  Data from the PAMELA satellite experiment were used to perform a detailed
measurement of under-cutoff protons at low Earth orbits. On the basis of a
trajectory tracing approach using a realistic description of the magnetosphere,
protons were classified into geomagnetically trapped and re-entrant albedo. The
former include stably-trapped protons in the South Atlantic Anomaly, which were
analyzed in the framework of the adiabatic theory, investigating energy
spectra, spatial and angular distributions; results were compared with the
predictions of the AP8 and the PSB97 empirical trapped models. The albedo
protons were classified into quasi-trapped, concentrating in the magnetic
equatorial region, and un-trapped, spreading over all latitudes and including
both short-lived (precipitating) and long-lived (pseudo-trapped) components.
Features of the penumbra region around the geomagnetic cutoff were investigated
as well. PAMELA observations significantly improve the characterization of the
high energy proton populations in near Earth orbits.
