Longitudinal Variation in the Thermospheric Superrotation: CHAMP Observation and TIE‐GCM Simulation

By means of 4 years of Challenging Minisatellite Payload (CHAMP) zonal wind observations and a Thermosphere‐Ionosphere Electrodynamics General Circulation Model simulation, longitudinal and seasonal variations of thermospheric superrotation at magnetic equator are investigated first. The superrotation shows four‐ and three‐peaked longitudinal patterns in March and September equinoxes, and a two‐peaked variation during solstices. The superrotation is stronger in December than in June solstice, and stronger in March than in September equinox. The mean correlation between the zonal variation of superrotation and nighttime eastward wind is about 0.8, while it is 0.6 with daytime westward zonal wind. The interaction between the neutral wind and geomagnetic field plays a more important role in the superrotation, rather than the F‐region electron density. The lower atmospheric tides tend to suppress the superrotation, but contribute to the longitudinal patterns of the superrotation. The viscous force is also important for the longitudinal modulation of the superrotation. Plain Language Summary The low altitude atmosphere rotates together with the Earth, resulting from the friction with the surface. Owing to the high viscosity, most parts of the atmosphere at different altitudes also rotate at the same rate as the Earth. Above 200 km, however, the low‐latitude atmosphere is found to rotate 10%–20% faster than the Earth. This phenomenon is more prominent on Venus, where the atmospheric circulation speed can reach 60 times of the rotation. Although this unexpected superrotation phenomenon was discovered in the 1960s in connection with satellite orbit evaluations, its physical mechanism has not been fully elucidated to date. This study reveals for the first time that the geomagnetic field configuration and the lower atmospheric tides have important effects on the superrotation of the upper atmosphere. Both can account for the longitudinal variation in the superrotation. This theoretical study is helpful for better understanding the generation mechanism of the superrotation on Earth and other planets. Key Points Superrotation of the thermosphere exhibits large longitudinal and seasonal variations Geomagnetic field geometry and lower atmospheric tides are vital for the longitudinal patterns of superrotation Viscous force also partly contributes to the longitudinal variation in superrotation