A mechanism is proposed to explain the cause of the global, semiannual thermospheric density variation. It is suggested that the global-scale, interhemispheric circulation at solstice acts like a huge turbulent eddy in mixing the major thermospheric species. The effect causes less diffusive separation of the species at solstice, which tends to raise molecular nitrogen and oxygen densities and reduce atomic oxygen density, compared with equinox. The increased mean mass, at solstice, reduces the density scale height at a given altitude. This compression of the atmosphere at solstice can explain the mean amplitude of the semiannual density anomaly. Since ionospheric loss rates are affected by neutral composition, the proposed mechanism also leads to a similar ionospheric density variation. ¿ 1998 American Geophysical Union