Benchmarking atomic data for astrophysics: Fe VII and other cool lines observed by Hinode EIS

Abstract

The EUV spectrum of ion{Fe}{vii} is reviewed, using new rates for electron impact excitation, atomic structure calculations, and experimental data. In particular, solar observations of a sunspot loop spectrum obtained from the Hinode EUV Imaging Spectrometer (EIS) are used. Previous line identifications, mostly based on laboratory data, have been assessed. Large discrepancies between observed and predicted line intensities and wavelengths are found for the decays from the 3s² 3p⁵ 3d³ configuration, which are strong EUV lines. We ascribe these discrepancies to incorrect line identifications. A number of new identifications are proposed. With these, very good agreement between theory and experimental data is found. A few transitions, in particular from the 3s² 3p⁶ 3d 4s configuration, are observed for the first time, and are shown to provide a new important diagnostic for measuring the electron temperature in the solar transition region. The temperatures obtained at the base of solar coronal loops are found to be close to the temperature of maximum abundance in ionization equilibrium (log T [K]= 5.4). The assessment of the ion{Fe}{vii} lines was done in conjunction with an assessment of all the strongest cool lines observed with EIS. This spectrum is rich in transition region lines. Some new identifications are presented, in particular for ion{Fe}{ix}. Most of the strongest transitions are identified, however a large number of lines still awaits firm identification.