Simultaneous measurements of ion and neutral wind velocities have been obtained at Arecibo, Puerto Rico, on several nights in both the summer and winter seasons. The diffusion velocity of O+, calculated in an analysis by using the accepted value for the ambipolar diffusion coefficient and the MSIS model atmosphere, is found to be in fairly close agreement with the observations. Incoherent scatter radar data have also been used to derive height profiles of the meridional neutral wind velocity. In the spring and summer months, the meridional wind is strongest near 2100 AST, with a southward velocity ranging from 50–100 m s−1. At about 0100 AST the ‘midnight descent’ of the F layer occurs, in association with a slackening, or sometimes a reversal, of the meridional wind. In general, the meridional wind is fairly constant with altitude. However, prior to midnight in the spring and summer months, contours of constant velocity are often observed to slope downward with increasing time. Some degree of vertical wind shear is often observed between 0200 and 0500 AST as well. In the winter months, meridional wind velocities are lower, and the nocturnal variation of the wind field is less pronounced than it is at other seasons.
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