We report on FAST observations of large amplitude (up to 500 mV m-1) envelope solitary waves at the edges of the AKR source region. These edges are characterized by the presence of two electron populations: a dominant hot (~keV) component and a minority cold (<60 eV) component. The nonlinear waves are recorded when the spacecraft passes the base of the parallel auroral acceleration region. They form intense packets of electron acoustic waves. The modulation is due to ion acoustic waves. These structures are electrostatic and propagate along the magnetic field at speeds of a few 100 km s-1. They may play a crucial role in the acceleration processes taking place in these regions. ¿ 1999 American Geophysical Union