Midlatitude thermospheric winds from incoherent scatter radar and theory

Ionospheric plasma vertical drifts measured by the Millstone Hill incoherent scatter radar are decomposed into estimates of meridional neutral particle motion by accounting for ion diffusion and semidiurnal electric fields. For two equinoctial days with relatively high F-region electron densities the resulting neutral circulation at 300 km is composed of equatorward winds of 100 to 150 m sec⁻¹ at night and poleward winds of 25 to 50 m sec⁻¹ during the day. Harmonic analysis of the wind pattern yields a diurnal northward component of 80 to 100 m sec⁻¹ with a maximum near noon, a semidiurnal component of 35 m sec⁻¹ at 0600 LT and a prevailing southward wind of about 20 m sec⁻¹. The observations are compared with tidal theory predictions and are interpreted by means of a simplified integration of the horizontal equations of motion through which effects of zonal advection, ion drag, viscosity, and pressure gradients are studied. Although the observed wind pattern is consistent in general with that derived from global models based on satellite drag observations, various dicrep-ancies concerning the amplitude and phase of the diurnal bulge are noted and discussed.