Longitudinal variation in zonal winds at subauroral regions: Possible mechanisms

Longitudinal differences in thermospheric zonal winds (Δ U y ) are investigated in the subauroral region for different seasons and under solar maximum and medium conditions by using Challenging Minisatellite Payload observations. Prominent wave‐1 longitudinal and diurnal variations of Δ U y are observed, along with an antiphase relationship between the Northern and Southern Hemispheres. These structures persist over the whole year and are independent of solar activity. Δ U y values are greater at nighttime than at daytime, and values in the south are greater than those in the north in local summer and winter. Model simulations confirm observed results in large‐scale structures, and the nonzero dipole tilt is found to be vital for the longitudinal variation of the zonal wind. The neutral air pressure gradient caused by the day‐night difference in solar heating is a major contributor to the observed Δ U y . The pressure effects are larger at nighttime than at daytime and larger in the Southern Hemisphere than in the Northern Hemisphere. Ion drag reduces the compatibility between the modeled and observed Δ U y as expected, with larger effects at nighttime than at daytime. Viscous force also reduces the compatibility between the modeled and observed Δ U y with greater effects at daytime, except at nighttime in the Southern Hemisphere. Similarly, the Coriolis force makes the difference between the modeled and observed Δ U y larger. The sum of these factors can explain, in general, the observed local time and hemispheric asymmetry features in longitudinal variation of the zonal wind.