Recent satellite observations have identified second harmonic auroral kilometric radiation, in part in source regions where the electron plasma frequency is known to be much smaller than the electron cyclotron frequency (&ohgr;pe≲0.2 &OHgr;e). These observations as well as certain observations of O mode fundamental radiation have raised some controversy because conventional discussions predict their generation to be more favorable in a high-density plasma (e.g., &ohgr;pe>0.2 &OHgr;e); the fundamental X mode dominating otherwise. In this work we reexamine the relativistic dispersion equation based on the cyclotron maser theory for a DGH distribution of energetic electrons. The purpose is to identify and understand the physical conditions under which fundamental O mode and second harmonic X mode radiation can dominate over fundamental X mode emission in low-density source regions. We find that the energy of the auroral electrons can play a significant role in determining the dominant wave mode. The temporal and spatial growth rates of both the fundamental O mode as well as the second harmonic X mode remain high for surprisingly low energies (several hundred electron volts) of the suprathermal electrons, while the fundamental X mode is suppressed for energies ≲1 keV due to its relativistic resonance condition. This finding can have significant implications to the theoretical study of magnetospheric radio environments in general. ¿ American Geophysical Union 1989