CN Tower Lightning Characteristics Based on Current-Recorded Flashes

Lightning strikes to the CN Tower have been observed since 1978. In 1991, measurement stations were operating to simultaneously record important parameters of lightning strikes to the tower, including the return-stroke current derivative, the associated lightning-generated electromagnetic pulse and optical images of the lightning channel. On August 24, 2011, in an unusual severe lightning storm, video records indicate that the CN Tower was struck with 52 lightning flashes within 84 minutes. This translates into an average inter-flash time of about 1.6 minutes. In a recent publication, CN Tower lightning characteristics - such as inter-flash time, flash duration and flash components - during this severe storm were thoroughly investigated, based on video records. This paper, however, focuses on investigating flash characteristics, such as flash multiplicity, inter-stroke time and flash duration, based on current derivative signals recorded at tower during the same storm. Before the advent of this severe storm of 2011, National Instrument PCI 5114 digitizer, featuring 64MB/channel memory with sampling rate of up to 250MS/s, was set to record the current derivative of CN Tower flashes. For the first time at the tower, this configuration enabled two seconds of continuous recording of the current derivative at 220ns resolution. During this severe storm, the current measurement system recorded 32 flashes, containing 159 return strokes. It is noted that 43% of these current-recorded flashes contain more than 8 return strokes each, with an overall average multiplicity of 4.97. Out of a total of 127 inter-stroke time intervals, 93% are found to be within 200ms, with an overall inter-stroke time average of 69.9ms. It is also noted that all inter-stroke time intervals, with the exception of two, are within 314ms. The maximum interstroke time recorded during this storm is found to be 726.3ms.