On the behaviour of wind systems in the upper mesopause region during the transition from summer to winter conditions

Ionospheric drift measurements in the LF range indicate that the phase of the semidiurnal tidal wind oscillation in the upper mesopause, while being approximately equal in summer and in winter, develops characteristic changes near the equinoxes. These changes may either take the form of a complete rotation or, alternatively, an advance and retardation of the phase. What happens in any one year is not predetermined by the initial conditions alone but depends on random processes the nature of which cannot be deduced from the results of wind measurements only.