Electron distributions obtained in the source regions of auroral kilometric radiation (AKR) by the Fast Auroral SnapshoT (FAST) satellite have revealed several free energy sources with positive gradients with respect to v⊥ superimposed on a board plateau with a radius close to the primary incident electron acceleration energy and covering pitch angles from near field-aligned all the way to the (upgoing) loss cone. Two-dimensional electromagnetic particle simulations are used to demonstrate that such a distribution arises as a quasi-steady feature of a process in which the increase of the perpendicular velocity of the electrons as they propagate into a region of increasing magnetic field strength is balanced by the diffusion to lower v⊥ caused by the electron-cyclotron maser instability. The maser radiation is emitted nearly perpendicular to the ambient magnetic field at frequencies between the relativistic and nonrelativistic cyclotron frequencies. In these circumstances, the entire primary auroral electron distribution can contribute to the resonant wave-particle interaction, leading to electric field intensities of the order of 500 mV/m. In contrast, a pure loss cone distribution is shown to produce much weaker electric fields, leads to emission at angles ≳10 ¿ away from perpendicular, and cannot produce the broad plateau observed in the electron distribution. The simulations and linear theory indicate that the maser instability in a uniform system produces an intrinsic bandwidth of the order of a few tenths of 1% of the cyclotron frequency (~0.5--1.0 kHz in the AKR source region). Any narrower spectra would appear to require some nonuniform or time-dependent feature in the source region. ¿ 1999 American Geophysical Union