A wide range of atmospheric monitoring instruments is employed at the Pierre Auger Observatory : two laser facilities, elastic lidar stations, aerosol phase function monitors, a horizontal attenuation monitor, star monitors, weather stations, and balloon soundings. We describe the impact of analyzed atmospheric data on the accuracy of shower reconstructions, and in particular study the effect of the data on the shower energy and the depth of shower maximum (X{sub max}). These effects have been studied using the subset of 'golden hybrid' events--events observed with high quality in the fluorescence and surface detector -- used in the calibration of the surface detector energy spectrum

Collaboration, for the Pierre Auger

Prouza, Michael

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30TH INTERNATIONAL COSMIC RAY CONFERENCESystematic study of atmosphere-induced influences and uncertainties on shower re-construction at the Pierre Auger ObservatoryMICHAEL PROUZA1, FOR THE PIERRE AUGER COLLABORATION21 Nevis Institute and Department of Physics, Columbia University, New York, N.Y., U.S.A.2 Observatorio Pierre Auger, Av. San Martı´n Norte 304, 5613 Malargu¨e, Argentinaprouza@nevis.columbia.eduAbstract: A wide range of atmospheric monitoring instruments is employed at the Pierre Auger Observa-tory : two laser facilities, elastic lidar stations, aerosol phase function monitors, a horizontal attenuationmonitor, star monitors, weather stations, and balloon soundings. We describe the impact of analyzedatmospheric data on the accuracy of shower reconstructions, and in particular study the effect of the dataon the shower energy and the depth of shower maximum (Xmax). These effects have been studied usingthe subset of “golden hybrid” events — events observed with high quality in the fluorescence and surfacedetector — used in the calibration of the surface detector energy spectrum.IntroductionThe Pierre Auger Observatory in Malargu¨e, Ar-gentina is a hybrid facility that uses its atmosphericfluorescence detector (FD) to obtain calorimetricestimates of shower energies. The atmosphere actsboth as a calorimeter and a scintillator. It affectsthe fluorescence yield of showers and attenuateslight between showers and the FD. The atmosphereover the observatory is also dynamic. Therefore,the state of the atmosphere must be continuouslymonitored to ensure reliable energy estimates.To an excellent approximation, the molecular andaerosol scattering processes that contribute to theoverall attenuation and scattering of light in the at-mosphere can be treated separately. In Malargu¨e,the molecular component is determined by regularmeasurements of several macroscopic parameters,including altitude profiles of air temperature, pres-sure, and density. It has been shown that daily vari-ations in these parameters have a small impact onshower energy estimates (∆E/E < 1%) and thedepth of shower maximum (∆Xmax ' 6 g cm−2);hence, the observations have been incorporatedinto monthly models for use in the FD reconstruc-tion [1]. A more important factor is the effect ofhumidity on the fluorescence yield, because theyield provides a scaling factor for the energy. Pre-liminary estimates suggest a 5− 10% effect on thefluorescence yield near the ground, and < 3% at4 km above sea level [2]. This effect has been in-corporated into the reported uncertainty of the flu-orescence yield [8]. Aerosols, unlike the molecu-lar component of the atmosphere, are much morevariable, and can change significantly in the courseof a few hours. Therefore, aerosols are systemat-ically measured at all FD sites, and the parame-ters most important for the FD reconstruction arerecorded hourly. The aerosol data and the facilitiesused to collect them are described in detail else-where [3]. In this paper, we discuss the effect ofaerosol measurements on energy and Xmax for animportant subset of observed showers.Aerosol MeasurementsThe presence of aerosols does not influence theair fluorescence yield, so their primary effect onthe shower reconstruction comes from their role inlight attenuation and scattering. Fluorescence lightis emitted isotropically, so a detector with a fieldof view ∆Ω observing shower light of intensity I0will observe a light levelI = I0 · Tm · Ta · (1 +H.O.) · ∆Ω4pi (1)arXiv:0706.1719v1  [astro-ph]  12 Jun 2007SYSTEMATIC STUDY OF ATMOSPHERE-INDUCED INFLUENCESIn this expression, Tm is the transmission factordue to molecular scattering; Ta is the transmissiondue to aerosol scattering; and H.O. is a higher-order correction that accounts for the single andmultiple scattering of photons into (or out of) thedetector field of view. Ignoring multiple scatteringeffects, Ta and H.O. may be fully characterizedusing three independent measurements: the heightprofile of the vertical aerosol optical depth (VAOD)τ(h); the wavelength dependence of the VAOD;and the normalized aerosol differential scatteringcross-section, or phase function, P (θ).The VAOD primarily affects light attenuation. Itis defined for each height above the ground level,so that aerosol transmission Ta(h) between theground and height h isTa(h) = e−τ(h). (2)The height profile τ(h) is measured independentlyby three elastic backscatter lidar stations and FD-reconstructed laser tracks provided by the CentralLaser Facility (CLF) [3, 4, 5]. With a typical mea-surement uncertainty of ±0.01, the CLF measure-ments are currently used for shower reconstruc-tions. However, both the lidars and the CLF usemonochromatic light sources, so the wavelengthdependence of the VAOD is measured by two otherindependent instruments: the Horizontal Attenua-tion Monitor (HAM) and the robotic astronomicaltelescope FRAM [6, 7]. This wavelength depen-dence can be parametrized in terms of the so-calledAngstrom exponent γ:τ(λ) = τ0 ·(λ0λ)γ, (3)where τ0 is measured at a reference wavelengthλ0 = 355 nm. Observations made between Juneand December 2006 indicate a typical value γ =0.7± 0.5 for the observatory location.Finally, aerosols not only attenuate light fromair showers, but also scatter Cherenkov light intothe FD field of view, contaminating the fluores-cence signal. The angular distribution of aerosol-scattered light is given by the aerosol phase func-tion, which we model using two free parameters(see [3]). The parameter f is sensitive to the rel-ative strength of forward and backward scatter-ing and g, the mean cosine of the scattering an-gle, is the measure of the asymmetry of scattering.Aerosol Phase Function monitors (APFs) performhourly measurements of these aerosol scatteringproperties at two FD locations [3].AnalysisThe showers of greatest importance to Auger arethose measured with high quality in both the FDand the surface detector (SD), because these areused to set the energy scale of the overall detector.For the analysis presented here, the set of eventspassing strong quality cuts presented in [8, 9] areused to determine the effect of aerosol measure-ments on energy and Xmax estimated by the FD re-construction. Only cloud-free measurements, iden-tified by a strict quality cut on the FD longitudinalprofile, were used in this study.Several different studies are of interest:1. The use of aerosols in the reconstruction, com-pared to the use of a pure molecular atmosphere.2. The propagation of measurement uncertaintiesin VAOD, γ, f , and g in the FD reconstruction,and in particular their effect on energy and Xmax.3. A test of the assumption of atmospheric hori-zontal uniformity used in the FD reconstruction.(1) Effects of the Presence of Aerosols on AirShower DetectionWe have compared the reconstruction of show-ers using real-time aerosol measurements with thesame events reconstructed using a purely molecu-Figure 1: Energy difference for events recon-structed with measured aerosol parameters (Eaero)and with assumption of no aerosols (molecular at-mosophere; Emol).30TH INTERNATIONAL COSMIC RAY CONFERENCEFigure 2: Spread in E and Xmax due to statistical fluctuations in aerosol measurements. ∆E+ and ∆X+ arecalculated using the +1 σ uncertainty (on aerosol measurement), ∆E− and ∆X− using the−1 σ uncertainty.lar atmosphere. As shown in Figure 1, neglectingthe presence of aerosols causes, on average, a 12%underestimate in shower energies. Moreover, thelong tail in the distribution indicates the enormouseffect of aerosol attenuation on the reconstructionof a significant fraction of all showers: 15% ofthe showers have an energy correction greater than25%; 6.5% of events more than 30%; and 3% morethan 40%. Especially for the highest energy events,where the statistics are poor, real-time atmosphericcalibration is essential.Having established the significant influence ofaerosols on the reconstruction, we can also askif simple parametric models of the aerosol con-tent are sufficiently accurate. For example, atwo-parameter exponential aerosol density profile,characterizing conditions at the site, has been con-sidered. Preliminary studies indicate that thisparameterization leads to a 4% overestimate inshower energies (with a large spread of 10%)compared to reconstructions performed with trueaerosol measurements.(2) Uncertainties Introduced by Aerosol Mea-surementsWe have propagated the measurement uncertain-ties in the VAOD, phase function, and wavelengthdependence in the hybrid reconstruction. Figure2 depicts the contribution of each measurement tothe uncertainty in energy and shower maximum.The VAOD provides the dominant contribution tothe shower uncertainties: 5.5% for the statisticaluncertainty in energy and 4 g cm−2 for Xmax. Thewavelength dependence and the phase function aresignificantly less important, contributing 1% and1.3%, respectively, to the uncertainty of the energy,and ∼ 2 g cm−2 to the uncertainty in Xmax.(3) Evaluation of the Horizontal Uniformity ofthe AtmosphereThe atmospheric measurements at the Auger Ob-servatory, while extensive, are only able to ob-serve conditions at several locations across the site.Therefore, during the reconstruction of events onemust assume that these limited measurements char-acterize broad regions of the atmosphere aroundeach FD, or in other words, that the atmosphereexhibits a large degree of horizontal uniformity.The assumption of horizontal uniformity and its ef-fect on the reconstruction can be tested for boththe molecular and aerosol components of the atmo-sphere. Molecular conditions are observed by bal-loon flights and two ground-based weather stationslocated at Los Leones and the Central Laser Facil-ity. Figure 3 indicates differences in the weatherconditions observed at these sites, and the corre-sponding effect of these differences on shower re-constructions is < 1% for the shower energy and∼ 1 g cm−2 for Xmax.SYSTEMATIC STUDY OF ATMOSPHERE-INDUCED INFLUENCESFigure 3: Comparison of monthly atmosphericdepth measurements at two sites separated by ∼30km.For the aerosol component of the atmosphere, it ispossible to estimate the effect of horizontal nonuni-formities by reconstructing showers using differentsets of aerosol measurements. For this study, wehave reconstructed a set of high quality air showersobserved by the Coihueco FD using CLF VAODprofiles measured concurrently at Coihueco andLos Leones (the distance between these two FDsites is about 40 km). As shown in Figure 4, thesystematic uncertainty on shower energies intro-duced by the assumption of uniformity is ∼ 2.5%,with measurement fluctuations contributing ∼ 7%to the statistical uncertainty. The shower maximumXmax is also shifted by ∼ 2.5 g cm−2, with typicalstatistical uncertainties of ∼ 9 g cm−2.DiscussionThe following table summarizes atmosphere-induced uncertainties in the hybrid reconstruction:Effect ∆ E / E ∆ XmaxMolecular:Horizontal uniformity < 1 % 1 g cm−2Variations in air density profile 1 % 6 g cm−2Aerosols:Horizontal uniformity [systematic] 2.5 % 2.5 g cm−2Horizontal uniformity [statistical] 7 % 9 g cm−2Vertical Aerosol Optical Depth 5.5 % 4 g cm−2Wavelength Dependence 1 % 1 g cm−2Differential Scattering Cross-section 1.3 % 2 g cm−2Table 1: Summary of atmosphere-induced uncer-tainties for the set of calibration eventsFigure 4: Comparison of energy difference (top)and of shift in position of shower maximum (bot-tom) due to the use of non-local aerosol atmo-spheric parameters.References[1] B. Keilhauer et al. [Pierre Auger Collabora-tion], Proc. 29th ICRC, Pune (2005), 7, 123[2] B. Keilhauer et al., Astropart. Phys. 25(2006), 259[3] S.Y. BenZvi [Pierre Auger Collaboration],these proceedings, #0399[4] B. Fick et al., JINST 1 (2006), P11003[5] S.Y. BenZvi et al., NIM A574 (2007), 171[6] R. Cester et al. [Pierre Auger Collaboration],Proc. 29th ICRC, Pune (2005), 8, 347[7] P. Tra´vnı´cˇek [Pierre Auger Collaboration],these proceedings, #0396[8] M. Roth [Pierre Auger Collaboration], theseproceedings, #0313[9] T. Yamamoto [Pierre Auger Collaboration],these proceedings, #0318[10] D.R. Longtin, Air Force Geophysics LabTechnical Report 88-0112 (1988).

Systematic study of atmosphere-induced influences and uncertainties on shower reconstruction at the Pierre Auger Observatory

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Systematic study of atmosphere-induced influences and uncertainties on shower reconstruction at the Pierre Auger Observatory

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