Particle scattering by magnetic fields

The need for a correct quantitative treatment of the interactions between c osmic rays and turbulent magnetic fields continues to be one of the fundame ntal problems of modern astrophysics. It is the aim of this paper to review new developments in the understanding of mechanisms involved in the scatte ring of charged particles by magnetic field fluctuations. Special emphasis is given to a comparison of transport parameters determined from the modeli ng of spacecraft and neutron monitor observation of solar particle events, with theoretical predictions derived from a spectral analysis of simultaneo usly measured fluctuation spectra. It appears that the traditional quasi-li near theory of particle scattering requires only a slight modification, and the major problem still is our lack of knowledge of the three-dimensional structure of the magnetic turbulence. Possibilities to better reconcile the theory with observations by properly taking into account the microphysics of wave and turbulence aspects of the fluctuations, and to use energetic pa rticles as probes to study certain properties of the magnetic turbulence, a re discussed.