Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...287..972l&link_type=abstract
Astronomy and Astrophysics 287, 972-981 (1994)
Astronomy and Astrophysics
Astrophysics
7
Sun: Chromosphere, Sun: Transition Region, Sun: Uv Radiation, Line: Formation
Scientific paper
The formation of the SiII resonance multiplets near 1814, 1530, 1306 and 1262 A 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 A 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 SiII(1807A) line is formed at deeper and cooler regions than the SiII(1808A) and SiII(1816A) 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 re-distribution approximation. The important diagnostic properties of these lines for the upper chromosphere and lower transition region are discussed. The intensities of SiII(1526.7A) and SiII(1533.4A) 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.
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