Cross-Field Particle Diffusion in a Collisionless Plasma: A Nonresonant and a Resonant Mechanism

Nonlinear transverse Alfven waves propagating in a medium with gradients, can lead to wave phase-steepening and wave damping. One damping process is perpendicular heating of ions through the Ponderomotive Force and the creation of local large magnetic field decreases (called MDs) through the diamagnetic effect. Thus in this case, evolution of transverse waves leads to power law and compressive 'spectra'. An analytical expression for nonresonant particle interactions with MDs is given. Energetic charged particles can diffuse across magnetic field lines at a rate almost equal to the Bohm rate through this process. The cross-field diffusion rate for a resonant wave-particle interaction mechanism is also given. The latter is applicable in cases where plasma instabilities are taking place