ABSTRACT
Lightning discharges to ground produce radiation fields which have structures that are surprisingly similar for different return strokes in the same discharge and also for different discharges. The first stroke begins with an initial portion or front which rises slowly for 2–8 μs to about half of the peak field amplitude. Return strokes subsequent to the first have initial fronts which last 0.5-1.0 μs, on the average, and which rise to about 20% of the peak field. Following the front, both first and subsequent stroke wave forms rise abruptly to peak with 10–90% rise times of 0.2 μs or less when the field propagation is over seawater. After the initial peak, most strokes have a small second peak or shoulder within about 2 μs, and all first strokes have several large subsidiary peaks at intervals of 10–30 μs following the first peak. The physical processes which produce these structures are not well understood, but the data suggest that there may be rather large currents in the upward connecting discharges that occur just prior to first strokes and that all return stroke currents contain a large submicrosecond component. The small second peak may be produced by a current oscillation or by a reflection of a traveling wave in the return stroke just after its onset. The large subsidiary peaks in first strokes are probably produced by the effects of branches. The similarity of subsequent stroke fields within a flash suggests that the currents and velocities of different subsequent strokes in the same discharge are often very nearly the same.
