The Pierre Auger Observatory, located on a vast, high-altitude plain in the Province of Mendoza in Argentina, is the world's largest cosmic ray observatory and measures the extensive air-showers produced by cosmic rays above ~10¹⁷ eV. The intensity of high energy cosmic rays (those above about 10¹⁴ eV) is only a few particles per square meter per day and thus too low to allow for direct measurement with satisfactory statistical precision from balloons or in space. Above 10¹⁹ eV the rate falls to about 1 event per km² per year. The phenomenon of extensive air-showers must be exploited to study cosmic rays at very high energies. An air-shower is a cascade of particles induced by the interaction of a single cosmic-ray with the Earth's atmosphere. These showers can be observed by telescopes that pick up the fluorescence radiation emitted from nitrogen molecules excited as the shower transverses the atmosphere, while the particles reaching the ground can be sampled by large arrays of detectors. The properties of these extensive air-showers are measured to determine the energy and arrival direction of each cosmic ray and to provide a statistical determination of the distribution of primary masses.

The Observatory features an array of 1600 water-Cherenkov particle detectors (WCD) spread over 3000 km² on a 1500 m triangular grid (SD1500, grey dots in the map). The array is overlooked by 24 air-fluorescence telescopes, grouped at four sites, located at Los Leones, Los Morados, Loma Amarilla and Coihueco (FD, colored markers in the map). Stable data-taking started on 1 January 2004 with 154 water-Cherenkov detector stations and two fluorescence sites partly operational. Installation of the FD and SD1500 array was completed in June 2008. The small number of holes visible in the array map are the result of difficulties in accessing the sites or with local landowners. After 2008, 50 additional WCDs were nested within the SD1500 array in the region close to Coihueco, forming an array of 71 stations with a spacing of 750 m (SD750) and covering an area of about 27 km². At the same time, three high elevation air-fluorescence telescopes (HEAT) overlooking the SD750 array were added. The installation of the SD750 array and of the HEAT telescopes was completed in 2011. The Observatory is located at a mean elevation of about 1400 m, corresponding to an atmospheric overburden of about 875 g cm^-2, between latitudes 35.0°S and 35.3°S and between longitudes 69.0°W and 69.4°W.

Each water-Cherenkov station is filled to a depth of 1.2 m with highly-purified water enclosed within a diffusively-reflective liner. The water is viewed from above by three 9-inch photomultiplier tubes (PMTs) in contact with it. These detect Cherenkov light emitted by charged particles that enter the detectors. Each PMT provides two signals which are tagged with GPS time stamps to an absolute accuracy of 12 ns and are digitized using 40 MHz, 10-bit Flash Analog-to-Digital Converters (FADCs). A low-gain signal is taken directly from the anode of the PMT, while a high-gain signal is provided by the last dynode and amplified to be nominally 32 times larger than the low-gain signal, enhancing the total dynamic range to span more than three orders of magnitude in integrated signal.

Information about the time and nature of station signals satisfying various trigger criteria are sent to a computer at the central campus via a purpose-built communications network at a rate of about 20 Hz. If spatial and temporal coincidences are identified, data from triggering stations are recorded and an event is reconstructed from the temporal and signal information.

The data from the fluorescence emission are collected by a set of six telescopes at each of the four FD sites, covering 30 degrees of elevation from the ground up and 6 x 30° over the array. The three telescopes of HEAT operate in the range of elevation angles from 30° to 60° over the FD-Coihueco field of view. Each fluorescence telescope consists of a camera with 440 photomultiplier tubes (pixels) recording the ultraviolet light received in each 100 ns time interval. At each site, an event is recorded whenever there are several pixels with signals above the night-sky background light, compatible with localized patterns on the camera. The GPS time is used to connect the fluorescence event to those seen simultaneously in other FD sites (multi-eye event) and to water-Cherenkov stations that have signals (hybrid event).

The Auger Observatory is operated by a Collaboration of more than 400 scientists, engineers, technicians and students from more than 90 institutions in 18 countries. You can find further information about the Observatory and the Collaboration in Nucl.Instrum.Meth.A 798 (2015) 172-213 (arXiv) and on the Auger website.

Atmospheric monitoring at the Pierre Auger Observatory

The Observatory makes use of the atmosphere as a giant calorimeter. This has required the implementation of an extensive program to monitor the atmosphere above the site, as detailed knowledge of the atmosphere is required for the accurate reconstruction of air showers observed by both fluorescence and surface detectors.

The atmospheric state-variables, such as temperature, pressure and humidity, are needed to reconstruct the amount of fluorescence light emitted by the air showers and hence to discover their longitudinal development. The measurements with the surface detector are also affected by the changes of atmospheric conditions. Varying air densities close to the ground modify the lateral spread of the electromagnetic component of the extensive air-showers. Varying air pressure affects the trigger probability and the rate of events detected above a fixed energy. The atmospheric conditions at the ground are monitored every five to ten minutes by a series of weather stations placed at the sites of each of the four fluorescence detectors, and at the center of the surface detector array.

Aerosols and clouds are also monitored, as they impact the atmospheric transmission of the optical signal from the air shower to the fluorescence detectors. The optical transmission must be taken into account, so as to reconstruct the light generated along the shower axis accurately, starting from the light recorded at the telescopes. At central positions within the surface detector array, two laser facilities are installed (CLF and XLF on the map). These instruments are used to fire beams of UV light into the atmosphere every 15 minutes, to measure the aerosol attenuation of the fluorescence light in the line-of-sight of each telescope. Four infra-red cloud cameras mounted on "pan-and-tilt" platforms are used to scan the night sky for clouds and provide the cloud coverage. In addition to cloud cameras, also satellite data (GOES) are used to infer the cloud coverage above the array. The cloud obscuration deduced from these measurements is used in the analysis of data from the air-fluorescence telescopes. Finally, elastic lidars are operating at the fluorescence detector sites to measure the altitude of the cloud layers and the uniformity of the aerosol distribution horizontally across the Observatory, and one Raman-lidar receiver is located at the CLF to provide the vertical aerosol attenuation and other measurements of the atmospheric properties.

Space-weather monitoring at the Pierre Auger Observatory

Although the Pierre Auger Observatory was conceived to study cosmic rays at the highest energies, it can also be used to monitor the phenomena that take place in the space surrounding the Earth, influenced by the variability of the Sun over periods ranging from hours to year. The so-called "space weather"

The solar activity, in particular, changes the intensity of cosmic rays of low energy arriving at the Earth. This intensity can be measured at the Observatory using the so-called 'single-particle technique'. This technique consists of counting all of the particles hitting the individual water-Cherenkov detectors, independently of whether they belong to a large shower or are the lonely survivors of small showers. Most of the events detected by the single detectors are, in fact, due to solitary particles, and are the residuals of showers generated by cosmic rays with energy between 10¹⁰ eV and 10¹² eV.

The flux of cosmic rays at these energies is modulated by solar activity because, after propagation in our Galaxy, they reach the heliosphere and interact with the magnetic field of the Sun embedded in the solar wind. The conditions of the solar wind are variable due to the modulation of the solar cycle and transient eruptions of solar ejecta. The solar magnetic field intensity and polarity change with time, following the cyclic solar activity: when sola activity is high, the flux of cosmic rays in the heliosphere is low; in turn, when the Sun is in a quiet state, the flux of cosmic ray at Earth is at its maximum. The solar activity can thus be monitored by analyzing the rate of particles detected by the individual water-Cherenkov detectors, which exhibits time variations such as those due to the short-term Forbush Decreases and to the longer-term modulations over 11- and 22-year solar cycles.

Atmospheric electricity studies at the Pierre Auger Observatory

Since 2014, the Pierre Auger Observatory is recording a special class of ionospheric phenomena called ELVES (Emission of Light and Very low frequency perturbations due to Electromagnetic pulse Sources). ELVES are flashes of light emitted by the base of the ionosphere, at an altitude of about 90 km, which occur when a strong electromagnetic pulse is produced during the evolution of a lightning strike. ELVES are quasi circular light fronts, with apparent propagation velocities which exceed the speed of light, which can be detected during the night by our fluorescence detectors. Most of the ELVES detected by the Auger Observatory are produced by strong thunderstorms occurring north-east and east of the observatory, at distances up to 1000 km, and are most frequently observed during the storm season, between November and April. The Auger Observatory is the first detector capable of producing 2-D animations in slow motion of the evolution of these light fronts, which sometimes are observed in pairs, or even multiplets (the Auger Observatory has observed the first triple ELVES). Recently, the ASIM experiment on the International Space Station has confirmed the direct connection between ELVES and Terrestrial Gamma Flashes (TGF), another very interesting phenomenon which occurs during lightning development, during which strong currents are capable of accelerating electrons to relativistic energies.