Simplified Grotrian diagram displaying the most important transitions involved in the N iv emission line problem. The horizontal line marks the N v ionization threshold. N ivλ4058 is formed by the transition 3d ¹D  →  3p ¹P⁰, while the absorption line at λ6380 originates from the transition 3p ¹P⁰  →  3s ¹S. An efficient drain of 3p is provided by the “two-electron” transitions 3p ¹P⁰  →  2p^2 1S, ¹D. Cascade processes from 3d′ ¹F⁰ and pumping from 2p ¹P⁰ are the major routes to overpopulate the 3d ¹D state. See text.