Diffused ?erenkov light measurements for the EUSO project

The aim of the EUSO (Extreme Universe Space Observatory) experiment is to measure from space the fluorescent light produced by the interaction of Extreme Energy Cosmic Rays (EECRs) with the Earth atmosphere. Besides the fluorescent signal, a huge amount of ?erenkov photons is emitted in a narrow cone hitting the Earth surface, where it is partially diffused. The detection of this diffused signal, in a delayed coincidence with the fluorescent signal, allows the absolute positioning of the EECR track, while the knowledge of the diffusing properties of the surface gives an independent indication of the shower energy. Measuring simultaneously on ground the electromagnetic component, the direct ?erenkov light, and the diffused ?erenkov light over different surfaces, we aim to characterize the emitted signal as a function of the energy and the arrival direction of the Extensive Air Shower (EAS), and to evaluate its possible detection from space. This is implemented by the ULTRA (Uv Light Transmission and Reflection in the Atmosphere) experiment composed by a small EAS array and a UltraViolet (UV) telescope. The experimental setups used in the first runs at sea level and at 1970 m a.s.l. are described and the first preliminary results are presented. © World Scientific Publishing Company.