The EUSO experiment is mainly devoted to the detection of Extreme Energy Cosmic Rays (EECRs, E > 5 ·1019 eV) and Neutrinos.
EECRs are produced and accelerated in unknown ways in the Universe, and an accurate measurement of the cosmic-ray spectrum in the region above the change in the spectral index at ~ 5 ·1018 eV (the so-called "ankle") can give valuable informations on the acceleration mechanism in the original sources, propagation in space or even a changement in the interaction process leading to their detection at Earth.
A particular aspect of the propagation is given by the energy loss mechanism of pion photo-production in the interaction of hadronic particles with the cosmic microwave background radiation at 2.7° Kelvin, leading to the Greisen-Zatsepin-Kusmin limit (GZK ~ 5 ·1019 eV).
This effect limits the distance of the sources of primary EECRs to less than 50-100 Mpc, imposing a boundary to the location of emitting sources.
The EUSO instrument uses a large Fresnel collecting wide-angle optics (FOV = 60°) concentrating the UV photons on a large focal surface made up of thousands of multipixel phototubes.
The detection is planned from the ISS, looking down to the night sky from an altitude of ~ 380 Km.
Atmospheric background measurement in the relevant wavelenght interval where EUSO will operate is of crucial importance to analyze and correctly interpret the data.
Balloon flight experiments BABY and BABY 2001 have been done to obtain over ground and over sea background radiance profiles, and other balloon flights are foreseen in the future from different latitudes and environment conditions.
Another important subject is related to the trasmission characteristics in atmosphere and the reflection/diffusion over land and clouds of the Cerenkov light produced by the Extensive Air Showers (EAS).
The Cerenkov light builds up with the shower front, highly beamed in the forward direction, and ends up landing on earth surface or clouds, where it is partially absorbed and partially reflectively diffused.
This signal together with auxiliary altimeter informations will allow the measurement of the atmospheric depth of the showew maximum (Xmax) with a precision of few tens of g/cm2, giving the unique possibility to distinguish high penetrating neutrinos from quick interacting hadron primary particles, and light from heavy nuclei.
The ULTRA (Uv Light Trasmission and Reflection in the Atmosphere) experiment is designed to provide quantitative measurements of the reflection /diffusion signal produced by the EAS impacting the earth surfaces, overcoming the lack of informations in this specific field.
A conventional array of scintillator detectors sampling the electro-magnetic component of the EAS (ETscope) and a light detector operating in the UV wavelenght interval of 300-400 nm (UVscope) will work simultaneously to detect in coincidence EAS from which all the relevant informations will be extracted.