Neutral thermotropic composition changes in response to several external inputs. Solar EUV radiation is absorbed throughout this region, and upward propagating tides and gravity waves are particularly important in the lower thermosphere. Coupling between the magnetosphere and the thermosphere/ionosphere system also affects neutral composition, both through the effects of joule heating and horizontal transport by ion-neutral momentum transfer. The major feature of these changes in composition at 300 km is the large enhancement of N2 density that occur over the magnetic polar cap within minutes to hours after the onset of the storm. These N2 enhancements can extend to low latitudes. Other changes in neutral composition include enhanced oxygen and NO densities, relative to N2 densities.