1.1. Experiments and future projects
The cosmic ray (CR) spectrum spans over roughly 11 decades of energy. Continuously running monitoring using sophisticated equipment on high altitude balloons and ingenious installations on the Earth's surface encompass a plummeting flux that goes down from 104 m-2 s-1 at ~ 109 eV to 10-2 km-2 yr-1 at ~ 1020 eV. Its shape is remarkably featureless, with little deviation from a constant power law across this large energy range. The small change in slope, from E-2.7 to E-3.0, near 1015.5 eV is known as the "knee". The spectrum steepens further to E-3.3 above the "dip" ( 1017.7 eV), and then flattens to E-2.7 at the "ankle" ( 1019 eV). Within statistical uncertainty of current observations, which is large above 1020 eV, the upper end of the spectrum is consistent with a simple extrapolation at that slope to the highest energies, possibly with a slight accumulation around 1019.5 eV (For recent surveys of experimental data the reader is referred to [1, 2, 3, 4, 5, 6]).
It is a lucky coincidence that at the energy (~ 1014 eV) where direct measurement of CRs becomes limited by detector area and exposure time, the resulting air showers that such particles produce when they strike the upper atmosphere become big enough to be detectable at ground level. There are several techniques that can be employed in the process of detection:
(i) Direct detection of shower particles is the most commonly used method, and involves constructing an array of sensors spread over a large area to sample particle densities as the shower arrives at the Earth's surface. After the pioneering measurements of ultra high energy cosmic rays (UHECRs) with the Volcano Ranch experiment in the 60's [7, 8, 9], several arrays have been constructed, such as Haverah Park in England , Yakutsk in Russia [11, 12], the Sydney University Giant Airshower Recorder in Australia (SUGAR) , and the Akeno Giant Air Shower Array (AGASA) in Japan [14, 15].
(ii) Another well-established method of detection involves measurement of the longitudinal development (number of particles versus atmospheric depth) of the extensive air shower (EAS) by sensing the fluorescence light produced via interactions of the charged particles in the atmosphere. The emitted light is typically in the 300 - 400 nm ultraviolet range to which the atmosphere is quite transparent. Under favorable atmospheric conditions, EASs can be detected at distances as large as 20 km, about 2 attenuation lengths in a standard desert atmosphere at ground level. However, observations can only be done on clear Moonless nights, resulting in an average 10% duty cycle. The fluorescence technique has so far been implemented only in the Dugway desert (Utah). Following a successful trial at Volcano Ranch  the group from the University of Utah built a device containing two separated Fly's Eyes [17, 18]. The two-eye configuration monitored the sky from 1986 until 1993. As an up-scaled version of Fly's Eye, the High Resolution (HiRes) Fly's Eye detector begun operations in May 1997 [19, 20]. In monocular mode, the effective acceptance of this instrument is ~ 350(1000) km2 sr at 1019 (1020) eV, on average about 6 times the Fly's Eye acceptance, and the threshold energy is 1017 eV. This takes into account a 10% duty cycle.
(iii) A more recently proposed technique uses radar echos from the column of ionized air produced by the shower. This idea suggested already in 1940 , has been recently re-explored [22, 23] as either an independent method to study air showers, or as a complement to existing fluorescence and surface detectors. A proposal has recently been put forth to evaluate the method using the Jicamarca radar system near Lima, Peru .
In order to increase the statistics at the high end of the spectrum significantly, two projects are now under preparation:
(i) The Pierre Auger Observatory (PAO), currently under construction in Argentina, is the first experiment designed to work in a hybrid mode incorporating both a ground-based array of 1600 particle detectors spread over 3000 km2 with fluorescence telescopes placed on the boundaries of the surface array . A second array will be set up in the Northern hemisphere to cover the whole sky. Such a full-sky coverage is very important to allow sensitive anisotropy analysis. The overall aperture (2 sites) for CRs with primary zenith angle < 60° and primary energy > 1019 eV is 1.4 × 104 km2 sr.
(ii) The mission "Extreme Universe Space Observatory" (EUSO) will observe the fluorescence signal of CRs, with energy > 4 × 1019 eV, looking downward from the International Space Station to the dark side of the Earth atmosphere [26, 27]. The characteristic wide angle optics of the instrument (with opening field of view ± 30° at an average orbit altitude of 400 km) yields a geometric aperture of 5 × 105 km2 sr, taking into account a 10% duty cycle. The monocular stand-alone configuration of the telescope will serve as a pathfinder mission to develop the required technology to observe the fluorescent trails of EASs from space.