TIME-GCM

The thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) is the most elaborate of the upper-atmospheric TGCM's and it solves for global distributions of neutral and plasma temperatures, velocities, and compositions, including all of the species that are photochemically important in the mesosphere, thermosphere, and ionosphere. The TIME-GCM includes all of the processes incorporated into the TIE-GCM. It is unique in its range of altitude coverage (30-500 km), with the critical mesosphere/lower thermosphere region in the center of its numerical grid, allowing dynamical, chemical, radiative, and electrodynamical couplings between the thermosphere and mesosphere to occur without major boundary influences. This model has been used in a number of studies made to analyze various ground- and satellite-based data.

Joe She (Colorado State University) and Roble (1995) examined the physics and chemistry controlling the seasonal variations of sodium lidar temperature observations over Fort Collins, Colorado. The TIME-GCM clearly revealed that the mesopause temperature structure is quite complex, with gravity wave heating, turbulent cooling, HOx chemical heating, and strong CO 15m cooling from O-CO vibrational quenching, all having a role in determining the temperature structure between about 70 to 110 km altitude. Both the observations and model simulations revealed a double mesopause structure during equinox conditions. During winter the mesopause has a single temperature minimum near 95 km, whereas during summer a double but distorted mesopause occurs at this mid-latitude station.

Gordon Shepherd (York University) and Roble used the TIME-GCM to help interpret the low latitude atomic oxygen green-line emissions observed by the WINDII instrument onboard the UARS satellite during March/April equinox conditions in 1992 and 1993. The results are quite striking and reveal a new aspect of equatorial aeronomy that had not been realized previously. If the diurnal tide is of sufficient magnitude to penetrate the atomic oxygen layer near 97 km, then it significantly alters the atomic oxygen distribution at low latitudes producing a strong green-line enhancement in the early evening hours, a pronounced green-line minimum near midnight, and a reformation of the green line at high altitude near the morning terminator. At latitudes near 20 deg N and S the opposite variation occurs indicating a global oscillation. If the tide does not penetrate the layer there is only a weak semi-diurnal variation of the nighttime green line similar to what is observed during solstice conditions.

The TIME-GCM is also being used to simulate data from the Upper Atmosphere Research Satellite (UARS) and various measurements made during CEDAR campaigns such as ALOHA, CADRE, etc.