Optical emissions associated with energetic electrons produced by stepping leaders in cloud-to-ground lightning discharges

Both natural cloud-to-ground and rocket-triggered lightning flashes have been found to be associated with intense and brief bursts of X-ray emissions. Using a full energy Monte Carlo model combined with an optical emission model, we quantify the optical emissions induced by the strong acceleration of thermal runaway electrons in the highly inhomogeneous electric field produced by stepping lightning leaders. The results demonstrate that this acceleration process is accompanied with not only high-energy X-ray emissions but also detectable levels of optical emissions. The fluorescence beam exhibits a conical shape and appears to be larger than the streamer zone associated with the lightning leader. Moreover, we emphasize that the size of the fluorescence beam largely depends on the electrical properties of the associated lightning leader and the intensity ratio between emissions from the second positive band system of N₂ and the first negative band system of N 2+ reflects the intrinsic difference in the energetics of electrons involved. Therefore, corresponding measurements compared to modeling results can provide useful information for understanding of the stepping mechanism of lightning leaders and associated X-ray production processes. Key Points Optical emissions from energetic electrons produced by lightning leaders Predictions on morphological and spectroscopic features of optical emissions Using optical emissions to evaluate the electrical properties of lightning leaders.