  Despite the significant progress achieved in recent years, the physical
mechanisms underlying the origin of solar energetic particles (SEPs) are still
a matter of debate. The complex nature of both particle acceleration and
transport poses challenges to developing a universal picture of SEP events that
encompasses both the low-energy (from tens of keV to a few hundreds of MeV)
observations made by space-based instruments and the GeV particles detected by
the worldwide network of neutron monitors in ground-level enhancements (GLEs).
The high-precision data collected by the Payload for Antimatter Matter
Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment offer a
unique opportunity to study the SEP fluxes between $\sim$80 MeV and a few GeV,
significantly improving the characterization of the most energetic events. In
particular, PAMELA can measure for the first time with good accuracy the
spectral features at moderate and high energies, providing important
constraints for current SEP models. In addition, the PAMELA observations allow
the relationship between low and high-energy particles to be investigated,
enabling a clearer view of the SEP origin. No qualitative distinction between
the spectral shapes of GLE, sub-GLE and non-GLE events is observed, suggesting
that GLEs are not a separate class, but are the subset of a continuous
distribution of SEP events that are more intense at high energies. While the
spectral forms found are to be consistent with diffusive shock acceleration
theory, which predicts spectral rollovers at high energies that are attributed
to particles escaping the shock region during acceleration, further work is
required to explore the relative influences of acceleration and transport
processes on SEP spectra.
