OREO: a balloon project to observe high-energy radiation in thunderstorms

Long-timescale gamma ray enhancements have been observed in regions inside thunderstorms by planes and balloons [e.g., Eack and Beasley, JGR, 120, 6887, 2015, and references therein; Kelley et al., Nature Comm., 6, 7845, 2015]. Until now, those observations have been mostly done using a few standalone instruments and the spatial extent and typical duration of gamma ray glows are still not well-known. However, gamma ray glows are crucially important to characterize in order to understand acceleration processes causing terrestrial gamma ray flashes (TGFs) [e.g., Smith et al., JGR, 123, 11,32111,332, 2018; Wada et al., Commun. Phys., 2, 67, 2019] and possibly thunderstorm electrification dynamics [Kelley et al., 2015]. In this contribution, we present the balloon project OREO funded by the French space agency (CNES). This project aims at launching clusters (3-4) of lightweight balloons into thunderstorms. The payload of each balloon consists of two instruments especially developed to observe gamma ray glows and TGFs: one gamma ray spectrometer with count-time-tagging capabilities and one field mill to characterize the local electrostatic field. We will present the specifications of each instrument and the launching strategies of the project.