Water-Group Pickup Ions From Europa-Genic Neutrals Orbiting Jupiter

Abstract

Water-group gas continuously escapes from Jupiter's icy moons to form co-orbiting populations of particles or neutral toroidal clouds. These clouds provide insights into their source moons as they reveal loss processes and compositions of their parent bodies, alter local plasma composition, and act as sources and sinks for magnetospheric particles. We report the first observations of H₂ ⁺ pickup ions in Jupiter's magnetosphere from 13 to 18 Jovian radii and find a density ratio of H₂ ⁺/H⁺ = 8 ± 4%, confirming the presence of a neutral H₂ toroidal cloud. Pickup ion densities monotonically decrease radially beyond 13 R _J consistent with an advecting Europa-genic toroidal cloud source. From these observations, we derive a total H₂ neutral loss rate from Europa of 1.2 ± 0.7 kg s^-1. This provides the most direct estimate of Europa's H₂ neutral loss rate to date and underscores the importance of both ion composition and neutral toroidal clouds in understanding satellite-magnetosphere interactions.

Figure 1

Count rates of H₂ ⁺ from 13 to 18 R _J (top) Ion energy per charge as a function of time‐of‐flight (TOF) for all species with nominal background subtraction (upper row) and with H₂ ⁺ isolated (lower row) for all observations within an averaging window of ±1 R _J from the magnetic equator. H₂ ⁺ is unambiguously observed when foregrounds and backgrounds are removed, in the vicinity of the second gray TOF trace for a mass per charge of 2. The horizontal line indicates the expected detected energy for a freshly created pickup ion at the spacecraft's position (bottom) Locations of each observation in cylindrical coordinates with the +z axis are aligned with Jupiter's magnetic dipole. H₂ ⁺ is observed with the strongest magnitude in the inner‐most detected locations.

Figure 2

Partial density of H₂ ⁺ pickup ions (gray) and H⁺ (orange), with average values every 1 R_J for ε = 1.2. The blue curves show expected H₂ ⁺ pickup ion densities for a neutral Europa loss H₂ rate of 6.3 kg s⁻¹ (Smyth & Marconi, 2006). The purple curves show the best fit solution to the observed H₂ ⁺ partial densities. These observations are consistent with a Europa neutral H₂ loss rate of 1.2 ± 0.7 kg s⁻¹. The orange dashed line shows an empirical model for proton densities based on Voyager observations (Bodisch et al., 2017).

Figure 3

Overview of Jupiter's magnetosphere in the vicinity of the Galilean satellites. H₂ ⁺ pickup ions (blue) originate from Europa's neutral toroidal cloud (brighter blue near Europa). Io and Europa contribute plasma pickup ions of different compositions to Jupiter's magnetosphere. Alfvén wings connected to the moons due to their interaction with corotating plasma are also shown in gray.