The nature of particle acceleration at the Sun, whether through flare
reconnection processes or through shocks driven by coronal mass ejections
(CMEs), is still under scrutiny despite decades of research. The measured
properties of solar energetic particles (SEPs) have long been modeled in
different particle-acceleration scenarios. The challenge has been to
disentangle to the effects of transport from those of acceleration. The Payload
for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA)
instrument, enables unique observations of SEPs including composition and the
angular distribution of the particles about the magnetic field, i.e. pitch
angle distribution, over a broad energy range (>80 MeV) -- bridging a critical
gap between space-based measurements and ground-based. We present high-energy
SEP data from PAMELA acquired during the 2012 May 17 SEP event. These data
exhibit differential anisotropies and thus transport features over the
instrument rigidity range. SEP protons exhibit two distinct pitch angle
distributions; a low-energy population that extends to 90{\deg} and a
population that is beamed at high energies (>1 GeV), consistent with neutron
monitor measurements. To explain a low-energy SEP population that exhibits
significant scattering or redistribution accompanied by a high-energy
population that reaches the Earth relatively unaffected by dispersive transport
effects, we postulate that the scattering or redistribution takes place
locally. We believe these are the first comprehensive measurements of the
effects of solar energetic particle transport in the Earth's magnetosheath.Comment: 21 pages, 4 figures. Accepted for publication in The Astrophysical
Journal Letter