Plasma chemistry of sprite streamers

1] 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 similar to 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 (tau similar to 10 mu s) ionizes the gas ( fractional ionization density similar to 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 N2O2+. 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 N2O2+. 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 similar to 10(-9)-10(-8)) of the metastable species O(D-1), O(S-1), N(D-2), O-2(a(1)Delta(g)), O-2(b(1)Sigma(+)(g)), N-2(A(3)Sigma(+)(u)), and N-2(alpha' (1)Sigma(-)(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 (similar to 1 kR), but possibly detectable, OI 557.7 nm and O-2(b(1)Sigma(+)(g) -> X-3 Sigma(-)(g)) Atmospheric airglow emissions. Neutral active species created in the greatest abundance (fractional densities > 10(-8)) are N-2(X-1 Sigma(+)(g), v), O(P-3), N(S-4), and O-2(alpha(1)Delta(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 (similar to 1-10 R) OH(X-2 pi, nu = 6 ... 9 - Delta nu) Meinel emissions produced by O(P-3)- enhanced OH catalysis and O-2(a(1)Delta(g) -> X-3 Sigma(-)(g)) Infrared Atmospheric emissions. Short-lived (similar to 100 s) populations of hydrated positive ions and negative ion clusters are also created in the streamer trail. Electron impact dissociated N(D-2) interacts with O-2 to create a long-lived (> 1000 s) increase (fractional enhancement similar to 75%) of the ambient NO density within the streamer channel, fo a net production of similar to 5 X 1(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.