Nature Geoscience, 2 (2009) 757-760; published online 25 October 2009; doi: 10.1038/ngeo662
Emergence of sprite streamers from screening-ionization waves in the lower ionosphere
Abstract
Sprite discharges above thunderclouds at altitudes of 40–90 km (refs 1, 2, 3, 4, 5) are usually created by a strong positive cloud-to-ground lightning flash6. Sometimes these sprite discharges emerge from a visible halo5, 7, 8, 9, and during the first stage they always propagate downwards and branch on their way5, 7, 9, 10, 11. Modelling efforts have been restricted to conditions of non-ionized air of constant density and show double-headed sprites12 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 grid13 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 observations9 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.
¹Centrum voor Wiskunde en Informatica (CWI), PO Box 94079, 1090 GB Amsterdam, The Netherlands
²Department of Applied Physics, Eindhoven University of Technology, The Netherlands
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