Nonlinear cosmic ray diffusion theories

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Charged particles moving through the interplanetary or interstellar medium interact with large-scale magnetic fields, like those of the Sun or the Galaxy. These fields, typically constant in time and space, can be approximated as homogeneous. In the absence of additional fields, particle trajectories form perfect helices at constant speeds. However, turbulent electric and magnetic fields from interstellar or solar wind plasma cause scattering of cosmic rays both parallel and perpendicular to the background field. These scattering effects, generally diffusive, can be described using spatial diffusion coefficients or mean free paths, which are crucial for understanding cosmic ray propagation and diffusive shock acceleration, the latter being responsible for the high energies of observed cosmic rays. The book begins with a general overview of the physical scenario, discussing fundamental processes like cosmic ray propagation and acceleration in various systems, including the solar system and interstellar space. These processes arise from the interaction between charged cosmic particles and astrophysical plasma turbulence, which is further explored in the second chapter.