SOLAR-CYCLE AND SEASONAL-VARIATIONS IN F-REGION ELECTRODYNAMICS AT MILLSTONE HILL

Incoherent scatter radar observations of ion drifts taken at Millstone Hill (42.6-degrees-N, 288.5-degrees-E) during 73 experiments in the p eriod February 1984 to February 1992 are used to construct, for the fi rst time at this station, average quiet-time ExB drift patterns for bo th solar cycle maximum and minimum, for the summer, winter, and equino x seasons. The daily variation of V(perpendicular-to N) shows a revers al from northward to southward drifts near noon, and a return to north ward drifts in the premidnight hours. The weaker southward drift in th e afternoon in summer noted by Wand and Evans (1981) is shown to occur only at sunspot minimum. The daily variation of V(perpendicular-to E) shows daytime eastward drifts and nighttime westward drifts, except i n summer when the usual daytime eastward maximum near 1200 LT is suppr essed. The daily mean drift is westward for all seasons, and is larges t in summer. The daytime eastward drift and nighttime westward drift t end to be stronger at solar maximum than at solar minimum. Average dri ft patterns are also constructed for equinox for both extremely quiet and geomagnetically disturbed periods. V(perpendicular-to N) is apprec iably mom northward under extremely quiet than under disturbed conditi ons in the postmidnight and morning periods. During extremely quiet pe riods, V(perpendicular-to E) turns slightly eastward in the evening ho urs, while it is strongly westward for disturbed conditions. This resu lt contrasts with the strong eastward drifts in the evening in summer reported for extremely quiet conditions at Millstone Hill by Gonzales et al. (1978). A strong anticorrelation is seen at Millstone Hill betw een V(perpendicular-to N) and V(parallel-to), as is found at lower lat itude stations. The quiet-time patterns are discussed in terms of the causative E and F region dynamo mechanisms. At Millstone Hill, conjuga te point electric fields are also important in winter when the conjuga te ionosphere is sunlit for much of the night.