F i Ionization and Excitation Equilibrium in the Solar Atmosphere

Computations for a 15-level model Fe I atom in the solar atmosphere indicate that many of the Fe I spectral lines are strongly influenced by departures from LTE. The strong lines originating from the ground term and low-lying metastable terms have a strong chromospheric contribution. Although for many of the lines the source functions saturate to the Planck function near the temperature minimum (τ_5000 ≈ 10-4) and lower, the ionization equilibrium does not saturate to the Saha condition until τ_5000 > 3 × 10-3. The low-lying levels in Fe I reach maximum absolute populations near τ_5000 = 3 × 10-3, and lines originating from these levels tend to be formed at atmospheric depths where τ_5000; ≤ 3 × 10-3. For high-excitation levels in Fe I, departures of the line source function from the Planck function extend deeper into the photosphere and may influence abundances derived from lines originating from these high lying levels. From the weakness of the principal Fe II lines in the far-ultraviolet (λ2599, in particular) relative to lines of Si II and Mg II it is suggested that the Fe II f-values of Corliss and Bozman are too high by a factor of the order of 10+2. Fe I photoionization from the lowest levels may be a significant source of opacity in the region 1600-2000 Å.