In the continuing effort to understand the behavior of lightning, arrays of radio receivers currently in use enable us to see the progression of negative leaders. Positive leaders, however, remain largely invisible outside clouds because they do not emit sufficiently strong radio pulses. To remedy this deficiency, this project will revive and improve the use of acoustic methods for locating positive (and negative) lightning leaders and apply these methods at Langmuir Laboratory in central New Mexico, where a Lightning Mapping Array and other instruments and facilities are already in place to provide the necessary complementary information.
One major goal of the study is to characterize positive leaders and positive cloud-to-ground lightning. A second major goal is to determine which of the acoustic lightning emissions come from the gas dynamic expansion of portions of the rapidly heated lightning channel and which from electrostatic field changes. A third major goal is to test the hypothesis that infrasonic signals are associated with and precede lightning initiation, and a fourth major goal is to image positive leaders and other events during triggered lightning experiments.
A networked array of acoustic transducers will be installed in the Magdalena Mountains of Central New Mexico, where Langmuir Laboratory already has a wide range of instruments to monitor thunderstorms and lightning. A recent pilot study in this area has shown co-incident infrasound acoustic sources and electromagnetic radiation sources. The present work will expand the pilot acoustic network so that more varied and extensive sources can be imaged in three dimensions plus time.
Intellectual Merit: Lightning has been investigated with different techniques, but an integrated approach involving simultaneous use of multiple techniques will yield new insights and better results. New technology has enabled distributed deployments of GPS-based acoustic networks that will provide complementary information to the already existing instrumentation systems from Langmuir Laboratory.
Broader Impacts: This project has immediate impact on scientific problems directly related to lightning hazards, as well as basic science of lightning dynamics. Undergraduate and graduate students will be involved in the scientific and applied aspects of the project. The work will enable integration and cross-disciplinary interaction between engineers, atmospheric, and earth scientists.
Acoustic emissions from lightning, called thunder, include audible and infrasonic ranges. The audible emissions are produced by the gas dynamic expansion of portions of the rapidly heated lightning channel, and infrasonic thunder by the conversion to sound of the energy stored in the electrostatic field of the thundercloud when lightning rapidly changes the electric field. One of the major goals of this project was to use acoustic measurements to determine which of the acoustic emissions are thermally produced and which are due to electrostatic changes. Acoustic characterization of near-field lightning was done by summer-long deployments of a network of acoustic arrays located in close proximity to the VHF lightning mapping array (LMA) from New Mexico Tech, and to other instruments (e.g., electric field meters) in the Magdalena mountains of central New Mexico where lightning events are frequent and isolated. The thunder is recorded by a set of sensor arrays developed in house at New Mexico Tech. Each acoustic array includes custom-designed microphones, with flat low frequency response (below 500 Hz) located at the center and vertices of an approximate 45-m equilateral triangle. At all arrays, broadband sound was recorded with GPS-based data loggers acquiring at high resolution. Since the sound will arrive at the nearest microphone first, the timing of the signals can be used to get its location. With a relatively inexpensive and simple to deploy acoustic array we can map the 3D location of lightning with good accuracy. For cloud-to-ground flashes we can also determine the time of the first return stroke. When the variation of the speed of sound with temperature is taken into account, the located acoustic sources achieve improved correlation with the VHF sources located by the LMA of New Mexico Tech. The LMA, which locates VHF sources over the network with an uncertainty of 6-12 m in the horizontal and 20-30 m in the vertical, is used as ground-truth to measure the location accuracy of the acoustic arrays.This shows that the rumbling heard in thunder is not from reverberations as commonly thought, but is due to sound coming from more distant branches of the lightning. In addition to being able to locate lightning with good accuracy, we have some scientific results coming from the measurement and analysis of acoustic emissions from lightning: a) The Electromagnetic Interference signal (EMI) signal picked up by unshielded cables can be used to determine the time of the first return stroke b) A new method was developed for precisely locating current flow in lightning strikes by joint inversion of VHF data from the LMA and recorded thunder signals c) In a paper recently submitted to Geohysical Research Letters, we show that infrasound (signature) pulses come from the same region near the lightning channel, are locatable and correlate well with VHF sources located by the LMA . Based on our analysis, we show that none of the currently proposed models can explain the mechanism by which lightning flashes produce infrasound pulses. Moreover, we supply a number of specific observations which provide constraints for future models, and indeed suggest a possible model which may explain the observed infrasound pulses from the analysis of acoustic, VHF, and electrical field measurements of two intra-cloud lightning flashes. This project has involved scientists from a number of different disciplines including: Electrical Engineers, Phycisists and Geophysicists. Three graduates students finished their Master's Degrees, a couple of graduate students collaborated with the project and several undergraduate students had research opportunities from the funding of this project.
