Neutral thermospheric winds deduced above Millstone Hill, 2. Seasonal wind variations, 1970–1971

Abstract

Millstone Hill (42.6°N) incoherent scatter data gathered during 1970–1971 were used as input for a local model of the neutral thermosphere. The pressure gradients in the model were adjusted so as to match the exospheric temperature and neutral wind parallel to the magnetic field observed above the radar. Large seasonal variations in the diurnal average horizontal winds were deduced. Annual average zonal velocities were 5 m/s toward the west with an annual variation of ±43 m/s. Meridional velocities were 21 m/s toward the equator in the annual average with annual variations of ±52 m/s. The mean winds were toward the southwest in the summer and the northeast in the winter and tended to be stronger in 1971 than in 1970. The average vertical velocities were found to be generally downward throughout the year. The mean equatorward winds, large annual variations, and average downward velocities are consistent with a global circulation model presented by Dickinson et al. (1977) and require the existence of significant heating of the thermosphere at high latitudes. Seasonal variations calculated using the MSIS (mass spectrometer/incoherent scatter) model pressure gradients were similar, except that in contrast to the observed behavior the intensity of the annual circulation appeared to decrease from 1970 to 1971, reflecting the decline in the solar flux. Since the magnetic activity tends to lag the sunspot cycle by about a year and remained at about the same level over the 2 years, it is suggested that this difference may be accounted for if the influence of the high‐latitude heating is increased.