  Data from the Payload for Antimatter Matter Exploration and Light-nuclei
Astrophysics (PAMELA) satellite experiment were used to measure the geomagnetic
cutoff for high-energy (>80 MeV) protons during the 14 December 2006
geomagnetic storm. The variations of the cutoff latitude as a function of
rigidity were studied on relatively short timescales, corresponding to
spacecraft orbital periods (94 min). Estimated cutoff values were compared with
those obtained by means of a trajectory tracing approach based on a dynamical
empirical modeling of the Earth's magnetosphere. We found significant
variations in the cutoff latitude, with a maximum suppression of about 7 deg at
lowest rigidities during the main phase of the storm. The observed reduction in
the geomagnetic shielding and its temporal evolution were related to the
changes in the magnetospheric configuration, investigating the role of
interplanetary magnetic field, solar wind and geomagnetic parameters. PAMELA's
results represent the first direct measurement of geomagnetic cutoffs for
protons with kinetic energies in the sub-GeV and GeV region.
