1 Introduction

Fexiv is one of the most important diagnostic ions in the solar corona. It is abundant at a temperature of about 2 10⁶ K (Arnaud & Raymond 1992). Transitions within Fexiv give rise to spectral lines in the visible (green line, 5303Å) and extreme ultraviolet (EUV) wavelength ranges. The visible line which arises from the forbidden transition (3s²3p, ²P $_{1/2}^{\rm o}-^2$P $_{3/2}^{\rm o}$) was discussed in Storey et al. (1996) (hereafter IP XIV). In this paper we consider the transitions between the ground configuration (3s²3p) and excited configurations (3s3p², 3s²3d and 3s3p3d). The EUV lines can be used to determine electron density in the solar atmosphere, as with recent observations from the Solar and Heliospheric Observatory (SOHO) (Mason et al. 1997).

In IP XIV, a review was given of the early calculations carried out for Fexiv, while Mason (1994) (hereafter M 94) has critically compared the existing calculations. She found that the results presented by Dufton & Kingston (1991) (hereafter DK91) were limited in two respects - they only used a 3 configuration target and the averaged collision strengths were obtained using LS-coupling collision strengths below 10 Ryd. She recommended using the DK91 atomic data with caution, but suggested that new calculations should be carried out with a more extensive target and in intermediate coupling. Bhatia & Kastner (1993) (hereafter BK93) provide distorted wave calculations for Fexiv with an extensive target. An analysis of the solar EUV lines using these new data was published by Bhatia et al. (1994) (hereafter B94) and contained new identifications for several spectral lines. There remain some longstanding inconsistencies between the theoretical and observed intensity ratios as detailed by Young et al. (1998) (hereafter Y98) indicating that more accurate close coupling (CC) calculations are required.

This work is part of the international collaboration known as the Iron Project (Hummer et al. 1993) whose aim is to make systematic calculations of electron scattering cross-sections and rate coefficients for ions of astronomical interest, using the best available methods. The principal tool of the project is the atomic R-matrix computer code of Berrington et al. (1974, 1978) as extended for use in the Opacity Project (Berrington et al. 1987). These codes have recently been further extended (Hummer et al. 1993) so that collision strengths can be calculated at low energies, where some scattering channels are closed, including the effects of intermediate coupling in the target. Previous calculations have always neglected such effects.

In Sect. 2, we discuss the target used in our Fexiv model. We give details of the electron scattering calculations in Sect. 3 and make a critical comparison with previous work in Sect. 4. The calculated line intensities and level populations are given in Sect. 5 and Sect. 6. Our theoretical intensity ratios are compared with those from solar observations in Sect. 7.

   

Table 1: Energies of target terms in Rydberg

Term		Exp.$^\dagger$	Basis 1	Basis 2
3s23p	2P$^{\rm o}$	0.	0.	0.
3s3p2	4P	2.03888	2.018	2.024
	2D	2.62454	2.619	2.625
	2S	3.20880	3.268	3.282
	2P	3.47445	3.488	3.502
3s23d	2D	4.20862	4.292	4.287
3p3	2D$^{\rm o}$	5.15891	5.154	5.173
	4S$^{\rm o}$	5.25285	5.259	5.281
	2P$^{\rm o}$	5.75764	5.744	5.763
3s3p3d	4F$^{\rm o}$	5.82870*	5.845	5.860
	4P$^{\rm o}$	6.23865	6.273	6.289
	4D$^{\rm o}$	6.26927	6.302	6.315
	2D$^{\rm o}$	6.42468	6.458	6.473
	2F$^{\rm o}$	6.75142	6.829	6.844
	2P$^{\rm o}$	7.26477	7.375	7.391
	2F$^{\rm o}$	7.34768	7.475	7.469
	2P$^{\rm o}$	7.56079	7.681	7.696
	2D$^{\rm o}$	7.56742	7.681	7.697
$\dagger$ Churilov & Levashov (1993), Redfors & Litzén (1989).
* Not all levels known experimentally.

   

Table 2: Weighted oscillator strengths, gf

Transition	Basis 1	Basis 2	Basis 3
3s23p2P$^{\rm o}$ - 3s3p22D	0.378	0.381	0.375
- 3s3p22P	0.320	0.325	0.319
- 3s3p22S	2.508	2.477	2.457
- 3s23d2D	2.942	2.877	2.833