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Astron. Astrophys. 357, 757-762 (2000)


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Detection and limb brightening of the H I n=20-19 Rydberg line in the submillimetre spectrum of the Sun

T.A. Clark 1, D.A. Naylor 2 and G.R. Davis 3

1 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada (taclark@ucalgary.ca)
2 Department of Physics, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada (naylor@uleth.ca)
3 Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada (g.r.davis@usask.ca)

Received 2 December 1999 / Accepted 9 March 2000

Abstract

Submillimetre spectra taken near to the solar limb with a polarizing interferometer on the James Clerk Maxwell Telescope have been compared with disk-centre spectra to reveal a limb-brightened feature whose peak intensity occurs at the predicted frequency of the n=20-19 Rydberg transition in HI at 29.622 cm-1. A shoulder on this peak, at 29.65 cm-1, has been tentatively assigned to the equivalent transition in MgI . The HI line exhibits limb brightening of up to 9% of the disk-centre continuum intensity. The intensity of the MgI line is about half of the HI line intensity across the observed region near to the limb. Widths of the HI line are between 0.020 and 0.027 cm-1, smaller than predicted by current models of this line in the Sun's spectrum. These measurements represent the highest-n Rydberg lines detected to date in the solar spectrum. The measured line intensity, line width, limb brightening and the relative heights of the contributions from HI and MgI place constraints upon further modelling of the solar atmosphere.

Key words: line: identification – Sun: general – Sun: atmosphere – Sun: infrared

Send offprint requests to: T.A. Clark

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000

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