Low-Density Cell of the Thermosphere at High Latitudes Revisited

Abstract

The density cell structure in the high-latitude thermosphere is referred to as the density enhancement or depletion with respect to the surrounding area. Previous simulation results showed that the density cells are only observed below about 350 km. In the present work, the global ionosphere-thermosphere model is used to investigate the mechanism for the absence of the isolated density cell in the high-altitude thermosphere during geomagnetic quiet time. The simulation results indicate that the ion convection tends to drive a neutral cyclonic flow on the dawnside of the Magnetic Pole in both the low- and high-altitude thermosphere. Around the center of the cyclonic flow, a downward wind is formed as a consequence of mass conservation. It is interesting that under the influence of the downward flow, a density depletion relative to the background density with the same magnitude is generated inside the cyclone, which is independent of altitude. In the low-altitude thermosphere, this density depletion is shown as an isolated low-density cell. However, in the high-altitude thermosphere, the ion drag-driven density depletion turns to be an insignificant structure, as compared to the strong day-night gradient of the background neutral density. Consequently, the isolated low-density cell does not stand out in the high-altitude thermosphere.