Optical observations of thermospheric neutral winds at Arecibo between 1980 and 1987

Since 1980, optical observations of nighttime thermospheric winds have been made using a Fabry-Perot interferometer at the Arecibo Observatory, Puerto Rico. High-resolution spectra of the O(1D) airglow emission at 630.0 nm are obtained by observing at eight equally spaced azimuth positions and a fixed zenith angle of 60 degree. The neutral wind field is inferred by assuming that each component of the wind velocity vector may be expanded in a linear Taylor expansion about a point directly above the observatory. Both the zonal and meridional components of the thermospheric wind field are observed to have well-defined seasonal and nocturnal variations. For each year between 1980 and 1987, eastward flow was observed in the evening hours, with an average peak velocity of about 100 m s-1 near 2,200 AST. In the winter months, the zonal wind generally remains eastward throughout the night. However, in summer, a reversal to westward flow is usually observed after local midnight. In the meridional direction, the largest equatorward velocities are observed in summer. A reduction (or reversal) in the meridional wind velocity is most often observed after midnight in the summer and equinoctial months. The authors find that the nocturnal and seasonal variations in the neutral wind field at Arecibo are remarkably unaffected by changes in the solar cycle