Dayeh, a research scientist at the Southwest Research Institute in San Antonio, Texas, designed the innovative experiment that produced the acoustic images. Thunder is created when lightning rapidly heats the air around it, causing it to expand faster than the speed of sound. In order to accurately record its acoustic signature, Dayeh had to know where the lightning would strike. “The first problem is that we do not know where lightning will hit. So we used what we call triggered lightning,” Dayeh told The Daily Star. In order to artificially induce lightning strikes, he and his team had to sit out in thunderstorm conditions and monitor electric fields, waiting for the right moment. “We knew that this thundercloud was coming overhead. So, right before, we launched a rocket into the sky which unspooled a copper wire behind it … it’s like bringing the ground up,” he explained. “Now you have this very large electric field, and all the charge goes to this one point. You create an ionic channel between the cloud and the tip of the rocket, and because the rocket is grounded, all of the charge goes down the copper wire.” The current through the wire is so high that it vaporizes in midair. But once it’s gone, the strike continues like any other, according to Dayeh. Triggering the strike allowed him and his team to approximate its location to within a few meters. Using a sophisticated array of microphones and a signal processing technique called beamforming, they were able to record its acoustic signature, creating the very first images of thunder. Dayeh compared the experience to a fireworks display, with fluorescent spectral emissions from the combusting copper and hydrogen. “It’s kind of nature’s fireworks really,” he said. Dayeh hails from Kefraya; his family home is less than a kilometer from the Bekaa Valley town’s eponymous winery. Most of his research concerns space physics, in which he holds a doctorate from the Florida Institute of Technology. But 12 years ago Dayeh was part of the group that first discovered that lightning produces X-rays; the research formed the basis of a master’s thesis in atmospheric electricity. Over the years he kept himself up to date with developments in lightning research. The emergence of new instruments and techniques in the field inspired Dayeh to write a proposal for the new experiment. There are a number of difficulties in the study of lightning, not least the unpredictability of strikes. The problems are so confounding that despite being such a common phenomenon, relatively little is known about how lightning forms. “You’d be surprised,” Dayeh said. “Lightning, thunder generation – we only know the general attributes. The microphysics of lightning strikes are almost unknown.” “It’s difficult. The environment in which lightning strikes is very noisy from an electromagnetic perspective. It’s also unpredictable. It’s not like a telescope where you can orient the telescope and just study [what you are looking at.]” “It’s a huge system, a very dynamic system.” Another problem facing researchers arises from the observer effect – sending sensors and instruments into the clouds can affect the environment, altering the very electric fields on which they collect data. “It’s really hard to [do] this instrumental observation. We are still dominated by mother nature.” But the techniques used by Dayeh and his team may have given scientists and researchers an important new tool with which to study lightning formation. “We showed a proof of concept, that you can study lightning using acoustic imaging. We are analyzing the sounds that come out from the lightning strike to investigate its microphysical properties.” “Now with this technique, we can tell the sources of the sound coming from the lightning strike and from that point we can investigate how much energy is created by the strike itself. We can actually tell more about the energy input from the storm.” The images show the release of acoustic energy from the entire lightning channel. Further exploitation of the technique could tell scientists even more about the physics behind how lightning forms. Dayeh said the overarching goal of the research is to be able to predict when and where strikes will occur. “If we understand how lightning works, we will be able to predict lightning strikes, we will be able to protect ourselves from lightning strikes,” he added. “If we are able to predict … say half a second ahead, we would be able to turn off our instrumentation, our power transformers, our high-tech computing. But right now we are at the stage where we are trying to understand how it even happens.” The research has been submitted to a scientific journal for review, and Dayeh said his team has already received positive feedback from the scientific community, including from a number of colleagues who were skeptical of whether it could work. He said he hoped to further develop the concept and design specific experiments to take full advantage of the instrument’s capabilities. Dayeh also plans to construct a more complex array of microphones in order to generate more detailed acoustic images. “It’s a new way to look at lightning,” he said. Ned Whalley| The Daily Star
