| Every second, lightning strikes the ground up to 200 times around the world with flashes occurring more frequently in some places than others. A variety of systems exist that detect and locate lightning flashes for a variety of commercial and scientific applications including air traffic control, climate modeling, and for use by the electric utility industry. These lightning detection systems have a wide range of capabilities in their detection accuracy and in their spatial and temporal coverage. For example, commercial detection systems provide very precise time and location information for lightning strikes but also require a densely packed array of sensors limiting their coverage to areas where those sensors can be placed. At the other extreme, the sensors aboard satellites can detect lightning all around the world yet any particular area is only visible for a few minutes each day as the satellite passes over it.; When lightning strikes it generates an impulsive broadband electromagnetic pulse known as a radio atmospheric (sferic). The radiation in the very low frequency and extremely low frequency (VLF/ELF) bands (300--30,000 Hz) of the sferic propagates through the waveguide formed by the Earth and the ionosphere with low attenuation. Because of this low attenuation sferics can be detected at distances in excess of 10,000 km from their source lightning locations.; A new technique of global lightning location is presented that takes advantage of propagation characteristics of the VLF/ELF frequency band. The technique employs a few widely (9,000 km) spaced receivers to provide continuous regional coverage of lightning activity. By combining magnetic direction finding and time of arrival difference measurements in the VLF/ELF band, individual lightning strokes are located to within ∼100 km of their source locations. The influences of the Earth/ionosphere waveguide on VLF wave propagation are discussed and lightning location results are compared with existing lightning detection systems. |
