  The spatial fluctuations of a superfluid flowing in a weak random potential
are investigated. We employ classical field theory to demonstrate that the
disorder-averaged nonequilibrium second-order correlation of the order
parameter at zero temperature is identical to the thermally averaged
equilibrium counterpart of a uniform superfluid at an effective temperature.
The physics behind this equivalence is that scattering of a moving condensate
by disorder has the same effect on the correlation function as equilibrium
thermal excitations. The correlation function exhibits an exponential decay in
one dimension and a power-law decay in two dimensions. We show that the
effective temperature can be measured in an interference experiment of
ultracold atomic gases.
