JEM-EUSO is a new type of observatory that uses the earth's atmosphere as a detector. JEM-EUSO will be on orbit on the International Space Station (ISS). It observes transient luminous phenomena taking place in the earth's atmosphere caused by particles coming from space. The sensor is a super wide-field telescope that detects extreme energy particles with energy above 31019 eV. This remote-sensing instrument orbits around the earth every 90 minutes on board of the International Space Station at the altitude of approximately 400km (Figure 1-1). The figure shows an extreme energy particle colliding with a nucleus in the earth's Atmosphere, where it produces an Extensive Air Shower (EAS), consisting of numerous electrons, positrons, and photons. JEM-EUSO captures the moving track, which is revealed by the fluorescent UV photons and reproduces the energy development of the EAS.

The JEM-EUSO telescope has a super-wide Field-of-View (±30°) with two double sided curved Fresnel lenses and records the track of an EAS with a time resolution of 2.5 microseconds and a spatial resolution of about 0.75 km (corresponding to 0.1 degrees). These time-segmented images allow the deermination of the energies and directions of the primary particles. The focal surface of the JEM-EUSO telescope is formed by about 6,000 multi-anode photomultipliers. The number of pixels is about two hundred thousand.

JEM-EUSO instrument can reconstruct the incoming direction of the extreme energy particles with accuracy better than several degrees. It's observational aperture of the ground area is a circle with 250 km radius and its atmospheric volume above it with a 60-degree field-of-view is about 1 tera-ton or more. The target volume for upward neutrino events exceeds 10 tera-tons. The instantaneous aperture of JEM-EUSO is larger than the Pierre Auger Observatory by a factor of 50 - 250 (Figures 1.1 and 1.2) when attached to ISS (Figure 1.3).

Figure 1-1: Principle of the JEM-EUSO telescope for detecting extremely energetic particles

Figure 1-2: Area observed by the JEM-EUSO telescope in one exposure

EUSO was originally selected by the European Space Agency (ESA) as a mission attached to the European Columbus module of the ISS. The phase-A study has been successfully completed in June 2004 under ESA. However, ESA postponed the start of phase-B, so Japanese and U.S. teams re-defined EUSO as a mission attached to the Japanese Experiment Module/ Exposure Facility (JEM/EF) of ISS. It was renamed as JEM-EUSO and started the preparation targeting the launch of 2013 in the framework of the second phase utilization of JEM/EF utilization.

Nadir mode

Titled mode

Figure 1-3: Artist's conception of the JEM-EUSO telescope attached to the Japanese Experiment Module of the International Space Station

JEM-EUSO reduces the threshold energy down to around 31019 eV and increases the effective area due to advances in technology and also the superior features of JEM/EF. The reduction in the threshold energy is realized by 1) new lens material and improved optical design, 2)detectors with higher quantum efficiency, 3) improved algorithm for the event trigger. The increase in effective area is realized by inclining the telescope from nadir which is named as tilted mode (figure1-3). In this tilted mode, the threshold energy gets higher since the mean distance to EAS and atmospheric absorption both increase. The first half of the mission lifetime is devoted to observe lower energy region in the nadir mode and second half of the mission to observe high energy region in the tilted mode. JEM-EUSO is planned to be attached to JEM/EF of ISS, which will be launched in FY 2016 by H2B rocket and conveyed by the HTV (H-II transfer Vehicle) to ISS.