Saturn's magnetosphere: Observations of ion cyclotron waves near the Dione L shell
Saturn's magnetosphere: Observations of ion cyclotron waves near the Dione L shell
复制标题
土星磁层：Dione L 壳附近离子回旋波的观测
DOI：
10.1029/ja088ia10p07831
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发表时间：
1983
影响因子：
--
通讯作者：
B. Tsurutani
中科院分区：
文献类型：
--
作者：
E. Smith;B. Tsurutani
土星磁层内的先驱者11号矢量氦磁力仪获得的高时间分辨率（0.75秒）测量结果显示，在L = 6.3和6.7之间的狄俄涅L壳附近存在准周期波。虽然土卫四离飞船很远，但当先锋号进入近心点和离开近心点时都观察到了这些波，并推测这些波与土卫四间接相关。波的特征周期为18 s，典型振幅为5 nT。在观测到波的区域，先锋11等离子体分析仪检测到一个峰值等离子体密度，可能与从土卫四表面溅射出来的重离子有关，并初步确定为O++。随后旅行者号在这个内部环面的观测似乎与0+一致。波的特征周期远低于质子的回旋周期。由于重离子是热的（~ 106 K），阿尔芬相速度和离子热速度几乎相同。波与主导离子的离子回旋共振似乎能够产生波。基于波的增长率的理论论据表明，O++更有可能是负责比O+，但涉及H+离子的共振能量的几个千电子伏不能排除。与损失锥相关的桨距角各向异性的存在是隐含的。对于O++，预测螺距角各向异性为0.70，对应于T θ/T θ = 1.7。波对重离子的俯仰角散射应该会导致降水，并可能在纬度67°附近产生极光。
High time resolution (0.75 s) measurements obtained by the Pioneer 11 vector helium magnetometer inside Saturn's magnetosphere show quasi-periodic waves to be present near the Dione L shell between L = 6.3 and 6.7. Although Dione was far from the spacecraft, the waves were observed when Pioneer was both inbound to, and outbound from, periapsis and are presumably associated indirectly with Dione. The waves have a characteristic period of 18 s and a typical amplitude of 5 nT. In the region in which the waves were observed, the Pioneer 11 plasma analyzer detected a peak plasma density associated with heavy ions presumably sputtered from Dione's surface and tentatively identified as O++. Subsequent Voyager observations in this inner torus appear to be consistent with 0+. The characteristic period of the waves is well below the proton gyroperiod. Because the heavy ions are hot (∼106 K), the Alfven phase speed and the ion thermal speeds are nearly the same. Ion cyclotron resonance of the waves with the dominant ions appears capable of generating the waves. Theoretical arguments based on the growth rates of the waves suggest that O++ is more likely to be responsible than O+ but that a resonance involving H+ ions with energies of a few keV cannot be excluded. The existence of a pitch angle anisotropy associated with a loss cone is implied. For O++, a pitch angle anisotropy of ≃0.70, corresponding to T⊥/T∥ ≃ 1.7, is predicted. Pitch angle scattering of the heavy ions by the waves should cause precipitation with the possible production of an aurora near 67° latitude.