THE THERMOSPHERIC SPOON - A MECHANISM FOR THE SEMIANNUAL DENSITY VARIATION

A mechanism is proposed to explain the cause off the global, semiannua l thermospheric density variation. It is suggested that the global-sca le, interhemispheric circulation at solstice acts like a huge turbulen t eddy in mixing the major thermospheric species. The effect causes le ss diffusive separation of the species at solstice, which tends to rai se molecular nitrogen and oxygen densities and reduce atomic oxygen de nsity, compared with equinox. The increased mean mass, at solstice, re duces 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 simila r ionospheric density variation.