Downward Terrestrial Gamma-ray Flashes at the Pierre Auger Observatory?

PIERRE AUGER COLLABORATION; A. MARIAZZI

Berlin

Conferencia; 37th International Cosmic Ray Conference (ICRC2021); 2021

At the Pierre Auger Observatory, designed primarily to study ultra-high-energy cosmic rays,phenomena related to atmospheric electricity are also observed. Particularly, events have beendetected with the surface detector, characterized by long-lasting signals (tens of microseconds)and event footprints much larger (up to 200 km2) than those produced by the highest energycosmic rays. Moreover, some of them appear to be accompanied by smaller events occurring inthe same area within about 1 ms and probably produced by the same phenomenon. A previouslyreported correlation with the World Wide Lightning Location Network, as well as the observationof very low-altitude clouds, confirm that such events are related to thunderstorms. An ad-hocreconstruction points to high-energy particles being produced very close to the ground, suggestingthat they originate from electrons accelerated to relativistic energies in strong electric fields insidelow clouds, as is the case for terrestrial gamma-ray flashes above thunderstorms.A clear explanation of the observed phenomenon is hindered by two facts. One is that the rate ofsuch events, detected serendipitously, is very small (less than 2 events/year) and decreases furtherafter optimization of the surface detector trigger for low-energy shower-events. The second is thatmost events show a puzzling lack of signals in the central part of the footprint. We have studiedin detail both effects and will present such studies here. We developed a strategy for a dedicatedtrigger to enhance the detection efficiency for these events associated with atmospheric-electricityevents.