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• #### Studies of compositional variations in the thermosphere and ionosphere using far-ultraviolet images from DE-1

The Dynamics Explorer mission returned a wealth of information from its two orbiting platforms. Of interest here are the three scanning photometers aboard the high-altitude platform DE-1, which obtained hundreds of thousands of global images of Earth, beginning in September of 1981, while using broad- and narrow-band filters to isolate particular terrestrial emissions. The far-ultraviolet (FUV) emissions include the line emissions of OI (130.4 and 135.6 nm) and the band emissions of $\rm N\sb2$ LBH, the brightness of each yielding information on the composition of the upper atmosphere. The OI emissions are related to the column density of atomic oxygen in the upper-atmosphere as well as the abundance of thermospheric $\rm N\sb2,$ both of which are affected by geomagnetic processes. This thesis presents a model of the DE-1 response to the OI emissions during periods of low geomagnetic activity and uses this model for studies of thermospheric response to geomagnetic storms and substorms. Variations in brightness observed after geomagnetic events are most often seen as decreases corresponding to reduced thermospheric O column densities. The relation between compositional variations in the morning sector at middle latitudes and the orientation of the magnetic field embedded in the solar wind is investigated. The orientation, which strongly affects the circulation of the thermosphere at high latitudes where these variations originate, is shown to be a significant parameter. Variations in brightness within the southern polar cap are investigated in the first study of its kind, demonstrating 20-30% decreases in brightness with the onset of magnetic activity and revealing structure in composition over distances on the order of ${\sim}300$ km. Compositional disturbances are observed immediately after heating takes place, demonstrating for the first time that an FUV instrument can detect changes in thermospheric composition on time scales under one hour. During these events, mid-latitude composition often remains relatively unperturbed. The first survey of FUV images to include ground-based measurements of ionospheric properties demonstrates that decreases in OI brightness correspond to decreases in peak F2 electron densities, known to be related to the ratio of the densities of O and $\rm N\sb2.$