We evaluate the exposure during nadir observations with JEM-EUSO, the Extreme Universe Space Obser-\ud
vatory, on-board the Japanese Experiment Module of the International Space Station. Designed as a mis-\ud
sion to explore the extreme energy Universe from space, JEM-EUSO will monitor the Earth’s nighttime\ud
atmosphere to record the ultraviolet light from tracks generated by extensive air showers initiated by\ud
ultra-high energy cosmic rays. In the present work, we discuss the particularities of space-based obser-\ud
vation and we compute the annual exposure in nadir observation. The results are based on studies of the\ud
expected trigger aperture and observational duty cycle, as well as, on the investigations of the effects of\ud
clouds and different types of background light. We show that the annual exposure is about one order of\ud
magnitude higher than those of the presently operating ground-based observatories
Data from the NUCLEON space observatory give a strong indication of the existence of a new universal cosmic ray "knee", which is observed in all groups of nuclei, including heavy nuclei, near a magnetic rigidity of about 10 TV. Universality means the same position of the knee in the magnetic rigidity scale for all groups of nuclei. The knee is observed by both methods of measurement of particles energy implemented in the NUCLEON observatory-the calorimetric method and the kinematic method KLEM. This new cosmic ray "knee" is probably connected with the limit of acceleration of cosmic rays by some generic or nearby source of cosmic rays. *
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