Pierre Auger Observatory (Mendoza, Argentina) — the largest cosmic-ray detector ever built: 1,660 water-Cherenkov stations over 3,000 km², overlooked by 27 fluorescence telescopes at four sites, plus underground muon detectors and a large radio array. Operating since 2004, Auger has measured the spectrum’s suppression, discovered the large-scale dipole anisotropy, and provided the world’s most precise composition data.
Telescope Array (TA) (Utah, USA) — the largest UHECR observatory in the northern hemisphere: ~500 plastic scintillator detectors over ~700 km² with three fluorescence stations. TA reported the “hotspot” excess near Ursa Major and, together with Auger, anchors full-sky coverage. TA recorded the 244 EeV “Amaterasu” event in 2021.
Joint working groups — Auger and TA operate joint analyses on spectrum, composition, and arrival directions to combine their complementary sky coverage and control systematic differences.
Upgrades underway
AugerPrime — scintillators and radio antennas on every Auger station, plus faster electronics, to separate the muonic and electromagnetic signals event-by-event and measure composition with the full-duty-cycle surface array up to the highest energies.
TA×4 — an expansion of the Telescope Array toward 4× its original area to accumulate statistics at the highest energies in the northern sky.
Next generation
GRAND (Giant Radio Array for Neutrino Detection) — a proposed set of radio arrays totalling ~200,000 antennas over 200,000 km², targeting ultra-high-energy neutrinos and cosmic rays; prototype arrays (GRANDProto300, GRAND@Auger) are running.
GCOS (Global Cosmic-ray Observatory) — a concept for a ~60,000 km² ground array, an order of magnitude beyond Auger, to do astronomy with the very highest-energy events.
POEMMA — a proposed pair of satellites observing fluorescence and Cherenkov light from orbit, monitoring huge atmospheric volumes for extreme-energy cosmic rays and tau neutrinos.
Historical milestones
1962 — John Linsley records the first ~10²⁰ eV event at Volcano Ranch.
1991 — the Fly’s Eye detector observes the “Oh-My-God” particle (~3×10²⁰ eV), still the highest-energy cosmic ray ever reported.
2007–2008 — HiRes and Auger establish the flux suppression above ~5×10¹⁹ eV.
2017 — Auger publishes the extragalactic dipole anisotropy (Science).