Comparison of Viking Ultraviolet Images with Low - Particle Precipitation

An investigation of global auroral images at far ultraviolet wavelengths from the Viking Ultraviolet Imager was carried out to assess their value for remote sensing of auroral electron precipitation. A large number of Viking orbits were searched for images obtained simultaneously with measurements of electron precipitation by low-altitude satellites within the UVI field of view. Sixteen such orbits were identified during April and May of 1986 when both cameras of the UVI were functioning, providing a total of 45 images. The images were corrected for various instrumental and nonauroral background contributions to the camera signal. The pixels in the images corresponding to the auroras excited by the measured low-altitude electron precipitation were determined. The auroral DN in these pixels were converted to equivalent signals for vertical viewing from Viking. The electron data were averaged over the spatial scale size of the image pixels. The linearity of the camera responses was investigated as a function of the average electron energy flux. The mean responses of the two cameras per unit electron energy flux were 11.1 +/- 0.9 and 15.2 +/- 3.1 DN per erg/cm^2 s, respectively, for Cameras 0 and 1, in the most common gain state. Current models of electron transport, auroral excitation, and radiative transport were used to estimate the camera responses for comparison with selected measurements. Good agreement was obtained with observations from Camera 0, but not with those from Camera 1. Ratios of auroral signal to electron energy flux, termed "effective sensitivities" and expressed in units of camera DN per erg/cm ^2 s, were plotted versus the mean energy of the electron flux to evaluate their dependence on electron energy. The effective sensitivities of both cameras showed large variations about the mean values quoted above, with no clear dependence on electron energy. The Camera 0 sensitivities tended to cluster between 6 and 20 DN per erg/cm ^2 cdot s, although several much larger values were measured during three orbits on May 2 and 3, 1986. The sensitivities from Camera 1 were mostly between 8 and 17 DN per erg/cm^2 cdot s. Simultaneous images from the two cameras showed little relative difference in signal strength in auroras. It was concluded that the ratio of signals from the two cameras is not very sensitive to the mean electron energy, and is thus not useful for remote sensing of auroral electron energies. Either camera could be used to estimate the distribution of total electron energy flux, within the uncertainties noted above. The Camera 0 images resolve the auroral distribution more accurately. This study is the first to make quantitative comparisons of large numbers of far ultraviolet auroral images with simultaneous measurements of the exciting particle flux. It provides the most extensive calibration of the Viking UVI cameras to date, and confirms the preflight calibration of the longer-lived camera (Camera 0) to within about 30%. It illustrates the possibilities and hazards of using images from the UVI to infer the energy flux precipitating into the auroral oval. The UVI obtained global snapshots of the auroral distribution at one-minute time intervals, and the results of this thesis allow more detailed characterization of the global energy input than ever before possible.