The Millstone Hill incoherent scatter radar was operated on May 30, 1984, to gather data on the effects of the annular solar eclipse on the structure and dynamics of the earth's ionosphere and thermosphere. Maximum obscuration of 86% was experienced at Millstone Hill at 1700 UT, and there was only low magnetic activity coincident with the event. The observations, which were made using both the zenith-fixed and the steerable antenna at Millstone Hill, indicated a drop of 60% in electron density shortly after peak obscuration time in the altitude region 130--250 km and a smaller decrease (15--30%) at higher altitudes. The electron temperature was found to decrease by 500--700 K in the F region during the eclipse, and relatively little variation was observed in the ion or neutral temperature. The measured three-dimensional ion drifts in the F region were synthesized in terms of a meridional wind velocity of about 25 m/s northward at eclipse time, which represents a drop of 40--50 m/s from conditions prior to the eclipse.

Electric field strengths were less than 1 mV/m and showed the same general patterns as quiet time fields at Millstone Hill. Comparisons with predictions of the neutral atmosphric response made for this eclipse by the NCAR thermospheric general circulation model (TGCM) and of the ionospheric response made by the NCAR ionospheric model indicate good agreement below 250 km for the electron density and temperature; at higher altitudes the predictions and observations can be reconciled when the effects of heat and ionization flux are taken into account for the boundary condition in the ionospheric model. The predicted neutral winds are within a factor of 2 of the measurements, but it is difficult to ascertain reliably from the data the magnitude of the perturbation fields converging onto the eclipse path.