Modeling the behavior of ionosphere above Millstone Hill during the September 21-27, 1998 storm

A theoretical ionospheric model is employed to investigate the ionospheric behavior as observed by the incoherent-scatter radar (ISR) at Millstone Hill during the September 21-27, 1998 storm. The observed NmF2 presented a significant negative phase on September 25, and a G condition (h(m)F(2) < 200 km) was also observed. The model results based on the standard input parameters (climatological model values) are in good agreement with the observed electron densities under quiet conditions, but there are large discrepancies during disturbed periods. The exospheric temperature T-ex, neutral winds, atomic oxygen density [O] and molecular nitrogen density [N-2], and solar flux are inferred from the ISR ion temperature profiles and from the electron density profiles. Our calculated results show that the maximum T-ex is higher than 1700 K, and an averaged decrease in [O] is a factor of 2.2 and an increase in [N-2] at 300 km is about 1.8 times for the disturbed day, September 25, relative to the quiet day level. Therefore, the large change of [N-2]/[O] ratio gives a good explanation for the negative phase at Millstone Hill during this storm. Furthermore, at the disturbed nighttime the observations show a strong NmF2 decrease, accompanied by a significant h(m)F(2) increase after the sudden storm commencement (SSC). Simulations are carried out based on the inferred T-ex. It is found that the uplift of F-2 layer during the period from sunset to post-midnight is mainly associated with the large equatorward winds, and a second rise in h(m)F(2) after midnight results from the depleted N-e in the bottom-side of F-2 layer due to the increased recombination, while the "midnight collapse" of h(m)F(2) is attributed to the large-scale traveling atmospheric disturbances. (C) 2004 Elsevier Ltd. All rights reserved.

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