A solar terminator wave is discovered in neutral thermosphere densities. The data originate from the accelerometer experiment on the CHAMP satellite between 2001 and 2007. During solar minimum conditions the phase fronts of the dusk terminator wave during Northern Hemisphere summer extend from about -60° to almost +30° latitude, at an angle of about 30° with respect to the terminator. The density amplitudes are of order ±3 - 6%, and the horizontal wavelength is of order 3,000 km. The dusk terminator wave is generally more well-defined than that near dawn, is more prominent during solar minimum than solar maximum, and during solstice as opposed to equinox. This wave is also found in similarly-analyzed output from the Kyushu University General Circulation Model that extends from the surface to the exobase. At solar minimum the model wave amplitude is similar to that observed, but with a horizontal wavelength close to 2,000 km. Analytic theory predicts a typical horizontal wavelength of 1,000 km. While there have been several reports of ionospheric waves in connection with the solar terminator, this appears to be the first such observation in the neutral thermosphere. In addition, the orientations of the dawn and dusk terminator waves are such that the maximum equatorial density perturbation occurs near midnight; therefore, some previous observations of the so-called "midnight temperature maximum" may contain contributions attributable to the terminator wave.
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