1 Introduction

Bright resonance lines due to strong electric dipole transitions in FeXII have been identified in solar spectra since the early days of UV rocket and space observations. The sensitivity to electron density of line ratios involving these resonance lines (Kastner $\&$ Mason 1978; Withbroe $\&$ Raymond 1984) has proved to be a powerful diagnostic technique for investigating the physical conditions in the solar corona. The FeXII ion is expected to attain peak abundance at $\log\,T_{\rm e} = 6.15$ in conditions of ionisation equilibrium (Arnauld $\&$Raymond 1992). The most recent Solar Heliospheric Observatory (SOHO) observations have provided a wealth of fascinating images and spectral data in the strongest FeXII optically allowed lines such as at 195 Å (EIT - Extreme ultraviolet Imaging Telescope) and at 364.5 Å (CDS - Coronal Diagnostic Spectrometer, Mason et al. 1997). Electric dipole transitions also play a key role in determining the relative populations of the excited levels of the ground configuration through collisional excitation and radiative cascade. This occurs because their excitation rates are considerably greater than the corresponding quantities relative to the forbidden transitions. The ratios of forbidden lines within the ground configuration of FeXII are important for diagnostic purposes (Feldman et al. 1983).

Past calculations of FeXII electric dipole atomic data include those of Flower (1977) (henceforward referred to as F77), Bromage et al. (1978); Fawcett (1986) and Tayal $\&$ Henry (1986), as far as structure and radiative data is concerned. Electron collisional data have been published by F77 and Tayal $\&$ Henry (1988) (henceforward referred to as TH88). In a previous paper in the IRON Project series (Binello et al. 1998, henceforward referred to as BMS) we presented atomic data for the forbidden transitions within the ground configuration of FeXII and discussed the limitations of a previous corresponding calculation by Tayal et al. (1987). In this paper we want to complement those results by presenting new electron scattering data for electric dipole fine-structure transitions between the ground 3s²3p³ and the first two excited 3s3p⁴, 3s²3p²3d configurations in FeXII. A schematic diagram of the relative position in energy of these three configurations is shown in Fig. 1. As the techniques employed in the atomic structure and electron scattering problems, and the computer codes used, have already been described in BMS, in the following sections we will discuss only the details related to the particular type of transitions considered here. In Sect. 2 an account of the electron scattering computation and related results will be given. A detailed comparison with previous results will be the subject of Sect. 3. Finally, discussion and conclusions will be presented in Sect. 4.

  

Figure 1: Energy distribution of the three lowest configurations in FeXII. Numbers in parenthesis indicate the number of levels in that configuration