Making waves with the Moon’s shadow

Making waves with the Moon’s shadow. As it sails across Earth during a solar eclipse, the lunar shadow creates a bow wave in the ionosphere like a moving boat does in water. That’s what researchers in Taiwan and Japan found when they analyzed data from more than 1400 GPS receivers on those islands during the long-lived eclipse of 22 July 2009. Predicted more than 40 years ago, the wave effect arises from the localized cooling within the darkest regions of the shadow; the temperature, density, and pressure differences set up acoustic gravity waves that ripple along, ahead of and outward from the faster-moving shadow’s leading edge. The researchers note that the receivers, each gathering data from 9 or 10 GPS satellites, give them about 13 000 lines of sight through Earth’s atmosphere, which allows them to tease out the spatial distribution of total electron content in the ionosphere with 30-second time resolution. The acoustic gravity waves they saw had periods of 2–3 minutes and 5 minutes and formed two wavefronts. The analogy to a moving boat is quite accurate in that the bow wavefront spreads at a much slower velocity than the boat is moving; the eclipse shadow was clocked at 954 m/s, the ripples at 300 m/s. What’s more, the Earth scientists also saw a stern wavefront consistent with the hydrodynamic predictions, separated from the bow wave by about 1700 km—the length of the shadow boat. (J. Y. Liu et al., Geophys. Res. Lett. 38, L17109, 2011.) —SGB