When the density ratio between the background plasma and energetic auroral electrons (plasma frequencies &ohgr;ec and &ohgr;eh, respectively) is of order unity or less, the dispersion relations derived from cold plasma theory are no longer valid, and new and interesting results may follow. Recently (Wong et al., 1989) we found that in low density source regions, the O mode and second harmonic X mode emissions can prevail over the X mode fundamental radiation if the energy of the suprathermal electrons is low (e.g., several hundred eV). Despite the low energy, the temporal and spatial growth rates may be substantial. The purpose of the present work is to discuss the generation of Z mode radiation in the light of these new results. Specifically, we address the questions whether the thermal effect on the Z mode dispersion can be equally important and whether the Z mode can compete for the available free-energy source. The numerical results are based on a DGH distribution of energetic electrons (characteristic velocity &agr;), which we have used previously to represent the loss-cone feature of the energetic electrons.

The Z mode growth rate strongly depends on the parameters that we consider, namely the density of the two electron populations, the ratio of electron plasma frequency to electron cyclotron frequency &OHgr;e, and the energy of suprathermal electrons. It will not normally prevail under conditions that are favourable for O mode and second harmonic X mode emissions. However, under suitable circumstances the growth rate of the Z mode can be substantial even for low energies of the suprathermal auroral electrons. Specifically, we find that growth generally maximizes for propagation perpendicular to the magnetic field and usually takes place at normalized wave numbers (ck/&OHgr;e)2>2 above a threshold (&ohgr;eh/&OHgr;e)2>2(&agr;/c)2 if the condition &ohgr;ec≲&ohgr;eh is satisfied. Below this threshold, growth takes place for (ck/&OHgr;e)2<2, and a (tenuous) cold plasma background can help sustain the instability. ¿ American Geophysical Union 1990