Response of the thermosphere and ionosphere to geomagnetic storms

Four numerical simulations have been performed, at equinox, using a coupled thermosphere‐ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12‐hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high‐velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large‐scale gravity wave with a phase speed of about 600 m s −1 . Behind the surge a global circulation of magnitude 100 m s −1 is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm‐induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.