Optical power and energy radiated by return strokes in rocket‐triggered lightning

Abstract

AbstractThe broadband optical radiation covering the visible and near‐infrared (VNIR) spectral regions (0.4–1.0 μm) has been measured from 70 negative return strokes (RS) in rocket‐triggered lightning; 17 events were recorded in 2011, and 53 were recorded in 2012. The radiometers were calibrated, and all measurements were time‐correlated with currents measured at the channel base. The risetime and peak of an irradiance waveform are determined primarily by the RS current and by the geometrical growth and total length of channel that is in the field of view of the sensor. Following an initial peak, the irradiance decays faster than the current until there is a plateau or secondary maximum 20 to 40 μs (median of 22 μs) after the peak current, a time when the current itself is steadily decreasing. Estimates of the space‐ and time‐average optical power per unit length (ℓo) that is emitted at the source during onset of RS have been computed using the measured slopes of 70 irradiance waveforms together with an assumption that the initial speed of propagation is 1.2 × 108 m/s. The values range from 0.25 to 9.5 MW/m, with a mean and standard deviation of 2.4 ± 1.7 MW/m, and they are in good agreement with prior estimates of ℓo that were made by Quick and Krider (2013) for the subsequent return strokes in natural lightning that reilluminate a preexisting channel. The values of ℓo also agree with numerical estimates of the VNIR power per unit length that were computed by Paxton et al. (1986). Estimates of the peak optical power per unit length (ℓR) that is radiated at the source have been derived from the peaks of 53 irradiance waveforms, and the values range from 0.4 to 11 MW/m with a mean and standard deviation of 4.2 ± 2.5 MW/m. Both ℓo and ℓR are approximately proportional to the square of the peak current at the channel base. Estimates of the total optical energy per unit length, Jo, that is radiated in the VNIR have been computed by integrating the irradiance waveforms over 2 ms. The values of Jo have a mean and standard deviation of 150 ± 140 J/m, and they are proportional to the total charge that is transported to ground in that interval.