Z‐mode maser instability

Although the cyclotron maser instability customarily involves the extraordinary (X) and ordinary (O) fast transverse electromagnetic wave modes, which directly escape and propagate long distances from the source region, the instability also excites the Z mode for a wide range of parameters. Even though Z mode is a nonescaping mode, it may convert to O mode in an inhomogeneous medium and be detected by remote means. This process is believed to explain radio emissions near cyclotron harmonics emitted both upward and downward from Earth's auroral ionosphere, as well as analogous emissions detected near Saturn. A more general analysis of the electron loss cone driven Z-mode maser instability than previously reported reveals that for certain plasma-to-cyclotron-frequency ratios, the unstable waves split into broadband and narrowband ranges of unstable modes and that these two bands are characterized by distinct polarizations. In previous studies of the Z-mode maser, the maximum growth rate was considered without distinguishing the two bands. Although the maximum temporal growth rate sometimes corresponds to the narrowband feature, the broadband feature with a significant O-mode-like polarization may be of importance, since it can more easily convert to an escaping radiation. This paper presents analysis of maximum growth rates, wave propagation angles, and wave frequencies for the two band features of the loss cone driven Z-mode maser instability as a function of the ratio of plasma-to-electron cyclotron frequency. The results are relevant to auroral radio emission phenomena, especially those observed near cyclotron harmonics at both Earth and Saturn.