Diagnostic Application of Highly Ionised Iron Lines in the Extreme Ultraviolet Spectrum of a Solar Flare

Recent atomic data have been used to analyze a solar flare spectrum obtained with the Goddard Space Flight Center's grating spectrometer on the OSO-5 satellite. There exist in the wavelength region 90-200 Å strong lines from each of the ions Fe XVIII-Fe XXIV. The Fe XXI lines can be used as an electron density diagnostic for the 10⁷ K plasma. From our analysis of a particular flare, we find a steep positive slope in the emission measure between 10^6.5 and 10^7.2 K and an electron density of ∼4 × 10¹¹ cm⁻³ at 10⁷ K. We emphasise the need for high spectral and spatial resolution observations of solar flares in this wavelength region, which has to date been largely neglected.