Investigation on HF-VHF Electromagnetic Emissions from Collision of Sprite Streamers

International audienceSprites are complex discharges that consist of many plasma filaments named streamers. They are produced high above thunderstorms (40-90 km), usually in association with positive cloud-to-ground lightning. It is known that sprites produce electromagnetic radiation observed typically in the extremely low (ELF) to ultra low (ULF) frequency bands [e.g., Cummer et al., GRL, 25, 1281, 1998]. More recently, sprites have been found to be associated with LF emissions in the range ~50 to 350 kHz [Fullekrug et al., JGR, 115, A00E09, 2010], which emissions have later been proposed to be related to streamer expansion processes [Qin, et al., GRL, 39, L22803, 2012]. In a different context, Ihaddadene and Celestin [JGR, 122, 1000, 2017] have shown that collisions between streamers with opposite polarities would lead to strong electric field variation on the order of a few picoseconds in air at ground-level. It is worth mentioning that collisions between sprite streamers are common. Moreover, use of similarity laws shows that those few picoseconds would turn into a timescale on the order of a fraction of microsecond at 50 km, which hence leads to the interesting possibility of HF-VHF emissions from sprites. These characteristic emissions might in turn carry a signature of given processes in sprite discharges. The future space mission TARANIS funded by CNES will measure electromagnetic emissions associated with Transient Luminous Events, such as sprites, from DC to 30 MHz. Quantifying possible emission processes and identifying corresponding signatures is therefore of prime importance for the preparation of TARANIS. Furthermore, the French radio telescope NenuFAR [Zarka et al., ICATT, 13-18, 2015] is sensitive in the frequency range 10-85 MHz and will attempt observation of sprites as part of one of its Key Programs. Using a streamer fluid model to simulate the collision of streamers in sprites and a simple antenna model, we present an effort on modeling relevant sprite streamer processes generating characteristic electromagnetic radiation in the HF-VHF range and investigate on its possible detection by TARANIS and ground-based instruments