Using measurements of magnetic-aspect-sensitive radar echoes from midlatitude sporadic E collected over a two-month period from Tanegashima, Japan, we show that while their occurrence duration from night to night did not exhibit any systematic variation, that of the so-called quasi-periodic (QP) echoes varied sinusoidally with a period of 5 days. We have interpreted this behavior in terms of effects produced by a planetary wave and identified its presence through neutral-wind measurements made with a partial-reflection drift radar located nearby at Yamagawa. We propose that the occurrence of QP echoes is affected both by a contribution of the wind to the dynamo electric field and by the direction of the neutral wind. We argue that because the wind vector of the planetary wave is elliptically polarized at midlatitudes, a preferred wind direction conductive to the generation of QP echoes occurs once every 5 days. On the other hand, this wave is linearly polarized and directed zonally over the geographic equator. The fact that QP echoes are most fully developed at midlatitudes and less so at lower latitudes suggests that zonal flow is not particularly favorable for QP echo production. ¿ 1998 American Geophysical Union