Plasma diagnostic techniques in the ultraviolet: the C III density-sensitive lines in the sun

Spectra and spectroheliograms of the C III transitions at 977 and 1176 A are obtained with the Harvard extreme-ultraviolet spectrometer on Skylab. Analysis of the intensities of these lines, and of their density-sensitive ratio, indicates a wide range of temperature gradients and electron densities in the transition region of various solar features. From values of the observed ratio, we suggest necessary revisions to the excitation rates, and propose a relationship between the ratio and density. The significantly higher ratio found in active regions indicates a density increase of about a factor 2 relative to the network. In the quiet Sun, there is no significant difference in density between network and cell interiors, but the uncertainty is as large as a factor 3. The very central 10 percent of the areas of cell interiors shows a significantly higher density than the mean value for cell interiors. We find that the temperature gradient in the region of C III formation is lowest over the network, with higher values both over active regions and cell interiors. Significant changes are found in the intensity of the lines and in their ratio, suggesting that actual variations in the ambient density and temperature profile occur on a time scale of 4 minutes or less