Emission measures and electron densities for the solar transition region

I analyze high spectral resolution ultraviolet spectra (1200-2000 Angstrom) recorded by the Naval Research Laboratory slit spectrograph (S082-B) on the Skylab space station. The spectra were obtained from the quiet Sun, a polar coronal hole, and two active regions. One goal of this work is to determine electron densities and emission measures for transition region lines formed between about 3 x 10(4) K and 2.5 x 10(5) K, using the best available atomic data. The majority of these data were not available during the Skylab era. Another goal of this work is to compare results using two independent instrument calibrations that differ at the extreme short- and long-wavelength ends of the spectrograph's useful wavelength range. The overall shape of the emission measure distribution with temperature derived in this analysis is similar to that found from other data sets. However, as found by other researchers recently, I find significant discrepancies in emission measures obtained for ions that should be formed at the same temperature. Most of these discrepancies are independent of the calibration used and are also independent of the solar region and element abundances adopted. Apart from inaccuracies in atomic physics, some of the discrepancies may be real and may reflect the unresolved fine structure of the transition region. No indication of non-Maxwellian distributions was found from the lines analyzed. The temperatures of formation for transition region ions seem to be close to their predicted temperatures in ionization equilibrium, although the sample of temperature-sensitive diagnostics is very limited over the S082-B wavelength range. Derived electron pressures (product of electron density and temperature) in the different solar regions range from 9.0 x 10(13) K cm(-3) in a coronal hole up to 2.4 x 10(16) K cm(-3) in an active region.