MODELING COMPOSITION CHANGES IN F-LAYER STORMS

A coupled thermosphere-ionosphere-plasmasphere model CTIP is used to s imulate storm changes in the ionosphere. The simulations cover a perio d of 72 hours, starting with imposed high-latitude energy inputs (part icle precipitation and electric fields) that represent a moderately se vere geomagnetic storm (K-p 5) lasting for 12 h. Equinox and solstice conditions are studied. We give particular attention to comparing chan ges in peak electron density, N(m)F2, to those of the [O/N-2] concentr ation ratio of the neutral air. During the first few hours of the stor m, large perturbations are produced by strong meridional winds. After that initial phase, we find that the changes of N(m)F2 and of [O/N-2] ratio correspond closely, the composition changes being produced by th e thermospheric ''storm circulation'', as in the ''composition bulge'' theory of Fuller-Rowell et al. (1994). The simulations reproduce the general form of the seasonal variations in the changes of N(m)F2 at mi d-latitudes as derived from worldwide ionosonde data. Some storm effec ts at sub-auroral latitudes are caused by movement and infilling of th e ionospheric trough. We conclude that the composition change theory a ccounts for the major features of F-layer storm behaviour at midlatitu des. (C) 1997 Elsevier Science Ltd. All rights reserved.