Air-density-dependent model for analysis of air heating associated with streamers, leaders, and transient luminous events

Blue and gigantic jets are transient luminous events in the middle atmosphere that form when conventional lightning leaders escape upward from the thundercloud. The conditions in the Earth's atmosphere (i.e., air density, reduced electric field, etc.) leading to conversion of hot leader channels driven by thermal ionization near cloud tops to nonthermal streamer forms observed at higher altitudes are not understood at present. This paper presents a formulation of a streamer-to-spark transition model that allows studies of gas dynamics and chemical kinetics involved in heating of air in streamer channels for a given air density N under assumption of constant applied electric field E. The model accounts for the dynamic expansion of the heated air in the streamer channel and resultant effects of E/N variations on plasma kinetics, the vibrational excitation of nitrogen molecules N-2(v), effects of gains in electron energy in collisions with N-2(v), and associative ionization processes involving N-2(A(3)Sigma(+)(u)) and N-2(a'(1)Sigma(-)(u)) species. The results are in excellent agreement with available experimental data at ground and near-ground air pressures and demonstrate that for the air densities corresponding to 0-70 km altitudes the kinetic effects lead to a significant acceleration of the heating, with effective heating times scaling closer to 1/N than to 1/N-2 predicted on the basis of similarity laws for Joule heating. This acceleration is attributed to a strong reduction in electron losses due to three-body attachment and electron-ion recombination processes with reduction of air pressure.