: Construction of the first stage of the Pierre Auger Observatory has begun. The aim of the Observatory is to collect unprecedented information about cosmic rays above 10(18) eV. The first phase of the project, the construction and operation of a prototype system, known as the engineering array, has now been completed. It has allowed all of the sub-systems that will be used in the full instrument to be tested under field conditions. In this paper, the properties and performance of these sub-systems are described and their success illustrated with descriptions of some of the events recorded thus far. (C) 2003 Elsevier B.V

We study the frequency and angular dependences of Cherenkov radio pulses originated by the excess of electrons in electromagnetic showers in different dense media. We develop a simple model to relate the main characteristics of the electric field spectrum to the properties of the shower such as longitudinal and lateral development. This model allows us to establish the scaling of the electric field spectrum with the properties of the medium such as density, radiation length, Molière radius, critical energy and refraction index. We normalize the predictions of the scaling relations to the numerical results obtained in our own developed GEANT4-based Monte Carlo simulation, and we give a unified parameterization of the frequency spectrum and angular distribution of the electric field in ice, salt, and the lunar regolith, in terms of the relevant properties of the media. Our parameterizations are valid for electromagnetic showers below the energy at which the Landau-Pomeranchuk-Migdal effect starts to be relevant in these media. They also provide an approximate estimate of radio emission in hadronic showers induced by high energy cosmic rays or neutrinos. PACS numbers: 95.85.Bh, 95.85.Ry,

The determination of the shower development in air using fluorescence yield is subject to corrections due to the angular spread of the particles in the shower. This could introduce systematic errors in the energy determination of an extensive air shower through the fluorescence technique.

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