Elsevier

Astroparticle Physics

Volume 35, Issue 9, April 2012, Pages 591-607
Astroparticle Physics

Description of atmospheric conditions at the Pierre Auger Observatory using the Global Data Assimilation System (GDAS)

https://doi.org/10.1016/j.astropartphys.2011.12.002Get rights and content

Abstract

Atmospheric conditions at the site of a cosmic ray observatory must be known for reconstructing observed extensive air showers. The Global Data Assimilation System (GDAS) is a global atmospheric model predicated on meteorological measurements and numerical weather predictions. GDAS provides altitude-dependent profiles of the main state variables of the atmosphere like temperature, pressure, and humidity. The original data and their application to the air shower reconstruction of the Pierre Auger Observatory are described. By comparisons with radiosonde and weather station measurements obtained on-site in Malargüe and averaged monthly models, the utility of the GDAS data is shown.

Highlights

► Confirmation of utility of GDAS data for the site of the Pierre Auger Observatory. ► reconstruction of extensive air showers as observed by the Pierre Auger Observatory. ► Reduction of reconstruction uncertainties due to atmospheric conditions.

Introduction

The Pierre Auger Observatory [1], [2] is located near the town of Malargüe in the province of Mendoza, Argentina. At the site, at the base of the Andes mountains, two well-established measurement techniques are combined to measure extensive air showers with energies above some 1017 eV. The hybrid detector consists of a surface detector (SD) array and five fluorescence detector (FD) buildings. Each of the slightly more than 1600 SD stations is a water-filled Cherenkov detector, measuring the secondary particles of air showers that reach the ground. The detectors of the array are spaced by 1.5 km (750 m in a small infill area in the western part of the array) and provide the lateral particle distribution around a shower core. Four FD buildings comprise six telescopes each and one FD enhancement installation consists of three telescopes. In each FD telescope, the UV light emitted by excited nitrogen molecules along the shower track is collected by a large segmented mirror and reflected onto a camera composed of 440 PMTs. With this measurement, the geometry and the longitudinal profile of the shower can be obtained.

For the reconstruction of extensive air showers, the optical properties of the atmosphere at the site of the observatory have to be known. This is particularly true for reconstructions based on data obtained with the fluorescence technique [3], but also impacts upon data collected with the surface detectors [4]. The detection of clouds is an important task of the atmospheric monitoring systems. Clouds can obstruct or – through scattering of the intense Cherenkov light – amplify the apparent fluorescence light before it reaches the FD. To eliminate data recorded in cloudy conditions from physics analyses, lidar stations and infrared cloud cameras are installed at each FD station of the Pierre Auger Observatory. These instruments scan the fields of view of the fluorescence detectors several times per hour during data taking periods to measure the cloud coverage and the base height of clouds [5]. The vertical profile of the aerosol optical depth is measured once every hour using vertical laser shots from two facilities near the center of the array. Using the calibrated laser energy and the amount of light scattered out of the beam towards the FDs, the amount of aerosols can be estimated [3]. Weather conditions near ground, and the height-dependent atmospheric profiles of temperature, pressure and water vapor pressure are relevant for several Auger Observatory measurements. E.g., these parameters affect the production of fluorescence light by excited nitrogen molecules at the shower track, and the Rayleigh scattering of the light between the air shower and detector. Atmospheric conditions are measured by intermittent meteorological balloon radio soundings. Additionally, ground-based weather stations measure surface data continuously. The profiles from the weather balloons were averaged to obtain local models, called (new) Malargüe Monthly Models [3]. Since March 2009, the atmospheric monitoring system has been upgraded with the implementation of a rapid monitoring system [6]. Part of the new program was the measurement of atmospheric profiles with radio soundings shortly after the detection of particularly high-energy air showers, a system called Balloon-the-Shower (BtS). This enables a high-quality reconstruction of the most interesting events.

However, performing radio soundings and applying these data to air shower analyses is not straightforward. Very critical aspects are the time of the weather balloon ascent and the data validity period. Furthermore, performing radio soundings, in particular within BtS, imposes a large burden on the collaboration. Therefore, we investigate the possibility of using data from the Global Data Assimilation System (GDAS), a global atmospheric model, for the site of the Auger Observatory. The data are publicly available free of charge via READY (Real-time Environmental Applications and Display sYstem). Each data set contains all the main state variables with their dependence on altitude with a validity period of 180 min for each data set.

Key aspects of the impact of the profiles of atmospheric state variables on the development and detection of extensive air showers are discussed briefly (Section 2). We motivate the necessity of more reliable atmospheric profiles by a discussion about the data validity period of weather balloons (Section 3), describe the content and processing of the GDAS data (Section 4) and compare them to local measurements (Section 5). The new atmospheric data are implemented in the data processing and simulation framework of the Auger Observatory for an analysis of reconstructed air showers (Section 6).

Section snippets

Impact of atmospheric state variables on the development and detection of extensive air showers

Varying atmospheric conditions in terms of state variables like temperature, pressure and humidity, may alter the development and, in particular, the detection of extensive air showers. Here, different aspects relevant to the analysis of air showers at the Pierre Auger Observatory are discussed.

The air fluorescence emission excited by the passage of an air shower depends on pressure, temperature, and humidity [7]. The collisional de-excitation of excited nitrogen molecules by other molecules of

Validity of radio soundings

Since August 2002, meteorological radio soundings have been performed above the Pierre Auger Observatory to measure altitude-dependent profiles of atmospheric variables, mainly pressure, temperature, and relative humidity. Regular measurements were done until December 2008 in order to collect data for all months. After applying selection criteria, 261 profiles from the middle of 2002 until the end of 2008 could be used to build the new Malargüe Monthly Models [3]. Starting in March 2009, the

Global Data Assimilation System (GDAS)

In the field of numerical weather prediction, data assimilation is the adjustment of the development within a model to the real behavior of the atmosphere as found in meteorological observations [16]. The atmospheric models describe the atmospheric state at a given time and position. Three steps are needed to perform a full data assimilation:

  • 1.

    Collect data from meteorological measuring instruments placed all over the world. These instruments include weather stations on land, ships, and airplanes

GDAS vs. local measurements

To validate the quality of the GDAS data and to verify its applicability for air shower reconstructions at the Auger Observatory, GDAS data are compared with local measurements – atmospheric soundings with weather balloons and ground-based weather stations. The new Malargüe Monthly Models (nMMM) are also shown in some comparisons as a reference since they were the standard profiles used in reconstructions until recently.

Air shower reconstruction

To study the effects caused by using the GDAS data, all air shower data from the Auger Observatory collected between June 1, 2005 and end of 2010 are used in a reconstruction analysis using the Off¯line̲ software framework of the Pierre Auger Observatory [24]. The change of atmosphere description will mainly affect the reconstruction of the fluorescence data, c.f. Section 2. Therefore, we concentrate on this part in the following. It is known that varying atmospheric conditions alter the

Conclusion

The reconstructions of air showers measured at the Pierre Auger Observatory have used a set of monthly mean profiles as the standard atmospheric description until recently. These profiles are averages from meteorological radio soundings performed at the site of the observatory over several years. The mean profiles describe the local conditions reasonably well, but cannot describe short-term variations in the atmosphere. Because of the large burden radio soundings impose on the collaboration,

Acknowledgments

The successful installation, commissioning and operation of the Pierre Auger Observatory would not have been possible without the strong commitment and effort from the technical and administrative staff in Malargüe.

We are very grateful to the following agencies and organizations for financial support: Comisión Nacional de Energía Atómica, Fundación Antorchas, Gobierno De La Provincia de Mendoza, Municipalidad de Malargüe, NDM Holdings and Valle Las Leñas, in gratitude for their continuing

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