  We study 2D fronts propagating up a co-moving reaction rate gradient in
finite number reaction-diffusion systems. We show that in a 2D rectangular
channel, planar solutions to the deterministic mean-field equation are stable
with respect to deviations from planarity. We argue that planar fronts in the
corresponding stochastic system, on the other hand, are unstable if the channel
width exceeds a critical value. Furthermore, the velocity of the stochastic
fronts is shown to depend on the channel width in a simple and interesting way,
in contrast to fronts in the deterministic MFE. Thus, fluctuations alter the
behavior of these fronts in an essential way. These affects are shown to be
partially captured by introducing a density cutoff in the reaction rate. Some
of the predictions of the cutoff mean-field approach are shown to be in
quantitative accord with the stochastic results.
