Development of a new system for determination of lightning locations via wave form observations of spherics

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We have developed a very-low-frequency (VLF) receiver to observe the intense electromagnetic pulses radiated from lightning discharges (VLF spherics). A set of crossed loop antennas is used to measure two horizontal (north-south and east-west) components of wave magnetic fields in the frequency range between 1 and 14 kHz. With a "pre-trigger" recording capability intense spheric pulses are automatically recorded onto a PC.We have tried to determine the locations of each individual lightning stroke, by calculating the direction and range to the lightning from the corresponding spheics data measured with this system. The wave form of each VLF spheric usually consists of a couple of sequential pulses. The first pulse comes directly from a lightning return stroke, and is used for direction finding of the stroke, as in a usual way. On the other hand, the second and later pulses are the multiple reflections of the first pulse inside the Earth-ionosphere waveguide. Since the time-of-arrival of each pulse is determined by its propagation path length in the waveguide, by using the observed difference in the time-of-arrival of two or more pulses, we can inversely estimate not only the reflection height of the ionosphere but also the range of the lightning stroke. Thus, it becomes possible to locate lightning strokes from just a single station.We installed this system at Kanazawa University in April 2000, and since then have been continuously observing lightning-generated spherics 24 hours a day. Compared with the lightning location data provided by a Hokuriku Electric power company, preliminary analysis has shown that this system can locate each lightning stroke within several hundred km with the error of about 10%. This technique would be applied to a portable lightning detector carried, for example, by airplanes or ships cruising over a remote area with no conventional lightning detection networks available.