Deceleration and acceleration of cosmic rays in the solar wind

Recent observations of the deceleration of low-energy solar cosmic rays in the solar wind are discussed in the context of cosmic-ray transport theory. It is concluded that the rate of deceleration is much slower than would be produced by adiabatic energy change due to the observed expansion of the plasma, so that a competing acceleration process must be operative. Second-order Fermi acceleration by hydromagnetic waves, acting together with adiabatic energy change, is shown to provide a natural interpretation of the observations.