(B) Effects of Large Diurnal Density Changes on Outer Atmosphere Circulation H. W. Church Sandia Laboratory Introduction Recent investigations 1, of upper~atmosphere density as derived from satellite orbital period data have shown diurnal density changes ranging from a few percent at 200 km to orders of magnitude above 700 km. To a rough degree of approximation, the density-versus-time curve can be pictured as a cosine function periodicity of 24 hours, with peak amplitude at 1400 hours true local time (Figure 1). A postulated cause for such a variation is the significant absorptional heating by solar ultraviolet radiation down to the F2 layer. It is further noted that absorption creates an integrated effect upward such that the ampli- tude of density variation increases with altitude. Such large density and accompanying pressure and temperature changes must have a profound effect on the motion of a neutral parcel of air (wind). Therefore, it is interesting to construct a simplified model in a coordinate system fixed to the earth and to perform some first approximation calculations on resulting winds caused by this fast-moving (500 meters per second) pressure system. Model From the density cosine function pictured in Figure 1, it is possible to construct a horizontal pressure field map using corresponding pressure computed by Kallmann-Bijl” for the 660-km level (Vanguard I perigee) on a mercator projection with time or longitude as abscissa (Figure 2). The simplest case to consider is that of a point fixed relative to earth near the equator coming under the influence of the pictured pressure system as the system moves with the sun at about 500 meters per second from east to west. An air parcel initially at rest at 300 E longitude is in a region of maximum pressure gradient (0800 hours) which decreases to zero at 1400 hours (local real time), at which time wind will be a maximum from east. The acceleration vector reverses direction, accelerates wind from the west, reversing its direction at 2000 hours until, at 0200 hours, when the low-pressure center is reached, it returns to zero acceleration. If the region of interest is displaced away from equator toward pole, e.g., to 20 N latitude, then the north-south component of pressure gradient must be considered. From Figure 2 it can be seen that the north wind component is a maximum at 0800 hours, decreases to zero at 1400 hours, is maximum southerly at 2000 hours, and returns to zero at 0200 hours. The resulting wind field is shown in Figure 3. 41