[1] In our recent paper, the sensitivity of infrasound to the upper atmosphere is investigated using impulsive signals from the Tungurahua volcano in Ecuador. We reported on the coherent variability of thermospheric travel times, with periods equal to those of the tidal harmonics. Moreover, it was shown that the error in predicted thermospheric travel time is in accord with typical uncertainties in the upper atmospheric wind speed models. Given the observed response of the infrasound celerities to upper atmospheric tidal variability, it was suggested that infrasound observations may be used to reduce uncertainty in the knowledge of the atmospheric specifications in the upper atmosphere. In this paper, we discuss the estimation of upper atmospheric wind model updates from the infrasound data described in the aforementioned paper. The parameterization of the model space by empirical orthogonal functions is described; it is found that the wind model in the upper mesosphere and lower thermosphere can be described by a four-parameter model. Due to the small dimensionality of the model space, a grid search method can be used to solve the inverse problem. A Bayesian method is used to assess the uncertainty in the inverse solution given the a priori uncertainty in the data and model spaces and the nonlinearity of the inverse problem at hand. We believe that this is the first study in which such methods are applied to real infrasound data, allowing for a rigorous analysis of this inverse problem. It is found that the complexity of the a posteriori model distribution increases for a larger dimensional model space and larger uncertainties in the data. A case study is presented in which the nonlinear propagation from source to receiver is simulated using an updated wind model and nonlinear ray theory. As nonlinear propagation effects further constrain the propagation path, this is a way to check the physical self-consistency of the travel time inversion approach. We obtain excellent agreement between the simulated and observed waveforms.

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We show that exposure of soda-lime glass to ultrafast laser pulses at 800 nm causes coloration ͑darkening͒. We have characterized this coloring with time-resolved measurements of the transmission of 633 nm light through the glass during laser exposure. Reverse processes ͑partial bleaching͒ operate on time scales of s to seconds. The competition between coloration after the femtosecond pulse and the subsequent transmission recovery limits the darkening that can be achieved at a given femtosecond pulse energy and repetition rate. The response of soda-lime glass to 400 and 267 nm ultrafast pulses is quite similar, although much lower pulse energies are required for darkening. We argue that darkening is due to absorption processes that produce mobile charge carriers, which then interact to produce trapped hole centers (H 3 ϩ) that absorb strongly at 633 nm. Trapped electrons ͑that form E centers͒ are the likely cause of the accompanying loss of transmission in the near ultraviolet. Finally, we show that diffraction gratings can be rapidly and easily produced in this material using holographic methods.

Experimental evidence shows that a liquid jet in air is an acoustic waveguide having a cutoff frequency inversely proportional to the jet diameter. Ultrasound applied to the jet supply liquid can propagate within the jet when the acoustic frequency is near to or above the cutoff frequency. Modulated radiation pressure is used to stimulate large amplitude deformations and the breakup of the jet into drops. The jet response to the modulated internal ultrasonic radiation pressure was monitored along the jet using (a) an optical extinction method and (b) images captured by a video camera. The jet profile oscillates at the frequency of the radiation pressure modulation and where the response is small, the amplitude was found to increase in proportion to the square of the acoustic pressure amplitude as previously demonstrated for oscillating drops [P.L. Marston and R.E. Apfel, J. Acoust. Soc. Am. 67, 27-37 (1980)]. Small amplitude deformations initially grow approximately exponentially with axial distance along the jet. Though aspects of the perturbation growth can be approximated from Rayleigh's analysis of the capillary instability, some detailed features of the observed jet response to modulated ultrasound are unexplained neglecting the effects of gravity.

This paper presents a relationship between the celerity and trace velocity of infrasound signals propagating in a stratified, windy atmosphere. Despite their importance, known celerity values have only been determined empirically. An infrasonic phase (I-phase) diagram is developed which is useful in identifying different I-phases. Such an I-phase diagram allows for the prediction of the range of values of the celerity and trace velocity for each I-phase. The phase diagram can easily be extended to underwater acoustic and acoustic-gravity waves. An I-phase diagram is compared with data obtained from a ground-truth event where qualitative agreement is obtained.