Chemical effects of weak electric fields in the trailing columns of sprite streamers

Abstract

The chemical effects of undervoltage electric fields in the trailing columns of sprite streamers are investigated. The existence of such fields is suggested by observations of ELF radiation associated with optical emissions of bright sprites that has been interpreted as evidence for intense electrical currents flowing in the body of the sprite. In this paper an earlier nonlinear coupled chemical kinetics modelling study involving 80+ species and 800+ reactions of the chemical response to the passage of a sprite streamer at 70 km altitude is extended to include undervoltage fields in the trailing region behind the compact streamer head that would be responsible for driving such currents. Recent high speed imaging observations combined with chemical modelling results suggest that the trailing field is less than ∼0.5Ek, where Ek is the threshold electric field for the ionization avalanche. Electron densities are slightly decreased on account of enhanced dissociative attachment in the undervoltage environment. The effects on the densities of O(3P) and the metastable species O(1D) and are enhancements by a modest factor of 2 or less compared with a field-free tail. O2(a 1Δg) shows slightly greater enhancements, by up to a factor of 3, and O− shows enhancements by up to a factor of 5.