Measured electric and magnetic fields from an unusual cloud-to-ground lightning flash containing two positive strokes followed by four negative strokes

[1] We present electric and magnetic fields measured at multiple stations between about 300 and 800 m of a cloud-to-ground ‘‘bipolar’’ lightning flash containing two initial positive strokes, separated in time by 53 ms and striking ground at two locations separated by about 800 m, followed by four negative strokes that traversed the same path as the second positive stroke. The leader electric field durations for the positive first, positive second, and negative third strokes were about 120 ms, 35 ms, and 1 ms, respectively. The first-stroke leader electric field changes measured at five stations ranged from about +16 to +35 kV/m, the second stroke +8 to +13 kV/m, and the third stroke 0.9 to 1.8 kV/m (atmospheric electricity sign convention). The microsecond-scale return stroke waveforms of the first and second (positive) strokes exhibited a similar ‘‘slow-front/ fast-transition’’ to those observed for close negative first strokes. The peak rate-of-change of the positive first stroke electric field normalized to 100 km was about 20 V m 1 ms , similar to the values observed for close negative first strokes. The positive second stroke was followed by a long continuing current of duration at least 400 ms, while the positive first stroke had a total current duration of only about 1 ms. All four negative strokes were followed by long continuing current, with durations ranging from about 70 ms to about 230 ms. The overall flash duration was about 1.5 s.