ENLIGHTEN · ExploriNg Lightning Initiation and Gamma-ray High-energy emission in Thunderstorm ENvironments

Lightning strikes earth 45 times each second, but we still don’t understand exactly what initiates it. The large electric fields in thunderclouds are also responsible for charged particle acceleration, ultimately leading to gamma-ray production. Energetic radiation is an essential component of thundercloud dynamics. Energetic radiation and lightning discharge appear tightly linked, but the nature of this link is yet to be understood. ENLIGHTEN posits a causal relationship between energetic radiation and lightning initiation. Combining novel time-resolved gamma-ray imaging of thunderclouds with simultaneous lightning discharge imaging in the radio band, we aim to establish the causal mechanisms which initiate lightning and, in doing so, address one of the longest standing unsolved questions in atmospheric electricity. Overarching hypothesis: lightning initiation is triggered by electron avalanches conducive to gamma-ray production. To test this hypothesis, we identify three research objectives:1. Mapping the spatio-temporal evolution of intense large-scale electric fields in tropical thunderclouds. 2. Imaging in gamma-rays of the newly discovered sub-second gamma-ray transients - flickering gamma-flashes and glow bursts. 3. Quantify the runaway electron avalanche discharge co-located with lightning initiation. ENLIGHTEN will achieve its objectives with a methodology which combines: a) time-resolved gamma-ray imaging of thunderclouds, b) time-resolved lightning imaging in the radio band, c) use of a high-altitude research aircraft, d) data analysis and modelling. Thunderclouds are the birthplace of a large variety of particle and gamma-ray phenomena, not to be considered just exotic and rare, but intrinsic and fundamental for thunderclouds. With 3 million lightning per day affecting life on planet Earth, unravelling the secrets of how gamma-ray emissions is linked to this common phenomenon is of great importance for the mankind.