Lightning return stroke

Distant lightning return stroke fields are often referred to as sferics (called atmospherics in the older literature). The peak in the sferics frequency spectrum is near 5 kHz due to the bipolar or ringing nature of the distant return-stroke electromagnetic signal and to the effects of propagation. 

Fig. 6.1. Mechanism of lightning initiation (a) stepped leader formation (b) initiation of an ascending leader (c) return stroke...

Figure 3. Photograph of lightning leader and return stroke...

Llewellyn ( ) in 1977 indicates that the current risetime may be much less than 1 M sec. Figure 5a shows seme risetimes for a selection of close and distant storms. For some storms, the average time to peak value was found to be of the order of 1/4 M sec. The effect of this term on lightning protection will be discussed later. Figure 5b shows normalized return stroke waveforms from over 2, 000 discharges in several storms. 

Thermal Considerations. The lightning leader stroke has a narrow cone which is surrounded by much larger corona. The return stroke current is concentrated in this central cone which is about 1-2 centimeters diameter and a maximum temperature... 

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