Highly ionized Fe X-ray lines at energies 7.7-8.6 keV

Fe XXV lines at 1.85 A (6.70 keV) and nearby Fe XXIV satellites have been widely used for determining the temperature of the hottest parts of solar flare and tokamak plasmas, though the spectral region is crowded and the lines are blended during flare impulsive stages. The aim of this work is to show that similarly excited Fe lines in the 7.7--8.6 keV (1.44--1.61 A) region have the same diagnostic capability with the advantage of not being so crowded. Spectra in the 7.7--8.6 keV range are synthesized using the CHIANTI spectral package for conditions (temperature, turbulent velocities) appropriate to solar flares. The calculated spectra show that the Fe lines in the 7.7--8.6 keV are well separated even when turbulent velocities are present, and Fe XXIV/Fe XXV line ratios should therefore provide valuable tools for diagnosing flares and tokamak plasmas. It is concluded that Fe lines in the 7.7--8.6 keV range are ideal for the measurement of flare temperature and for detecting the presence of low-energy nonthermal electrons present at flare impulsive stages. An indication of what type of instruments to observe this region is given.