Densities in the 400--500 km height region inferred from accelerometer measurements on the CHAMP and GRACE satellites are utilized to study the response to the isolated and severe geomagnetic storm of 20--21 November 2003. The CHAMP and GRACE satellites provide data at approximate local times of 1110/2310 and 1430/0230 hours, respectively. In a global sense, density increases of order 300--800% occur during this storm, with relatively little time delay at high latitudes and with about 4-hour delay at the equator. Significant latitudinal asymmetries in the response are discussed in the context of neutral wind patterns and enhanced summer versus winter Joule heating rates at high latitudes. Comparisons with the NRLMSISE-00 empirical model densities during this period show marked differences in amplitude of the response, as well as latitudinal and temporal structures. Evidence for a tight coupling between solar wind and neutral density variability is found for the high-latitude summer hemisphere near noon, in that densities near 410 km are very responsive to increases in solar wind dynamic pressure, even during periods when both Bz and By are near zero or positive. Filtering of the data reveals regional-scale (~1000--2000 km) density anomalies that are common between the CHAMP and GRACE measurements. During the geomagnetic disturbance, alternating regions of density enhancements (~50%) and depressions (~50%) exist between the pole and low to middle latitudes that have the appearance of a standing wave pattern. During magnetically quiet intervals before and after the storm, daytime density depressions (~4--8%) are seen that track the magnetic equator, while density enhancements (~10%) track the latitudes of the ionospheric Appleton anomaly peaks. Large-scale (≥1000 km) wave-like structures are also prevalent during both day and night during the magnetic disturbance and extend throughout both hemispheres, consistent with the concept that these are associated with the so-called "direct" gravity waves forced in the auroral regions and propagating far from the source. Substantial variability is also present at medium scales (~300--500 km), but these structures are confined mainly to middle to high latitudes, in accord with theoretical expectations. GRACE-A and GRACE-B data are used in a feasibility study of the detection of small-scale wave activity by taking advantage of the 30-s separation of the satellites in the same orbital plane. Despite the large amount of maneuvers, which are not always recorded, this first analysis is promising. Amplitude variations of order 10--20% and wave speeds of 1000--1500 m s-1 are observed.