Seasonal variations of the ionospheric E x B drifts above Saint-Santin on quiet days

A seasonal model of the F region electric-field-induced plasma drifts prevailing during magnetically quiet periods, and of their day-to-day variability, has been established on the basis of 3 years of measurements (1973--1975) above Saint-Santin (440 39'N, 20 12'E) with the French quadristatic incoherent scatter facility. It is expressed analytically with the help of a steady component and of the first four diurnal harmonic oscillations and can be used both as a base line for magnetospheric disturbance studies and in itself as a source of information on the morphology of ionospheric wind dynamo electric fields. For all seasons these drifts appear to be dominated by a semidiurnal oscillation in the north-south direction and by a diurnal one in the east-west direction. During the daytime the flow is northwestward in the morning and then southeastward in the afternoon. Seasonal effects are dominated by a marked contrast between summer and the two other seasons. Summer is characterized by a disappearance of the morning northward drifts and a more complex harmonic content of the east-west drifts, with a larger contribution of the secondary semidiurnal and terdiurnal components. Comparison with recent dynamo calculations shows an improved agreement on the north-south component of the drifts, even though it still leaves a significant gap between theory and experiment. The results provide evidence that the attempt to realize a good theory/experiment coincidence meets several difficulties of a fundamental nature: one is the lack of a simple summer/winter symmetry in the observed drifts, which reveals the importance of longitude effects introduced by the real earth's magnetic field; another is the large day-to-day variability within each season, which is comparable to the diurnal amplitude itself. Both show the need of worldwide coordinated efforts for the acquisition of simultaneous incoherent scatter data as well as for their interpretation