VARIATIONS IN THE FUV DAYGLOW AFTER INTENSE AURORAL ACTIVITY

A localized similar to 55% decrease is observed in the brightness of E arth's FUV dayglow in the morning sector at 130.4 nm after an interval of intense geomagnetic activity. This large decrease is interpreted a s being the consequence of auroral-associated heating which reduces th e thermospheric column density of O relative to that of N-2. Spatial e xtent of the observed decrease exceeds 10(7) km(2) at the -25% level, and the depth of the decrease lessens during more than two hours of ob servations. These remote observations provide the first instantaneous, two-dimensional measurement of the large-scale spatial extent of such a change in thermospheric composition.