1. Introduction

Many astrophysical emission line spectra are produced by the collisional excitation of a variety of ionization stages of elements commonly found in cosmic plasmas. Their interpretation requires an accurate knowledge of the atomic structure of these ions and their excitation and emission rates (Mason & Monsignori Fossi 1994). In the past a number of spectral codes have been constructed to aid in the interpretation of astrophysical spectra. Examples include those of Landini & Monsignori Fossi (1970, 1990), Tucker & Koren (1971), Mewe (1972), Mewe & Gronenschild (1981), Mewe et al. (1985), Kato (1976), Raymond & Smith (1977), Stern et al. (1978), and Gaetz & Salpeter (1983). A comparison and critique of different plasma emission codes for X-ray and UV spectra is published by Mason (1996a). Many of these codes were assembled at a time when the necessary atomic data such collision strengths were often lacking. Often the formula (Van Regemorter 1962), or variations, were used to estimate electron excitation rates. Because of the increasing power of computer technology, reliable calculations of many of these excitation rates are continually becoming available.

Recent papers have attempted to provide more accurate representations for the atomic parameters, in particular for the iron ions (cf. Brickhouse et al. 1995; Monsignori Fossi & Landini 1994a; Mewe et al. 1995).

The basic goal for the CHIANTI database is to construct a database that includes the best available calculations of atomic parameters for analyzing astrophysical emission line spectra. In implementing the database, we also tried to achieve several other goals: 1. the database could be readily updated, 2. the database would be easy to distribute, 3. it would be transparent to the end user, 4. accuracy would be maintained by visually examining as much of the input data as possible, 5. it would use a data and programming structure that would facilitate the development of programs by end users.

The basic unit of the data base is the individual ion. For each ion there is a directory that contains a file specifying its energy levels, a file providing wavelengths and radiative rates, and a file providing fits to the collision strengths which give the electron excitation rates. Each file contains information on the source of the data and other relevant comments. The energy level information is largely taken from the NIST database of observed energy levels (Martin et al. 1995), updated by more recent observed values and supplemented by our best theoretical estimates where energy levels are not known. Radiative data are obtained from available publications and supplemented where necessary by new calculations. Collision strengths ('s) and upsilons ('s - collision strengths averaged over a Maxwellian velocity distribution) have all been visually inspected and scaled according to the formulation of Burgess & Tully (1992). Elemental abundances can be freely specified and the ionization equilibrium is determined from steady state calculations, for which the Arnaud & Rothenflug (1985) calculation is supplied.

At the present time, the database is capable of reproducing the optically thin emission line spectrum at wavelengths greater than 50 Å for electron densities less than about 10¹⁵ cm^-3. The ions necessary to calculate the spectrum below 50 Å will be developed in the near future. There is no long wavelength limit to the database but, in a practical sense, it becomes less comprehensive at longer wavelengths because neutrals are not included. The scaling of the upsilons should be accurately reproducible for nearly any range of temperature. Astrophysically abundant elements from hydrogen through nickel are included. Tables 1 (click here) and 2 (click here) show which ions are currently included in the CHIANTI database.

   

   

A package of programs written in Interactive Data Language (IDL) are also supplied. These allow the calculation of level populations, synthetic spectra, and density and temperature sensitive line ratios. Because of the nature of the database, further capabilities and interfaces to other programming languages, such as Fortran or C, can be readily implemented in the future. The package is freely available for downloading over the internet. The CHIANTI database and accompanying IDL routines have been incorporated into the scientific analysis software for the Coronal Diagnostic Spectrometer onboard the Solar and Heliospheric Observatory (SOHO) by C.D. Pike and G. Del Zanna.