© Pierre Auger Collaboration/Tobias Schulz

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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. Using a grid of focusing mirrors to collect the light, cameras can view the air shower up to 15 kilometers away.

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A builiding of the Fluorescence Detector hosts 8 fluorescence detectors. 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.
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The Auger Observatory is a "hybrid detector," employing two independent methods to detect and study high-energy cosmic rays. One technique detects high energy particles through their interaction with water placed in surface detector tanks. The other technique tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere.
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The Pierre Auger Project is the largest scientific enterprise ever conducted in the search for the unknown sources of the highest-energy cosmic rays ever observed.
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Each 3,000-gallon (12,000 liter) tank, separated from each of its neighbors by 1.5 kilometers, is 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.

Each 3,000-gallon (12,000 liter) tank, separated from each of its neighbors by 1.5 kilometers, is 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.
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