The knowledge of the composition of the corona is an important key to our understanding of the physical processes in the solar atmosphere. The coronal elemental abundances can help to investigate the mechanisms that transport, accelerate and heat the solar coronal plasma and wind. The composition of the corona must also be known in order to interpret observations of coronal emission spectra. Elements' relative abundances must be known also to determine temperatures from the intensity ratios of the coronal lines of different elements, while the absolute abundances must be known to determine emission measure and radiation losses. Obviously, elemental abundances are among the most fundamental sets of parameters in the solar and stellar atmospheres. Extensive research regarding the abundances of elements in the solar atmosphere has been reviewed by Feldman (1992), and an update on the most recent results can be found in the recent review by Feldman & Laming (2000).
The status of coronal abundances relative to hydrogen is not entirely settled. There is a significant body of evidence that abundances are correlated with the first ionization potential, giving rise to the so-called FIP effect. This effect consists of systematic differences between photospheric and coronal element abundances. The latter show enhancements of a factor around 4 in the so-called low-FIP elements (FIP 10 eV), while abundances for the high-FIP elements (FIP 10 eV) remain constant between the photosphere and the corona. So far, no theoretical model has been able to satisfactorily explain this effect.
Dwivedi et al. (1997, 1999a) carried out an observing sequence based on a theoretical study by Dwivedi & Mohan (1995), with intercombination and forbidden and lines, which are formed at essentially the same temperature (4x105 K), according to Arnaud & Rothenflug (1985). The FIPs of Ne and Mg are 21.6 and 7.6 eV, respectively: they form a low-FIP/high-FIP pair. This study provided new observational facts in transition region emission lines (Dwivedi et al. 1999a,b). In the present paper, we extend this investigation taking account of other low-FIP/high-FIP pairs such as K/Ar, Si/Ar and S/Ar present in the spectra. The use of the recently identified line at 994.58 Å (Mohan et al. 2000) allows us to investigate the amount of FIP bias also for the very low-FIP element K (FIP = 4.3 eV), so that it is possible to investigate whether the FIP bias of the elements with FIP 10 eV is constant or depends on the FIP value of each element, as suggested by Feldman & Laming 2000.
The FIP bias of the S, Si and K elements is also investigated as a function of the height above the photosphere. Also, the Mg/Ne ratio is re-examined and correlated with structures in the field of view.
We briefly describe the observations in Sect. 2 and plasma diagnostics in Sect. 3. Density and temperature measurements are discussed in Sect. 4. We present the relative element abundances in Sect. 5 and discuss them in Sect. 6.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001