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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.

Drob, DP, Broutman D, Hedlin MA, Winslow NW, Gibson RG.  2013.  A method for specifying atmospheric gravity wavefields for long-range infrasound propagation calculations. Journal of Geophysical Research-Atmospheres. 118:3933-3943.   10.1029/2012jd018077   AbstractWebsite

Two important challenges in infrasound propagation physics are (1) to explain frequently observed infrasound signals in the classical near-field shadow zones and (2) to accurately predict observed waveform amplitude and signal duration. For these problems, the role that small-scale internal atmospheric gravity wave fluctuations play has recently been realized. This paper provides a methodology for representing small-scale internal gravity wave fluctuations which is suitable for infrasound propagation calculations. Adapted from the numerical weather prediction and climate modeling communities, the resulting stochastic gravity wave noise field model is three-dimensional, time dependent, and self-consistent with the atmospheric background state. To illustrate the methodology the resultant gravity wavefields are applied to ray-trace simulations of observed infrasound travel times for a dense seismic network in the Western United States which recorded infrasound signals from a large surface explosion.

Hedlin, MAH, Walker KT.  2013.  A study of infrasonic anisotropy and multipathing in the atmosphere using seismic networks. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 371   10.1098/rsta.2011.0542   AbstractWebsite

We discuss the use of reverse time migration (RTM) with dense seismic networks for the detection and location of sources of atmospheric infrasound. Seismometers measure the response of the Earth's surface to infrasound through acoustic-to-seismic coupling. RTM has recently been applied to data from the USArray network to create a catalogue of infrasonic sources in the western US. Specifically, several hundred sources were detected in 2007-2008, many of which were not observed by regional infrasonic arrays. The influence of the east-west stratospheric zonal winds is clearly seen in the seismic data with most detections made downwind of the source. We study this large-scale anisotropy of infrasonic propagation, using a winter and summer source in Idaho. The bandpass-filtered (1-5 Hz) seismic waveforms reveal in detail the two-dimensional spread of the infrasonic wavefield across the Earth's surface within approximately 800 km of the source. Using three-dimensional ray tracing, we find that the stratospheric winds above 30 km altitude in the ground-to-space (G2S) atmospheric model explain well the observed anisotropy pattern. We also analyse infrasound from well-constrained explosions in northern Utah with a denser IRIS PASSCAL seismic network. The standard G2S model correctly predicts the anisotropy of the stratospheric duct, but it incorrectly predicts the dimensions of the shadow zones in the downwind direction. We show that the inclusion of finer-scale structure owing to internal gravity waves infills the shadow zones and predicts the observed time durations of the signals. From the success of this method in predicting the observations, we propose that multipathing owing to fine scale, layer-cake structure is the primary mechanism governing propagation for frequencies above approximately 1 Hz and infer that stochastic approaches incorporating internal gravity waves are a useful improvement to the standard G2S model for infrasonic propagation modelling.

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.