This project is a theoretical investigation of the fundamental properties of streamer discharges in transient luminous events. Transient luminous events are recently discovered large scale optical phenomena occurring at stratospheric and mesospheric/lower ionospheric altitudes in the Earth's atmosphere, which are directly related to electrical activity in underlying thunderstorms. Although a typical transient luminous event may occupy a large overall volume of the upper atmosphere, high spatial resolution imagery of these events reveals many internal and not yet fully understood small scale features, including bright filamentary channels of ionization, or streamers, with transverse spatial scales ranging from tens to hundreds of meters. These streamers are plasma structures which can initiate spark discharges in relatively small gaps in air which is near ground pressures. Streamers are commonly observed with branching structures, but the experimental and theoretical work on understanding the branching mechanism in both transient luminous events and in high pressure applications is still in the preliminary stage. The exact mechanism of streamer branching will be explored and quantified as part of this project, as well as the microphysics of streamers and transient luminous events and their effects on the Earth's atmosphere. This project undertakes a focused effort, including numerical simulations and modeling, which will attack a limited set of current outstanding questions on the fundamental properties of streamer discharges in transient luminous events. The specific scientific questions to be investigated are: (1) What is the initiation mechanism for streamers in low applied electric fields? (2) What are the minimum fields required for the propagation of streamers in air at different pressures? (3) What is the mechanism or mechanisms involved in streamer branching? Answering these questions will quantify more accurately the total volume of atmosphere affected by transient luminous events and the ability of these events to establish a direct path of electrical contact between the troposphere and the mesosphere/lower ionosphere regions. The broader impacts of the proposed activity include the education and training of a graduate student, the support and further training of a post-doctoral scholar, the involvement of undergraduate REU students in research activities related to this project during summer time periods.
by means of numerical simulation and modeling. Transient luminous events and sprite discharges in particular are large-scale optical events occurring at mesospheric/lower ionospheric altitudes in the Earth's atmosphere, which are directly related to the electrical activity in underlying thunderstorms. Although a typical sprite may occupy a large overall volume of the upper atmosphere in excess of thousands of cubic kilometers, high spatial resolution imagery of these events reveals many internal and not yet fully understood small scale features, including bright filamentary channels of ionization called streamers, with transverse spatial scales ranging from tens to a few hundreds of meters. Streamers are filamentary plasma structures, which can initiate spark discharges in relatively short (several cm) gaps at near ground pressures in air, and which are commonly utilized in such applications as ozone production, pollution control and surface processing. In ground air pressure applications a typical transverse scale of individual streamer filaments is a fraction of millimeter. 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. The project was largely motivated by the need to better interpret and place the transient luminous event discharges in the proper context with respect to previous experimental and theoretical studies of gas discharges at higher pressures, emphasizing similarity relationships and understanding deviations from these relationships. The completed project took full advantage of a significant progress that has been achieved in recent years in high spatial and temporal resolution video imaging of streamers at low air pressures in transient luminous events in the Earth's atmosphere. Specific scientific contributions made under this project include: (1) Systematic explanation of roles of lightning charge moment change, ambient electron density profile, and plasma inhomogeneities in inception of sprite streamers; (2) Development of understanding of mechanisms of the most commonly observed sprite morphologies, namely column and carrot sprites; (3) Explanation of the observed lightning polarity asymmetry in producing sprites. This project resulted in one Ph.D. dissertation. Additionally, this project provided training to two post-doctoral fellows and five undergraduate students at Penn State. The project results have been published in seventeen refereed journal papers, an invited topical review article, and a book chapter.
