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A theoretical study of the effect of geomagnetic fluctuations and solar tides on the propagation of infrasonic waves in the upper atmosphere



Propagating infrasonic waves are sensitive to changes in the atmospheric conditions. A theoretical acoustic wave propagation model is integrated with time-varying atmospheric models to investigate in more detail the effects of mesospheric and lower thermospheric variability on the propagation of infrasonic waves. We focus on the effects of the solar tides and geomagnetic perturbations on acoustic propagation in Alaska during winter of 1989–90, which corresponds to a period of solar maximum. Solar-driven tides are the strongest source of variability in mesospheric and lower thermospheric winds. These tides can alter the direction and magnitude of azimuth deviations, as well as the turning heights of ducted waves. Geomagnetic changes generally depress the turning heights of rays by increasing the thermospheric temperatures, and may either improve or deteriorate the azimuth deviations according to whether the geomagnetic-induced winds act with or against the tidal-induced winds. Observed arrivals with a low apparent horizontal phase velocity may be refracted in the thermosphere or the stratosphere. Unless the source epicentre and origin time are known, unambiguous phase identification can only be produced with knowledge of the stratospheric winds.