Minimum inter-stroke time between consecutive return strokes in a flash observed by the Japanese Lightning Detection Network

摘要

Authors have observed E-field waveforms associated with lightning discharges by the Lightning E-Field Waveform Recording Device (LEFWRD) where the LPATS-IV sensor is used as the measuring devices since spring of 2015. The LEFWRD has capabilities to observe E-field waveforms associated with lightning discharges at a radius of approximately 30 km to 200 km from the observation site. The Japanese Lightning Detection Network (JLDN) consists of 30 LF sensors and observes lightning discharges in the whole area of Japan. We found out that there were lightning flashes whose inter-stroke time between return strokes in a flash was less than 500 microseconds in the results of observation by using the JLDN. Approximately 85% of inter-stroke time between return strokes in a flash observed by the JLDN was less than 100 microseconds in the population of return strokes whose inter-stroke time was less than 500 microseconds. We found out that the subsequent peak, which existed in a waveform of the initial peak with inter-stroke time of less than 100 microseconds, was misclassified as other strokes of the identical flash by the JLDN. Therefore, subsequent strokes detected by the JLDN should be eliminated when the inter-stroke time was less than 100 microseconds. On other hand, the strokes occurring at the inter-stroke time more than 200 microseconds from the preceding strokes were correctly categorized as subsequent strokes in the identical flash. This indicates that the minimum inter-stroke time between return strokes in a flash is in the range of 100 - 200 microseconds. We analysed data sets observed by the JLDN and simultaneously observed the waveforms of lightning E-field by using the LEFWR in the time period from April to August, 2015. In the results, we found out negative subsequent strokes whose inter-stroke time was approximately 130 microseconds. This inter-stroke time was between two return strokes of ten ones in a flash observed by the JLDN. This flash was negative and hit to a mountain whose altitude was approximately 700 m when the altitude of negative electric charge was existing between 5700 m (-5C degree) and 6700 m (-10 C degree). This means the length of the return strokes whose inter-stroke time was 130 microseconds was approximately 5500 m. The time for the return stroke of the preceding stroke was approximately 37 microseconds when the speed of return stroke was 0.5c (c: the speed of light). We assumed that the time for the dart leader initiated the subsequent stroke was approximately 93 microseconds. Therefore, the speed of the dart leader for it was estimated approximately 5.9 × 10~7 m/s. The various values regarding the speed of dart leader have been reported for long time. Generally, it is said that the speed of dart leader is 1.0 - 2.0 × 10~7 m/s. However, a report said that the speed of the dart leader was 4.9 × 10~7 m/s (estimated error -20% and +40%) and there was a tendency to increase the speed of the dart leader with decreasing preceding inter-stroke time. Therefore, we can safely say the minimum inter-stroke time between the return strokes in a flash was 130 microseconds.