Cosmic rays can be studied indirectly by detecting the billions of high-energy particles they produce in interactions with the Earth's atmosphere. The secondary particles propagate through the atmosphere re-interacting and increasing their number finally resulting in a cascade, called an "extensive air shower"(EAS), and arrive at ground level with billions of energetic particles extending over an area as large as 20 square kilometers. The Auger Observatory is a "hybrid detector," employing two independent methods to detect and study high-energy cosmic rays. One technique detects the high energy particles which arrive at earth. The other technique tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere .

Image 1: Sketch of the hybrid technique for EAS detection

The first detection method uses the Observatory's main visible feature - the 1,600 water tanks that cover an enormous section of the Pampa (3000 km 2 ) and serve as particle detectors. Each tank is filled with 1 2,000 litres of ultra pure water , separated from each of its neighbors by 1.5 kilometers, and completely dark inside - except when particles from a cosmic ray air shower pass through it. These energetic particles are traveling faster than the speed of light in water when they reach the detectors; therefore, their electromagnetic shock waves produce Cherenkov light that can be measured by photomultiplier tubes mounted on the tanks. Extensive air showers contain billions of secondary particles and can cause nearly simultaneous bursts of light in several (at least three) tanks. Scientists can determine the energy of the primary cosmic ray particle by first measuring the amount of light detected in the tanks that are hit by secondary particles, and afterwards reconstructing the total number of secondary particles that are sampled by the surface array. Slight differences in the detection times at different tank positions are used to determine the trajectory of the incoming cosmic ray.

Image 2: Water Cerenkov tank in the Pampa Amarilla. EAS detection.

The charged particles in an air shower also interact with atmospheric nitrogen, causing it to emit ultraviolet light via a process called fluorescence, which is invisible to the human eye - but not to the Auger Observatory's optical detectors.

The observatory's second detection method uses these detectors to observe the trail of nitrogen fluorescence and tracks the development of air showers by measuring the brightness of the emitted light. To the 4 fluorescence detectors (eyes), a cosmic ray looks like a UV light bulb passing through the atmosphere at the speed of light, with an ever-increasing brightness that can reach up to four watts as the cascade grows to its maximum size. Using a grid of large telescopes to collect the light, cameras can view the air shower up to 15 kilometers away.

The Auger Observatory's fluorescence detectors are much more sensitive than the human eye and can "see" distant air showers development. Occasionally, a cascade will occur in a place where two fluorescence detectors can record it, which allows for very precise measurements of the direction the cosmic ray came from. From the total amount of light emitted in the atmosphere it is possible to determine the energy of the primary particle, when the effect of the propagation of the light in the atmosphere is properly taken into account.

Image 3: Fluorescence detector mirror and camera

Image 4: View of Los Leones fluorescence building(eye).

Using Lidar Systems, located near each fluorescence eye, the quality of air is monitored during the nights of fluorescence detector data taking. Also near the center of the array there are two central laser facilities, used to fire laser shots into the sky at night to calibrate the response of the nitrogen fluorescence detectors.

Image 5: Mirrors of the Lidar system used for the atmospheric monitoring.

Image 6: Central Laser Facility located in the center of the SD array

An observatory campus is located in the town of Malargüe, at the edge of the array, with an office building,, a computer center that performs data collection and storage, and assembly building (where surface detectors are prepared prior to deployment in the field).

Image 7: View of the Observatory Campus located in the town of Malargue

Employing these two complementary observation methods provides the Auger Observatory with high quality information about the types of particles in the primary cosmic rays. Comparing results from the different types of detectors also helps scientists reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays

Image 8: Hybrid detection display of a Cosmic Ray Shower

Image 9: Longitudinal Shower profile as seen by the Auger fluorescence detector

The fluorescence detectors are able to detect the total energy of an air shower, which is approximately equal to the energy of the primary cosmic ray. Total cosmic ray energy is more difficult to determine with the surface detectors, which sample a small fraction of the energy of an air shower. Comparing data from the two methods allows scientists to better understand data from both detection methods and work on increasing the accuracy of both techniques. While the fluorescence detectors only work on clear, moonless nights, the surface detectors are always operating regardless of atmospheric conditions.

The Auger Observatory is in the final stages of construction and has begun to collect data near Malargüe, Argentina, a town in Mendoza Province that lies just east of the Andes Mountains. A matching site will also be built in southeastern Colorado, providing nearly uniform coverage of the skies in the northern and southern hemispheres. If cosmic rays are found to arrive from specific directions, the Auger Observatories will be able to identify and study possible cosmic ray sources all over the sky with equal sensitivity. If discrete sources are not found, the full-sky coverage provided by the two sites will be essential for determining whether cosmic ray arrival directions are characterized by subtle large-scale patterns in the universe, or whether they are completely arbitrary.