1 Introduction

Extreme ultraviolet (XUV) solar emission lines corresponding to n=2, $\Delta n=0$ transitions of highly ionized calcium, have been recorded in the past by space-born spectrometers (OSO-5, OSO-6, Skylab NRL/ATM-S082A) and sounding rocket-born instruments (Aerobee 150, 200 rockets), and have been used for wavelength identifications, temperature and density estimates in solar flares and for development of solar atmosphere models (Lawson & Peacock [1984]; Feldman et al. [1988]; Mason & Monsignori Fossi [1994] and references therein). The XUV lines emitted by L-shell calcium ions constitute a unique diagnostic tool for studying plasma conditions in the solar flares. This is because many of the spectral line ratios due to 2s²2p^k-2s2p^k+1transitions are density and temperature sensitive, and because of the high formation temperature and density limits of these ions (logarithmic temperatures between 6.25 and 6.75, electron density range $10^{9} \div 10^{13}$ cm^-3). There are few solar abundant elements which provide useful flare diagnostics based on the XUV line ratios in this range of densities and temperatures. Most observed Fe IX - Fe XV XUV line intensity ratios are sensitive to electron density $\le 10^{11}$ cm^-3 (Dere et al. [1979]; Brickhouse et al. [1995]), most of the Fe XVI - Fe XXIV XUV line ratios are density-sensitive at $n_{\rm e} \ge 10^{13}$ cm^-3 (Feldman et al. [1992]), and their formation temperatures are high. The ions of the third period elements (Si, Mg, S) appear to be less abundant in the flares, and their observed lines, useful for $T_{\rm e}$ and $n_{\rm e}$ diagnostics, are in the longer wavelength region (above 300 Å), inaccessible to a typical grazing incidence XUV spectrometer. Previously, the Skylab NRL/ATM-S082A solar flare observations have been the main source of measured calcium line intensities. Recently flown solar missions (e.g. SERTS, SOHO) have produced a wealth of new spectroscopic data. Its interpretation relies on the accuracy of the available atomic data. High quality spectra of the solar abundant elements, obtained from a laboratory plasma can provide a test for the validity of atomic data and collisional-radiative (CR) models. This is possible because the spectra are less contaminated by emission lines from other elements and local plasma parameters (such as electron density and electron temperature) are independently measured (Finkenthal et al. [1987]).

This work presents a re-evaluation of the laboratory study of beryllium- through oxygen-like calcium spectra performed by our group several years ago (Lippmann et al. [1987]). The spectra, recorded at the TEXT tokamak (University of Texas, Austin), have been re-analyzed, previously unpublished lines have been added and the analysis was extended to lithium- and fluorine-like charge states. We also analyze new calcium spectra, recorded at the FTU tokamak (Frascati, Italy). In the previous work (Lippmann et al. [1987]; Huang et al. [1987]), the difficulty in interpreting the experimental results was mainly due to the accuracy of the available atomic data. Atomic transition rates were taken from the literature, or extrapolated from other ions of the same isoelectronic sequence. The complexity of the model, i.e. the types of the processes considered and the number of levels included, was limited. In several cases these factors precluded full analysis of the experimental data. In this work, a CR model, based on ab initio calculated transition rates, is used.

Figure 1: Time histories of electron temperature (top) and density (bottom) spatial profiles of the FTU tokamak plasma

For Be I-, B I- and C I-like calcium ions, a detailed model is constructed in order to study the role of collisional and radiative processes from excited states in application to electron temperature and density diagnostic potential of these ions. We also compare our calculations and measurements with the predictions based on the atomic data from the astrophysical database CHIANTI (v. 2.0, Dere et al. [1997]; Landi et al. [1999]). CHIANTI contains the best available atomic data for the ions of astrophysical interest and has been used in SERTS and SOHO data analysis (e.g. Young et al. [1978]; Mason et al. [1997]; Landi & Landini [1997]). Throughout the paper, the units of eV are used for the temperature $k\,T_{\rm e}$, unless noted otherwise, and line intensity ratios are expressed in photon units.

Figure 2: Schematic view of temperature ($T_{\rm e}$), density ($n_{\rm e}$) and impurity density (nq) profiles (upper part of the drawing). The procedure of obtaining $T_{\rm e}$ and $n_{\rm e}$ at the maximum abundance (MA) location of each ion is shown. Lower part: bold dashed line indicates different lines of sight (LOS) of the spectrometer (1) - central, (2) - MA, (3) - plasma periphery

Figure 3: Line-integrated, time-averaged XUV spectrum of highly ionized calcium in the TEXT tokamak. The spectrum was obtained using six reproduceable discharges