[1] The weakly (or mildly) relativistic cyclotron maser instability has been successfully applied to explain the Earth's auroral kilometric radiation and other radio sources in nature and laboratory. Among the most important physical parameters that determine the instability criteria is the ratio of plasma-to-electron cyclotron frequencies, ! p / . It is therefore instructive to consider how the normalized maximum growth rate, max / , varies as a function of ! p / . Although many authors have already discussed this problem, in order to complete the analysis, one must also understand how the radiation emission angle corresponding to the maximum growth, Â max , scales with ! p / , since the propagation angle determines the radiation beaming pattern. Also, the behavior of the frequency corresponding to the maximum growth rate at each harmonic, (! max -s )/ , where s = 1, 2, 3, : : : , as a function of ! p / is of importance for a complete understanding of the maser excitation. The present paper computes these additional quantities for the first time, making use of a model loss cone electron distribution function.

[1] 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.

The recent Parker Solar Probe observations of type III radio bursts show that the effects of the finite background magnetic field can be an important factor in the interpretation of data. In the present paper, the effects of the background magnetic field on the plasma-emission process, which is believed to be the main emission mechanism for solar coronal and interplanetary type III radio bursts, are investigated by means of the particle-in-cell simulation method. The effects of the ambient magnetic field are systematically surveyed by varying the ratio of plasma frequency to electron gyrofrequency. The present study shows that for a sufficiently strong ambient magnetic field, the wave–particle interaction processes lead to a highly field-aligned longitudinal mode excitation and anisotropic electron velocity distribution function, accompanied by a significantly enhanced plasma emission at the second-harmonic plasma frequency. For such a case, the polarization of the harmonic emission is almost entirely in the sense of extraordinary mode. On the other hand, for moderate strengths of the ambient magnetic field, the interpretation of the simulation result is less clear. The underlying nonlinear-mode coupling processes indicate that to properly understand and interpret the simulation results requires sophisticated analyses involving interactions among magnetized plasma normal modes, including the two transverse modes of the magneto-active plasma, namely, the extraordinary and ordinary modes, as well as electron-cyclotron-whistler, plasma oscillation, and upper-hybrid modes. At present, a nonlinear theory suitable for quantitatively analyzing such complex-mode coupling processes in magnetized plasmas is incomplete, which calls for further theoretical research, but the present simulation results could provide a guide for future theoretical efforts.

Electromagnetic ion cyclotron (EMIC) waves generated by hot anisotropic (T⊥ > T∥) protons (∼10–100 keV), play an important role in accelerating cold (<1 eV) protons (H+) and helium (He+) ions in the magnetosphere. Using a hybrid code with parameters found in the inner magnetosphere, we examine when and how cold H+ and He+ ions are energized by EMIC waves. Hybrid simulations show that the energization of the cold particles occurs in two steps. In the first step, EMIC waves, which are linearly excited in the early stage of the simulation, interact with cold H+ and He+ ions, resulting in energization mostly in the direction perpendicular to the background magnetic field. The energization in this step is mainly contributed by enhanced bulk motion of these ions as a result of the linear response, consistent with recent observations in the inner magnetosphere. In the second step, nonlinear evolution of energized cold H+ and He+ ions are confirmed in the parallel direction, which is seen after about 200 proton gyroperiods (∼8.5 s). Throughout the simulation run, cold He+ ions are much more energized in the perpendicular direction than in the parallel direction. However, the cold protons are more energized in the parallel direction than in the perpendicular direction after 500 proton gyroperiods (∼21.3 s). By comparing recent observations and the present simulation results, we suggest that the cold particle energization by EMIC waves occurs at an early stage of wave generation when the nonlinear evolution of EMIC waves is not dominant in the inner magnetosphere.