The hidden power of lightning: studying the most explosive events in thunderstorms with the world’s largest cosmic-ray observatory

The Pierre Auger Observatory, designed to study ultra-high energy cosmic rays, has accidentally observed, with its 3000 km² surface array of water-Cherenkov detectors, several events that are very likely downward TGFs. Their morphology, as well as the signals observed in the detectors, are totally different from what observed when an extensive air shower strikes the array. The TGF-like events are characterized by large footprints (~200 km²) and long signals (~ 10 µs), if compared to cosmic-ray showers. They happen in coincidence with lightning as demonstrated by the correlation with WWLLN data and with strong variations observed in the electric fields measured by the E-mills available at the Observatory. Other events within 1 ms of these peculiar events were observed in the same zone of the array. This time interval is about the time taken by the steeped leader to reach its full length. Finally, from a first reconstruction, the source altitude of this events is estimated to be very close to ground, at about 1-2 km. From lidar measurements, we know that there were low clouds at altitudes compatible with the estimated source location at the time of many events. The source altitudes can be used as input parameter for the REAM simulation to start a campaign to compare our experimental results with TGF models. The rate of TGF-like events per year is very low, less than 2 events per year. To increase the statistics, a modification to the read-out logic to give priority to events which contain long signals was implemented ans is under test.