Measurement of the aerosol phase function at the Pierre Auger Observatory

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 Malargüe, 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.

A study of the effect of molecular and aerosol conditions in the atmosphere on air fluorescence measurements at the Pierre Auger Observatory

“…In general, the angular distribution of light scattered by aerosols is very strongly peaked in the forward direction, reaches a minimum near 90°, and has a small backscattering component. It is reasonably approximated by the parameterization [22,44,45] …”

“…Two Aerosol Phase Function Monitors (APFs) are used to determine the aerosol scattering properties of the atmosphere using collimated horizontal light beams produced by Xenon flashers [22]. Two optical telescopes -the Horizontal Attenuation Monitor (HAM) and the (F/ph)otometric Robotic Telescope for Atmospheric Monitoring (FRAM) -record data used to determine the wavelength dependence of the aerosol attenuation [23,24]. Finally, clouds are measured hourly by the lidar stations, and infrared cameras on the roof of each FD building are used to record the cloud coverage in the FD field of view every 5 min [25].…”

“…These measurements are augmented by data from lidar stations located near each FD building [21], a Raman lidar at one FD site, and the eXtreme Laser Facility (or XLF, named for its remote location) deployed in November 2008. Two Aerosol Phase Function Monitors (APFs) are used to determine the aerosol scattering properties of the atmosphere using collimated horizontal light beams produced by Xenon flashers [22]. Two optical telescopes -the Horizontal Attenuation Monitor (HAM) and the (F/ph)otometric Robotic Telescope for Atmospheric Monitoring (FRAM) -record data used to determine the wavelength dependence of the aerosol attenuation [23,24].…”

The importance of atmospheric monitoring at the Pierre Auger Observatory

“…[2,4]. Optical telescopes called HAM and FRAM are used to measure the wavelength dependence of the aerosol scattering, while the FRAM is also used to examine the sky in the directions of significant air shower events to search for unusual absorption by clouds or aerosol layers [2,5].…”

Atmospheric monitoring and model applications at the Pierre Auger Observatory

“…The Xenon light crosses horizontally the field of view of these telescopes and the scattered part is recorded in dependence on the scattering direction. The Henyey-Greenstein function is used for the description [17] and an average value of the phase function asymmetry parameter g of 0.56±0.10 is found [18]. The concentration, size distribution and chemical composition of the local aerosols are further investigated with an aerosol sampler and spectrometer [7,8].…”