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

Abstract

Using electric and magnetic field and field derivative sensors arrayed over an area of about 1 km2, 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 μs 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 μs was about 35 kV m−1; and the average peak electric field derivative was about 15 kV m−1μs−1, the largest unsaturated measurement being about 20 kV m−1μs−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.