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 period February 1984 to February 1992 are used to construct, for the first time at this station, average quiet-time ExB drift patterns for both solar cycle maximum and minimum, for the summer, winter, and equinox seasons. The daily variation of V(perpendicular-to N) shows a reversal from northward to southward drifts near noon, and a return to northward drifts in the premidnight hours. The weaker southward drift in the 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 in summer when the usual daytime eastward maximum near 1200 LT is suppressed. The daily mean drift is westward for all seasons, and is largest in summer. The daytime eastward drift and nighttime westward drift tend to be stronger at solar maximum than at solar minimum. Average drift patterns are also constructed for equinox for both extremely quiet and geomagnetically disturbed periods. V(perpendicular-to N) is appreciably mom northward under extremely quiet than under disturbed conditions in the postmidnight and morning periods. During extremely quiet periods, V(perpendicular-to E) turns slightly eastward in the evening hours, while it is strongly westward for disturbed conditions. This result 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 between V(perpendicular-to N) and V(parallel-to), as is found at lower latitude stations. The quiet-time patterns are discussed in terms of the causative E and F region dynamo mechanisms. At Millstone Hill, conjugate point electric fields are also important in winter when the conjugate ionosphere is sunlit for much of the night.