Techniques for Measuring Aerosol Attenuation using the Central Laser Facility at the Pierre Auger Observatory

TL;DR

This paper presents two methods for measuring aerosol attenuation at the Pierre Auger Observatory using laser shots from the Central Laser Facility, crucial for accurate cosmic ray energy measurements.

Contribution

The paper introduces two novel procedures for assessing aerosol attenuation, improving atmospheric correction accuracy for fluorescence detector data.

Findings

Both methods produce compatible aerosol attenuation results.

Uncertainties in measurements are well characterized.

The procedures are used in actual data reconstruction at the observatory.

Abstract

The Pierre Auger Observatory in Malarg\"ue, Argentina, is designed to study the properties of ultra-high energy cosmic rays with energies above 1018 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.