Line ratios (in phot cm⁻² s⁻¹) for key diagnostic lines of Ar XII as a function of density at a fixed temperature of 2.5 × 10⁶ K. The solid curve is calculated with the present R-matrix data, while the dashed curve used the distorted wave data of Eissner et al. (2005), as incorporated in CHIANTI v9.0.1. The upper panel is the line ratio between the forbidden transition at 1054.7 Å and the resonance transition at 224.2 Å, while the lower panel is the line ratio between two forbidden transitions (Table 4). The measurement (square) in the lower panel is from a post-flare solar SUMER observation (Landi et al. 2003).