Studying downward TGFs with the largest ground array of gamma-ray detectors

Abstract

The Pierre Auger Observatory, the largest cosmic-ray detector in the world, has beenobserving peculiar events which are very likely downward TGFs. Their experimentalsignature and their time evolution are very different from those of a shower producedby an ultra high energy cosmic ray. The TGF-like events happen in coincidence withlightning and low clouds and their deposited energy at the ground is compatible withthat of a standard downward TGF with the source at few kilometers above theground. The surface detector (SD) of the Auger Observatory consists of 1660 water-Cherenkov detectors (WCDs) spread over 3000 km2 in the Argentinian pampa. TheWCD height of 1.2 m makes them highly sensitive to gamma rays and the large areacovered with SD allows us to sample the TGF beam from different points. Thetiming shape of WCD signals can be very important to constrain different TGF sourcemodels. Cold runaway from the high fields near the leader tips or relativisticfeedback produce the same energy spectrum but predict a different rise and fall of thecounts versus time, and they could produce a different angular distribution.Comparisons between simulations and data will be shown.Moreover, first results from a preliminary analysis of the available meteorologicaldata at the time of Auger TGF-like events will be presented. Little is known about theTGF-producing storms. The characteristics of these thunderstorms are beinginvestigated by studying meteorological data in coincidence with upward TGFs. Asimilar analysis is important to better understand downward TGF productionmechanisms and investigate if are the same as those producing upward TGFs.