The vast majority of the UVCS observations will pertain to a small number of spectral lines (Ly-, O VI, Mg X, Si XII and [Fe XII]). To provide a consistent basis for the comparison of different analyses, it is useful to have a ''standard" set of atomic rates for these lines. We list suggested rates here, along with estimates of their accuracy.
We need the concentrations of the relevant ions as a function of temperature. At large distances from the sun, the assumption of ionization equilibrium fails, and we need ionization and recombination rates. For hydrogen, we recommend the ionization rate of Scholz and Waters (1991) and the radiative recombination rates of Burgess and Summers (1976). The former should be good to 10%, and the latter to 3%. For O, Mg and Si, we recommend the ionization balance of Arnaud and Rothenflug (1985). The ionization rates should be good to around 10% (based on comparison of laboratory and theoretical cross sections), and the radiative and dielectronic recombination rates relevant to O VI, Mg X and Si XII should be accurate to 30% (based on comparison among theoretical dielectronic recombination rate calculations). For [Fe XII], the ionization rates of Arnaud and Raymond (1992) should be accurate to 15%, but the dielectronic recombination rates are only reliable at the 30% level.
For resonant scattering, we require the oscillator strengths of the observed transitions. The NBS values (Weise, Smith and Glennon 1966; Weise, Smith and Miles 1969) are accurate to about 1% for Ly- and better than 10% for the Li-like ion resonance lines. The oscillator strengths given by Zhang et al. (1990) for the Li-like ions agree with these values to about 6% for O VI and 1% for Mg X and Si XII. Note that the resonant scattering cross section of the [Fe XII] line is negligibly small, but that resonant scattering of Fe XII EUV lines might pump the 1240 line (Withbroe and Raymond 1984). The Ly scattering cross section is given in Section 3.2.1.
We recommend collisional excitation rates from Scholz and Waters (1991) for Ly- (see also Callaway and Unnakrishnan 1994). We include excitations to both the 2s and 2p levels, as 2s 2p collisions are more rapid than 2-photon decay of the 2s level at densities above about K. Cochrane and McWhirter (1982) present excitation rates for the Li-like ions based on Close-Coupling calculations which agree to about 20% with laboratory measurements. Burgess, Mason and Tully (1989) present CC calculations for Mg X. For Si XII, we adopt the Zhang, Sampson and Fontes (1990) collision strengths. For H I and the Li-like ions, there may be contributions from cascades following excitation to higher levels, but they should only increase the excitation by a few percent. For [Fe XII], the Close Coupling calculations of Keenan, Tayal and Henry (1990) are the best available. The uncertainty is difficult to estimate due to the complexity of the ion and the complicated cascades which feed the 1242 line, but we estimate 30%. Much of this results from uncertainties in 3p-3d excitations which can feed the levels. This line is somewhat density sensitive, however, and the level populations should be computed for the case at hand. Therefore, one needs the full set of collision strengths and A values.
Table 1 presents absorption oscillator strengths and disk emission line intensities from the Quiet Sun spectrum of Vernazza and Reeves (1978). Estimated line widths are based on Gouttebroze et al. (1978) for Ly (which in fact has a self-reversed profile, so the 1/e width is given only for rough estimates) and Noci et al. (1987) for O VI. For Mg X and Si XII we assume velocity widths measured by Hassler et al. (1989) for Mg X. Abundances are based on Meyer (1993). Note that these coronal abundance estimates assume that the high First Ionization Potential elements have nearly photospheric values and that low FIP elements are enhanced relative to hydrogen, but this is far from certain. Tables 2 and 3 present the recommended ion fractions and excitation rate coefficients at several temperatures. Excitation rates pertain to the total excitation of the doublets of O VI, Mg X and Si XII. According to the calculations of Zhang et al (1990), the collision strengths of the individual lines of the Li-like doublets follow the expected 2:1 ratio to within 1.5%.
Note that some of these values differ somewhat from those used in the estimates of Chapter 3.
Ion Concentrations ()
Table 3. Excitation Rate Coefficients* ()
* Excitation rates for the totals for the O VI, Mg X, and Si XII doublets.