Parametrization and Characterization of Multi-ELVES Events at the Pierre Auger Observatory

The Fluorescence Detector (FD) at the Pierre Auger Observatory has been observing ELVES since 2013 with a dedicated trigger and a temporal resolution of 100 ns. With this high resolution, we obtain the photon traces of the ELVES in great detail and we have observed events with multiple peaks, known as multi-ELVES. The origin of multi-ELVES is not fully understood, but it is believed that intra-cloud lightning occurring at a certain altitude ($H_{bolt}$) can produce double and even quadruple-peaked ELVES. According to this mechanism, the first peak of multi-ELVES is produced by the electromagnetic pulse (EMP) traveling directly from the source to the ionosphere and the second peak is produced by the EMP bouncing off the ground, reaching the ionosphere with a time delay. The time difference between peaks ($\Delta T$) has a functional dependence on $H_{bolt}$. This report shows that during the period 2014-20 about 27% of the events detected with the FD are multi-ELVES. Only a few events follow the EMP ground reflection mechanism while a group of multi-ELVES has another behavior. Other observables, e.g. the ratio between the first and second peak, can be analyzed to better understand the origin of this behavior. We also correlated the observables of multi-ELVES events with the lightning waveforms data from Earth Networks.