CEDAR: Modification of Ionosphere by Lightning ElectroMagnetic Pulse (LEMP): Dependence on Lightning Type, Stroke Order, and Other Factors

大气闪电是雷暴期间非常常见的现象。然而，在低层大气中看到的雷击也可以作为电磁脉冲向上传播，从而改变高层大气的特性，这一点尚不为人所知。对闪电的研究对理解电离层特别有用，电离层是海拔50英里以上的大气层，具有丰富的带电粒子，如离子和电子。闪电的累积效应可能会改变电离层，进而影响无线电通信和全球导航。本项目将开展雷电电磁脉冲（LEMP）对电离层影响的跨学科研究。在很大程度上决定电离层反射高度的大气电导率剖面预计将由于LEMP引起的电离而发生改变。这种变化将通过比较从同一闪电中发生的单个闪电的电磁场特征（包括天波）推断的电导率曲线来检测。有证据表明，多次冲程在降低电离层视反射高度方面具有累积效应。将首次比较不同极性的云内闪电脉冲（高架源）和云对地闪光（CGs）（近地源）的回冲脉冲估算的电离层反射高度。本项目将尝试回答以下与CEDAR计划主要目标相关的新科学问题，重点关注从低层大气向上传播的扰动（LEMPs）：在单个闪电的时间尺度上，大气电导率廓线是如何变化的？2. 广泛应用于研究闪电与电离层相互作用的赫子偶极子模型的适用性有哪些限制？为什么在同一雷暴中，正闪电的电离层反射高度大于负闪电？4. 在干扰低层电离层方面，闪电过程比cg中的负回击更重要吗？宽带LEMPs将在佛罗里达州完善的设施（盖恩斯维尔的闪电观测站和高尔夫球场站）中记录，在不同的季节，在陆地和海上，白天和夜间条件下，距离从50到500公里不等。获取的数据将用于解决问题1、3和4。本文将采用球坐标系下地球-电离层波导中LEMP传播的时域有限差分（FDTD）模型来求解问题1至问题3。sq1将使用一个优化程序，从由单个闪光产生的测量LEMP波形（包括天波）中找到等效指数大气电导率剖面的最佳拟合参数。该模型将采用真实的闪电源，并基本覆盖感兴趣的全部频率带宽（高达数百千赫兹）。研究成果将用于佛罗里达大学流行的高级本科/研究生课程“闪电”。该项目的研究人员将继续致力于高中科学教育和提高公众对闪电危害的认识。这些研究将继续促进FDTD建模方面的国际合作。该项目的结果将对其他学科产生影响，包括无线电通信、电离层遥感、评估人工改变电离层的方法、电离层化学和电离层等离子体物理学。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Atmospheric lightning is a very familiar phenomenon seen during thunderstorms. However, it is not well-known that lightning strokes seen in the lower atmosphere could also travel upward as an electromagnetic pulse, modifying the properties of the upper atmosphere. The study of lightnings is especially useful for the understanding of the ionosphere, which is the layer of atmosphere above 50 miles altitude that has abundance of charged particles such as ions and electrons. Cumulative effects of lightnings could potentially modify the ionosphere, which in turn affect radio communication and global navigation. This project will carry out a cross-disciplinary study of the modification of ionosphere by Lightning Electro Magnetic Pulse (LEMP). The atmospheric conductivity profile that largely determines the apparent ionospheric reflection height is expected to be modified as a result of ionization caused by LEMP. This modification will be detected via comparison of conductivity profiles inferred from electromagnetic field signatures (including skywaves) of individual lightning strokes occurring within the same flash. There is some evidence of cumulative effect of multiple strokes in lowering the apparent ionospheric reflection height. For the first time, ionospheric reflection heights estimated using in-cloud lightning pulses (elevated sources) and return-stroke pulses in cloud-to-ground flashes (CGs) (near-ground-level sources) of different polarity will be compared. This project will attempt to answer the following new scientific questions (SQs) that are relevant to the primary objective of the CEDAR Program, with emphasis on perturbations (LEMPs) that propagate upward from the lower atmosphere: 1. How is the atmospheric conductivity profile changing on the time scale of individual lightning flash? 2. What are the limits of applicability of the Hetzian dipole model that is widely used in studying lightning interaction with the ionosphere?3. Why is the apparent ionospheric reflection height for positive lightning larger than for negative lightning in the same thunderstorm? 4. How important are lightning processes other than negative return strokes in CGs in disturbing the lower ionosphere? Wideband LEMPs will be recorded at the well-established facilities (Lightning Observatory in Gainesville and Golf Course Station) in Florida, at distances ranging from 50 to 500 km, in different seasons, over land and over sea, under both daytime and nighttime conditions. Data acquired will be used to address SQs 1, 3 and 4. A Finite-Difference Time-Domain (FDTD) model of LEMP propagation in the Earth-ionosphere waveguide in the spherical coordinate system will be used to address SQs 1 to 3. An optimization procedure to find best-fit parameters of equivalent exponential atmospheric conductivity profile from measured LEMP waveforms (including skywaves) produced by individual strokes of a flash will be used for SQ 1. The model will employ realistic lightning sources and cover an essentially full frequency bandwidth of interest (up to hundreds of kilohertz). Research results will be used in the popular University of Florida senior undergraduate/graduate course “Lightning”. The project researchers will remain committed to high-school science education and to enhancement of public awareness of lightning hazards. The studies will continue to foster international collaboration on FDTD modeling. The results of this project will have implications for other disciplines, including radio communication, ionospheric remote sensing, assessment of methods for artificial modification of the ionosphere, ionospheric chemistry, and ionospheric plasma physics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.