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

A self-consistent method for day-time F2region modelling was applied to the analysis of Millstone Hill incoherent scatter observations during the storm period of March 16-22, 1990. The method allows us to calculate in a self-consistent way neutral composition, temperature and meridional wind as well as the ionized species height distribution. Theoretically calculated Ne(h) pro®les ®t the observed daytime ones with great accuracy in the whole range of heights above 150 km for both quiet and disturbed days. The overall increase in Tex by 270 K from March 16 to March 22 re ̄ects the increase of solar activity level during the period in question. A 30% decrease in [O] and a twofold increase in [N2] are calculated for the disturbed day of March 22 relative to quiet time prestorm conditions. Only a small reaction to the ®rst geomagnetic disturbance on March 18 and the initial phase of the second storm on March 20 was found in [O] and [N2] variations. The meridional neutral wind inferred from plasma vertical drift clearly demonstrates the dependence on the geomagnetic activity level being more equatorward on disturbed days. Small positive F2-layer storm e€ects on March 18 and 20 are totally attributed to the decrease in the northward neutral wind but not to changes in neutral composition. A moderate (by a factor of 1.5) O/ N2 ratio decrease relative to the MSIS-83 model prediction is required to describe the observed NmF 2 decrease on the most disturbed day of March 22, but virtually no change of this ratio is needed for March 21.