The concentration and altitude distribution of metastable N(2P) atoms has been measured in a diffuse IBC &Pgr;+ auroral arc. The dominant N(2P) source is shown to be the dissociative excitation of N2 by electron impact with a minor contribution from the dissociative recombination of N2+ ions. The possibility that an ion-molecule process involving atomic oxygen and vibrationally excited N2+ ions, (N2+)vib+O→NO++N(2P), is a significant N(2P) source is also explored. Atomic oxygen is found to be the principal N(2P) quenching agent, with O2 contributing significantly below 120 km. By combining these observational results with recent laboratory studies on N2 dissociative excitation the proportional yield of N+, N(2P), N(2D), and N(4S) atoms from electron-impact dissociation of N2 under optically thick conditions is found to be 0.135:0.165 0.30:0.40, respectively, at 100 eV.