Determination of the formation temperature of Si IV in the solar transition region

Using spectra obtained with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer flown on the Solar and Heliospheric Observatory spacecraft, we deduce the temperature of formation of the Si IV ion in the solar transition region from the Si IV ultraviolet spectral line intensity ratio, 3p(2)P(3/2)-3d(2)D(3/2,5/2)/3s(2)S(1/2)-3p(2)P(1/2) and compare the result to the temperature predicted under the assumption of ionization equilibrium. The wavelengths are as follows: D-2(3/2,5/2), 1128.325, 1128.340 Angstrom; 2P(1/2), 1402.770 Angstrom. Ratios are derived for typical features of the quiet Sun, such as cell center and network, and are systematically higher than those predicted at the 6.3 x 10(4) K ionization equilibrium temperature of formation of Si IV. For most solar features the ratios imply a temperature of formation of about 8.5 x 10(4) K. The ratios for the faintest features imply a temperature of formation of up to 1.6 x 10(5) K. It is not clear, however, that all the discrepancies between the measured and theoretical ratios are due to a temperature effect. Accurate temperature measurements are important since a large discrepancy from ionization equilibrium has significant implications for the physics of the transition region, such as the possible presence of nonthermal electrons.