HR: 1340h
AN: AE23A-0893    [Abstracts]
TI: Modeling the Slow-Tail of Atmospheric Waves to Approximate the Distance of Propagation
AU: * Le Cocq, C
EM: cisou@stanford.edu
AF: Department of Electrical Engineering, Stanford University, 350 Serra Mall, Stanford, CA 94305, United States
AU: Fraser-Smith, A C
EM: acfs@alpha.stanford.edu
AF: Department of Electrical Engineering, Stanford University, 350 Serra Mall, Stanford, CA 94305, United States
AB: A lightning strike emits an electromagnetic wave known as an atmospheric or sferic, which propagates through the earth-ionosphere waveguide. Sferics can be recorded by extremely low and very low frequency, ELF and VLF, receiver systems. The recorded signal is composed of two segments, a pulse containing VLF frequencies, followed by a slow-tail, containing the ELF components. The slow-tail is essentially a single cycle wave, which is delayed with respect to the rest of the sferic due to the dispersive nature of the ionosphere. The recorded time- domain slow-tail varies with the lightning strike's current moment, and the waveguide's media characteristics. It is possible to approximate the location of the lightning source with measurements of the sferic. Many methods require measurements from multiple stations, however the goal of this work is to approximate the distance a sferic propagated with a single station. J.R. Wait developed a mode theory where propagating ELF radio are characterized by the first mode. The research reported here uses the first mode equations to model a slow-tail that propagated a certain distance. We include a comparison to measurements on slow-tails observed at widely variable distances from their causative lightning, and analyze the accuracy of our model. Using the inverse of this method along with sferics from known locations, we approximate the form of the current moment at the source and use an average of this waveform to improve our slow-tail model. With an accurate computed slow-tail we can approximate the distance of propagation by fitting the computed waveform to the observed slow-tail. An analysis is given of the effectiveness of this method. As expected, since this method uses data from only one station, the estimation error from this method are larger than those of the traditional multiple station estimation method. However, in most instances our method was accurate to within hundreds of kilometers. With such accuracy, this method can be used to predict abnormalities in the conditions of the ionosphere along the path of propagation. In addition, it is a powerful tool that can be used to validate multiple station estimates and determine any errors due to single station faults.
DE: 0624 Guided waves
DE: 2487 Wave propagation (0689, 3285, 4275, 4455, 6934)
DE: 6964 Radio wave propagation
DE: 6969 Remote sensing
SC: Atmospheric and Space Electricity [AE]
MN: 2007 Fall Meeting