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Nuclear Instruments and Methods in Physics Research A 518 (2004) 183–185

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doi:10.1016

The LIDAR systems for atmospheric monitoring in Auger

R. Mussaa,*, S. Argir !oa, R. Cestera, M. Chiossoa, A. FilipWiWb, M. Horvatc,J. Matthewsd, M. Mostaf!aa,d, M. Robertsd, G. Sequeirosa, D. VeberiWb,

D. Zavrtanikc, M. Zavrtanikb

aDip. di Fisica Sperimentale, dell’Universit "a and INFN Torino 10125, ItalybJomef Stefan Inst., SI-1001 Ljubljana, Slovenia

cNova Gorica Polytechnic, SI-5000 Nova Gorica, SloveniadUniversity of New Mexico, Albuquerque, NM 87131, USA

Abstract

A LIDAR network is being built for the measurement and online monitoring of the atmospheric optical parameters,

which play a central role in the energy measurement of ultra-high-energy cosmic rays. Four LIDAR systems, each one

equipped by an Nd:YAG UV laser and three parabolic mirrors with PMTs for the detection of the backscatter photons,

are scheduled to be installed in the proximity of the four fluorescence detectors of the Pierre Auger Observatory

(Malarg .ue, Argentina). In this paper a report describing hardware components, commissioning and shooting strategies

of the LIDAR systems is given.

r 2003 Elsevier B.V. All rights reserved.

1. Atmospheric monitoring in Auger

The Pierre Auger Observatory will studyUHECR using two complementary techniques:muon detection at ground level, and atmosphericfluorescence. The fluorescence detector (FD) sub-stantially uses our atmosphere as a calorimeter, todetect the EM component of the cosmic airshowers. Given the variability of its parameters,atmospheric monitoring is crucial [1] for ourunderstanding of the energy corrections. A multi-fold plan of attack [2] is therefore in preparation toget the most thorough understanding of theatmospheric conditions on site. A central role willbe played by four LIDAR stations, located atapproximately 150 m from each FD site.

onding author.

ddress: mussa@to.infn.it (R. Mussa).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.10.055

The first system has been installed by the FDSouth Site (Los Leones) in March 2002. It startedtaking data soon after, to test its impact on the FDactivity, define optimal running conditions, andimplement controls for full remote operation.Installation of the second system, by the FD WestSite (Coihueco) has started in April 2003, and willbe completed during the current austral winter. Anidentical LIDAR setup [3] is also mounted at thePino Torinese Astronomical Observatory, nearTorino, to test possible upgrades.

2. The LIDAR telescopes

The workhorse of such system will be four fullysteerable alt-altazimuthal frames built in 1993 forthe EAS-TOP [4] experiment. Each mount isequipped with a UV laser source and three

d.

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Fig. 1. The LIDAR setup in Los Leones.

R. Mussa et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 183–185184

parabolical glass mirrors. These mirrors arealuminum coated, having 0:8 m diameter, andf =0:5:Mirrors’ axes were aligned within 0:1� to theUV laser source.Two DC servomotors steer the frame axes to

speeds up to 2�=s; active feedback is provided byrelative encoders. The absolute pointing directionis known with 0:2� accuracy. The whole set-up,mounted on a 200 container (as shown in Fig. 1), isprotected by a fully retractable motorized cover,and custom built by a local company. Both theframe-steering and cover movements are con-trolled by the MC-204 motion controller, madeby control techniques, which allows full remoteoperation of the system, via Ethernet.The LIDAR shooting activity will follow two

schemes: (1) a continuous sky scan on a B50�

cone around the zenith direction, outside the FDfield of view; and (2) a shoot-the-shower fast sweepof the sky region where an interesting event hasbeen spotted by the FD. A Linux PC, connectedvia serial ports to all the sub-components, controlsthe whole system. A GPSY module [5] providesthe trigger to the laser source, with a fixed delaywith respect to the GPS clock. The NdYAGflashlamp with pumped laser source emits 5 nspulses at l ¼ 355 nm; at a repetition rate up to20 Hz: The pulse energy is limited to 0:1 mJ; tomatch the PMT and DAQ dynamic range. Thecurrent source is planned to be replaced by a diodepumped YAG laser, to raise the repetition rateabove 1 kHz:The back-scattered UV light is focused by each

mirror on a Hamamatsu R7400 PMT. To suppress

the sky background, we use a UG-1 filter with60% transmittance at 355 nm; FWHM ¼ 50 nm:The PMT signals are digitized by a LICELtransient recorder TR40-160 with a 12 bit resolu-tion, 40 MHz sampling rate, 16 k trace length. TheLICEL can be also operated in single photoncounting mode at 250 MHz: Each raw LIDARevent from 3 PMTs will be 300 kb wide.

3. Data analysis

The back-scattered power observed at the PMTfrom distance R is given by the LIDAR equation

PðRÞ ¼ KbðRÞR2

e�2tðRÞ

where bðRÞ is the back-scattering coefficient andthe optical depth tðRÞ ¼

RR

0 aðrÞ dr is the integralof the extinction coefficient aðrÞ along the path.Both these quantities depend linearly on thedensity of scattering centers, and are sums of theaerosol and molecular contributions: aðRÞ ¼amolðRÞ þ aaerðRÞ; bðRÞ ¼ bmolðRÞ þ baerðRÞ:The molecular contribution can be evaluated

with standard models, knowing pressure andtemperature vs. height from atmospheric measure-ments. The extraction of the aerosol coefficientsfrom the LIDAR equation uses either Klett’s [6] orFernald’s [7] inversion algorithms to extract a andb from LIDAR shots at a given polar angle y:Both methods need a priori assumptions on themolecular vs. aerosol parameters functional rela-tion. As an alternative, in case of smoothhorizontal variation of atmospheric patterns, amulti-angle inversion technique can be used [8],which fully exploits the steering capabilities ofAuger LIDAR systems. The optical depth tðRÞ isthen used to calculate the atmospheric lighttransmission coefficient TBe�tðRÞ; to correct thecosmic ray shower energy measurement.

Acknowledgements

The authors wish to warmly thank all the staffof the Auger Observatory, as well as D. Melo, A.Risi, M. Santander, A. Tonachini for their

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contribution to the installation of the two LIDARsystems in Los Leones and Coihueco.

References

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