Hybrid Simulations of Positively and Negatively Charged Pickup Ions and Cyclotron Wave Generation at Europa

TL;DR

This study uses hybrid simulations to analyze how positively and negatively charged pickup ions at Europa generate specific Alfvén-cyclotron waves, providing new insights into local ion densities and wave behaviors.

Contribution

The paper introduces the first hybrid simulation model that reproduces both LH and RH wave growth from positive and negative pickup ions at Europa.

Findings

Simulations match observed wave power near ion gyrofrequencies.

Negative ions can produce observable RH wave signals.

Constraints on chlorine ion densities at Europa are established.

Abstract

In the vicinity of Europa, Galileo observed bursty Alfv\'en-cyclotron wave power at the gyrofrequencies of a number of species including K$^+$, O$_2^+$, Na$^+$, and Cl$^+$, indicating the localised 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 Alfv\'en-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 localised regions at Europa.