Further characterization of the luminous variation events that occurred during the initial stage of upward positive leaders

Temporary increase phenomena in luminosity of the lightning luminous channel (optical pulses) often occur regularly (or periodically) during the initial stage of upward leaders. Some authors called these phenomena the steps of positive leaders, similar to that occurred in negative stepped leaders. However, the phenomena observed in upward positive leaders usually have a rise time more than ten times longer than the negative leader steps. In order to underscore such difference, we have simply termed the phenomena occurred in the upward positive leaders luminous variation events (LVEs) (Wang et al., 2007). As far as authors know, LEVs’ mechanism remains unknown. Using a high speed optical imaging system (ALPS) at a time resolution 100 ns and a spatial resolution of about 14 m, in this study we have documented and analyzed 38 LVEs occurred in the initial stages of 5 upward positive leaders from a wind turbine and its lightning protection tower. This presentation is to report our results. For all those LVEs, we are not able to identify at what heights they initiated and how they propagated. After comparing the LVEs’ light signals at different heights, we found that those LEVs can be apparently classified into the following four groups. Group 1: the optical pulses are more or less similar in their wave shapes at different heights of the lightning channels and they account for 63.2%. Group 2: the pulses differ only in their rising fronts and they account for 10.5%. Group 3: the pulses differ only in their decaying tails and they account for 10.5%. Group 4: the pulses differ both in their rising fronts and decaying tails and they account for 15.8%. For different groups of LVEs, their rising times, durations, peak currents, and electrical charges transferred by them are compared. It appears that group 3 and group 4 LVEs tend to have bigger current peak and transfer more charges than group 1 and group 2 LVEs. References D. Wang, N. Takagi, and T. Watanabe, Observed characteristics of luminous variation events during the initial stage of upward positive leaders, J. Atmos. Electr., Vol.27, No.1, 61-68 (2007).