Effects of superthermal ring current ion tails on the electromagnetic ion cyclotron instability in multi-ion magnetospheric plasmas
Effects of superthermal ring current ion tails on the electromagnetic ion cyclotron instability in multi-ion magnetospheric plasmas
复制标题
DOI：
10.1029/2010ja016393
复制
发表时间：
2011-05
影响因子：
--
通讯作者：
R. Mace;R. Sydora;I. Silin
中科院分区：
文献类型：
--
作者：
R. Mace;R. Sydora;I. Silin
[1]一个重要的等离子体源的风暴增强环电流是等离子体片。在近地等离子体片中，离子的速度分布具有幂律尾，在各种地磁条件下都能很好地拟合kappa分布。出于这些想法，我们调查的电磁离子回旋（EMIC）的不稳定性驱动的热环电流离子具有速度分布，表现出热各向异性和幂律尾不同程度的硬度（小的功率指数）的参数一致的内磁层。除了少数例外，硬幂律尾环电流离子物种的速度分布的存在下，观察到显着提高不稳定性增长率相对于双麦克斯韦环电流模型。对于一个环电流组成的只有热质子，所有的EMIC分支是不稳定的，与氦分支表现出最快的增长率的热各向异性考虑。在环电流等离子体中加入等数密度的氦离子和氧离子，对质子和氦分支有显著的稳定作用。在这种情况下，经常只有EMIC波色散关系的氧分支是不稳定的。由一个物种产生的回旋阻尼和由另一个物种产生的生长之间的详细平衡提高了速度分布谱指数的重要性，使得它可以作为打开某些分支的不稳定性的“开关”。
[1] An important plasma source for the storm-enhanced ring current is the plasma sheet. Ion species in the near-Earth plasma sheet have been observed to have power law tails on their velocity distributions, which can be well fitted with kappa distributions under a variety of geomagnetic conditions. Motivated by these ideas, we investigate the electromagnetic ion cyclotron (EMIC) instability driven by hot ring current ions having velocity distributions that exhibit thermal anisotropy and power law tails of varying degrees of hardness (smallness of power index) for parameters consistent with the inner magnetosphere. With few exceptions, the presence of hard power law tails on the velocity distributions of the ring current ion species is observed to significantly enhance instability growth rates relative to a bi-Maxwellian ring current model. For a ring current composed of only hot protons, all EMIC branches are unstable, with the helium branch exhibiting the fastest growth rate for the thermal anisotropies considered. The addition of equal number densities of helium and oxygen ions to the ring current plasma has a dramatic stabilizing effect on the proton and helium branches. In this case it is frequently only the oxygen branch of the EMIC wave dispersion relation which is unstable. The detailed balance between cyclotron damping produced by one species and growth produced by another elevates the importance of the velocity distribution spectral index so that it can serve as a “switch” to turn on instability of certain branches.