| dc.description.abstract | A population-inversion particle distribution usually causes some instabilities in space plasmas. The theoretical study of Yoon et al. (2007) shows that a ring-beam particle distribution would lead to the amplification of X-mode, O-mode, and Z-mode waves. However, in the present study, we find that the beam component of energetic particles may cause a strong excitation of whistler waves. Only when the pitch angle of the average drift velocity of energetic particles is nearly 90 degree, the X-mode waves can be intensively amplified with little interference of whistler waves, which may destroy the initial particle distribution rapidly. In the pure ring cases (90-degree pitch angle), we also find that whistler waves with a parallel propagation can have considerably high growth rate, which is still lower than that of X-mode. In the 60-degree ring-beam cases, the near 60-degree propagation angle leads to a strong amplification of Z-mode waves, and the growth rate and saturation energy of these Z-mode waves are comparable to those of whistler waves with nearly parallel propagation. As the beam component of energetic particles increases, the excitation rate of whistler waves increases, and the whistler mode becomes the dominant wave mode. In ring-beam cases with 30-degree pitch angle, whistler waves become quite strong, and we cannot find any obvious excitation of other wave modes. | en_US |