Cosmic ray propagation in interplanetary space

The validity of the test particle picture and the approximation of static fields, as well as the spatial diffusion approximation, are discussed in a general way before specific technical assumptions are introduced. It is argued that the spatial diffusion equation for the intensity per unit energy has a much wider range of applicability than the kinetic (Fokker‐Planck) equation it is derived from. This gives a strong weight to the phenomenological propagation theory. Its general success (and possible failure at small energies) for the modulation of galactic cosmic rays and solar events is described. Apparent effects like the ‘free boundary’ are given disproportionate weight, since through them the connection to the detailed plasma physics of the solar wind is established. Greatest attention in detail is paid to the pitch angle diffusion theory. A general theory is presented that removes the well‐known secularities of the quasi‐linear approximation. The possible breakdown of any pitch angle diffusion theory at very small energies is perhaps connected to the observed ‘turn up’ of the spectrum at low energies. A first attempt to derive the spatial dependence of the diffusion coefficient in the solar cavity, using such a divergence free scattering theory, is described and compared with recent observations out to 5 AU.