The electron pressure in the solar lower transition region determined from O v and Si III density-sensitive line ratios

We determine the electron density at the temperatures of formation of O+4 and Si+2 ions, which are about 2.5 x 10(5) and 3.2 x 10(4) K in ionization equilibrium, respectively. These temperatures occur in the lower transition region of the Sun's atmosphere and allow a test of the often invoked assumption of constant pressure in quiet-Sun models. The O+4 density is determined from a density-sensitive spectroscopic O v line ratio involving 2s2p P-3-2p(2) P-3 transitions that fall near 760 Angstrom. The Si+2 density is determined from a density-sensitive Si III line ratio within the 3s3p P-3-3p(2) P-3 multiplet near 1300 Angstrom. There are few available line ratio techniques for determining the density and hence electron pressure in the quiet-Sun and coronal hole transition regions using lines emitted by the same ion, and determining these quantities is the principal motivation for this work. The spectra used in our analysis were obtained from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) experiment on the Solar and Heliospheric Observatory (SOHO). We determine the electron density and pressure in typical quiet-Sun/coronal hole regions, and densities in active region brightenings and in an explosive event. Our O v and Si III results indicate that constant pressure is valid or nearly valid in quiet-Sun lower transition regions, although there are complications arising from the weakness of a key Si III line in the quiet-Sun disk spectra. We also discuss our results in light of other density measurements and theories regarding the structure and heating of the transition region.