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 ( τ ∼ 10 μ s) 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 + , v ), 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, v = 6…9 − Δ v ) 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 × 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.