  Plasma turbulence can enhance the diffusion of both magnetic fields and
plasmas. But, any theory must be consistent with their evolution equations
having the mathematical form of advection-diffusion equations.
Advection-diffusion equations have a remarkable feature. When the diffusion is
extremely weak but non-zero, its magnitude makes only a small difference even
when varied over sixteen orders of magnitude. This is true when the advective
velocity is chaotic -- the exponential separation of neighboring streamlines --
as natural flows generally are. Even highly turbulent flows have in addition to
eddies large-scale coherent flows, such as the Gulf Stream in the Atlantic,
which dominates their transport effects on longest scales. Basic physics and
easily understood mathematics place important constraints, which are explained.
