Detection of multiple terrestrial gamma-ray flashes from thunderstorm systems

Since their discovery, Terrestrial Gamma ray Flashes (TGFs) exhibited an evident correlationwith thunderstorms and lightning activity. The fleeting nature of these events and the heavy absorptionof gamma rays in the lowest atmospheric layers severely hamper the observation of this phenomenon,making us reveal just a small fraction of a probably much wider population. As each thunderstorm producesa large amount of lightning discharges during its lifetime, it is reasonable that even a large amount of TGFsare produced during the same event. However, detection of multiple TGFs coming from the same stormis difficult to perform, as it requires the constant monitoring of a spatially limited geographic region:this is not an easy task to perform for satellites on high-inclination orbits that make them experiencenonnegligible latitudinal shifts at each orbital passage over a certain region, preventing the monitoringof a limited geographic region throughout successive overpasses. In this perspective, the quasi-equatorial(2.5?) orbit of the Astrorivelatore Gamma ad Immagini LEggero (AGILE) satellite ensures a minimal latitudinalshift when flying over the same region at successive passages, allowing for the follow-up of thunderstormsin time. We exploit this feature of the AGILE satellite to search for multiple TGFs coming from the samegeographic region and, in particular, from the same thunderstorm. We carry out this search on the AGILETGF database (2009-2016), ending up with a sample of 79 systems producing more than one TGF, bothduring the same overpass and up to four overpasses after. Data acquired by geostationary meteorologicalsatellites and cross correlation with radio sferics detected by World Wide Lightning Location Network areused to support this investigation. The AGILE satellite for the first time clearly establishes the multipleoccurrences of TGFs from convective thunderstorms, both on timescales of minutes to several hours.