The primary diagnostic tool for UVCS will be the resonantly scattered Ly- profile. Its width is a direct measure of the velocity distribution of neutral hydrogen atoms along the line of sight. Except at very great heights, this will be the same as the proton velocity distribution due to rapid charge transfer. The Ly- intensity relative to the disk intensity is primarily sensitive to the velocity of the observed gas away from the sun, which shifts the absorption profile of coronal away from the chromospheric Ly- emission profile (doppler dimming). Knowledge of the electron density from the UVCS White Light Channel, LASCO, or other white light coronagraph is needed for interpretation of the Ly- intensity, as well as a knowledge of the hydrogen neutral fraction.
The next most important diagnostic will be the intensities of O VI 1032 and 1037 and their ratio. Close to the sun these lines are collisionally excited, their ratio is 2:1, and their intensity is proportional to . At greater heights, resonant scattering of transition region emission begins to dominate, and the line ratio and intensities change. Because 1037 is pumped by a nearby C II line, the line ratio is velocity-sensitive up to about 250 km/s. The intensity relative to the disk intensity drops as the bulk velocity increases. Similar methods can be used with the Mg X and Si XII doublets.
Third, UVCS will measure the profile of electron-scattered Ly-. Its width determines the electon temperature, .
Other spectral lines will also be important, particularly [Fe XII] 1242. This will be combined with the other strong lines and occasionally with fainter lines such as Ly-, He II 1085, and O V 630 to determine the electron temperature and elemental abundances.