Correlated luminosity and magnetic field peaks produced by canton tower-strokes

Simultaneously measured luminosity and magnetic field induced by the return strokes in typical upward and downward flashes occurring on the 600 m-tall Canton Tower were presented. The correlation between peak relative luminosity and peak relative magnetic field induced by return strokes were examined for 4 upward and 4 downward flashes. All the flashes contained 3 or more return strokes and all the return strokes in a same flash followed the same path. Luminosity of the lightning channel close to the top of the Canton Tower exhibited characteristics of pronounced initial peak with 20–90% rise time of about 5.2 μs (arithmetic mean value, based on 43 samples) for subsequent strokes, which was inferred to be related to the secondary (typically largest) peak current at the top of the Canton Tower. The magnetic field induced by subsequent strokes exhibited characteristics of pronounced initial peak with 20–90% rise time of about 1.2 μs (arithmetic mean value, based on 47 samples). The initial peak magnetic field was modified to proportionally represent the largest peak current at the top of the Canton Tower. Roughly linear and quadratic relation between the initial peak luminosity and modified peak magnetic field were found for subsequent strokes in both upward and downward flashes. The quadratic relation fit the data slightly better than the linear relation. The ratio of peaks of luminosity to magnetic field of the downward first stroke were considerably larger than those of the subsequent strokes, the possible reasons were also discussed.