Modelling composition changes in F-layer storms

A coupled thermosphere-ionosphere-plasmasphere model CTIP is used to simulate storm changes in the ionosphere. The simulations cover a period of 72 hours, starting with imposed high-latitude energy inputs (particle precipitation and electric fields) that represent a moderately severe geomagnetic storm (K-p 5) lasting for 12 h. Equinox and solstice conditions are studied. We give particular attention to comparing changes in peak electron density, N(m)F2, to those of the [O/N-2] concentration ratio of the neutral air. During the first few hours of the storm, 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 the 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 mid-latitudes as derived from worldwide ionosonde data. Some storm effects at sub-auroral latitudes are caused by movement and infilling of the ionospheric trough. We conclude that the composition change theory accounts for the major features of F-layer storm behaviour at midlatitudes. (C) 1997 Elsevier Science Ltd. All rights reserved.