On the sensitivity of infrasonic traveltimes in the equatorial region to the atmospheric tides

Abstract

In comparison to the lower atmosphere where comprehensive global atmospheric specifications are commonplace, the ability to perform routine global measurements of the properties of the atmosphere above the stratopause continues to be an active area of scientific research. In this paper, we revisit the use of infrasound as a remote sensing technique for horizontal winds in the upper atmosphere. To use infrasound to monitor the winds in the upper atmosphere, a sufficiently strong and persistent infrasound source is required. In addition, because of the highly attenuating nature of the thermosphere, the signals used must have low enough frequency content to survive. In this paper, impulsive signals from the Tungurahua volcano in Ecuador are used. This volcano has gone through periods of intense activity during which impulsive signals of dominant periods between 3 and 5 s are emitted. Depending on the atmospheric conditions, stratospheric, mesospheric, and thermospheric arrivals are observed. It is found that the traveltimes of the thermospheric arrivals exhibit a coherent variability with periods equal to those of the tidal harmonics. As such, these observations suggest a means of passive atmospheric remote sensing that can be utilized in conjunction with other techniques to routinely measure the state of the upper atmosphere.