Abstract
The Pierre Auger Observatory in Malargüe, Argentina, is designed to study the properties of ultra-high energy cosmic rays with energies above 1018eV. It is a hybrid facility that employs a Fluorescence Detector to perform nearly calorimetric measurements of Extensive Air Shower energies. To obtain reliable calorimetric information from the FD, the atmospheric conditions at the observatory need to be continuously monitored during data acquisition. In particular, light attenuation due to aerosols is an important atmospheric correction. The aerosol concentration is highly variable, so that the aerosol attenuation needs to be evaluated hourly. We use light from the Central Laser Facility, located near the center of the observatory site, having an optical signature comparable to that of the highest energy showers detected by the FD. This paper presents two procedures developed to retrieve the aerosol attenuation of fluorescence light from CLF laser shots. Cross checks between the two methods demonstrate that results from both analyses are compatible, and that the uncertainties are well understood. The measurements of the aerosol attenuation provided by the two procedures are currently used at the Pierre Auger Observatory to reconstruct air shower data.
Highlights
Direct measurements of primary cosmic rays at ultra-high energies above the atmosphere are not feasible because of their extremely low flux
The Pierre Auger Observatory [1] in Argentina combines two well-established techniques: the Surface Detector, used to measure photons and charged particles produced in the shower at ground level; the Fluorescence Detector, used to measure fluorescence light emitted by air molecules excited by secondary particles during shower development
If the aerosol attenuation is not taken into account, the shower energy reconstruction is biased by 8 to 25% in the energy range measured by the Pierre Auger Observatory [3]
Summary
Direct measurements of primary cosmic rays at ultra-high energies (above 1018 eV) above the atmosphere are not feasible because of their extremely low flux. The fluorescence technique to detect EAS makes use of the atmosphere as a giant calorimeter whose properties must be continuously monitored to ensure a reliable energy estimate. Atmospheric parameters influence both the production of fluorescence light and its attenuation towards the FD telescopes. At the Pierre Auger Observatory, molecular and aerosol scattering in the near UV are measured using a collection of dedicated atmospheric monitors [3]
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