  The plasticity transition at the yield strength of a crystal typically
signifies the tendency of dislocation defects towards relatively unrestricted
motion. For an isolated dislocation the motion is in the slip plane with
velocity proportional to the shear stress, while due to the long range
interaction dislocation ensembles move towards satisfying emergent collective
elastoplastic modes. Such collective motions have been discussed in terms of
the elusively defined backstress. In this paper, we develop a two-dimensional
stochastic continuum dislocation dynamics theory that clarifies the role of
backstress and demonstrates a precise agreement with the collective behavior of
its discrete counterpart, as a function of applied load and with only three
essential free parameters. The main ingredients of the continuum theory is the
evolution equations of statistically stored and geometrically necessary
dislocation densities, which are driven by the long-range internal stress, a
stochastic flow stress term and, finally, two local diffusion-like terms. The
agreement is shown primarily in terms of the patterning characteristics that
include the formation of dipolar dislocation walls.
