Luminous propagation of lightning attachment to CN tower

Using an eight-channel photodiode system, we resolved in time and space the luminous propagation of lightning attachment to the CN tower. Approximately 60 lightning discharges were recorded during 1991 and 1992. Most of these discharges demonstrated simultaneous, bidirectional propagation starting at several tens of meters above the top of the CN tower. Propagation speeds ranged from 8×10° to 287×106 m/s. The observed bidirectional propagation suggests the presence of space leaders or the reilluminations following these space leaders during the initial stage of development of upward leaders from the CN tower. A few of the bidirectional discharges were found to have the return stroke speeds and these might be deduced to be manifestations of the bidirectional propagation processes of return strokes which occurred after the connection of a downward leader and an upward connection leader. We also recorded portions of seven ordinary dart leader—return stroke sequences. The dart leader speeds ranged from 7×106 to 23×106 m/s; the velocities of the return strokes ranged from 88×106 to 143×106 m/s. These values are in excellent agreement with the previous results by other authors. Positive correlations are found among the return stroke velocity, the leader velocity, and the leader light intensity. However, it also is observed that even in the case of a very tall structure, when a dart leader approaches the CN tower, it is highly possible that no apparent connecting leader is initiated before the ensuing return stroke.

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