The bursts of Langmuir waves observed in the auroral ionosphere are interpreted as localized wave packets of tens to a hundred wavelengths along the geomagnetic field, which interact with streaming, energetic electrons. We investigate the interaction at a kinetic level and show by means of self-consistent, particle-in-cell simulations with open boundary conditions that the interaction itself can lead to wave localization. Ballistic perturbations of the distribution function and nonlinear orbit corrections combine to modulate the perturbed density that drives the waves. The resulting kinetic localization takes place within shorter timescales and for smaller wave amplitudes than the focusing via ponderomotive force. ¿ American Geophysical Union 1995