A climatology of middle and upper atmosphere radar observations of thermospheric winds

Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period Septemb er 1986 to September 1996 are reported as climatological averages in the fo rm of time-of-day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizonta l wind model (HWM) anti with winds measured at Saint Santin and Millstone H ill. The dominant feature of the MU wind behavior is its mean diurnal varia tion of northward flow by day and southward flow by night, with the nightti me wind smoothly approaching and receding from a midnight maximum, while th e daytime wind tends to show two peaks, a strong. one in the early daylight hours and a weak one in the afternoon-evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted abo ut 2 hours later in time. The amplitude of the diurnal harmonic decreases f rom 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 remarkab ly stable with season but is superposed on a steady willd of 41 m/s southwa rd in summer, 15 m/s northward in winter, and midway between these limits a t the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summe r and 30 m/s northward ill winter. Ion drag appears to be the main regulato r of wind speed, and the seasonal wind patterns have a profound effect on t he seasonal behavior of the ionosphere.