A Re-evalua(ion of (he Sokes Drif in (he Polar Summer Mesosphere

of 10 smaller than the mean June and July downward vertical velocities we have verified from 4-year averaging of the Poker Flat, Alaska, radar data base. In this extended analysis we also find that the upward winter mean vertical velocity is smaller than previously reported but still in apparent conflict with the mesospheric winter circulation theory. We suggest, as an alternative to the Stokes drift idea, that the observed sununer velocity might be the terminal velocity of charged aerosols. Such an explanation would have the advantage of relating the mean velocity observations to the large VHF radar cross section exhibited by the polar sununer mesosphere. This possible connection is discussed further in a companion paper. agating waves pass through this region and deposit their momentum in the upper mesosphere. This wave-momentum deposition represents an eastward force acting on the mean flow and causes a reduction in the magnitude of the west- ward jet. The Coriolis force approximately bMances this eastward force due to waves, resulting in a meridional mo- tion away from the pole. Continuity then requires an upward motion near the pole. This upward motion should be of the order of I cm/s to explain the observed low temperatures. For examplethe model of Holton (1983), which includes the zonal drag due to breaking gravity waves, produces an up- ward motion in the upper summer mesosphere of about 1 In order to make statistically significant measurements of small mean verticM velocities in the presence of large fluc- tuationslong-term observations are necessary. The radar observations from Poker FlatAlaska (Ecklund and Balsley 1981) for 1979-1985 provide such a data set. Using a portion of this data, Balsley and Riddle (1984) reported that during the period of May 1980 through July 1981 a downward av- erage velocity of 20-30 cm/s wdetected in the summer