Abstract :
Measurements of electron densities, Ne, and temperatures, Te, at Millstone Hill are compared with results from the time-dependent mathematical model of the Earthʹs ionosphere during 5–12 April, 1997. To agree the measured and modelled hmF2 an additional plasma drift is added to compensate inaccuracies of the model input parameters. The height variations in plasma drifts decrease NmF2 up to 24 % in agreement with early results of Schlesier and Buonsanto (1999). The comparison of the measured and modelled altitude profiles of Ne shows that anomalously low values of the main F-region peak altitude (< 200 km) seen as a “G condition” on ionograms and observed on April 11 exist in the ionosphere due to a decrease of O+ ion production rates resulting from low values of [O] and an increase of molecular ions resulting from high values of [N2] and [O2] in comparison with the MSIS-86 neutral densities. The G conditions observed on 7 April can be explained by deviations of [O], [N2], and [O2] from the MSIS-86 neutral densities, and the solar flare and post-flare increase of the incident solar flux. The ionospheric storm dusk effect observed on 10 April, 1997, can result from (1) the F2 layer uplifting to more high altitudes due to an enhanced equatorward neutral wind, resulting in an decreased O+ recombination rate and an electron density enhancement, and (2) increase by a factor of 1.9 in [O] from 17:00 LT to 18:30 LT on 10 April in comparison with the MSIS-86 [O]. The explanation of the observed negative phases in Ne is presented.
