On the preferred source location for the convective amplification of ion cyclotron waves
On the preferred source location for the convective amplification of ion cyclotron waves
复制标题
DOI：
10.1029/92ja02972
复制
发表时间：
1993-06
影响因子：
--
通讯作者：
R. Horne;R. Thorne
中科院分区：
文献类型：
--
作者：
R. Horne;R. Thorne
利用HOTRAY射线追迹程序研究了Pc 1频率范围内电磁离子回旋波的传播、增长和吸收。热等离子体（H+、He+和O+）的实际分布假定在等离子体层顶外的低密度区处于扩散平衡状态，而在等离子体层顶外的低密度区处于无碰撞状态。L模式波增长的自由能由高能H+和O+的双麦克斯韦分布提供，温度和密度模拟卫星观测。热等离子体色散关系的解决方案表明，内部的等离子体层顶的空间增长率是小的，而它们增加外的等离子体层顶与增加L壳。光线追迹表明，在等离子体波导中，L模波只在一次穿越磁赤道时增长，并且只获得很小的路径积分波增益（≤ 2 e-折叠）。在等离子体层顶的密度梯度，使导模波在几个赤道交叉和净路径积分增益大得多（10.8.7 e-折叠）。对于在L = 4时观测到的最大环电流密度4 × 106 m−3，增益高于放大到可观测水平的临界水平（10个e折叠）。在等离子体层顶之外，波只在第一次穿越赤道时增长，增益较小。在不存在非对流不稳定性的情况下，导模的路径积分放大率随L壳层的增大而增大，并且只有在外磁层（L ≥ 7）才能达到可观测波的临界水平。非导波L模式波具有非常小的波增益.当L ≥ 7时，等离子体β变大（β β> 1），并将导致非对流不稳定性的开始。然而，我们认为，介质中的不均匀性和准线性散射将防止绝对不稳定性的发生，在现实中，波传播的群速度非常低。我们认为，安德森等人（1990，1992 a，B）在L = 7以上近当地正午观测到的波，受到这些极低群速度波所引起的增强波增益的影响。
The propagation, growth and absorption of electromagnetic ion cyclotron waves in the Pc 1 frequency range is investigated using the HOTRAY ray tracing program for a realistic distribution of thermal plasma (H+, He+ and O+) that is assumed to be in diffusive equilibrium inside the plasmasphere and collisionless in the low-density region outside the plasmapause. Free energy for L- mode wave growth is provided by a bi- Maxwellian distribution of energetic H+ and O+ with a temperature and density modelled on satellite observations. Solutions to the hot plasma dispersion relation show that inside the plasmasphere the spatial growth rates are small whereas they increase outside the plasmapause with increasing L shell. Ray tracing shows that inside the plasmasphere guided L- mode waves only grow during one crossing of the magnetic equator and only achieve small path-integrated wave gain (≤ 2 e- foldings). At the plasmapause the density gradient enables guided mode waves to grow during several equatorial crossings and the net path-integrated gain is much larger (≃ 8.7 e-foldings). For the largest observed ring current densities of 4 × 106 m−3 at L = 4 the gain is above the critical level (10 e-foldings) for amplification to observable levels. Just outside the plasmapause the waves only grow during the first equatorial crossing and the gain is smaller. In the absence of nonconvective instabilities the path- integrated amplification of the guided mode tends to increase with L shell and reaches the critical level for observable waves only in the outer magnetosphere (L ≥ 7). Unguided L- mode waves have very small wave gain. For L ≥ 7 the plasma beta becomes large (β⊥ > 1) and should lead to the onset of nonconvective instabilities. However, we suggest that inhomogeneities in the medium and quasi-linear scattering will prevent absolute instabilities from occurring and that in reality the waves are propagating with very low group velocities. We suggest that the waves observed by Anderson et al. (1990, 1992a, b) beyond L = 7 near local noon are influenced by the enhanced wave gain due to these very low group velocity waves.