Until recently, the background state of the middle atmosphere and its natural variability have not been well characterized because this region was difficult to study observationally. During the past decade, the development of sophisticated remote sensing techniques, and the impressive evolution of numerical models have provided the tools necessary to begin understanding this complex and important region of the atmosphere and to clarify its interactions with the lower atmosphere. In recent years much of our knowledge of the middle atmosphere has come from observational studies employing multiple remote sensing instruments in coordinated campaigns (e.g. AIDA-89, ALOHA-90, ALOHA/ANLC-93, and CADRE). By building upon this prior experience and employing a variety of instruments, including a Rayleigh/Aerosol/Na lidar, Fe temperature/Ca+ lidar, and airglow imager, all developed by our group and deployed at South Pole, we are studying key scientific issues related to the following middle atmosphere phenomena:

Polar Stratospheric Clouds and Aerosols Coordinated balloon/lidar observations of the region between 3 and 30 km are being conducted to study the formation and evolution of PSCs throughout the Antarctic winter and spring. New compact lidars, developed by NASA-Goddard collaborators, are being operated at South Pole and deployed in Autonomous Geophysical Observatories (AGOs) and used to study the formation and advection of PSCs over the Antarctic ice cap.

Atmospheric Temperature Structure High altitude balloonsondes (3-28 km), a Rayleigh lidar (28-80 km), and a new Fe temperature lidar (80-100 km) are being used to measure the temperature profile above South Pole from the ice to 100 km. The data are being used to study the seasonal and diurnal variation of the atmospheric thermal structure and the relationship to PSC formation and dissipation. These data are providing an important calibration point for validating global circulation models and for studying the temperature dependence of middle atmospheric chemistry, especially the chemistry of mesospheric Na and Fe. The data also represent an important baseline against which to compare future measurements to quantify secular changes in temperature that may be related to global climate change. Another important goal is to establish the amplitudes and phases of the tidal perturbations in middle atmosphere temperatures at South Pole.

Gravity Wave Dynamics Balloonsondes, lidars, and airglow instruments are being used to study gravity wave activity throughout the middle atmosphere at South Pole. The total wave variances and wave spectra and their seasonal and altitude variations are being derived from the observations. Monochromatic gravity waves, their intrinsic parameters, and their sources are being explored using lidar and airglow observations in the mesosphere. While wave activity appears to be small in the lower atmosphere over Antarctica relative to mid- and low-latitude sites, wave effects at mesopause heights are comparable to that observed elsewhere. A major goal of this work is to identify the lower atmosphere sources of the mesospheric waves observed over the South Pole.

Trace Species Na, Fe, and Ca⁺ lidars and various airglow instruments are being used to study mesopause region chemistry and sporadic metal layer phenomena. The temperature dependence of the chemical reactions that play key roles in establishing the seasonal variations of Na and Fe are also being explored as well as the effects of tides and planetary waves on the emission of trace species such as excited OH and O₂.