# Measurement of the Aerosol Phase Function at the Pierre Auger Observatory

## TL;DR

This paper details the design, operation, and performance of the aerosol phase function measurement system at the Pierre Auger Observatory, crucial for accurate cosmic ray energy estimation through atmospheric correction.

## Contribution

It introduces a novel hourly aerosol phase function measurement system using UV light sources, enhancing atmospheric correction accuracy for cosmic ray detection.

## Key findings

- Successful implementation of the APF system at Pierre Auger Observatory
- Regular hourly aerosol phase function measurements achieved
- Improved atmospheric correction for fluorescence light signals

## Abstract

Air fluorescence detectors measure the energy of ultra-high energy cosmic rays by collecting fluorescence light emitted from nitrogen molecules along the extensive air shower cascade. To ensure a reliable energy determination, the light signal needs to be corrected for atmospheric effects, which not only attenuate the signal, but also produce a non-negligible background component due to scattered Cherenkov light and multiple-scattered light. The correction requires regular measurements of the aerosol attenuation length and the aerosol phase function, defined as the probability of light scattered in a given direction. At the Pierre Auger Observatory in Malargue, Argentina, the phase function is measured on an hourly basis using two Aerosol Phase Function (APF) light sources. These sources direct a UV light beam across the field of view of the fluorescence detectors; the phase function can be extracted from the image of the shots in the fluorescence detector cameras. This paper describes the design, current status, standard operation procedure, and performance of the APF system at the Pierre Auger Observatory.