Electric and magnetic fields and field derivatives from lightning stepped leaders and first return strokes measured at distances from 100 to 1000 m

Using electric and magnetic field and field derivative sensors arrayed over an area of about 1 km 2, we measured the close fields of stepped leaders and first return strokes in 18 negative cloud-to-ground lightning flashes at distances to individual sensors ranging from about 100 m to about 1 km. We present examples of the close field waveforms along with their statistical characterization as a function of the distance to the lightning. Statistical data are presented for the half-peak width of the stepped-leader/return-stroke electric field waveform; the stepped-leader electric field change; the return-stroke electric field change at 20, 100, and 1000 mu s after return-stroke initiation; the peak electric field derivative; the risetime of the electric field derivative waveform; and the magnetic field initial peak, largest peak, risetime, and half-peak width. For example, in the 100-200 m range, the average half-peak width of the leader/return-stroke electric field change was about 0.8 ms; the average observed leader electric field change was about 40 kV m(-1); the average return-stroke electric field change at 20 ms was about 35 kV m(-1); and the average peak electric field derivative was about 15 kV m(-1) ms(-1), the largest unsaturated measurement being about 20 kV m(-1) ms(-1). Peak derivative values observed at close range are consistent with derivative measurements made for return strokes over salt water at distances of some tens of kilometers if an inverse-distance dependence is assumed for the field amplitude.