Auroral electron precipitation can be attributed energetic electrons which have been scattered into the loss cone by the cyclotron maser instability, whereupon the structure of the diffuse aurora can then be attributed to the enhanced scattering which should then occur in density depletions as a result of the increased wave gain of the cyclotron maser instability at lower densities. The resulting precipitation, moreover, then presumably enhances the density depletions in which it occurs, thus causing arc-shaped density depletions as a result of particle drifts which then account for the formation of diffuse auroral arcs. Discrete arcs, on the other hand, have previously been attributed to radio lasing caused by wave feedback within such depletions, and the onset of discrete arcs at substorm onset can then be attributed to the onset of radio lasing within the arc-shaped density depletions caused by the diffuse aurora prior to substorm expansion. Substorm onset should therefore be delayed depending upon the amplitude of the incoming waves being amplified by the cyclotron maser instability to produce the diffuse aurora, thus accounting for the observed triggering of auroral kilometric radiation (AKR) by external waves and also suggesting that the similar triggering of adjacent discrete arcs by the resulting AKR then accounts for the subsequent latitudinal expansion of the aurora during a substorm. ¿ American Geophysical Union 1995