  We investigate the deceleration of Solar Energetic Particles (SEPs) during
their propagation from the Sun through interplanetary space, in the presence of
weak to strong scattering in a Parker spiral configuration, using relativistic
full orbit test particle simulations. The calculations retain all three spatial
variables describing particles' trajectories, allowing to model any transport
across the magnetic field. Large energy change is shown to occur for protons,
due to the combined effect of standard adiabatic deceleration and a significant
contribution from particle drift in the direction opposite to that of the solar
wind electric field. The latter drift-induced deceleration is found to have a
stronger effect for SEP energies than for galactic cosmic rays. The kinetic
energy of protons injected at 1 MeV is found to be reduced by between 35 and
90% after four days, and for protons injected at 100 MeV by between 20 and 55%.
The overall degree of deceleration is a weak function of the scattering mean
free path, showing that, although adiabatic deceleration plays a role, a large
contribution is due to particle drift. Current SEP transport models are found
to account for drift-induced deceleration in an approximate way and their
accuracy will need to be assessed in future work.
