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All available data of the thermospheric wind meridional component, deduced from the red line OID Doppler shift as measured by WTNDII on board UARS, have been sorted to plot their averaged diurnal variations. A comparison is presented here of these variations of the poleward wind for high solar flux and mid latitude, at each season, with similar diurnal variations deduced from the set of IS radar data obtained at Saint-Santin.

Simulations from the National Center for Atmospheric Research thermosphere-ionosphere general circulation model constitute controlled numerical experiments which may be used to assess current understanding of Earth's upper atmosphere. Comparisons with a long term data base are particularly valuable in this regard. Accordingly model simulations of geomagnetically quiet and active periods are compared with an observational database from Saint Santin. The simulations and observations are for equinox and northern summer and winter during solar cycle minimum. The observations consist of the diurnal variation of the meridional neutral winds near 300 km; harmonic analysis yielded the mean components and the 24-, 12-, 8-, and 6-hour waves. The model/data comparisons for the diurnal variations are good to excellent: differences are generally ≤ 25 m/s with largest differences typically occurring between 1800 and 0600 UT. In the observations, the diurnal component is approximately 60 m/s in amplitude and 12 hours in phase during quiet periods. These values persist during active periods except in summer when the diurnal amplitude falls to 47 m/s. The model predicts a weaker diurnal amplitude in winter than the observations indicate; it also does not predict the observed decrease of the diurnal amplitude in summer with increasing activity. Harmonic analysis of the data indicated that (1) 12-r and 8-hour waves are important in summer and equinox; in winter the variation is largely diurnal; (2) the semidiurnal and terdiurnal waves vary with season during quiet periods; and (3) the largest effect of geomagnetic activity is in the 12- and 8-hour waves. In the model, the semidiurnal and terdiurnal tides do not vary with season. Further, the effects of varying geomagnetic activity are predominantly in the diurnal component; the semidiurnal and terdiurnal tides in the TIGCM are relatively unaffected in marked contrast to the observations. The differences between the modeled and observed winds illustrate the pervasiveness and importance of variability in the atmosphere: in the high-latitude energy and momentum sources, in the solar forcing, and in the waves that originate in the lower atmosphere and penetrate the thermosphere.

The MICADO instrument has been built to measure temperature and wind in the E - and F -regions. It employs a thermally stable field-compensated Michelson interferometer to allow wind measurements. During the winter of 1988–1989, the MICADO instrument was operated at Sodankyla (67°22′N, Finland). Measurements were made by observing the O 1 S (low thermosphere) and the O 1 D lines (high thermosphere) emission. Two co-ordinated campaigns were organized with the EISCAT radar, which operated in special modes. Neutral wind and temperature are derived from EISCAT data. Results of the two instruments are shown. The differences between the two sets of results are discussed and show that most of the discrepancy is due to the presence of vertical winds during the observations where the magnetic activity was high.

Upper thermospheric meridional wind data at midlatitudes and for low magnetic activity are examined for solar activity variations following an analysis scheme suggested by a Coordinated Analysis of the Thermosphere workshop. Wind data from incoherent scatter, Fabry-Perot, and F2 peak heights show decreasing diurnal amplitudes with increasing solar activity during all seasons, except for Saint Santin data, which show a slight increase in summer. Equivalent winds from F2 peak height data have strong decreases in diurnal amplitude in all seasons. The coupled thermosphere ionosphere model and thermosphere ionosphere global circulation model predictions of diurnal amplitude, while differing considerably in magnitude, also show decreasing amplitudes during all seasons except summer, while the HWM90 empirical model amplitudes increase slightly with solar activity during all seasons. The diurnal mean wind trends with solar activity are fairly weak, except for Millstone Hill incoherent scatter radar, which shows a shift from strong southward to near zero or northward wind with increasing activity. Model results for the mean generally fall within the band of measurements. Near midnight, most of the data also show that the typically southward winds weaken with increasing solart activity in all seasons except summer, when results are mixed. There are significant differences between the trends and between absolute values for the various data sets and models which need further investigation.

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.