Simultaneous Wind and Temperature Observations of Tides in the Mesosphere and Lower Thermosphere (MLT) From Orbit

The TIMED satellite, launched in December 2001, has acquired a unique set of synoptic observations from space of both neutral winds and neutral temperatures in the mesosphere and lower thermosphere (MLT) with the TIDI and SABER instruments, respectively. Though the measurements are simultaneous, the fields of view of the two instruments are sufficiently different that the limb airglow is sampled at different Local Solar Times at any common Universal Time. Given measurements over a complete yaw cycle, it is possible to reconstruct an `average-day' weather map of temperature and neutral wind for the MLT throughout the mission. The orbit of TIMED is repeatable on an annual basis providing a capability to study the differences in the MLT weather over a 5-year period. The maps show the dominance of the diurnal tide at low latitudes, peaking in strength at the equinoxes, and the variation of the amplitude of the diurnal tide from one year to the next roughly following the phase of the stratospheric Quasi-Biennial Oscillation (QBO). Similar observations in the MLT were conducted during the UARS mission by the HRDI experiment. This instrument had sufficient resolution to extract both neutral wind and temperature from individual limb airglow spectra. The HRDI temperature data have not been widely used but show similar tidal and interannual patterns to those derived from TIMED-SABER. Combining HRDI wind and temperature data into TIMED-like yaw cycles permits the evolution of the MLT to be displayed for the period 1992 to present on a common basis. This paper will compare the MLT neutral temperature/wind patterns obtained by the UARS and TIMED satellites. It is important to document the basic state parameters of the MLT over the past 15 years to ensure that changes in the upper atmosphere are considered in the context of available data sets. Only by combining observations over several satellite missions is it possible to show that the MLT exhibits strong seasonal patterns that are mixed with longer period oscillations.