The relative concentrations of different ionization stages of iron are measured using the spectral emission of plasmas formed during solar flares. This is an extension of a study on the ionization balance of heavy elements, initiated with the analysis of calcium solar spectra (Antonucci et al., 1984). The data consist of a large set of iron spectra in the wavelength range from 1.84 to 1.88 Å, detected during the recent maximum of activity with the X-ray Polychromator Bent Crystal Spectrometer (BCS) on the NASA Solar Maximum Mission satellite and on the Soft X-ray Crystal Spectrometer (SOX) on the Hinotori satellite.
At the low densities typical of the solar corona, in the steady state the ionization balance of an element is a function of the plasma electron temperature. Hence, it can be measured for plasmas of known temperature and in slowly varying physical conditions, and in most cases, solar flare plasmas can be considered to be in such conditions.