The angular distribution of energetic electron and X‐ray emissions from triggered lightning leaders

Abstract

We investigate individual X‐ray bursts from lightning leaders to determine if energetic electrons at the source (and hence X‐rays) are emitted isotropically or with some degree of anisotropy. This study was motivated by the work of Saleh et al. (2009), which found the falloff of X‐rays in concentric radial annuli, covering all azimuthal directions in each annulus, from the lightning channel to be most consistent with an isotropic electron source. Here we perform a statistical analysis of angular and spatial distributions of X‐rays measured by up to 21 NaI/PMT detectors at the International Center for Lightning Research and Testing site for 21 leader X‐ray bursts from five leaders (including four dart‐stepped leaders and one dart leader). Two procedures were used to complete this analysis. Procedure 1 found the first‐order anisotropy, and procedure 2 tested whether or not the angular distribution was consistent with an isotropic distribution. Because higher‐order anisotropies could be present in the data, a distribution that is not isotropic does not necessarily have a significant first‐order anisotropy. Using these procedures, we find that at least 11 out of 21 X‐ray bursts have a statistically significant first‐order anisotropy, and hence those 11 are inconsistent with an isotropic emission. The remaining 10 bursts do not have significant first‐order anisotropy. However, of those 10 bursts, 9 are inconsistent with isotropic emission, since they exhibit significant higher‐order anisotropies. Since Saleh et al. (2009) did not consider anisotropies in the azimuthal direction, these new measurements of anisotropy do not necessarily contradict that work. Indeed, our analysis supports the finding that the X‐ray emissions from lightning are inconsistent with a vertically downward beam. The level of anisotropy of the runaway electrons is important because it provides, in principle, information on the streamer zone in front of the leader and the electric field near the lightning leader tip.