The solar-cycle-dependent response of the thermosphere to geomagnetic storms

Geomagnetic storms cause large, global-scale changes in the neutral thennosphere. But both these changes and the mechanisms that produce them are affected strongly by the background "quiet-time" thermospheric conditions. For example, the extent of the area of enhancements in the storm-time O/N-2 ratio is much more restricted in winter than in summer. Seasonal changes are not the only variations in the background thermosphere that affect the storm-time response: great changes also occur as a result of the variations of solar EUV radiation over the solar cycle. Thus, the response of the neutral thermosphere to geomagnetic storms should also be expected to vary with the phase of the solar cycle. In this paper, we describe the nature of these varying responses of neutral composition and neutral temperature using runs of the National Center for Atmospheric Research's Thermosphere-Ionosphere-Electrodynamic General Circulation Model. For both solar maximum and minimum conditions, the model runs were initiated with the same background level of geomagnetic forcing (the hemispheric power was 11 GW). The power was then increased to 50 GW for 12 h to simulate a geomagnetic storm. By keeping the geomagnetic inputs the same, we have been able to isolate the effects of the changes in incoming solar EUV radiation on the storm response for the two extremes of a solar cycle. The following conclusions have been drawn from this study: (1) the regions in which the greatest increases of N-2 mass mixing ratio in winter are found to occur at much lower geomagnetic latitudes during solar minimum than during solar maximum; (2) this different distribution of changes occurs because horizontal advection is more important during solar minimum than during solar maximum; (3) horizontal advection is more important during solar minimum both because equatorward winds are stronger, and because the high-latitude air is relatively more enhanced in molecular species than it is during solar maximum; (4) stronger equatorward winds occur during solar minimum because the pressure gradients driven by solar EUV heating are weaker, and thus the pressure gradient set. up by high-latitude Joule heating is relatively more important; (5) in contrast to the compositional variations, storm-time temperature increases (numerical values not percentages) are greater during solar maximum than during solar minimum; (6) the initial thermal recovery from storm-time perturbations is faster during solar maximum, but then recovery rates are roughly equivalent after the first 12 h of the recovery period; (7) initial compositional recovery occurs much more rapidly at solar maximum than at solar minimum. (C) 2003 Elsevier Ltd. All rights reserved.