The seasonal variation of the diurnal thermospheric winds over Millstone Hill during solar cycle maximum

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density and vertical plasma drift are used to determine the diurnal variation of neutral winds at F-layer heights over Millstone Hill (42.6°N, 71.5°W) for different seasons. The technique to derive the thermospheric winds from incoherent scatter radar data and the results for equinox conditions have been discussed by Roble et al. (1974). Data for three summer and three winter days in 1969 and 1970 are examined for seasonal characteristics during geomagnetic quiet time at solar cycle maximum conditions. The derived diurnal variation of the neutral winds for summer months shows a later morning transition from equatorward to poleward winds and an earlier evening transition from poleward to equatorward winds than during winter months. These times of transition have a pronounced influence on the ionospheric structure and are partly responsible for the characteristic ‘summer’ and ‘winter’ type behavior of the ionosphere observed over Millstone Hill. The diurnally averaged values of the thermospheric winds at 300 km also have a seasonal variation. During summer, the zonal winds are westward at about 15 m⁻¹ and are eastward at about the same velocity during the winter. The diurnally averaged meridional winds show a strong equatorward flow during summer of about 50 m s⁻¹ whereas during winter months the winds are considerably weaker and irregular, with an average equatorward flow on some days and poleward flow on others. The zonally averaged values are consistent with a mean meridional circulation from the summer hemisphere to the winter hemisphere at F-layer heights. However the irregular meridional winds in the winter indicate a weakening of this circulation near Millstone Hill.