A Comprehensive Analysis of Optical Emissions, Production of NOx, HOx, and Other Chemical Species by Lightning

Abstract

We analyze lightning optical emissions and their production of different chemical species with particular emphasis on ${\text{NO}}_x$ and ${\text{HO}}_x$. We calculate synthetic lightning spectra with inputs from an electrodynamical model of hot air lightning plasmas. The electrodynamical model calculates the temporal and radial evolution of plasma parameters and chemical species. We analyze 26 lightning-like discharges modeled between 0 and 16 km altitude, varying the input energy between 4 and 200 J/cm, the initial radius, the initial mass, and the humidity. We use a simplified model to estimate the peak current corresponding to each input energy. The ratio of the production of ${\text{HO}}_x$ to ${\text{NO}}_x$ obtained in this study for saturated air ranges between $5\times 10^{-5}$ and $2.3\times 10^{-1}$ strongly influenced by air humidity, which is in better agreement with laboratory measurements than previous modeling results. We calculate the synthetic optical emissions of an exposed lightning section. We compare the simulated peaks in the 777.4 nm and the 337.0 nm photon fluxes with observations of shallow and exposed lightning by the Atmosphere–Space Interactions Monitor (ASIM). Our simulations of hot lightning plasmas result in negligible thermal (due to ions) 337.0 nm optical emissions below 30 kA peak current. This indicates that 337.0 nm optical emissions detected by ASIM are mainly produced by lightning streamers instead of by thermally produced ionic lines in lightning. Finally, we found that the hot lightning channel produces a significant amount of Vacuum Ultraviolet photons that can travel outside the channel to produce non-negligible amounts of OH by photodissociation of water.