Shigaraki MU radar observations of horizontal thermospheric winds in the
magnetic meridian plane over the period September 1986 to September 1996
are analyzed to give climatological averages in the form of time-of-day
variations for several combinations of seasonal and solar-activity
conditions. The dominant feature of the MU wind behavior is its mean
diurnal variation of northward flow by day and southward flow by night,
with the nighttime wind smoothly approaching to and receding from a
midnight maximum while the daytime wind tends to show two peaks, a strong
one in the early daylight hours and weak one in the afternoon-evening.
HWM shows the same unimodal nighttime and bimodal daytime behavior, but
the HWM pattern is shifted about two hours later in time. The amplitude
of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s
at solar maximum while HWM shows a corresponding increase from 53 to 62
m/s. The diurnal amplitude is remarkably stable with season but is
superposed on a steady DC wind of 41 m/s southward in summer, 15 m/s
northward in winter, and midway between these limits at the equinoxes.
HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s
northward in winter. Ion drag appears to be the main regulator of wind
speed, and the seasonal wind patterns have profound effect upon the
seasonal behavior of the ionosphere.
Kawamura, S., Y. Otsuka, S.-R. Zhang, S. Fukao and W. L. Oliver,
A Climatology of MU radar observations of thermospheric winds, J.
Geophys. Res., 105, 12,777-12,788, 2000
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