F-region neutral winds and temperatures at equatorial latitudes: Measured and predicted behaviour during geomagnetically quiet conditions

Abstract

Nighttime thermospheric winds and temperatures were measured over Roi Namur Island, Kwajalein Atoll, Marshall Islands, with a Fabry-Perot interferometer; measurements were obtained during August and September 1977, August 1978, July 1979, and March and July 1980. The winds and temperatures were determined from the Doppler shifts and line profiles of the 630.0 nm twilight and nightglow line emission. All of the measured wind vectors lie in the South-East quadrant, however, there is considerable variation in magnitude and direction within that quadrant. During 1977 all of the data exhibit the same qualitative behaviour. At evening astronomical twilight the wind vector is directed eastward with velocities ranging between 50 and 120 m s −1. The wind vector turns clockwise, increasing in magnitude to 130–160 m s −1 at 23 L.T. in an East South-East direction and then decreasing to < 50 m s −1 at 04 L.T. in a southward direction. During 1978 the wind vector generally turns counterclockwise during the night, with considerable day-to-day variability. The measured wind vector variation during July 1979 and March and July 1980 is quite irregular, but the wind vector consistently remains directed toward the South-East quadrant. The measured nighttime variation of the neutral gas gemperature indicates a 200–300 K temperature drop between sunset and sunrise. The average temperature, however, increases by about 400 K from 1977 to 1980. The NCAR thermospheric general circulation model (TGCM) is used to calculate the nighttime variation of winds and temperatures over Kwajalein Atoll. Numerical experiments made with the TGCM indicate that F-region winds in the equatorial region are strongly influenced by variations of ion drag caused by E × B drifts in the equatorial ionosphere, upward-propagating tides from the lower atmosphere, and high-latitude magnetospheric convection. With a reasonable prescription of the three effects, agreement can be obtained between the calculated and observed variation of winds over Kwajalein. By altering the timing between the interaction of tides and E × B drifts it is possible to derive either a clockwise or counterclockwise wind vector rotation during the night. TGCM calculations suggest that much of the observed variability of winds can be attributed to variations of E × B drifts, tides and to a lesser extent high-latitude magnetospheric convection.