A High-Speed Time-Resolved Spectroscopic Study of the Lightning Return Stroke. Part III. A Time-Dependent Model

Abstract

A model of the lightning return stroke is developed using the experimental data presented in Parts I and II and Gilmore's tables for the composition of dry air in thermodynamic equilibrium. The temperature, electron density, pressure, relative mass density, per cent ionization, and specie concentration in a 10-m section of a model-return stroke are given with 5-?sec resolution from 0?30 ?sec. During this period, the temperature decreases from 30,000 to 16,000 K and the electron density decreases from 1018 to 1.5 ? 1017 cm?3. The channel is characterized by an average pressure of 8 atm in the first 5 ?sec and attains atmospheric pressure at approximately 20 ?sec. A minimum relative mass density of 3?10?2 is attained at the same time. Per cent ionization is on the order of 100 in the first 15 ?sec and then decreases. The largest nitrogen specie concentration in the 0-15 ?sec period is NII, followed by N-III, which in turn is followed by NI, All three concentrations decrease in the 0-15 ?sec time period. In the 15-30 ?sec period the NI concentration increases, the NII concentration attains a quasi-equilibrium, and the NIII concentration continues to decrease rapidly. The salient characteristics of the model return stroke are discussed and related to spectral observations.