This project conducts a modeling and observational study to investigate optical properties of sprite streamer discharges and their chemical effects on the upper atmosphere. Sprite discharges represent one of four known types of transient luminous events occurring in the Mesosphere and Lower Thermosphere-Ionosphere (MLTI) regions, which are directly related to the lightning activity in underlying thunderstorms. Although a typical sprite event may occupy a large overall volume of the upper atmosphere, high spatial resolution imagery of sprite events reveals many internal and not yet fully understood small scale features, including bright filamentary channels of ionization. It is quite remarkable that the filamentary structures observed in sprites are the same phenomenon known as streamer discharges at atmospheric pressure, only scaled by reduced air density at higher altitudes. Practical applications of streamers at near ground pressures include ozone production, pollution control and surface processing. However, present knowledge about the chemical effects of sprite streamers on the upper atmosphere is very limited. Driven by lightning discharges during thunderstorms, sprites are one of the manifestations of the upward coupling of electromagnetic energy from tropospheric altitudes to the mesospheric and lower ionospheric regions. In particular, the electrostatic energy provided by lightning discharges in the upper atmosphere is converted into other forms of energy. Some of them obtain sufficient energies to excite, dissociate and ionize neutral molecules in air (N2 and O2). As a result, chemically active species (e.g., atomic N and O) are generated, and they can initiate a chain of reactions leading to production of several important constituents (e.g., NO) in the upper atmosphere. Excited neutral or ionized molecules radiate photons which are subsequently observed by optical imaging instruments as spectacular sprite discharges. The emissions from sprites are a key indicator of electron energetics and the energy conversion process, and the current knowledge about sprites is mostly obtained by analyzing sprite emission data. Intense ionization occurs in sprites as well, which results in the local modification of atmospheric conductivity. Therefore, sprites are an important component within the context of energy budget and chemical composition of the upper atmosphere, and this project addresses the need to characterize and interpret optical properties of sprite streamers and their effects on the upper atmosphere. The specific scientific questions addressed are: (1) What are the chemical effects of sprite streamers on the upper atmosphere? (2) What is the dynamics of optical emissions produced by sprite streamers? The primary result will be better understanding of emissions and chemical effects of sprite streamers, including short and long term effects. The investigation will provide a theoretical basis for a better and more complete analysis of existing and future sprite data (e.g., far-UV and optical data collected by the ISUAL instrument on the FORMOSAT-2 satellite). The broader impacts include collaborations between modelers and experimentalists and the support and further training of a postdoctoral scholar.
