Four numerical simulations have been performed, at equinox, using a co
upled 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 1
2-hour period; each storm commences at a different universal time (UT)
. The initial response at high latitude is that Joule heating raises t
he temperature of the upper thermosphere and ion drag drives high-velo
city neutral winds. The heat source drives a global wind surge, from b
oth polar regions, which propagates to low latitudes and into the oppo
site 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 mani
festations of the same phenomena. A dominant feature of the response i
s the penetration of the surge into the opposite hemisphere where it d
rives poleward winds for a few hours. The global wind surge has a pref
erence 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 pha
se of the meridional circulation develops after the wave interaction b
ut is also restricted, in this case by the buildup of zonal winds via
the Coriolis interaction. Conservation of angular momentum may limit t
he buildup of zonal wind in extreme cases. The divergent wind field dr
ives upwelling and composition change on both height and pressure surf
aces. The composition bulge responds to both the background and the st
orm-induced horizontal winds; it does not simply rotate with Earth. Du
ring the storm the disturbance wind modulates the location of the bulg
e; during the recovery the background winds induce a diurnal variation
in its position. Equatorward winds in sunlight produce positive ionos
pheric changes during the main driving phase of the storm. Negative io
nospheric phases are caused by increases of molecular nitrogen in regi
ons of sunlight, the strength of which depends on longitude and the lo
cal time of the sector during the storm input. Regions of positive pha
se in the ionosphere persist in the recovery period due to decreases i
n 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 bu
lge; this variation explains the ac component of the local time variat
ion of the observed negative storm phase.