A theoretical interpretation of Forbush decreases

It is suggested that the most important mechanism that produces Forbush decreases is simply additional adiabatic cooling of cosmic ray particles due to prolonged containment behind the compressed field region of a flare shock wave. Using a technique for studying diffusive propagation by trajectory simulation, we show that adiabatic cooling satisfactorily accounts for the main intensity vs time profile of Forbush decreases and for their dependence on particle rigidity. Moreover, we find that the diffusion coefficient also plays a significant role in determining the amplitude of the effect as the containment time is strongly affected by the choice of scattering mean free path. Isolated Forbush decreases are concerned for relativistic cosmic rays at the earth, related to the passage of a model flare compression wave bounded by forward and reverse shocks and limit the discussion to changes in isotropic intensity