Particle diffusion on vortices in nearly incompressible magnetohydrodynamics

Standard theory of energetic particle transport in magnetic irregularities treats the latter as weak random fluctuations described as linear MHD wave modes and does not include nonlinear coherent structures such as vortices. To address this shortcoming, a study of energetic particle diffusion in the heliosphere, taking into account scattering on vortices, is made. We consider vortices as nonlinear coherent planar two-dimensional structures orthogonal to the background magnetic field. They can be derived in the framework of nearly incompressible two-dimensional MHD for a subsonic plasma in the fluid frame with plasma beta betaless than or equal to1 and correspond to "zero-frequency'' modes in the linear regime. Transport coefficients for energetic particles are evaluated using the Hamiltonian formalism and variational principles. Further applications for study of cosmic-ray transport in the heliosphere are discussed.