Benchmarking atomic data for astrophysics: Fe VIII EUV lines

Abstract

The EUV spectrum of is reviewed, using new solar observations from the Hinode EUV Imaging Spectrometer (EIS), together with older solar and laboratory data. The most up-to-date scattering calculations are benchmarked against these experimental data, with the use of a large atomic structure calculation. Once adjustments are made to the excitation rates, good agreement is found between calculated and observed line intensities. All previous line identifications have been re-assessed. Several lines are identified here for the first time, most notably the strong decays from the 3s<sup>2<sup/> 3p<sup>5<sup/> 3d<sup>2<sup/> <sup>4<sup/>D<sub><i>j<i/><sub/> levels. It is shown that they provide a new, important diagnostic of electron temperature for the upper transition region. The temperatures obtained at the base of solar coronal loops are lower (log <i>T<i/> [K] = 5.5) than those predicted by assuming ionization equilibrium (log <i>T<i/> [K] = 5.6), however firm measurements will only be possible once better scattering calculations are available, and the EIS radiometric calibration is properly assessed.