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A self-consistent method for day-timeF2-region modelling was applied to the analysis of Millstone Hill incoherentscatter observations during the storm period of March 16-22, 1990. The methodallows us to calculate in a self-consistent way neutral composition, temperatureand meridional wind as well as the ionized species height distribution.Theoretically calculated Ne(h) profiles fit the observeddaytime ones with great accuracy in the whole range of heights above 150 km forboth quiet and disturbed days. The overall increase in Tex by270 K from March 16 to March 22 reflects the increase of solar activity levelduring the period in question. A 30% decrease in [O] and a two-fold increase in[N2] are calculated for the disturbed day of March 22 relativeto quiet time prestorm conditions. Only a small reaction to the firstgeomagnetic disturbance on March 18 and the initial phase of the second storm onMarch 20 was found in [O] and [N2] variations. The meridionalneutral wind inferred from plasma vertical drift clearly demonstrates thedependence on the geomagnetic activity level being more equatorward on disturbeddays. Small positive F2-layer storm effects on March 18 and 20 are totallyattributed to the decrease in the northward neutral wind but not to changes inneutral composition. A moderate (by a factor of 1.5) O/N2ratio decrease relative to the MSIS-83 model prediction is required to describethe observed NmF2 decrease on the most disturbed day of March22, but virtually no change of this ratio is needed for March 21

annales geophysicae

Day-to-day thermosphere parameter variation as deduced from Millstone Hill incoherent scatter radar observations during March 16-22, 1990 magnetic storm period