Recently, a linear dependence has been found between the polar cap potential and the log of ULF power in the solar wind for Bz north. We argue here that this is very strong evidence that Alfvén waves propagating from the solar wind to the polar cap drive two-dimensional turbulence in agreement with other work along these lines. In brief, two-dimensional (2-D) turbulence is characterized by an inverse cascade of energy from intermediate to large scales, eventually leading to two large counter-rotating vortices filling the available volume and, we think, creating two of the four vortices in the Bz north polar cap. In this model, the Alfvén waves create velocity shear at the 500 or so kilometer scale associated with polar cap arcs. This energy/vorticity then inverse cascades to fill the polar cap.