The DE 1 far ultraviolet (FUV) imaging instrument recorded a large thermospheric disturbance over Europe and Russia on October 14, 1981, during a geomagnetic storm commencing near the end of the previous day. The disturbance is quantified in terms of O/N2, the column density ratio referenced to a fixed depth in N2. The ratio is derived from DE 1 dayglow observations with a filter whose signal transmission is dominated by O I 130.4-nm emission. Within the disturbed region, O/N2 is less than that in surrounding regions, with minimum values being more than a factor of 2 smaller than undisturbed values. Images of O/N2 are compared to ground-based ionosonde data from 13 sites, some within and others outside of the disturbed region. The data are in the form of Nmax, the maximum electron density, which in the presence of an F2 layer is equivalent to NmF2. The data exhibit negative ionospheric storm effects (reduced Nmax) within the disturbed region and normal behavior elsewhere. The degree of correlation between O/N2 and Nmax suggests that the approximate geographical extent of dayside regions experiencing negative ionospheric storm effects can be determined from global FUV imaging of the disturbed and surrounding regions. The analysis is extended to the following day to address the recovery of the disturbed region. The O/N2 ratio has returned to undisturbed values except in a smaller disturbance region containing weak reductions in O/N2. This is either a remnant of the strong disturbance from the day before or a newly formed disturbance from substorm activity several hours prior to the new observations. Only the ionosonde sites within this region recorded values of Nmax below their monthly median values. ¿ 2001 American Geophysical Union