Sensitivity of JEM-EUSO to Ensemble Fluctuations in the Ultra-High
  Energy Cosmic Ray Flux
  The flux of ultra-high energy (UHE) cosmic rays (CRs) depends on the cosmic
distribution of their sources. Data from CR observations are yet inconclusive
about their exact location or distribution, but provide a measure for the
average local density of these emitters. Due to the discreteness of the
emitters the flux is expected to show ensemble fluctuations on top of the
statistical variations, a reflection of the cosmic variance. This effect is
strongest for the most energetic cosmic rays due to the limited propagation
distance in the cosmic radiation background and is hence a local phenomenon. In
this work we study the sensitivity of the JEM-EUSO space mission to ensemble
fluctuations on the assumption of uniform distribution of sources, with local
source density \sim 10^{-5} Mpc^{-3}. We show that in 3 years of observation
JEM-EUSO will be able to probe ensemble fluctuations if the nearest sources are
at 3 Mpc, and that after 10 years orbiting the Earth, this pathfinder mission
will become sensitive to ensemble fluctuations if the nearest sources are 10
Mpc away. The study of spectral fluctuations from the local source
distributions are complementary to but independent of cosmic ray anisotropy
studies.
