Physical and chemical processes of the inner coma observed in mid-ultraviolet cometary spectra

Accurate measurement of cometary H{dollar}sb2{dollar}O is essential for comparing the compositions and understanding the properties of cometary nuclei. Water production rates {dollar}Qsb{lcub}rm Hsb2O{rcub}{dollar} are easily derived from observations of OH solar resonance fluorescence with the International Ultraviolet Explorer (IUE) satellite. Variable solar activity and several inner coma processes affect the estimates of {dollar}Qsb{lcub}rm Hsb2O{rcub}{dollar} using IUE.; Water photodissociation by solar Lyman-{dollar}alpha{dollar} radiation creates OH(A{dollar}sp2Sigmasp+){dollar} radicals, which quickly decay to OH(X{dollar}sp2Pi){dollar} through spontaneous emission. This "prompt" emission can provide a direct measure of {dollar}Qsb{lcub}rm Hsb2O{rcub},{dollar} if observed at sufficiently high spatial resolution. Spectral observations of comets IRAS-Araki-Alcock (1983 VII) and Tuttle (1980 XIII), differing only in effective spatial resolution, show evidence for OH prompt emission in IRAS-Araki-Alcock.; The scale length of the short-lived molecule S{dollar}sb2{dollar} is re-evaluated from IUE spectra of Comet IRAS-Araki-Alcock. Spectra of other comets are analyzed to determine S{dollar}sb2{dollar} abundances relative to H{dollar}sb2{dollar}O and reveal no other detections of S{dollar}sb2,{dollar} but upper limits comparable to values observed in IRAS-Araki-Alcock. The sole exception is significant: the upper limit on S{dollar}sb2/{dollar}H{dollar}sb2{dollar}O in Comet Austin (1989c{dollar}sb1){dollar} indicates true variation in the relative S{dollar}sb2{dollar} abundance between comets.; Solar resonance fluorescence can create a population inversion of the {dollar}Lambda{dollar}-states of the OH molecule, but ions collisionally quench the inversion to affect both the UV fluorescence and the 18 cm maser emissions. Previous models of the quenching of OH radio emission do not account for both kinds of observations simultaneously, and do not include solar variability or a Swings effect. A quenching model based upon Giotto observations of Comet P/Halley appropriate for IUE observations is developed and compared to results from high resolution IUE spectra.; The solar UV flux determines the photodestruction rates of H{dollar}sb2{dollar}O and OH and affects their spatial density distributions. Lifetimes appropriate to varying levels of solar activity are calculated from cross section measurements and a solar UV flux model. Spatial mapping by IUE of the OH comae of several comets observed at different epochs of solar activity are matched by modeled density distributions. The evolution of {dollar}Qsb{lcub}rm Hsb2O{rcub}{dollar} with respect to heliocentric distance of comets Bradfield (1979X) and Austin (1989c{dollar}sb1){dollar} is determined.