Daytime thermosphere above Millstone Hill during severe geomagnetic storms

A self-consistent method for daytime F region modeling was applied to Millstone Hill radar observations during the severe geomagnetic storm of April 6-12, 1990, when the F-2 layer maximum practically disappeared at usual heights and the F-1 layer constituted the ionospheric maximum during daytime hours. Neutral composition and temperature changes are shown to be the main reason for the observed tenfold decrease of electron concentration at the F-2 layer heights. The decrease in [O] at 300 km is more than a factor of 6, and the [N-2] and [O-2] increases are 3 and 16 times, respectively, with respect to the prestorm quiet time level. Such changes of neutral composition lead to a strong decrease in O+ production rate and increase in O+ loss rate resulting in a complete disappearance of the F-2 layer maximum. Our calculations for 300 km altitude indicate a nineteenfold O/N-2 ratio decrease for April 10 with respect to quiet day of April 7. Horizontal plasma transfer due to the observed E-W drift is insufficient to account for the observed density changes during daytime hours. The calculated exospheric temperatures T-ex are close to both the mass spectrometer incoherent scatter (MSIS) 83 model predictions and the Millstone Hill estimates for all days except for April 10, when T-ex is higher than 2000 degrees K, which may be attributed to Joule heating due to strong electric fields. The calculated meridional thermospheric wind V-nx is more equatorward on disturbed days. The strong V-nx (similar to 90 mis northward) on April 11 can explain the fast recovery of the local thermospheric parameters to the April 9 level. Calculations indicate that the ionosphere was molecular-ion-dominated up to 350 km on April 10, requiring a correction to the routinely derived T-e(h), T-i(h), and N-e(h) radar profiles.