Techniques for measuring aerosol attenuation using the Central Laser Facility at the Pierre Auger Observatory

Abstract

The Pierre Auger Observatory in Malargüe, Argentina, is designed to study the properties of ultra-high energy cosmic rays with energies above 10 18 eV. 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.

The Pierre Auger Cosmic Ray Observatory

“…Laser tracks from the CLF and XLF (Section 4.4.2) are recorded by the 4 FD sites. They are used to obtain hourly measurements of the aerosol optical depth profiles [88] that are used in the reconstruction of each FD air shower event. Sets of 50 vertical shots are measured every 15 minutes by the FD telescopes throughout each night.…”

Measurement of the Radiation Energy in the Radio Signal of Extensive Air Showers as a Universal Estimator of Cosmic-Ray Energy

“…Fluorescence detectors measure the calorimetric energy in the electromagnetic cascade of air showers, which allows an accurate determination of the energy of the primary particle [2]. However, fluorescence light detection is only possible at sites with good atmospheric conditions, and precise quantification of scattering and absorption of fluorescence light under changing atmospheric conditions requires extensive atmospheric monitoring efforts [3][4][5][6].…”

Measurement of the average shape of longitudinal profiles of cosmic-ray air showers at the Pierre Auger Observatory

“…They included the impact of possible patches of clouds in the sky, differences found when separating data by the seasons of the year, and uncertainties in the overall aerosol content and its height dependence. The atmospheric aerosol attenuation, τ A (h), is measured hourly with the central laser facility [13,14]. The measurement compares the number of photons detected on the FD (as a function of height) in a given laser run with the one detected on a clear reference night.…”