Atmospheric Aerosol Measurements at the Pierre Auger Observatory

Atmospheric Aerosol Measurements at theAtmospheric Aerosol Measurements at thePierre Auger ObservatoryPierre Auger Observatory

The Pierre Auger Observatory operates an array of monitoring devices to record the The Pierre Auger Observatory operates an array of monitoring devices to record the atmospheric conditions. Most of instruments are used to estimate the atmospheric conditions. Most of instruments are used to estimate the hourly aerosol hourly aerosol transmission transmission between the point of production of the fluorescence light and the between the point of production of the fluorescence light and the Fluorescence Detectors: if not properly taken into account, these dynamic conditions Fluorescence Detectors: if not properly taken into account, these dynamic conditions

can bias the showers reconstruction. can bias the showers reconstruction.

Main sources of uncertainties on energy and Xmax are the Main sources of uncertainties on energy and Xmax are the aerosol aerosol optical depthoptical depth and and cloudsclouds.The aerosol optical depth .The aerosol optical depth aa(h) contributes to (h) contributes to

the uncertainty in energy from 3.6% at E = 10the uncertainty in energy from 3.6% at E = 1017.517.5 eV to 7.9% at E = eV to 7.9% at E = 10102020 eV, and to the uncertainty in Xmax from 3.3 g eV, and to the uncertainty in Xmax from 3.3 g··cmcm-2 -2 to 7.3 g to 7.3 g··cmcm-2-2 . . Clouds can distort the light profiles of showers, and give a significant Clouds can distort the light profiles of showers, and give a significant contribution to the hybrid exposure of the detector and therefore to the contribution to the hybrid exposure of the detector and therefore to the hybrid spectrum.0hybrid spectrum.0

CENTRAL LASER FACILITY CENTRAL LASER FACILITY Laser light is attenuated on its travel towards FDs as the fluorescence light emitted by Laser light is attenuated on its travel towards FDs as the fluorescence light emitted by a shower. The analysis of the amount of CLF light that reaches the FD building can be a shower. The analysis of the amount of CLF light that reaches the FD building can be used to infer the attenuation due to aerosols, once the nominal energy is known.used to infer the attenuation due to aerosols, once the nominal energy is known.Clouds along the laser beam and between the laser and the FDs can be identified.Clouds along the laser beam and between the laser and the FDs can be identified.

Data Normalised AnalysisData Normalised Analysis : iterative procedure that compares hourly average profiles : iterative procedure that compares hourly average profiles to reference profiles chosen in extremely clear nightsto reference profiles chosen in extremely clear nights

Laser Simulation AnalysisLaser Simulation Analysis : compares quarter-hour CLF profiles to simulated laser : compares quarter-hour CLF profiles to simulated laser events generated varying the aerosol conditions to find the best compatibility. events generated varying the aerosol conditions to find the best compatibility. A parametric model of the aerosol attenuation is adopted.A parametric model of the aerosol attenuation is adopted.

INFRARED CLOUD CAMERAS (IRCCs)INFRARED CLOUD CAMERAS (IRCCs)IRCCs record the cloud coverage making a photograph of the field of view IRCCs record the cloud coverage making a photograph of the field of view of each telescope every 5 minutes during FD DAQ. Images are processed of each telescope every 5 minutes during FD DAQ. Images are processed and a coverage “mask” is created for each pixel of the telescope to identify and a coverage “mask” is created for each pixel of the telescope to identify cloud covered pixels to be removed from the shower reconstruction. cloud covered pixels to be removed from the shower reconstruction. A database is filled with the coverage for each pixel of the map.A database is filled with the coverage for each pixel of the map.

LIDARSLIDARSIn cloud detection mode, In cloud detection mode, LIDAR telescopesLIDAR telescopes sweep the sky with a continuous scan in sweep the sky with a continuous scan in two orthogonal paths with fixed azimuthal angle, one of which is along the central FD two orthogonal paths with fixed azimuthal angle, one of which is along the central FD azimuth angle, with a maximum zenith angle of 45◦. Clouds are detected as strong azimuth angle, with a maximum zenith angle of 45◦. Clouds are detected as strong localised scattering sources, and the timing of the scattered light is related to the cloud localised scattering sources, and the timing of the scattered light is related to the cloud heightheight.. LIDARs provide hourly information on cloud coverage and height. LIDARs provide hourly information on cloud coverage and height.

L. Valore for the Pierre Auger CollaborationL. Valore for the Pierre Auger Collaboration

• Laser Facilities (CLF, XLF)Laser Facilities (CLF, XLF) aerosol, cloudsaerosol, clouds• Infrared Cloud Cameras (IRCCs)Infrared Cloud Cameras (IRCCs) cloudsclouds• 4 Elastic + 1 Raman LIDARs4 Elastic + 1 Raman LIDARs aerosol, clouds aerosol, clouds • Sky Background Data AnalysesSky Background Data Analyses cloudsclouds

The results produced with the two analyses The results produced with the two analyses over a period of 4 years are fully compatible over a period of 4 years are fully compatible

A seasonal variation of A seasonal variation of the vertical aerosol the vertical aerosol

optical depth is optical depth is observed over 4 years.observed over 4 years.

II - AEROSOL OPTICAL DEPTH MEASUREMENTS

HE 1.4HE 1.4

III - CLOUDS DETECTION

I - THE AEROSOL MONITORING SYSTEM

In addition to CLF, are used to provide a local estimate of In addition to CLF, are used to provide a local estimate of αα(h) and (h) and (h) outside the FD field of view(h) outside the FD field of view to avoid triggering the detector.to avoid triggering the detector.

SKY BACKGROUND DATA ANALYSESSKY BACKGROUND DATA ANALYSESThe variance of the baseline fluctuation recorded every 30 s provides a reasonable estimate of the The variance of the baseline fluctuation recorded every 30 s provides a reasonable estimate of the changes in the brightness of the sky. Two methods have been developed :changes in the brightness of the sky. Two methods have been developed :Star Visibility MethodStar Visibility Method : stars are visible in the FD background data. A null detection of a given star at : stars are visible in the FD background data. A null detection of a given star at the predicted time indicates the presence of a cloud in the field of view of the viewing pixel.the predicted time indicates the presence of a cloud in the field of view of the viewing pixel.Background Model MethodBackground Model Method : sudden drops in the brightness of a part of the sky can be due to the : sudden drops in the brightness of a part of the sky can be due to the presence of a cloud in the FOV of the telescope.presence of a cloud in the FOV of the telescope.

LIDARSLIDARS

~30 km

FD telecope CLF

attenuation

~30 km

FD telecope CLF

attenuation