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{sub 2} combined with a decrease in the atomic oxygen to molecular composition ratio of {approx}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 duringmore » 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. 58 refs., 12 figs., 1 tab.« less