We compare measurements of the ionospheric F region at Millstone Hill during the severe geomagnetic storm of July 15--16, 2000, with results from a time-dependent mathematical model of the Earth's ionosphere and plasmasphere. Use of [O> correction factors with the Mass Spectrometer Incoherent Scatter (MSIS-86) model was found to bring the modeled and measured F region main peak electron densities into agreement. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the nighttime plasmasphere region above 5000 km to bring the measured and modeled electron temperatures into agreement. We conclude that anomalously low values of the F peak altitude seen as a G condition on ionograms and observed on July 16 exist in the ionosphere due to a decrease in the production rates of oxygen ions resulting from low values of atomic oxygen density. The resulting effect of vibrationally excited N2 and O2 on NmF2 is the decrease of the calculated daytime F region main peak electron density by up to a factor of ~3. The modeled electron temperature is very sensitive to the electron density, and this decrease in electron density results in an increase in the calculated daytime electron temperature of ~930 K at the F region main peak altitude, giving closer agreement between the measured and modeled electron temperatures. The model without the inclusion of vibrationally excited N2 and O2 does not reproduce both the daytime and nighttime electron densities and temperatures. Inclusion of vibrationally excited N2 and O2 brings the model and data into better agreement. Without the contribution of vibrationally excited N2 and O2 in the loss rate of O+(4S) ions, the model calculations result in a very large disagreement between the measured and modeled electron density altitude profiles during the G condition daytime period on July 16. In order to bring the modeled and measured electron density altitude profiles into agreement, it would be necessary to decrease the MSIS-86 model [O> by a factor of 5.5. It is unlikely that the MSIS-86 atomic oxygen number density correction factor is so large. This finding provides an additional argument for the inclusion of the effects of vibrationally excited N2 and O2 in ionospheric models. ¿ 2001 American Geophysical Union