![]() ![]() Figure 1 shows the still picture of the lightning flash in question that hit a house about 820 m away from our observation instruments at 0829:31, 17 July 2009 (local time). The digital oscilloscope is triggered via a trigger-out signal from ALPS each time when ALPS recorded a lightning flash. The outputs of the slow and the fast antenna are recorded with a digital oscilloscope which is operated at a sampling rate of 10 MHz and a recording time of 0.8 s for each triggered event. The fast antenna is about 100 times more sensitive than the slow antenna. The fast antenna has a frequency bandwidth from several kHz to around 5 MHz. The slow antenna has a frequency bandwidth from several Hz to around 3 MHz. On the roof of the building, a slow antenna with a time constant of 2.2 s and a fast antenna with a time constant of 1.5 ms were set up to record electric field changes of lightning discharges. Alongside ALPS, an ordinary home video camera was set up to record lightning flashes. The ALPS was installed in the seventh floor hallway of a building within the campus of Gifu University of Japan with an elevation angle of 12.5 degrees and a height of 24 m above the ground level. The interframe interval used in the present study was 100 ns, and thus the resulting total recording time per event was 1.6 ms. ![]() The ALPS can operate at a time resolution (interframe interval) from 100 ns to 50 ms with either internal or external trigger and can record up to 16,000 frames for each event with up to 16,000 frames of pretrigger. Each of the diodes operates at wavelengths from 400 to 1000 nm with a response time of less than 3 ns. A lightning channel imaged onto the sensor surface falls mostly (nearly 90%) onto active sensing surface. The photodiode array module consists of 256 (16 × 16) pin photodiodes, each 1.3 × 1.3 mm 2 in size, with separation of 1.5 mm between the centers of individual diodes. The ALPS consists of a conventional camera lens, a photodiode array module, 256 amplifiers with large dynamic range, a multichannel digitizer, and a personal computer system. Optical data presented in this paper were acquired using the same digital imaging system ALPS (Automatic Lightning Progressing Feature Observation System) that has been described before. From the recording, one can see how the step optical pulses look like and how the pulse discharges propagate backward along a positive leader channel. This paper presents the first optical recording which has temporally and spatially resolved the steps contained in a downward positive leader. However, compared to a negative stepped leader, since no high-speed recordings have ever been documented for a downward-moving positive stepped leader, much less is known about its stepping. recently reported a downward positive leader that is apparently a stepped leader. concluded that more than 25% of downward positive leaders could be stepped leaders. Increasing evidence has shown that some downward-moving positive leaders that precede positive first return strokes could propagate in stepped modes. 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. 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 × 10 8 m/s. 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 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. The leader propagated at a speed of around 1.0 × 10 6 m/s over the height from 272 m to 93 m and then accelerated to a speed of 2.5 × 10 6 m/s at the height of about 45 m. 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.
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