Plasma chemistry of sprite streamers

A study is conducted of the principal chemical effects induced by the passage of a single sprite streamer through the mesosphere at an altitude of 70 km. Recent high-speed imaging of sprite streamers has revealed them to comprise bright (1-100 GR), compact (decameter-scale) heads moving at ∼10 7 m s -1 . On the basis of these observations, a quantitative model of the chemical dynamics of the streamer head and trailing region is constructed using a nonlinear coupled kinetic scheme of 80+ species and 800+ reactions. In this initial study, chemical processes related to currents in the trailing column and to vibrational kinetics of N 2 and O 2 are not included. The descending streamer head impulsively (T ∼ 10 μs) ionizes the gas (fractional ionization density ∼10 -9 ), leaving in its trail a large population of ions, and dissociated and excited neutral byproducts. Electrons created by ionization within the head persist within the trailing column for about 1 s, with losses occurring approximately equally by dissociative attachment with ambient O 3 , and by dissociative recombination with the positive ion cluster N 2 O + 2 . The ion cluster is produced within the trailing channel by a three-step process involving ionization of N 2 , N + 2 charge exchange with O 2 , and finally three-body creation of N 2 O + 2 . On the basis of simulation results, it is concluded that the observed reignition of sprites most likely originates in remnant patches of cold electrons in the decaying streamer channels of a previous sprite. Relatively large populations (fractional densities ∼10 -9 -10 -8 ) of the metastable species O( 1 D), O( 1 S), N( 2 D), O 2 (a 1 Δ g ), O 2 (b 1 Σ + g ), N 2 (A 3 Σ + u ), and N 2 (a' 1 Σ - u ) are created in the streamer head. The impulsive creation of these species initiates numerous coupled reaction chains, with most of the consequent effects being of a transient nature persisting for less than 1 s. These include weak (∼1 kR), but possibly detectable, OI 557.7 nm and O 2 (b 1 Σ + g → X 3 Σ - g ) Atmospheric airglow emissions. Neutral active species created in the greatest abundance (fractional densities > 10 -8 ) are N 2 (X 1 Σ + g , ν), O( 3 P), N( 4 S), and O 2 (a 1 Δ g ), which, because of the absence of readily available chemical dissipation channels, persist for longer than 100s of seconds. Other long-lived (>1000 s) effects are very weak (∼1-10 R) OH(X 2 Π, ν = 6...9 - Δν) Meinel emissions produced by O( 3 P)-enhanced OH catalysis and O 2 (a 1 Δ g → X 3 Σ - g ) Infrared Atmospheric emissions. Short-lived (∼100 s) populations of hydrated positive ions and negative ion clusters are also created in the streamer trail. Electron impact dissociated N( 2 D) interacts with O 2 to create a long-lived (>1000 s) increase (fractional enhancement ∼75%) of the ambient NO density within the streamer channel, for a net production of ∼5 x 10 19 NO molecules for the streamer as a whole. It is suggested that in addition to the optical emissions from electron-impact excited electronic states of N 2 , a substantial portion of the spectrum may be due to chemiluminescent processes derived from vibrational kinetics of nitrogen.