CEDAR: Modeling Studies of Infrasonic Waves from Thunderstorms and Aurora

本计画为期三年，目的在理论上探讨雷暴与极光所产生的次声波的机制与影响。 次声波对应于频率范围为0.02-10 Hz的声波。 有强有力的实验证据表明，雷暴和极光是次声波活动的重要来源，其范围从对流层到热层，高度跨度很大。 这一证据包括雷暴产生0.1-1赫兹次声波的静电，最近发现中间层闪电引起的称为精灵的瞬时发光事件产生的次声，以及超音速极光弧和脉动极光产生的次声波辐射。 该项目旨在通过进行集中的二维数值模拟工作来表征和量化观察到的次声信号。 它将解决一系列与雷暴和极光产生次声波有关的未解决问题。 有待解决的科学问题是：（1）与闪电放电有关的周期为0.5 ~ 1秒的压缩和稀疏次声脉冲的机制是什么？ 当这些脉冲从雷暴云向下和向上传播，并在地面和热层反射后到达遥远的观测者时，它们的命运和预测特征是什么？(2)最近发现的来自精灵的次声辐射的机制是什么？ 哪些物理因素（包括次声产生和传播因素）有助于形成观察到的类似啁啾的特征？ 观测到的波形有哪些具体特征，使远程观测者能够区分来自低空雷雨云的次声辐射和来自精灵的次声辐射？(3)在以超音速移动的极光弧中，作用于中性空气的体积力的时空结构是什么，导致在地面上观察到极光次声波？ 最近观测到的脉动极光次声信号的机制是什么？ 具体活动包括利用时域有限差分技术开发一个新的有效的二维次声传播模型。 新的模型将是完全非线性的，并将考虑到地球大气中的实际温度和风结构以及与高度有关的耗散效应。 该模型将遵循次声波包在大气中传播，反射和消散的命运，以解释现有的已发表的观测结果，并对未来的观测做出具体的预测。

This project is a three year effort to theoretically investigate mechanisms and effects of infrasonic waves generated by thunderstorms and aurora. The infrasonic waves correspond to acoustic waves in the frequency range 0.02-10 Hz. There is strong experimental evidence that thunderstorms and aurora represent significant sources of infrasonic wave activity spanning a broad altitude range from the troposphere up to the thermosphere. This evidence includes electrostatic production of 0.1-1 Hz infrasonic waves from thunderstorms, recent discovery of infrasound from lightning induced transient luminous events in the mesosphere called sprites, and radiation of infrasonic waves from supersonic auroral arcs and pulsating aurora. The project aims to characterize and quantify the observed infrasonic signals by undertaking a focused two-dimensional numerical modeling effort. It will address a limited set of unsolved problems related to infrasonic wave production by thunderstorms and aurora. The outstanding scientific questions to be pursued are: (1) What is the mechanism for the compression and rarefaction infrasonic pulses with periods 0.5 to 1 sec produced in association with lightning discharges? What is the fate and predicted signature of these pulses as they propagate downward and upward from thunderclouds and reach a remote observer after reflections from the ground and thermosphere? (2) What is the mechanism of the recently discovered infrasound radiation from sprites? Which physical factors, including both infrasound generation and propagation factors, contribute to formation of the chirp-like signatures that are observed? What are the specific features of observed waveforms, which would allow a remote observer to discriminate between infrasound radiation coming from thunderclouds at low altitudes and those from sprites? (3) What is the temporal and spatial structure of the body force acting on neutral air in auroral arcs that move with supersonic speeds, leading to auroral infrasound waves observed on the ground? What is the mechanism of the recently observed infrasound signals from pulsating aurora? Specific activities include development of a new efficient two-dimensional model of infrasound propagation using finite-difference time-domain techniques. The new model will be fully non-linear and will account for realistic temperature and wind structure in the Earth's atmosphere as well as for altitude dependent dissipation effects. The model will follow the fate of infrasonic wave packets at they propagate, reflect, and dissipate in the atmosphere in order to interpret existing published observations and to make specific predictions for future observations.