The Fluorescence Detector (FD) at the Pierre Auger Observatory has triggered on numerous elves since the first observation in 2005, and it has potential for simultaneous Terrestrial Gamma ray Flashes (TGF) detection. In addition, the Surface Detector (SD) observed peculiar events with radially expanding footprints, which are correlated with lightning strikes reconstructed by the World Wide Lightning Location Network (WWLLN).
Emissions of Light from Very low frequency perturbations due to Electromagnetic pulse Sources (elves) expand radially up to 300 km (in ~1 ms) at the base of the ionosphere. With the 100 ns time resolution of the FD, Auger provides the community with a detailed structure of the emission region, necessary for the study of various lighting discharges (ie: compact intra-cloud discharges and energetic in-cloud pulses) possibly associated with the current hot topic in atmospheric electricity physics, TGF’s. In 2014, we improved the elves trigger for the Auger FD to allow the acquisition of photon traces up to 300 us and better our reconstruction of the lightning bolt position. In addition, the 30 degree field of view of individual FD telescopes is wide enough to capture lightning-related phenomena happening just above thunderstorms (~20 km altitude) and correlated elves at the base of the ionosphere (~90 km altitude).
Also in 2005, Auger found the first peculiar SD event with long-lasting traces (~10 us) compared to typical muon signals (~0.1 us). Since then, approximately 30 sporadic events were found to have similar, radially expanding footprints and time structures. The footprints vary from 2 to 8 km. Using the reconstruction of the events, we found that the observed timing is consistent with a spherical front expanding at the speed of light with an origin point very close to ground. In addition to the presence of triggered SD stations with high-frequency noise caused by lighting RF signals, many events are in coincidence with WWLLN. More recent focus has been on potential trigger improvements for the detection of future events.