The Ar / Ca relative abundance in solar coronal plasma

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 4412Å, seen in eclipse observations, is compared to the Ca xiii 4086Å and Ca xv 5445Å & 5694Å lines to yield an Ar/Ca abundance of 0.85 ± 0.20. In the extreme ultraviolet (EUV), the Ar xiv lines at 187.94Å and 194.41Å can be compared with Ca xiv 193.87Å—flare data from Skylab giving values of 1.10 ± 0.25 and 0.55 ± 0.21. Analysis of previous work indicates a pho-tospheric Ar/Ca abundance of 1.31 ± 0.30, hence supporting the conclusion that elements with high FIP have lower coronal abundances.