The excitation and collisional deactivation of metastable N(²<i>P</i>) atoms in auroras

Abstract

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