Grotrian diagram displaying the transitions involved in the N iii emission lines problem. The horizontal line marks the N iii ionization threshold. The N iiiλλ4634 − 4640 − 4642 triplet is formed by the transitions 3d  →  3p, while the absorption lines at λλ4097-4103 are due to the transitions 3p  →  3s. An effective drain of 3p is provided by the “two electron transitions” 3p  →  2pS, ²P, ²D). The levels above the ionization limit are the autoionizing levels that feed level 3d via dielectronic recombination (Sect. 3.1). Cascade processes (4f  →  3d) can overpopulate the 3d state as well. The Swings mechanism involves the resonance transitions 2p  →  3s and 2p  →  3d. Note that the (energetic) positions of the autoionizing niveaus have been shifted upwards for clarity.