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Astron. Astrophys. 323, 243-249 (1997)


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The Ar/Ca relative abundance in solar coronal plasma

P.R. Young 1, H.E. Mason 1, F.P. Keenan 2 and K.G. Widing 3

1 Department of Applied Mathematics and Theoretical Physics, Silver Street, Cambridge CB3 9EW, UK
2 Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, UK
3 E.O. Hulburt Center for Space Research, Naval Research Laboratory, Code 7674L, Washington DC 20375, USA

Received 8 August 1996 / Accepted 29 October 1996

Abstract

The relative abundances of elements with low and high first ionisation potentials (FIP) is a subject of much recent debate. In situ measurements of the solar wind reveal a clear pattern of low FIP enhancement that has been followed up with various spectroscopic measurements of the solar corona.

Argon is unique amongst the more abundant high FIP elements in retaining several of its electrons at the high temperatures seen in flares. This allows emission lines of different ions to be compared with more confidence than for, say, hydrogen-like high FIP ions such as O VIII and Ne X.

In this paper we look at emission lines of the boron-like ion Ar XIV ; in particular, the optical line at 4412 Å, seen in eclipse observations, is compared to the Ca XIII 4086 Å and Ca XV 5445 Å & 5694 Å lines to yield an Ar/Ca abundance of [FORMULA]. In the extreme ultra-violet (EUV), the Ar XIV lines at 187.94 Å and 194.41 Å can be compared with Ca XIV 193.87 Å-flare data from Skylab giving values of [FORMULA] and [FORMULA]. Analysis of previous work indicates a photospheric Ar/Ca abundance of [FORMULA], hence supporting the conclusion that elements with high FIP have lower coronal abundances.

Key words: Sun: abundances – Sun: corona – Sun: UV radiation

Send offprint requests to: P.R. Young

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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