A localized ~55% decrease is observed in the brightness of Earth's FUV dayglow in the morning sector at 130.4 nm after an interval of intense geomagnetic activity. This large decrease is interpreted as being the consequene of auroral-associated heating which reduces the thermospheric column density of O relative to that of N2. Spatial extent of the observed decrease exceeds 107 km2 at the -25% level, and the depth of the decrease lessens during more than two hours of observations. These remote observations provide the first instantaneous, two-dimensional measurement of the large-scale spatial extent of such a change in thermospheric composition. ¿ American Geophysical Union 1994