Determine the appropriate closure for stochastic-drift perpendicular transport

Determine the closure for the particle-displacement characteristic function that yields a reliable description of perpendicular transport including stochastic gradient and curvature drifts in isotropic magnetic turbulence.

Background

The generalized nonlinear guiding-center treatment expresses the perpendicular diffusion coefficient through a displacement-correlation factor whose evaluation requires a closure assumption. The paper compares a fully diffusive closure with a telegraph-equation closure that accounts for the transition from quasi-ballistic to diffusive parallel transport.

The authors find that both closures improve agreement with simulations, while the telegraph closure is quantitatively more accurate. However, the choice of closure remains the principal theoretical uncertainty in the calculation, leaving unresolved which prescription is generally appropriate for stochastic-drift transport.