Lightning Mapping with Inexpensive Acoustic Arrays

Lightning mapping systems which record the high frequency radio emission from lightning have been successfully used for the study of storm dynamics and lightning physics. While these systems provide very high quality data, they are expensive and require many sensors to be deployed over a large area. Lightning can also be mapped using an array of low frequency microphones. In this case, the mapping can be done with only 4 microphones, spread out in a 50 m aperture. These maps are also 3-dimensional. The direction to a source is determined from the relative time difference of arrival of the sound at each microphone. The range to the source is found by measuring the time delay from the onset of the lightning flash (measured using EMI pickup on the cables of the microphones) to the sound arriving at the array. This can be done with good source location accuracy of about 150 meters. The limitation of these systems is that the resulting maps are not time resolved. However, the low frequency acoustic data can be recorded at digitization rates of as low as 1000 Hz. This results in a very modest cost associated with deploying an array capable of 3-dimensional lightning location. Currently, a pair of arrays containing 6 microphones are deployed side by side to form a large, dense array. This should allow improved sensitivity, location accuracy, and possibly the ability to distinguish between sources arriving at the array simultaneously.