QUASI-PERIODIC RADAR ECHOES FROM MIDLATITUDE SPORADIC-E AND ROLE OF THE 5-DAY PLANETARY WAVE

Using measurements of magnetic-aspect-sensitive radar echoes from midl atitude sporadic E collected over a two-month period from Tanegashima, Japan, we show that while their occurrence duration from night to nig ht did not exhibit any systematic variation, that of the so-called qua si-periodic (QP) echoes varied sinusoidally with a period of 5 days. W e have interpreted this behavior in terms of effects produced by a pla netary wave and identified its presence through neutral-wind measureme nts made with a partial-reflection drift radar located nearby at Yamag awa. 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 dire ction of the neutral wind. We argue that because the wind vector of th e planetary wave is elliptically polarized at midlatitudes, a preferre d wind direction conducive to the generation of QP echoes occurs once every 5 days. On the other hand, this wave is linearly polarized and d irected zonally over the geographic equator. The fact that QP echoes a re most fully developed at midlatitudes and less so at lower latitudes suggests that zonal flow is not particularly favorable for QP echo pr oduction.