VHF and UHF Electromagnetic Radiation Produced by Streamers in Lightning
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In this letter, we report simulation results of streamer propagation and collision that produce electromagnetic radiation in the very high frequency (VHF) and ultra high frequency (UHF) bands. The streamers are initiated in overbreakdown field conditions, 1.5E(k)and2E(k), respectively, which may be found during the corona flash stage of negative leader stepping processes. We find that while streamer propagation produces stronger VHF radiation, the head-on collision of streamers dominates UHF, and even higher-frequency radiation. Analysis of the energy spectral densities obtained from different simulation cases shows that the total length and radii of colliding streamers, as well as the ambient field, are important parameters for the UHF radiation produced by streamer collisions. The larger those parameters are, the stronger UHF radiation produced. Finally, by comparing with the measured spectral magnitude of lightning field in the VHF range, it is found that there are probably 10(5)-10(7) streamers involved during the lightning corona flash stage. Plain Language Summary Despite being a familiar phenomenon, the physics of lightning initiation and propagation is not well understood. An effective approach to study lightning is to observe their radio frequency (RF) signals, which is especially critical for understanding the lightning activities inside thunderstorms, because clouds are opaque for other signals. The RF signals with frequencies above about 10MHz are commonly used to map/image lightning development. They are believed to be produced by the physical process electrically breaking down virgin air. Previous work has shown that electrical breakdown processes known as streamers, which are the precursors of lightning, can produce RF radiation below hundreds of megahertz. Our study investigates a physical process that enables lightning to produce RF radiation above hundreds of megahertz. We find that collisions between streamers can generate rapid increases of electrical current to produce RF emissions extending to tens of gigahertz. The results will be helpful for understanding and interpreting RF observations/measurements of lightning and will generate impact in the field of atmospheric and space electricity.