Numerical simulations of the ion-ion instability carried out with parameters suitable to the description of the plasma in the high-altitude auroral region of the Earth show a very fast evolution of the electron distribution function that cannot be explained by the quasi-linear theory. A detailed analysis of this phenomenon has been carried on. A model based on the reflection of electrons against potential structures created during the linear phase of the ion-ion instability is discussed. The lifetime of the potential structures, which is longer than one ion gyroperiod, can be explained by the linear theory of the ion-ion instability. A simple statistical model of these potential structures enables us to reproduce accurately the evolution of the electron distribution function. ¿ American Geophysical Union 1996