THE POLAR LOWER THERMOSPHERE

The Earth's mesosphere and lower thermosphere are the least explored r egions of the Earth's atmosphere. The observations that have been made in this region, however, indicate that it is a dynamically active reg ion, 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 a re important dynamic, chemical, radiational. and electrodynamic coupli ngs that occur between the lower thermosphere and the middle atmospher e. There are also couplings with the upper thermosphere, ionosphere, a nd magnetosphere. A brief review of the available observations importa nt for understanding global dynamic processes in the lower thermospher e is given. Results of simulations made with the NCAR thermosphere/ion osphere general circulation model (TIGCM) are presented to illustrate interactions of polar lower thermospheric chemistry and dynamics. The calculations show that major neutral gas constituents' (O2, N2, and O) number densities in the lower thermosphere are influenced primarily b y dynamics, whereas the minor neutral constituents of the odd nitrogen system are influenced by both dynamics and chemistry. Furthermore, bo th major and minor constituent number density changes, as well as temp erature changes, have an important influence on the electron number de nsity distributions in the thermosphere. Results of time-dependent sim ulations with the TIGCM also suggest that a significant amount of NO i s generated in the lower thermosphere during disturbed geomagnetic con ditions which could be transported to the middle atmosphere. It is imp ortant to consider both chemistry and dynamics in determining the tran sport of constituents between the mesosphere and thermosphere.