A Climatology of MU Radar observations of thermospheric winds
S. Kawamura, Y. Otsuka, S.-R. Zhang, S. Fukao, and W. L. Oliver
J. Geophys. Res.,
Vol. 105, No. A6, Pages 12,777-12,788, June 1, 2000.
Abstract
Shigaraki MU radar observations of horizontal thermospheric winds
in the magnetic meridian plane over the period September 1986 to
September 1996 are reported as climatological averages in the form of
time-of-day variations for several combinations of seasonal and
solar-activity conditions and are compared with winds predicted by the
horizontal wind model (HWM) and with winds measured at Saint Santin and
Millstone Hill.
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.