Emergence of sprite streamers from screening-ionization waves in the lower ionosphere

Sprite discharges above thunderclouds at altitudes of 40-90 km (refs 1-5) are usually created by a strong positive cloud-to-ground lightning flash(6). Sometimes these sprite discharges emerge from a visible halo(5,7-9), and during the first stage they always propagate downwards and branch on their way(5,7,9-11). Modelling efforts have been restricted to conditions of non-ionized air of constant density and show double-headed sprites(12) or sprites starting from metal electrodes, but they do not explain why observations exclusively record sprites that propagate downwards. Here we present simulations with a numerical discharge model on a non-uniform, dynamically adapted computational grid(13) to capture the wide range of emerging spatial scales, and we use realistic air and electron densities that vary with altitude. Our model shows a downward-propagating screening-ionization wave in the lower ionosphere that sharpens and collapses into a sprite streamer as it propagates farther down. Streamer velocity, diameter and length until branching agree with observations(9) within measuring accuracy. We speculate that sprites generically emerge through the collapse of a wide screening-ionization wave into a sprite streamer, although this wave is only sometimes visible as a luminous halo.