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

The Pierre Auger Observatory, the largest cosmic-ray detector in the world, has been
observing peculiar events which are very likely downward TGFs. Their experimental
signature and their time evolution are very different from those of a shower produced
by an ultra high energy cosmic ray. The TGF-like events happen in coincidence with
lightning and low clouds and their deposited energy at the ground is compatible with
that of a standard downward TGF with the source at few kilometers above the
ground. The surface detector (SD) of the Auger Observatory consists of 1660 water-
Cherenkov detectors (WCDs) spread over 3000 km² in the Argentinian pampa. The
WCD height of 1.2 m makes them highly sensitive to gamma rays and the large area
covered with SD allows us to sample the TGF beam from different points. The
timing shape of WCD signals can be very important to constrain different TGF source
models. Cold runaway from the high fields near the leader tips or relativistic
feedback produce the same energy spectrum but predict a different rise and fall of the
counts 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 meteorological
data at the time of Auger TGF-like events will be presented. Little is known about the
TGF-producing storms. The characteristics of these thunderstorms are being
investigated by studying meteorological data in coincidence with upward TGFs. A
similar analysis is important to better understand downward TGF production
mechanisms and investigate if are the same as those producing upward TGFs.