C) 2016 Acoustical Society of America.

Walker, KT, Le Pichon A, Kim TS, de Groot-Hedlin C, Che IY, Garces M.  2013.  An analysis of ground shaking and transmission loss from infrasound generated by the 2011 Tohoku earthquake. Journal of Geophysical Research-Atmospheres. 118:12831-12851.   10.1002/2013jd020187   AbstractWebsite

The 2011 M(w)9.0 Tohoku earthquake generated infrasound that was recorded by nine infrasonic arrays. Most arrays recorded a back azimuth variation with time due to the expanse of the source region. We use ray tracing to predict group velocities and back azimuth wind corrections. A Japan accelerometer network recorded ground shaking in unprecedented spatial resolution. We back projected infrasound from arrays IS44 (Kamchatka) and IS30 (Tokyo) to the source region and compare these results with acceleration data. IS44 illuminates the complex geometry of land areas that experienced shaking. IS30 illuminates two volcanoes and a flat area around the city of Sendai, where the maximum accelerations occurred. The arrays and epicentral region define three source-receiver profiles. The observed broadband energy transmission loss (TL) follows an exponential decay law. The best fitting model, which has parameters that are interpreted to include the effects of geometric spreading, scattering, and the maximum ratio of the effective sound speed in the stratosphere to that at the ground (accounts for stratospheric wind speed), yields a 65% variance reduction relative to predictions from a traditional TL relationship. This model is a simplified version of the model of Le Pichon et al. (2012), which yields an 83% variance reduction for a single frequency, implying that fine-scale atmospheric structure is required to explain the TL for stratospheric upwind propagation. Our results show that infrasonic arrays are sensitive to ground acceleration in the source region of megathrust earthquakes. The TL results may improve infrasonic amplitude scaling laws for explosive yield.

Hedlin, MAH, Walker K, Drob DP, De Groot-Hedlin CD.  2012.  Infrasound: Connecting the Solid Earth, Oceans, and Atmosphere. Annual Review of Earth and Planetary Sciences, Vol 40. 40( Jeanloz R, Ed.).:327-354., Palo Alto: Annual Reviews   10.1146/annurev-earth-042711-105508   Abstract

The recently reinvigorated field of infrasonics is poised to provide insight into atmospheric structure and the physics of large atmospheric phenomena, just as seismology has shed considerable light on the workings and structure of Earth's solid interior. Although a natural tool to monitor the atmosphere and shallow Earth for nuclear explosions, it is becoming increasingly apparent that infrasound also provides another means to monitor a suite of natural hazards. The frequent observation of geophysical sources-such as the unsteady sea surface, volcanoes, and earthquakes-that radiate energy both up into the atmosphere and down into the liquid or solid Earth and transmission of energy across Earth's boundaries reminds us that Earth is an interconnected system. This review details the rich history of the unheard sound in the atmosphere and the role that infrasonics plays in helping us understand the Earth system.