Mean meridional winds in the ionosphere at 70°N

Abstract

Values of the meridional wind at 70°N, 19°E are calculated from EISCAT field-aligned measurements in the solar minimum years 1985–1987. Full allowance is made for convolution effects, to obtain accurate mean diurnal variations for summer, winter and equinox. In all seasons the equatorward wind varies almost linearly from about −80 m s −1 at 13:00 L.T. to 170 m s −1 at 23:30 L.T. There is an overall equatorward shift in summer of about 30 m s −1 near noon and midnight, decreasing to 10 m s −1 near 06:00 and 18:00. The observed winds are about half as large as those calculated from the recent wind model HWM87. Discrepancies are greatest near 08:00 L.T. when small equatorward winds are consistently observed, rather than the strong poleward winds predicted by the model. Day to day variations in the calculated winds are least near 06:00 L.T. and largest near midnight. On about half the nights in summer there is a large surge which increases the equatorward wind from about 120 m s −1 at 21:00 to 260 m s −1 at 22:00 L.T. Independent calculations were made at heights of 250, 350 and 450 km. To obtain agreement between the three sets of results it was necessary to (1) increase the commonly accepted value of the O +-O collision cross-section by 80%, and (2) reduce thermal diffusion to much less than that given by the often-used term ( 0.37 T ) dT/ dh, where T = 1 2 (T i + T n ) . Investigation of (2) showed that the first value of T should 1 2 (T i + T e ) , and that allowance for ion-ion collisions reduces thermal diffusion by a further factor of typically about 4 at 300 km and 50 at 500 km. This makes thermal diffusion negligible at all heights in the F-region of the ionosphere (in the absence of strong convection electric fields).