Abstract
We derive an amplitude equation for a stochastic partial differential equation of Swift-Hohenberg type under the simplifying assumption that the noise acts uniformly on the whole system. Due to the natural separation of timescales, solutions are well approximated by a stochastic differential equation, the so called amplitude equation, describing the evolution of the dominant pattern. Although the slow dominant modes are not forced directly, via the nonlinearity the noise gets transmitted through the system to those modes, too, and multiplicative noise appears in the amplitude equation. Moreover, additional linear and cubic terms appear due to averaging. This leads to either noise-induced stabilization or destabilization effects in the dominating pattern.