The evolution of the drifting electron distribution responsible for exciting electrostatic ion cyclotron waves in a magnetized collisionless plasma is studied by means of a computer code. Quasi-linear equations including cyclotron damping on the ions are used for investigating two situations: (1) initial value problem in an infinite homogeneous plasma and (2) open configuration with fresh electrons streaming along the field lines into a region of instability. In both situations a partial plateau is progressively dug through the top of the distribution by velocity diffusion of the electrons along the magnetic field. Cyclotron damping limits the extension of the plateau toward small velocities whereas the high velocity end terminates with a strong positive slope. The asymptote relaxed flat-topped distribution can be directly calculated from an equation based on marginal stability. Due to wave pileup the level of ion cyclotron waves is greatly enhanced in the inhomogeneous situation. For an isothermal plasma the critical drift is 19 vi (with the vi the ion thermal velocity); for drifts 25, 30 vi, the waves are found to reach eϕ/T=0.02, 0.06, respectively. In the first case the resonant heating of the ion tail is negligible. Consequence for ion cyclotron waves and hydrogen energization in the downward and upward current regions of the topside ionosphere are discussed. It is argued that localized parallel electric fields may play an important role in the generation of the observed large waves. ¿ American Geophysical Union 1990