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Monday, 4 November 2019

Listen to the energy of lightning

Everyone knows that counting the seconds between the appearance of a lightning and the arrival of thunder gives the distance at which the lightning falls. Researchers at the Jean Le Rond Institute in Alembert and CEA have shown that this sound can also be used to estimate the energy of a lightning bolt. In this work published in Geophysical Research Letters , scientists deduce the geometry of lightning through a network of microphones, then calculate the energy.

© Institut Jean le Rond d'Alembert The acoustic energy of thunder as a function of the observation distance in km at the point of impact on the ground of the lightning. Squares represent simulated flashes and triangles measure HyMeX. Both follow a similar behavior.

Lightning rebalances the electrostatic charge between two clouds, or between a cloud and the ground. This electrostatic discharge locally increases the temperature of several tens of thousands of degrees, causing a shock wave that spreads in the atmosphere : thunder. If we know how to use this sound to estimate the distance at which a lightning struck, researchers at the Jean-Rond d'Alembert Institute (CNRS / Sorbonne University) and the CEA managed to use it to measure the distance power of lightning. A parameter that suffers from an uncertainty of up to three orders of magnitude.

The team used the data measured in 2012 as part of the European project HyMeX, which studies the Mediterranean climate . Four microphones recorded for two months , and continuously, the sound emanating from the sky of the Cevennes, a region particularly struck by the storms. These recordings were first used to reconstruct the geometry of lightning, proving the correlation between the location of acoustic and electromagnetic sources. Then, the researchers used them again to isolate, within the thunder, the signalacoustics from some of its branches, including the main channel that connects the storm cloud and the ground. Now we can calculate the thunder of a flash from its geometry and its energy. The researchers compared field-collected thunders to a simulated thunderstorm database of 72 virtual flashes, statistically consistent with true lightning. This has shown that acoustic measurements give very good results in estimating the energy of negative-lightning flashes, which represent 90% of the cloud-to-cloud discharges, and at a distance of between three and twelve kilometers from the pickups.


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