  The existence of a significant flux of antiprotons confined to Earth's
magnetosphere has been considered in several theoretical works. These
antiparticles are produced in nuclear interactions of energetic cosmic rays
with the terrestrial atmosphere and accumulate in the geomagnetic field at
altitudes of several hundred kilometers. A contribution from the decay of
albedo antineutrons has been hypothesized in analogy to proton production by
neutron decay, which constitutes the main source of trapped protons at energies
above some tens of MeV. This Letter reports the discovery of an antiproton
radiation belt around the Earth. The trapped antiproton energy spectrum in the
South Atlantic Anomaly (SAA) region has been measured by the PAMELA experiment
for the kinetic energy range 60--750 MeV. A measurement of the atmospheric
sub-cutoff antiproton spectrum outside the radiation belts is also reported.
PAMELA data show that the magnetospheric antiproton flux in the SAA exceeds the
cosmic-ray antiproton flux by three orders of magnitude at the present solar
minimum, and exceeds the sub-cutoff antiproton flux outside radiation belts by
four orders of magnitude, constituting the most abundant source of antiprotons
near the Earth.
