Lightning current and luminosity at and above channel bottom for return strokes and M-components

We measured current and luminosity at the channel bottom of 12 triggered lightning discharges including 44 return strokes, 23 M‐components, and 1 initial continuous current pulse. Combined current and luminosity data for impulse currents span a 10–90% risetime range from 0.15 to 192 µs. Current risetime and luminosity risetime at the channel bottom are roughly linearly correlated (τr,I = 0.71τr,L1.08). We observed a time delay between current and the resultant luminosity at the channel bottom, both measured at 20% of peak amplitude, that is approximately linearly related to both the luminosity 10–90% risetime (Δt20,b = 0.24τr,L1.12) and the current 10–90% risetime (Δt20,b = 0.35τr,I1.03). At the channel bottom, the peak current is roughly proportional to the square root of the peak luminosity (IP = 21.89LP0.57) over the full range of current and luminosity risetimes. For two return strokes we provide measurements of stroke luminosity vs. time for 11 increasing heights to 115 m altitude. We assume that measurements above the channel bottom behave similarly to those at the bottom and find that (1) one return stroke current peak decayed at 115 m to about 47% of its peak value at channel bottom, while the luminosity peak at 115 m decayed to about 20%, and for the second stroke 38% and 12%, respectively; and (2) measured upward return stroke luminosity speeds of the two strokes of 1.10 × 108 and 9.7 × 107 ms−1 correspond to current speeds about 30% faster. These results represent the first determination of return stroke current speed and current peak value above ground derived from measured return stroke luminosity data. Key Points A new experiment‐based measurement of return stroke current speed At channel bottom, the peak impulse luminosity is roughly proportional to the peak current squared At channel bottom, current‐luminosity sub to hundreds of microsecond risetimes correlate linearly