Model/data comparison of F region ionospheric perturbation over Millstone Hill during the severe geomagnetic storm of July 15–16, 2000 (original) (raw)

2001, Journal of Geophysical Research

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 N: and O: on NmF: 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 N: and O: does not reproduce both the daytime and nighttime electron densities and temperatures. Inclusion of vibrationally excited N: and O: brings the model and data into better agreement. Without the contribution of vibrationally excited N: and O: 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 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 N: and O: in ionospheric models. 29,051 29,052 PAVLOV AND FOSTER: F REGION IONOSPHERIC PERTURBATION and model simulations are needed to distinguish between these mechanisms. The reviews of Prolss [1995] and Buonsanto [1999] summarize papers addressing ionospheric storms. One of the aims of this paper is to study the physics of the negative phase of the ionospheric storm observed on July 15-16, 2000, and, in particular, a situation that is known as a G condition, when the measured F2 layer peak electron density, NmF2, drops below that of the F• layer (seen as a "G condition" on ionograms) . As a result of this anomalous structure of the ionospheric F region, a very low main peak altitude (below 200 km) is observed in ionograms, so that no information is obtainable above this height from ground-based ionosonde data. To our knowledge the altitude distribution of the electron density with NmF2<NmF• was deduced for the first time by Norton [ 1969].