Abstract
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.
Highlights
The Alfvén-cyclotron instability is driven by a T⟂∕T∥ > 0 anisotropy in the distribution function of a given ion species, where ⟂ and ∥ are defined with respect to the ambient magnetic field
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
Electromagnetic ion cyclotron waves (ICWs) associated with this ion pickup instability have been observed in the solar wind at comets, Mars, and Venus (Barabash et al, 1991; Delva et al, 2008; Thorne & Tsurutani, 1987), in the Earth’s polar wind (Le et al, 2001), and within the Jovian and Kronian magnetospheres (Kivelson et al, 1996; Leisner et al, 2006)
Summary
The Alfvén-cyclotron instability is driven by a T⟂∕T∥ > 0 anisotropy in the distribution function of a given ion species, where ⟂ and ∥ are defined with respect to the ambient magnetic field. In supra-Alfvénic plasma flows such as the solar wind, this can cause LH ICWs to be observed as right-hand (RH) polarized in the spacecraft frame (e.g., Jian et al, 2009; Wicks et al, 2016). It is possible for obliquely propagating ICWs to become linearly polarized and to reverse polarization if they attain and pass through the crossover frequency of the multicomponent plasma in which they are generated (Petkaki & Dougherty, 2001; Rauch & Roux, 1982)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
