Cyclotron maser emission of auroral Z mode radiation

Z mode radiation has been observed in the auroral zones where the plasma frequency ωp is less than the electron cyclotron frequency Ωe. We explore the possibility that this radiation is generated in the same way as the X mode radiation of the auroral kilometric radiation (AKR), i.e., by cyclotron maser emission driven by a loss cone distribution, specifically by electrons reflected at a lower height and propagating upward. We calculate the growth rate for the Z mode by using a method developed for the X mode and the O mode. We find: (1) Growth occurs in a small crescent‐shaped region of ω‐θ space just outside a forbidden zone near θ = 90° with ω between Ωe and the upper hybrid frequency. (2) The temporal growth rate for the Z mode is less than that for the (unsuppressed) X mode but comparable with that of the O mode; for ωp/Ωe ≳ 0.3 the X mode is suppressed and the growth of the Z mode and the O mode compete for the available free energy. Because of the low group speed of the Z mode its spatial growth rate is higher than that of the O mode, giving it an advantage. (3) The product of the spatial growth rate and the bandwidth of the growing waves for the Z mode is comparable with that for the (unsuppressed) X mode and is much greater than that of the O mode. (4) Although all growing Z mode waves have slightly upward directed wave normals (θ ≳ 90°), most have downward directed rays, many at angles θg between 50° and 70°, and so can propagate toward regions where ω < Ωe. We argue that these properties suggest that loss cone driven cyclotron emission may be the mechanism generating the observed auroral Z mode radiation.