Density dependence of forbidden transitions within 3s 23p 53d of the coronal ion Fe IX

The relative populations of metastable levels within the configuration 3s²3p⁵3d of the ion Fe ix have been calculated as a function of electron density N_e. The results are used to derive emissivities of the most intense lines originating from these levels. Comparison with observations confirms the proposed identifications. The density dependence of some line ratios may be used to determine N_e.