Thermospheric dynamics above Saint-Santin: Statistical study of the data set

The meridional neutral winds at 300 km altitude have been calculated from the data obtained during 15 years of measurements by the incoherent scatter facility at Saint-Santin (France). The analysis is based on the classical technique: the meridional neutral wind is given by the difference between the measured ion velocity and the computed diffusion velocity. This relies on knowledge of the O+-O collision frequency, which is still the subject of some controversy. A simple but reasonable expression is given and has been agreed upon with workers at Millstone Hill, so that comparisons may be made. This expression gives values consistent with the equations used elsewhere, which are more complicated but no more reliable. The whole set of data gives 223 twenty-four-hour periods of measurements from which the average meridional winds are computed for a range of conditions. A statistical analysis shows how the diurnal variations depend on season, solar flux, and geomagnetic activity. It is shown that these variations are well represented by four harmonic components, of which amplitudes and phases are given.