Visco-resistive magnetohydrodynamic turbulence, driven by a two-dimensional unstable
shear layer that is maintained by an imposed body force, is examined by decomposing it into
dissipationless linear eigenmodes of the initial profiles. The down-gradient momentum flux,
as expected, originates from the large-scale instability. However, continual up-gradient
momentum transport by large-scale linearly stable but nonlinearly excited eigenmodes is
identified and found to nearly cancel the down-gradient transport by unstable modes. The …