Detection and triggering of ELVES at the Pierre Auger Observatory

We report about the observation of sixty ELVES events using the Fluorescence Detector of the Pierre Auger Observatory, located in Malargue (Argentina), currently the world's largest facility for the study of ultra high energy cosmic rays ( E ≥ 10¹⁸ EeV). The Fluorescence Detector (FD) consists of 24 telescopes, each with a field of view of 30°x30°, grouped in units of 6 at four locations overlooking an array of 1600 water Cerenkov detectors laid out over an area of 3000 km². Exploiting a time resolution of 100 ns and a space resolution of about 1^o , and stereo viewing capabilities, the Fluorescence Detectors can provide 3D imaging of elves, originating at distances of several hundred kilometers, with unprecedented accuracy. Fits of the time evolution of the light front are based on a simple geometric model. The ELVES events are extracted from a dataset taken with large prescaling factor, which we used to develop a trigger strategy, described in this report. The upgraded trigger will allow to acquire ELVES data with much higher efficiency in the coming years of operation.;