Luminosity waveforms versus height for triggered-lightning return strokes and M-components

Abstract:

We present and analyze luminosity waveforms versus height from triggered lightning return strokes and M-components in conjunction with our measurements at the International Center for Lightning Research and Testing (ICLRT) of channel-base current and close electric field. In summer 2014 we constructed a system consisting of 11 Thorlabs APD120A2 avalanche photodiode modules, with bandwidths of 50 MHz (corresponding to about 10 ns 10-90% risetime to a step function of light) and an optical bandwidth from 200 nm to 1,000 nm with a peak response at 600 nm. Each of the 11 photodiodes observed different heights ranging from roughly 4 m to 115 m above the bottom of the triggered lightning channel. In summer 2015, we expanded the 2014 system to monitor altitudes ranging from roughly 4 m to about 1 km above the lightning channel bottom. The new system consists of 12 Thorlabs APD120A2 and 20 Thorlabs APD130A, both types having the same frequency response with a -3 dB high frequency cutoff around 50 MHz, but having different optical bandwidths, the latter photodiode from 400 nm to 1,000 nm with a peak response around 800 nm. All 32 photodiodes were mounted on a platform and positioned 293 m from the ICLRT rocket-launcher. The 32 channels are digitized at 100 MHz with time accuracy better than 10 ns. Here we show data from the 2014 system (see also Carvalho et al., 2014 and Carvalho et al., 2015 (under review)) and the first measurements of luminosity versus height to 1 km from the 2015 device. The first dataset measured with the 32-photodiode-system was acquired on July 19^th, 2015 for an altitude-triggered flash. At the time of this Abstract submission, data have been acquired for 11 dart-leader/return stroke sequences in four 2015 triggered flashes, some containing M-components. We (1) show that different optical bandwidths produced different measured luminosity signal for the same triggered-lightning channel height, (2) measure the luminosity speed and other characteristics of return strokes and M-components, and (3) infer upward current speed for return strokes.