Mesospheric electric breakdown and delayed sprite ignition caused by electron detachment

Abstract

The electric discharge of a thundercloud in the troposphere is often accompanied by upper-atmospheric electric discharges such as sprites^1,2 or halos³. Present models assume that the net change in the density of free electrons during these discharges is determined by the competition between electron impact ionization and electron dissociative attachment to oxygen molecules^4,5,6,7, and that balance is achieved in an electric field termed the conventional breakdown field. According to these models, free electrons are removed in an electric field whose strength is below the breakdown field, but multiply in a field that is stronger than the breakdown field. Here we use a simple model of the electric response of the mesosphere at timescales of tens of milliseconds to show that in the upper atmosphere, electrons multiply also under field strengths significantly below that of the conventional breakdown field because, at low pressure, electron associative detachment from atomic oxygen ions^8,9 counteracts the effect of dissociative attachment. Our model couples chemical kinetics to dielectric relaxation, and our simulations indicate that associative detachment could explain recent measurements of the time and altitude of the inception of delayed sprites¹⁰. We conclude that associative detachment is a fundamental process in upper-atmospheric electrodynamics.