CEDAR: Modeling Studies of Infrasonic Waves from Thunderstorms and Aurora

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.