Grant: Discovery Grants Program - Individual

Simultaneous measurement of the lightning current and its associated electric and magnetic fields, and the determination of lightning channel characteristics are necessary for the quantitative assessment of lightning return-stroke models. Such assessments will enhance our understanding of the lightning return-stroke process, help in the formulation of new models that can be more successfully used in lightning detection algorithms, and permit the development of more sophisticated measures for lightning protection. Simultaneous measurements of the lightning current and its radiated electromagnetic pulse are possible using instrumented tall structures, or rocket- or laser-triggered lightning. The CN Tower, with its extreme height, presents one of the best options in the world to carry out this investigation. This fact led to the initiation of an important phase of the CN Tower lightning studies. By the start of the summer of 1991, five measurement stations were operational to simultaneously capture all relevant lightning parameters including the lightning current derivative and its associated electromagnetic field. Many lightning return-stroke current models have been evaluated on the basis of unrelated statistical information about lightning currents and lightning-generated electromagnetic fields; notable exceptions include the use of instrumented towers or rocket-triggered lightning. During the proposed project, a detailed and rigorous analysis of this transient electromagnetic field problem will be carried out with the aim of developing a more experimentally verifiable return-stroke model. Although CN Tower lightning flashes are mostly of the upward-initiated type, downward-initiated flashes, similar to ordinary ground flashes, have been observed at the tower. Therefore, the developed model can be evaluated using the two types of lighting. In present models, it is assumed that lightning channels are straight and vertical, contrary to most observations. In this project, a three-dimensional image of the lighting channel will be constructed from video records and used, in addition to the measured (height-dependent) return-stroke velocity, for the development of an experimentally verified lightning return-stroke model.