Climatology and latitudinal gradients of quiet time thermospheric neutral winds over Millstone Hill from Fabry-Perot interferometer measurements

Midlatitude nighttime thermospheric neutral winds are strongly dependent on season, solar activity, and latitude. We use an extensive database of wind measurements made during 1989-2001 by the Millstone Hill Fabry-Perot interferometer to study the detailed climatology of quiet time neutral winds near an altitude of 250 km. To facilitate the analysis of these data, we develop a local time, day-of-year, solar flux, and latitude-dependent empirical model, with the latitude dependence obtained by considering north looking and south looking observations separately. Our results show that the zonal winds are predominantly eastward after dusk and westward before dawn, with the strongest eastward winds occurring in the winter and with an east-to-west transition that occurs earliest in the summer. The zonal winds exhibit weak-to-moderate latitudinal gradients, with more westward values to the north. The zonal wind magnitudes decrease with increasing solar flux; the strongest trends occur during winter. The meridional winds are predominantly equatorward in all cases and exhibit strong latitudinal gradients, with larger values to the north. The maximum nighttime equatorward winds decrease with increasing solar flux, except during summer, when there is no significant solar activity variation. They are largest during the summer, except at solar minimum when a semiannual variation is observed and the peak winds occur during the equinoxes. Earlier studies of midlatitude wind measurements are generally consistent with our data, with our results providing a considerably more detailed description of the nighttime wind climatology at midlatitudes.