CEDAR Postdoc: Modeling Studies of Optical Properties of Sprite Streamers and their Chemical Effects on the Upper Atmosphere

本项目进行了模拟和观测研究，探讨了精灵飘流放电的光学特性及其对高层大气的化学效应。Sprite放电是发生在中间层和下热层-电离层（MLTI）区域的四种已知瞬态发光事件之一，它与潜在雷暴中的闪电活动直接相关。尽管一个典型的精灵事件可能占据了上层大气的大量空间，但精灵事件的高空间分辨率图像揭示了许多内部的、尚未完全理解的小尺度特征，包括明亮的离子化丝状通道。值得注意的是，在精灵中观察到的丝状结构与大气压下被称为流光放电的现象是相同的，只是在高海拔地区空气密度降低。在近地压力下，拖缆的实际应用包括臭氧生产、污染控制和表面处理。然而，目前关于精灵飘带对上层大气的化学影响的知识非常有限。在雷暴期间由闪电驱动的精灵是电磁能量从对流层高度向上耦合到中间层和较低电离层区域的表现之一。特别是，上层大气中闪电放电所提供的静电能量被转换成其他形式的能量。它们中的一些获得足够的能量来激发、解离和电离空气中的中性分子（N2和O2）。因此，产生了化学活性物质（如原子N和O），它们可以引发一系列反应，导致上层大气中几种重要成分（如NO）的产生。受激发的中性或电离分子辐射光子，随后由光学成像仪器观察到，这是壮观的精灵放电。精灵排放物是电子能量学和能量转换过程的关键指标，目前对精灵排放物的认识大多是通过对精灵排放物数据的分析获得的。在精灵中也会发生强烈的电离，这导致大气电导率的局部改变。因此，精灵是高层大气能量收支和化学成分背景下的重要组成部分，本项目解决了表征和解释精灵飘带的光学特性及其对高层大气影响的需要。具体的科学问题是：(1)精灵飘带对高层大气的化学效应是什么？(2)精灵飘带产生的光发射动力学是什么？主要结果将是更好地了解精灵飘带的排放和化学效应，包括短期和长期影响。这项调查将为更好和更完整地分析现有和未来的精灵数据（例如，由FORMOSAT-2卫星上的视觉仪器收集的远紫外和光学数据）提供理论基础。更广泛的影响包括建模者和实验家之间的合作，以及博士后学者的支持和进一步培训。

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.