Abstract
Intermittent deviation from average dispersion curves reveals differences between the dynamics of magnetohydrodynamic turbulence and hydrodynamic turbulence. Turbulent dispersion is typically measured by the average separation of a group of Lagrangian tracer particles from each other as the particles move in a turbulent flow. On average, the particles will increase their separation as time evolves. However, due to the intermittent nature of turbulence, radical deviations from this behavior can be observed when considering individual groups of particles. When the group consists only of two particles, those two particles sometimes can move toward each other for short periods of time, decreasing their separation. When the group consists instead of many particles that are initially densely packed, similar results can be obtained using the convex hull of the group of particles. We examine this intermittency of many-particle dispersion in incompressible isotropic hydrodynamic turbulence, incompressible isotropic magnetohydrodynamic turbulence, and incompressible anisotropic magnetohydrodynamic turbulence. We interpret periods of decreasing separation physically in terms of anisotropic dispersion due to a large-scale static magnetic field, interactions with local magnetic fields and current sheets, and interactions with Alfvénic fluctuations.
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