NEUTRAL WINDS IN THE THERMOSPHERE AT MIDLATITUDES OVER A FULL SOLAR-CYCLE - A TIDAL DECOMPOSITION

The magnetic meridional component of the thermospheric neutral wind at the F2 peak at Boulder, Colorado, is derived for days with magnetic index Ap less-than-or-equal-to 20 over a full solar cycle (July 1985 - June 1986). The servo model technique and ionosonde measurements of hmF2 are used, and the results are corrected for ionization drifts due to electric fields. A Fourier decomposition of the mean daily variation is carried out for each month. This reveals significant components with wave periods of 24, 12, 8, and 6 hours. The importance of ion drag in causing seasonal and solar cycle variations in the winds is inferred. Specifically, the conclusion of previous work that the diurnal amplitude is larger at solar minimum than at solar maximum is confirmed by the present study. The diurnal amplitude is generally in the range 60-80 m s-1 at solar minimum but only 25-45 m s-1 at solar maximum. The diurnal phase maximum of the northward wind lies in the range 1115-1625 LT with latest values in winter and at solar maximum, when the daytime effects of ion drag are greatest. The amplitude of the semidiurnal component shows little solar cycle variation but tends to be smaller in summer compared to other seasons. It has a mean value of 41 m s-1 over the solar cycle. The semidiurnal phase maximum is remarkably close to 0800 LT for all seasons throughout the solar cycle. The amplitudes of the 8 and 6 hours waves are both in the range 4-25 m s-1. The 8 and 6 hour waves both have later phase maxima in summer than in winter. The present results are generally consistent with current models of atmospheric tides, except for the large solar cycle variation in the diurnal component.