One of the most challenging issues in Astroparticle Physics is represented
by the observation of the energy spectrum of the EECR
Cosmic Rays of Extreme Energy (E>1019
The very existence of such energetic particles
and of neutrinos of comparable energy
raises fundamental scientific questions in connection
with their origin and propagation in the interstellar/intergalactic space.
The EECR particles can be detected through the EAS (Extensive Air Shower)
they produce along their path in the Earth Atmosphere.
The shower development is accompanied by
emission of fluorescence, in particular that induced
in Nitrogen with characteristics spectral lines in the UV band.
Viewed at some instant from a distance,
an EAS appears as a relatively small disc-shaped luminous object.
When it is viewed continuously, the object moves on a straight
path with the speed of light. As it does so, the disc luminosity changes
continuously from so faint to be undetectable, up to a maximum
followed by a gradual fading.
Because the amount of UV light generated from an
EAS is relatively faint, the collecting area of the optical system
must be correspondingly large.
Furthermore, air showers occur at unpredicted locations and with
extremely low rate in the atmosphere; a wide field of view is then
necessary in order to observe a statistically significant number
John Linsley and Livio Scarsi pioneered the field of EECRs in
Volcano Ranch, New Mexico, in 1957.
Following a first suggestion by J.Linsley in the early 1980`s, taken over
by Y.Takahashi, the fluorescence observation can be advantageously
carried out by space. By using wide angle optics with large
we can monitor a target area of atmosphere of the
order of millions km2 sr,
and corresponding mass above 1013 tons,
allowing the detection of the very small flux values typical of the EECR
and making possible the search of the elusive high energy neutrinos.
JEM-EUSO, acronym for "Extreme Universe Space Observatory on the Japanese Experiment Module"
on board the International Space Station, is the first Space Mission devoted to the exploration
of the outermost bounds of the Universe through the detection of the ultra high energy
(above 1020 eV) cosmic rays and neutrinos.
JEM-EUSO will utilize the Earth atmosphere as a giant detector of the EECRs,
the most energetic particles coming from the Universe.
Looking downward the Earth from Space, JEM-EUSO
will detect the extreme energy cosmic rays particles observing the fluorescence signal produced
during their pass in the atmosphere; the Cherenkov signal, diffused when the shower hits ground or the top of a cloud, will also be imaged.
The main objective of JEM-EUSO is the possibility of doing astronomy and astrophysics through the particle channel
with extreme energies so extending, with a significant statistical evidence, the measurement of the energy spectrum
of the cosmic radiation beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off.
Moreover, using the atmosphere as a giant detector, JEM-EUSO could observe extremely high energy neutrinos,
so opening the field of high energy neutrino astronomy.
Furthermore, JEM-EUSO will contribute to the investigation of phenomena intrinsic
to the Earth's atmosphere or induced by the flux of meteoroids incoming from Space.
Firstly proposed as a free-flyer in the framework of the AirWatch program, the observatory was selected by the European Space Agency (ESA)
as a mission attached to the Columbus module of the ISS.
The phase-A study for the feasibility of that observatory, named EUSO, was successfully completed in July 2004.
Nevertheless, because of financial problems in ESA and European countries,
the green-light to start the EUSO phase-B was postponed for a long time.
In 2006, Japanese and U.S. teams redefined the mission as an observatory attached to the Japanese Experiment Module/Exposure
Facility (JEM/EF) of the International Space Station (ISS). They renamed it as JEM-EUSO
and started with a renewed two-year-long Phase A study, whose results were firstly reported in December 2008.
The Phase A study has then been continued with extensive simulations, design, and prototype hardware developments
that have significantly improved the JEM-EUSO mission profile,
targeting the launch of 2017 in the framework of a next phase of JEM/EF utilization.
JEM-EUSO is designed to operate for more than 3 years on board the ISS.
JEM-EUSO is a worldwide collaborating effort of 72 research groups from 15 countries.
JEM-EUSO in brief
instrument of Flight Segment basically consists of an EECR telescope
assisted by an atmosphere monitoring device and controlled by a calibration system.
The JEM-EUSO telescope has a super-wide
(60o) full Field-of-View with optics composed by Fresnel lenses;
the telescope 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 0.1o) in nadir mode.
These time-segmented images allow determining energy and direction of the primary particles.
The focal surface of the JEM-EUSO telescope is formed by about 6000 multi-anode photomultipliers;
the number of pixels is of the order of two hundred thousand.
With respect to the original EUSO, first design of such an observatory, JEM-EUSO
reduces the threshold energy down to around 1019 eV and
increases the effective area by means of advances in technology and
taking advantage of specific features of the JEM/EF module.
The reduction in the threshold energy is realized thanks to new lens material and improved optical design,
detectors with higher quantum efficiency, and improved algorithm for event trigger.
The increase in effective area, as schematized in the figure representing JEM-EUSO passing over Japan,
is realized by inclining the telescope from nadir;
under this so-called "tilted mode", the threshold energy gets higher since the mean distance
to EAS and atmospheric absorption both increase.
First few years of the mission lifetime will be devoted to observe lower energy region in "nadir mode" and
then later to observe high energy region by "tilted mode".
The JEM-EUSO telescope can reconstruct the incoming direction of the EECRs with accuracy better than few degrees.
Its observational aperture of the ground area is a circle with 250 km radius,
and its atmospheric volume above it, with a 60o FoV,
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 Southern Observatory
by a factor ranging from 65 to 280, depending on its observation mode (nadir or tilted).
As concerns the atmosphere monitoring, JEM-EUSO will use an infrared camera and a Lidar
with ultraviolet laser to observe the conditions of the atmosphere in the field of view
of the main EECR telescope, with the objective of determining effective observation time,
and of increasing the reliability of the events around the energy threshold.
JEM-EUSO will be calibrated through instrumentation both onboard and on ground.
The onboard calibration system will be composed of a set of LEDs with different wavelengths (from 300 to 500 nm)
to be loacted in the telescope cylinder as diffusive light sources. Moreover, Xenon flasher lamps will be installed
in a dozen of sites on the ground and, when JEM-EUSO passes over them, once a day or so,
it will detect such lights and measure the total atmospheric UV absorption and, therefore, calibrate the device.
In order to estimate the systematic error in the energy and arrival direction of the primary cosmic rays,
JEM-EUSO telescope observes the ultraviolet laser from ground Lidar stations.
This also will allow to estimate the transmittance of the atmosphere as a function of the altitude.
The design and the construction of the JEM-EUSO telescope is a real technical challenge,
as it involves the use of new technologies from the laboratories of both industrial and research laboratories
in areas as diverse as optical large and accurate Fresnel lenses, a technique of photo-detection
highly sensitive and good resolution, and very innovative analog and digital electronics.
In view of 2017, when JEM-EUSO should be accommodated on the ISS,
a reduced version of the EECR telescope will be tested through specific experiment,
both on ground (EUSO-TA) and on board of stratospheric balloons (EUSO-Balloon).
Moreover, a Mini-EUSO would be mounted inside the pressurized ISS, with access from astronauts.
(more details in the
JEM-EUSO Italian Collaboration Webpages.
The JEM-EUSO Team at IASF-Palermo
Our Institute has been strongly involved in the definition, design and realization of the original EUSO
Nowadays, with different commitments and tasks, the following IASF-Palermo people is part of the JEM-EUSO
working on topics as science, telescope, atmospheric monitoring, cloud coverage analysis algorithms, communication and outreach:
Osvaldo Catalano |
Anna Anzalone |
Giovanni La Rosa |
Maria Concetta Maccarone |
Alberto Segreto |
Osvaldo.Catalano at iasf-palermo.inaf.it
Several papers have been and are currently produced in the framework of the JEM-EUSO Collaboration.
As an example of the EUSO and JEM-EUSO activity at IASF-Palermo Institute,
you can give a look at the
list of selected publications
we produced during the last years.
Some useful links