A generalized non-Markovian diffusion model which describes the interaction of Langmuir waves with field-aligned electrons in the auroral ionosphere is found to be relevant to the large amplitude Langmuir waves measured by UC Berkeley sounding rockets and by the Freja satellite. This model is valid for any ordering of the diffusion time and the autocorrelation time (the standard quasilinear diffusion model requires the diffusion time to be much longer than the autocorrelation time). We demonstrate that, while the quasilinear diffusion approximation is valid for the lower altitudes studied by the Berkeley sounding rockets, the non-Markovian model is needed for the intense waves observed at the higher altitudes probed by Freja. A test particle simulation is employed to quantify our estimates of the relevant timescales.¿ 1997 American Geophysical Union