Features of the first and the subsequent return strokes in positive ground flashes based on electric field measurements

This paper presents the characteristics of the electric fields produced by the first and the subsequent return strokes observed in positive ground flashes in Sweden. Fifty one positive ground flashes containing 60 return strokes recorded during 2014 summer thunderstorms were analyzed. In our analysis, only 12% of the cases were multiple-stroke while 88% were single-stroke. On average, the number of strokes per flash was 1.20 and the highest number of strokes per flash recorded was four. The geometric mean (GM) value of the interstroke interval was 60 ms while the distance between the first and the subsequent strokes ranged between 4.9 and 46.4 km. We found that the average duration of the subsequent strokes parameters were smaller than that of the first strokes. For the first strokes, the GM values of the slow front duration, the fast transition 10-to-90% risetime, the zero crossing time, the zero-to-peak risetime and the 10-to-90% risetime were 8.7 μs, 1.4 μs, 29 μs, 11 μs and 5.7 μs, respectively while for the subsequent strokes, the values were 4.0 μs, 0.91 μs, 11 μs, 5.8 μs and 3.2 μs, respectively. Possible reasons for the shorter duration of the subsequent return strokes parameters were discussed.