MILLSTONE-HILL INCOHERENT-SCATTER F-REGION OBSERVATIONS DURING THE DISTURBANCES OF JUNE 1991

The extreme geomagnetic disturbances of early June 1991 were accompanied by dramatic effects in the Earth's thermosphere and ionosphere which are the subject of coordinated analysis by the Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Storm Study project. F region observations taken at Millstone Hill over the interval June 5-11 using a nine-position experiment allow estimation of spatial gradients in ionospheric parameters above the station. From these gradients the motion term in the F region continuity equation is obtained and combined with calculations of rates of production and loss of the major ions in a semiempirical model of the F region ionosphere above Millstone Hill. Electron densities calculated from this semiempirical model using MSIS-86 are generally much smaller than the measurements. This discrepancy could be explained by the presence of vibrationally excited N-2 combined with a decrease in the atomic oxygen to molecular composition ratio of approximate to 2 or more on the most disturbed days. Neutral winds calculated by using the incoherent scatter line-of-sight ion drifts show a strong equatorward surge during the day on June 5 and strong equatorward surges during all the disturbed nights, Synoptic observations taken from Millstone Hill by using elevation and azimuth scans during the nights of June 12, 13, and 14 illustrate the large-scale structure of electron density and its relation to neutral winds and electric fields observed over much of eastern North America.