Desai R T, Cowee M M, Wei H, Fu X, Gary S P, Volwerk M, Coates A J
Mullard Space Science Laboratory University College London London UK.
Centre for Planetary Sciences University College London/Birkbeck London UK.
J Geophys Res Space Phys. 2017 Oct;122(10):10408-10420. doi: 10.1002/2017JA024479. Epub 2017 Oct 26.
In the vicinity of Europa, Galileo observed bursty Alfvén-cyclotron wave power at the gyrofrequencies of a number of species including K, O 2+, Na, and Cl, indicating the localized pickup of these species. Additional evidence for the presence of chlorine was the occurrence of both left-hand (LH) and right-hand (RH) polarized transverse wave power near the Cl gyrofrequency, thought to be due to the pickup of both Cl and the easily formed chlorine anion, Cl. To test this hypothesis, we use one-dimensional hybrid (kinetic ion, massless fluid electron) simulations for both positive and negative pickup ions and self-consistently reproduce the growth of both LH and RH Alfvén-cyclotron waves in agreement with linear theory. We show how the simultaneous generation of LH and RH waves can result in nongyrotropic ion distributions and increased wave amplitudes, and how even trace quantities of negative pickup ions are able to generate an observable RH signal. Through comparing simulated and observed wave amplitudes, we are able to place the first constraints on the densities of Chlorine pickup ions in localized regions at Europa.
在木卫二附近，伽利略观测到了包括钾、氧离子、钠和氯等多种离子回旋频率处的突发阿尔文 - 回旋波功率，这表明这些离子存在局部拾取现象。氯存在的额外证据是在氯回旋频率附近出现了左旋（LH）和右旋（RH）极化的横向波功率，据认为这是由于氯以及易于形成的氯离子的拾取所致。为了验证这一假设，我们针对正、负拾取离子使用了一维混合（动力学离子，无质量流体电子）模拟，并与线性理论一致地自洽再现了左旋和右旋阿尔文 - 回旋波的增长。我们展示了左旋和右旋波的同时产生如何导致非陀螺ropic离子分布和波幅增加，以及即使是微量的负拾取离子如何能够产生可观测的右旋信号。通过比较模拟和观测到的波幅，我们能够对木卫二局部区域中氯拾取离子的密度施加首个限制。
