A downward positive leader that radiated optical pulses like a negative stepped leader

[1] Using a high-speed optical imaging system (Automatic Lightning Progressing Feature Observation System (ALPS)) operated at a time resolution of 100 ns, we recorded a downward positive leader that radiated more than 20 optical pulses during its downward progression over the height from 299 m to 21 m above the ground like a negative stepped leader. The leader propagated at a speed of around 1.0 × 106 m/s over the height from 272 m to 93 m and then accelerated to a speed of 2.5 × 106 m/s at the height of about 45 m. The positive leader pulses show a 10–90% risetime ranging from 1.2 to 3.8 μs with a geometric mean (GM) value of 2.0 μs, and a half-peak width ranging from 1.8 to 5.1 μs with a GM value of 3.4 μs. On average, the GM risetime and GM half-peak width of these positive leader pulses are about 5 times (2.0 μs versus 0.4 μs) and 3 times (3.4 μs versus 1.1 μs) larger, respectively, than those of negative leader pulses. The step luminosity pulses apparently originate in the leader tip region, which is unresolved with our limited spatial resolution of about 25 m, and propagate upward over distances from several tens of meters to more than 200 m (undetectable beyond that distance) with a speed close to 1.0 × 108 m/s. The pulses appear to attenuate significantly during their initial upward propagation of several tens of meters and then exhibit a tendency to be more or less constant in their luminosity.