Effect of Adiabatic Deceleration on the Focused Transport of Solar Cosmic Rays

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.