The initiation of lightning in thunderclouds: The possible influence of metastable nitrogen and oxygen molecules in initiating lightning streamers

Abstract

An examination is made of the possible role of metastable molecules of oxygen and nitrogen to contribute to the difference between the observed breakdown field in air of ~25 kV/cm and the electric fields observed in thunderclouds which generally have a maximum of ~3 kV/cm, which suggest much lower breakdown fields. Analysis of steady state equations for electron, ion, and metastable densities indicates that reductions on the order of a factor of 5 are possible in the electric field necessary to sustain a steady state discharge because of (1) electron detachment from negative ions by the oxygen metastable molecules and (2) increased ionization through deexcitation collisions of electrons with metastable nitrogen vibrational states, assuming high vibrational temperatures of the order of the electron temperature. However, solution of the time-dependent equations for the particle densities indicates that for a uniform discharge, times on the order of 0.1 s are required for the production of sufficient densities of the metastable particles from initial electron densities of ~1011 cm-3. In such times, electron diffusion would have dispersed electrons from any initial narrow electron pulse postulated to be produced by cosmic rays. It is concluded that the role of the metastable molecules, particularly the nitrogen vibrational states, is unlikely to be significant in the initiation of lightning streamers.