ELVES are being studied since 2013 with the twenty-four FD Telescopes of the Pierre Auger
Observatory, in the province of Mendoza (Argentina), the world’s largest facility for the study
of ultra-high energy cosmic rays. This study exploits a dedicated trigger and extended readout.
Since December 2020, this trigger has been extended to the three High Elevation Auger Telescopes
(HEAT), which observe the night sky at elevation angles between 30 and 60 degrees, allowing a
study of ELVES from closer lightning. The high time resolution of the Auger telescopes allows
us to upgrade reconstruction algorithms and to do detailed studies on multiple ELVES. The origin
of multiple elves can be studied by analyzing the time difference and the amplitude ratio between
flashes and comparing them with the properties of radio signals detected by the ENTLN lightning
network since 2018. A fraction of multi-ELVES can also be interpreted as halos following ELVES.
Halos are disc-shaped light transients emitted at 70-80 km altitudes, appearing at the center of the
ELVES rings, due to the rearrangement of electric charges at the base of the ionosphere after a strong
lightning event.