Direct measurements of nighttime thermospheric winds and temperatures, 1. Seasonal variations during geomagnetic quiet periods

Abstract

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory (39.9°N, 105.5°W) to determine the nighttime thermospheric temperature and winds from November 1973 through February 1975. The thermospheric temperature and winds at a height near the F2 peak are determined from the Doppler broadening and shift, respectively, of the atomic oxygen line emission at 15867.852 K (6300.308 Å). The experimentally derived temperatures and winds during geomagnetically quiet conditions are compared with a three-dimensional semiempirical model of the neutral thermosphere. This model in turn uses global empirical models of neutral temperature, composition, and electron density to determine the pressure forces and ion drag which generate and modify the wind fields. The large-scale details of measured and calculated nighttime meridional wind components are in general agreement, showing maximum equatorward winds during the summer months. Measured and calculated zonal winds agree for the equinoctial and winter months; however, the measured nighttime zonal winds are westward during summer months in contrast to model calculations that indicate a midnight eastward to westward transition. The measurements also indicate that both the temperature and the equatorward component of the meridional wind measured north of the observing station are greater than those measured south of the station. These positive latitudinal temperature and wind gradients are greatest during the summer months.