The dependence of atmospheric conditions on altitude and time have to be
known at the site of an air shower experiment for accurate reconstruction of
extensive air showers and their simulations. The height-profile of atmospheric
depth is of particular interest as it enters directly into the reconstruction
of longitudinal shower development and of the primary energy and mass of cosmic
rays. For the southern part of the Auger Observatory, the atmosphere has been
investigated in a number of campaigns with meteorological radio soundings and
with continuous measurements of ground-based weather stations. Focussing on
atmospheric depth and temperature profiles, temporal variations are described
and monthly profiles are developed. Uncertainties of the monthly atmospheres
that are currently applied in the Auger reconstruction are discussed.Comment: To be published in Proceedings of 29th International Cosmic Ray
  Conference (ICRC) 2005, Pune, Indi

Bluemer, J.

Engel, R.

Gora, D.

Homola, P.

Keilhauer, B.

Klages, H.

Pekala, J.

Risse, M.

Unger, M.

Wilczynska, B.

Wilczynski, H.

English

arXiv

The dependence of atmospheric conditions on altitude and time have to be known at the site of an air shower experiment for accurate reconstruction of extensive air showers and their simulations. The height-profile of atmospheric depth is of particular interest as it enters directly into the reconstruction of longitudinal shower development and of the primary energy and mass of cosmic rays. For the southern part of the Auger Observatory, the atmosphere has been investigated in a number of campaigns with meteorological radio soundings and with continuous measurements of ground-based weather stations. Focusing on atmospheric depth and temperature profiles, temporal variations are described and monthly profiles are developed. Uncertainties of the monthly atmospheres that are currently applied in the Auger reconstruction are discussed

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29th International Cosmic Ray Conference Pune (2005) 00, 101–106Atmospheric Profiles at the Southern Pierre Auger Observatory andtheir Relevance to Air Shower MeasurementJ. Blümer, R. Engel, D. Gora, P. Homola, B. Keilhauer, H. Klages, J. Pekala, M. Risse,M. Unger, B. Wilczynska, H. Wilczynski for the Pierre Auger Collaboration Presenter: B. Keilhauer (bianca.keilhauer@ik.fzk.de), ger-keilhauer-B-abs2-he14-posterThe dependence of atmospheric conditions on altitude and time have to be known at the site of an air showerexperiment for accurate reconstruction of extensive air showers and their simulations. The height-profile ofatmospheric depth is of particular interest as it enters directly into the reconstruction of longitudinal showerdevelopment and of the primary energy and mass of cosmic rays. For the southern part of the Auger Obser-vatory, the atmosphere has been investigated in a number of campaigns with meteorological radio soundingsand with continuous measurements of ground-based weather stations. Focussing on atmospheric depth andtemperature profiles, temporal variations are described and monthly profiles are developed. Uncertainties ofthe monthly atmospheres that are currently applied in the Auger reconstruction are discussed.1. IntroductionThe Pierre Auger Observatory measures extensive air showers (EAS) induced by ultra-high energy cosmic raysusing a hybrid technique [1]. One detection method is the registration of secondary particles of EAS at groundwith water Cherenkov tanks [2]. The second technique is the observation of the longitudinal development ofEAS with fluorescence telescopes [3]. Especially for the detection and reconstruction of fluorescence lightemission of EAS, knowledge of actual atmospheric conditions at the site of the experiment is necessary.Therefore, several meteorological systems have been installed at the site of the Pierre Auger Observatory andadditionally, data from publicly available databases of atmospheric measurements are used. The data recordedat the site are compared to the US Standard Atmosphere 1976 (US-StdA), which had been commonly used.The longitudinal development of EAS can be described by the number of particles at a given atmosphericdepth. The atmospheric depth at which a shower exhibits its maximum,  , is well correlated with the massof the primary particle. However, using the fluorescence technique for detecting EAS, these quantities cannotbe observed directly. The fluorescence telescopes (FD) observe the light within a fixed field of view. Thus,the simulated shower profiles have to be transformed from a description based on vertical atmospheric depthto geometrical height. For a physical interpretation of detected EAS events, the conversion has to be donevice versa. Therefore, the transformation between atmospheric depth and geometrical altitude is a crucial pointin the simulation and reconstruction of EAS and the relation between atmospheric depth and height followsfrom the air density profile [4]. Atmospheric conditions also have a major impact on the fluorescence emissionprocess itself, and on the details of light propagation from the emitting region to the telescope.2. Atmospheric conditions at the southern Pierre Auger ObservatorySince August 2002, meteorological radio soundings have been performed in several campaigns near Malargüe,Argentina. The radiosondes are launched above the site of the experiment on helium-filled balloons. A set ofdata is taken about every 20 m during ascent up to 25 km a.s.l. in average. Despite changing wind conditions,the radiosondes stay mostly directly above the array up to 10 km a.s.l., covering the more interesting part of the Observatorio Pierre Auger, Av. San Martin Norte 304, 5613 Malargüe, Argentina, http://www.auger.org/auger-authors.pdfFERMILAB-CONF-05-292-E-TD2 B. Keilhauer et al. for the Pierre Auger Collaborationprofiles for EAS development. More than 100 atmospheric profiles, including data for temperature, pressure,relative humidity, and wind speed and direction, were collected. Day-night variations are very small in thisarea. Only temperature changes up to 10 K may occur in the lowest 1000 m above ground which is roughly at1420 m a.s.l. in our case. The important profile of atmospheric depth hardly changes on day-night time scales.From day to day, the extent of variation is strongly seasonal dependent. During austral summer, the conditionsare much more stable than during winter. At ground, differences in atmospheric depth up to 5 g cm   havebeen found which are related to pressure variations. At higher altitudes, between 6 and 12 km a.s.l., evenvariations of 10 - 15 g cm  are recorded. Over a period of days, the temperature may shift by 15 K. Seasonaleffects are, of course, the largest. For individual days, the difference in atmospheric depth between summerand winter can be as large as 20 g cm  at ground and reach approximately 30 g cm  at altitudes between 6and 9 km a.s.l.Apart from these intermittent profile measurements, ground-based weather stations record temperature, pressure, relativehumidity, and wind data every 5 min. At completion of thesouthern Observatory stations will be located at every FD build-ing and the central laser facility. Up to now, data from 2 stationsare available. While the atmospheric profiles are mainly usedfor simulation and reconstruction of the longitudinal showerdevelopment, the continuous ground-based data are applied inthe calculation of trigger efficiencies of the water Cherenkovtanks. An example of the pressure distribution within a singleday is given in Fig. 1. In Figures 2 and 3, atmospheric depth andtemperature data of the year 2004 are shown. Additionally tothese local measurements, information from public databasesare included in our analyses. The UK Met Office, through themin. of day0 200 400 600 800 1000 1200 1400pressure (hPa)854855856857858859860861862Figure 1. Pressure distribution at the FD buildingLos Leones at April 3rd, 2004.British Atmospheric Data Centre [5], maintains a database of atmospheric radio soundings worldwide. Thestations closest to Malargüe are Cordoba and Santa Rosa, at a distance of 500 - 650 km.day0 50 100 150 200 250 300 350)2atmos. depth (g/cm865870875880885890895900905black - midnight red - noonline - Malargue monthly modelsFigure 2. Atmospheric depth data at the FD Los Leonesfrom the year 2004. The plotted data are recorded at noonand midnight local time. The line indicates the values ofthe Malargüe Monthly Models at the same altitude.day0 50 100 150 200 250 300 350temperature (K)260265270275280285290295300305310black - midnight red - noonline - Malargue monthly modelsFigure 3. Temperature data at the FD Los Leones fromthe year 2004. The plotted data are recorded at noon andmidnight local time. The line indicates the values of theMalargüe Monthly Models at the same altitude.The investigation of locally measured atmospheric profiles and the comparison of them to data from the UKMet Office shows that monthly parameterisations can give a good description of the atmospheric variations.To obtain such parameterisations, once-a-day measurements of Cordoba and Santa Rosa are averaged withineach month and also between these two stations. The resulting monthly profiles describe the atmosphere nearAtmospheric Profiles at the Southern Pierre Auger Observatory 3temperature (K)200 220 240 260 280 300altitude (km a.s.l.)051015202530US-StdAJanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecemberFigure 4. Temperature profiles of the Malargüe MonthlyModels.altitude (km a.s.l.)0 5 10 15 20 25 30)2difference in atmos. depth to US-StdA (g/cm-50510152025JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecemberFigure 5. Profiles of atmospheric depth of the MalargüeMonthly Models in difference to the US-StdA.Malargüe already quite well. Further adjustments are calculated using the information from the local radiosoundings. The obtained monthly models are shown in Figures 4 and 5. These profiles, called MalargüeMonthly Models, are employed in the Auger simulation and reconstruction of EAS events [6].3. Effects on longitudinal profilesFor the physical interpretation of EAS, the position of shower maximum and the energy of the primary particleare important features. Using the fluorescence technique, the shower maximum can be observed directly whilethe energy of the primary particle has to be deduced from the deposited ionisation energy in the atmosphere.In Figure 6, the shower development represented by the energy deposit profile can be seen for an averageof 100 Fe-ind. showers with 10 eV and 60Æ incidence. The profiles are plotted versus altitude developingin the US-StdA, Malargüe February, August, and annual average atmospheres. The figure reveals that theposition of shower maximum is shifted due to atmospheric conditions. To clearly demonstrate the effect, theinclination angle  is chosen to be 60Æ, since the extent of the shift is enlarged by a factor  . In particular,the Malargüe atmospheres give a systematic shift of the position of    towards higher altitudes which isheight a.s.l. (km)energy deposit (GeV/(5g/cm2 ))Fe, 1019 eV, 60o US-StdA(Max. at 8.4 km)Malargue Year(Max. at 8.6 km)Malargue Febr.(Max. at 8.8 km)Malargue Aug.(Max. at 8.5 km)20040060080010001200140016001800x 10 50 2 4 6 8 10 12 14 16 18 20Figure 6. Energy deposit profile for an average of 100EAS. The black-solid line shows the profile expected in theUS-StdA. The coloured curves are for the two extrema ofthe Malargüe Monthly Models and the annual average.height a.s.l. (km)rel. difference in energy depositFe, 1019 eV, 60oUS-StdAMalargue Feb.Malargue Aug.-0.02-0.0100.010.020.030.040.050.060 2 4 6 8 10 12 14 16 18 20Figure 7. Difference of the profiles shown in Fig. 6 tothe US Standard atmosphere divided by the energy depositat shower maximum. The profiles are shifted in height tobring all shower maxima to the same position (see text).4 B. Keilhauer et al. for the Pierre Auger Collaborationequivalent to smaller values of atmospheric depth. However, not only the average position of the showermaximum is a measure of the type of the primary particle. Also the width of the distribution of this position fora large number of EAS is systematically different for proton and iron induced showers. The daily variationsof the atmosphere lead to a broadening of the    distribution by about 25% for iron but only 4% for protoninduced showers, again   Æ, as compared to the expectation for the time-independent US-StdA [4].For estimating changes in the reconstruction of the primary energy of EAS, the distortion of profiles due toatmospheric variations has to be checked. In Fig. 7, differences of two Malargüe profiles to the US-StdA areplotted. To remove obvious differences, the profiles are shifted in height such that all maxima are at the positionof the maximum for the US-StdA. The variation of the total energy is negligible since the integral over eachcurve is small in comparison to the entire amount of deposited energy of the EAS.4. Discussion of remaining uncertaintiesFirstly, the influence of applying Malargüe Monthly Models instead of US-StdA is analysed with about 2700hybrid events. The position of shower maximum is shifted on average by  -15 g cm  , see Fig. 8, right panel.The change of the reconstructed primary energy is only 0.7%, see Fig. 8, left panel. E /E∆-0.15 -0.1 -0.05 0 0.05 0.1 0.15entries0100200300400500600700800mean 0.007 RMS 0.026]2 X [g/cm∆-50 -40 -30 -20 -10 0entries0501001502002502mean -15.1 g/cm2RMS   5.8 g/cmFigure 8. Difference of reconstruction using MalargüeMonthly Models to reconstruction in US-StdA is shown.Left: primary energy. Right: position of shower maximum. E /E∆-0.15 -0.1 -0.05 0 0.05 0.1 0.15entries02004006008001000120014001600mean 0.005 RMS 0.014]2 X [g/cm∆0 5 10 15 20 25entries01002003004005006007008002mean   6.0 g/cm2RMS   2.3 g/cmFigure 9. Comparison between normal reconstruction andreconstruction applying an one-  error to the atmosphericprofiles. Left: primary energy. Right:   position.Secondly, the remaining uncertainties due to day-to-day variations of the atmospheric conditions within eachmonth have been studied using the same set of events. For this purpose, modified monthly profiles are used inthe analysis that represent the uncertainty bound of one standard deviation of the Malargüe Monthly Models.The position of   , Fig. 9, right panel, and the primary energy, Fig. 9, left panel, are compared between thenormal reconstruction and the modified reconstruction. The uncertainty of the depth of maximum of EAS isabout 6 g cm  and of the reconstructed primary energy 0.5%. Finally it should be noted that the calculation offluorescence photon profiles is included in the standard Auger reconstruction chain. The fluorescence photonyield is dependent on temperature and pressure according to the used atmospheric models.References[1] Mostafa, M., for the Pierre Auger Collaboration, usa-mostafa-M-abs1-he14-oral[2] Bertou, X., for the Pierre Auger Collaboration, arg-bertou-X-abs1-he14-oral[3] Bellido, J., for the Pierre Auger Collaboration, aus-bellido-J-abs1-he14-oral[4] Keilhauer B. et al., Astropart. Phys. 22 (2004) 249[5] British Atmospheric Data Center, http://badc.nerc.ac.uk/data/radiosglobe/radhelp.html[6] Paul, T. et al., for the Pierre Auger Collaboration, usa-paul-T-abs1-he15-poster

Atmospheric profiles at the southern Pierre Auger Observatory and their relevance to air shower measurement

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Atmospheric Profiles at the Southern Pierre Auger Observatory and their
  Relevance to Air Shower Measurement

Atmospheric Profiles at the Southern Pierre Auger Observatory and their Relevance to Air Shower Measurement

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