Spatial and energy distributions of X-ray emissions from leaders in natural and rocket triggered lightning

Energetic radiation is known to be produced by lightning. To investigate these emissions, ground-based observations are being conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL where measurements of energetic radiation from both natural and rocket-triggered lightning discharges are recorded. In the present study, data from two natural negative cloud-to-ground stepped leaders and one rocket-triggered "chaotic" dart leader are analyzed in detail to investigate X-ray energy spectra and spatial X-ray distributions around the source. These measurements are compared with Monte Carlo simulations of runaway electron propagation with the goal of understanding the underlying mechanism of runaway electron production and their role in lightning initiation and propagation. We show that the energetic electrons that produce X-rays exhibit a characteristic energy less than 3MeV for two natural and one rocket-triggered leaders investigated. In addition to studying these three leaders, energetic electron luminosity, total energy, and energetic electron per meter are compared to the following return stroke currents for 28 leaders from 12 different triggered flashes. Electron luminosity is found to increase exponentially with return stroke current up to about 10 kA and to be roughly constant for larger currents. The maximum electron luminosity, which is determined indirectly through X-ray luminosities, is on the order of 10¹⁷ electrons/s, which is less than the value found from theoretical calculations.