The Solar Heliospheric Observatory (SOHO), an ESA/NASA space
mission for the study of the solar upper chromosphere, transition
region, inner and extended corona, was successfully launched in
December, 1995. The analysis of SOHO
spectra has generated a great deal of interest in
the atomic physics related to coronal ions.
In particular the IRON Project, an international
collaboration aimed at providing the most accurate set of atomic data to
date for all the iron ions (Hummer et al. 1993), is an
ideal framework for the calculation of new atomic data for
astrophysical applications.
A complete list of IRON Project publications
and papers in press can be found at http://www.am.qub.ac.uk.
The FeXII ion gives rise to spectral lines observed by several
different spectrometers on board SOHO: at 195 Å (EIT
- Extreme ultraviolet Imaging Telescope), at 1242 Å and
1349 Å (UVCS - Ultraviolet Coronagraph and
Spectrometer and SUMER - Solar Ultraviolet Measurement of
Emitted Radiation) and various lines in the CDS (
Coronal Diagnostic Spectrometer) wavelength range 
.
The first calculations for FeXII were carried out
by Flower (1977), who computed radiative and collisional data
for transitions between the ground 
and the first two
excited 
and 
configurations. The
distorted wave technique was employed in that work for the calculation of
the scattering data. Bromage et al. (1978)
and Fawcett (1986) provided improved excitation energies and
oscillator strengths by incorporating strong CI
(configuration interaction) and relativistic
effects in the structure problem. Subsequently new and more extensive
calculations were performed of energy levels and oscillator strengths
for the same three lowest configurations (Tayal & Henry 1986)
and of collision strengths, effective collision strengths and electron
impact excitation rates for fine-structure transitions either within
the ground configuration (Tayal et al.
1987) or between this and the first excited
configuration (Tayal & Henry 1988). The use of these atomic
data together with FeXII lines identifications (Svensson
1971; Jordan 1971) has made possible a considerable
amount of diagnostic work on physical parameters of the solar plasma.
Electron temperature, density
and iron abundance have been derived by analysing various spectra from
early solar eclipse observations (Gabriel & Jordan 1975) to
recent Solar EUV Rocket Telescope and Spectrograph SERTS (Thomas &
Neupert 1994). The atomic data from Tayal & Henry
(1986, 1988) and Tayal et al. (1987) are the most
widely used in these astrophysical applications. However Mason
(1994), in her assessment of theoretical electron excitation data for
various iron ions, pointed out some unusual features in their FeXII
collisional data, which have not yet been explained. This has
prompted the new set of atomic calculations which are described in
this paper.
In Sect. 2 (click here) we discuss the atomic structure problem and the
code used for the computation of the radiative data there
presented. Section 3 (click here) will be devoted to the analysis of the 
scattering problem and to the resulting collisional data
relating to the fine-structure forbidden transitions within the ground
configuration. Corresponding results for allowed and
intercombination transitions from the ground to the excited
and configurations will be presented in a subsequent
paper. Discussion and conclusions will be given in Sect. 4 (click here).