An investigation of Fe emission lines in solar flare spectra

Previously, large discrepancies have been found between theory and observation for Fe  emission line ratios in solar flare spectra covering the 224–327 Å wavelength range, obtained by the Naval Research Laboratory’s S082A instrument on board Skylab. These discrepancies have been attributed to either errors in the adopted atomic data or the presence of additional atomic processes not included in the modelling, such as fluorescence. However our analysis of these plus other S082A flare observations (the latter containing Fe  transitions between 321–482 Å), performed using the most recent Fe  atomic physics calculations in conjunction with a  synthetic flare spectrum, indicate that blending of the lines is primarily responsible for the discrepancies. As a result, most Fe  lines cannot be employed as electron density diagnostics for solar flares, at least at the spectral resolution of S082A and similar instruments (i.e. ∼0.1 Å). An exception is the intensity ratio I(3s3p P2–3p P1)/I(3s3p P2–3p D2) = I(321.8 Å)/I(327.0 Å), which appears to provide good estimates of the electron density at this spectral resolution.