SI-II RESONANCE MULTIPLETS IN THE SUN

The formation of the Si II resonance multiplets near 1814, 1530, 1306 and 1262 angstrom in the Sun is investigated by means of non-LTE radiative transfer computations. Use has been made of recent calculations for electron-impact excitation rates and absorption oscillator strengths. The lines of the multiplet near 1814 angstrom are found to be effectively thin, but their depth of formation embraces regions of widely differing gas pressures, essentially invalidating the coronal approximation. Furthermore, the Si II (1807 angstrom) line is formed at deeper and cooler regions than the Si II (1808 angstrom) and Si II (1816 angstrom) lines. The lines of the other multiplets are found effectively thick, so that an accurate evaluation of their emerging flux requires a non-LTE radiative transfer treatment, ideally under partial redistribution approximation. The important diagnostic properties of these lines for the upper chromosphere and lower transition region are discussed. The intensities of Si II (1526.7 angstrom) and Si II (1533.4 angstrom) are compared with observational data from the High-Resolution Telescope and Spectrograph (HRTS), while, for the other lines, line ratios are compared with observations from HRTS and from the Naval Research Laboratory slit spectrograph on Skylab.