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Signatures of the non-Maxwellian $κ$-distributions in optically thin line spectra \subtitle{I. Theory and synthetic Fe IX--XIII spectra}

We investigate the possibility of diagnosing the degree of departure from the Maxwellian distribution using single-ion spectra originating in astrophysical plasmas in collisional ionization equilibrium. New atomic data for excitation of Fe IX-XIII are integrated under the assumption of a kappa-distribution of electron energies. Diagnostic methods using lines of a single ion formed at any wavelength are explored. Such methods minimize uncertainties from the ionization and recombination rates, as well as the possible presence of non-equilibrium ionization. Approximations to the collision strengths are also investigated. The calculated intensities of most of the Fe IX-XIII EUV lines show consistent behaviour with kappa at constant temperature. Intensities of these lines decrease with kappa, with the vast majority of ratios of strong lines showing little or no sensitivity to kappa. Several of the line ratios, especially involving temperature-sensitive lines, show a sensitivity to kappa that is of the order of several tens of per cent, or, in the case of Fe IX, up to a factor of two. Forbidden lines in the near-ultraviolet, visible, or infrared parts of the spectrum are an exception, with smaller intensity changes or even a reverse behaviour with kappa. The most conspicuous example is the Fe X 6378A red line, whose intensity incerases with kappa. This line is a potentially strong indicator of departures from the Maxwellian distribution. We find it possible to perform density diagnostics independently of kappa, with many Fe XI-XIII line ratios showing strong density-sensitivity and negligible sensitivity to kappa and temperature. We also tested different averaging of the collision strengths. It is found that averaging over 0.01 interval in log(E/Ryd) is sufficient to produce accurate distribution-averaged collision strengths at temperatures of the ion formation in ionization equilibrium.