1 Introduction

Analysis of the soft X-ray and EUV spectra from non-solar astronomical sources in the past has generally made use of the several spectral synthesis codes that are available in the literature. The MEKAL code, used via the SPEX spectral software package ([24, Mewe et al. 1995]; [16, Kaastra et al. 1996]), has been one of the most frequently used of these, and contains a considerable amount of data relating to atomic transitions that give rise to both line and continuous spectra. The original MEKA code ([22, Mewe et al. 1985]; [15, Kaastra & Mewe 1993]) was modified to allow for the fact that comparison of theoretical spectra with those observed by the ASCA spacecraft of the central regions of galaxy clusters ([11, Fabian et al. 1994]) showed a discrepancy with the intensity ratio of the two groups of spectral lines due to n=3-2 and n=4-2 (n= principal quantum number) transitions in various ions of Fe (specifically Fe XVII-Fe XXIV). The addition of more than 2000 of the strongest lines from the Fe L complex in the 7-19 Å range using data calculated with HULLAC (the Hebrew University/Lawrence Livermore Atomic Code) (e.g. [17, Klapisch et al. 1977]) has enabled this discrepancy to be largely removed ([18, Liedahl et al. 1994, 1995]). The latest version of the code was implemented as "MEKAL'' (Mewe-Kaastra-Liedahl) in XSPEC (version 8.3) (cf. [24, Mewe et al. 1995]).

Previously, most spectra of non-solar sources have been of low resolution (e.g., ASCA spectra have an energy resolution $\Delta E/E \approx 0.1$ at 1 keV), but with the imminent launch of the NASA spacecraft Advanced X-ray Astrophysics Facility or AXAF (now baptized as Chandra X-ray Observatory), and that of the ESA spacecraft X-ray Multi-Mirror Mission (XMM), we shall soon have available soft X-ray spectra with very high spectral resolution: the Low-Energy Transmission Grating Spectrometer (LETGS) (4-155 Å) on Chandra, for example, will have a wavelength resolution of $\sim 50$ mÅ (e.g. [23, Mewe et al. 1991]). Also the Reflection Grating Spectrometer (RGS) (5-35 Å) on XMM has a spectral resolution of about 50 mÅ. For examples of simulated Chandra and XMM spectra see, e.g., [21, Mewe (1991)].

This makes it much more important than in the past that accurate information about line excitation rates and wavelengths is contained in the spectral code.

Very high resolution soft X-ray spectra have been obtained in the past using crystal spectrometers dedicated to solar active regions and flares. Among these are spectra from the Flat Crystal Spectrometer (FCS), part of the X-ray Polychromator which was on board the Solar Maximum Mission spacecraft (SMM), which operated between 1980 and 1989. This paper is concerned with a benchmark study of the MEKAL code using solar flare spectra from the FCS instrument in the wavelength range 5-20 Å and a spectral resolution of between 1 and 22 mÅ. This wavelength range covers, for example, a large part of that of the RGS on XMM. In the following sections, we describe the FCS instrument and the data obtained from it, then discuss the fitting of two solar flare spectra chosen for detailed analysis to the MEKAL model. The adjustments to the atomic data included in MEKAL are outlined, as are the identifications of lines. Details are given in Appendices I and II to this paper. For the identifications, we used not only SMM FCS data but data from high-resolution spectra obtained from the Lawrence Livermore Electron Beam Ion Trap (EBIT) and the Princeton Large Torus (PLT) tokamak (also briefly described). We regard this analysis as a considerable advance in obtaining an improved spectral code, and we expect the code to contain much more precise data than before to enable satisfactory comparisons with Chandra and XMM spectra when they become available.