  In the framework of focused transport theory, adiabatic deceleration arises
from adiabatic focusing in the solar wind frame and from differential solar
wind convection. An explicit formula is given for the deceleration of
individual particles as a function of the pitch angle. Deceleration and other
first-order effects of the solar wind, including convection, are incorporated
into a numerical code for simulating the transport of energetic particles along
the interplanetary magnetic field. We use this code to model the transport of
solar flare protons. We find that including deceleration can increase the decay
rate of the near-Earth intensity by 75\% more than would be expected based on
advection from higher momenta, due to an interplay with diffusive processes.
Improved response functions are derived for the impulsive injection of
particles near the Sun, and it is found that neglecting deceleration leads to
incorrect estimates of the scattering mean free path based on the intensity
decay alone, especially for lower-energy particles.
