The polar lower thermosphere

Abstract

The Earth's mesosphere and lower thermosphere are the least explored regions of the Earth's atmosphere. The observations that have been made in this region, however, indicate that it is a dynamically active region, especially the polar lower thermosphere where most of the auroral energy that impacts the Earth's atmosphere is deposited. This energy is redistributed globally by the thermospheric wind system and there are important dynamic, chemical, radiational, and electrodynamic couplings that occur between the lower thermosphere and the middle atmosphere. There are also couplings with the upper thermosphere, ionosphere, and magnetosphere. A brief review of the available observations important for understanding global dynamic processes in the lower thermosphere is given. Results of simulations made with the NCAR thermosphere/ionosphere general circulation model (TIGCM) are presented to illustrate interactions of polar lower thermospheric chemistry and dynamics. The calculations show that major neutral gas constituents' (O 2, N 2, and O) number densities in the lower thermosphere are influenced primarily by dynamics, whereas the minor neutral constituents of the odd nitrogen system are influenced by both dynamics and chemistry. Furthermore, both major and minor constituent number density changes, as well as temperature changes, have an important influence on the electron number density distributions in the thermosphere. Results of time-dependent simulations with the TIGCM also suggest that a significant amount of NO is generated in the lower thermosphere during disturbed geomagnetic conditions which could be transported to the middle atmosphere. It is important to consider both chemistry and dynamics in determining the transport of constituents between the mesosphere and thermosphere.