A New Look at the Transport in 2D MHD Turbulence*

We revisit the study of transport of mean magnetic potential - an active scalar field - in 2D MHD turbulence with a large scale magnetic field B, using numerical simulation. Previous studies found that the turbulent transport is suppressed even when the large scale B field is much weaker than the equipartition level. We further notice that the real space structure of the B field contains important information, which is ignored by previous studies that mainly focused on spatially averaged quantities. The B field in the suppression stage is strongly localized in some linearly extended domains. These strong B domains are effectively transport barriers. The barrier width is studied. One characteristic of the existence of transport barriers is found in the PDF of the magnetic potential field A: this PDF is bimodal when there are transport barriers in the suppression stage. If the PDF of A field is designed to be unimodal in the initial condition while <|B|> is held fixed, barriers never form. Therefore, the suppression stage is significantly shorter than the case with bimodal initial condition. This result implies that local transport barriers play an important role in the suppression of transport in 2D MHD.