In Situ Ion Composition Observations of Ganymede's Outflowing Ionosphere

Abstract

On 7 June 2021 the Juno spacecraft passed through the Ganymede magnetosphere, with a closest approach altitude of 1,046 km. The Jovian Auroral Distributions Experiment‐Ion (JADE‐I) sensor observed ionospheric ions, consisting of O2+, O+, H2+, H+, and H3+. These ions were outflowing, with no bi‐directional flow except possibly near the magnetopause. Relative ion densities with respect to time agree the electron density determined by the Waves instrument, but are ∼2.5 times larger. The light ions appear to be in hydrostatic equilibrium because the altitude profile is generally symmetric between inbound and outbound legs of the flyby. H3+ ions are an exception to this, with the ratio of H3+/H2+ being ∼a factor 4 lower on the outbound than the inbound leg. The heavy ions have higher densities outbound than inbound. The outflowing flux of light ions peak near closest approach, but the heavy ions peak outbound of the flyby.

Plasma Observations During the 7 June 2021 Ganymede Flyby From the Jovian Auroral Distributions Experiment (JADE) on Juno

“…For comparison, the ram speed is ∼18 km/s (Hansen et al, 2022) and the relative plasma disk speed is ∼140 ± 20 km/s (Kivelson et al, 2022). A detailed analysis of the ion flow direction, velocity distribution, and density profiles in the region C-D is given in Valek et al (2022).…”

“…from ∼0.013 to 46 keV/q at 2 s time resolution. The details of the ion observations are given in Valek et al (2022). Evidence for magnetic reconnection near the upstream magnetopause is given in Ebert et al (2022) and Romanelli et al (2022).…”

Evidence for Magnetic Reconnection at Ganymede's Upstream Magnetopause During the PJ34 Juno Flyby

“…The white pixels in Figure 2b are due to the fluxes exceeding the color bar upper limit. The protons and heavy ions with energies of ≤∼0.1 and ≤∼0.5 keV/q, respectively, observed by JADE inside Ganymede's magnetosphere come from Ganymede's ionosphere (e.g., Valek et al., 2022). The ions in that region above ∼0.1 keV/q for protons and ∼1 keV/q for heavy ions are from Jupiter's plasma disk.…”

“…The ions in that region above ∼0.1 keV/q for protons and ∼1 keV/q for heavy ions are from Jupiter's plasma disk. The cold, low energy protons and heavy ions show a slight energy increase between ∼16:59:15 and 17:00:00 UT as Juno approaches the MCL that is driven by increased ion velocities, primarily in the field‐aligned direction (Valek et al., 2022). Cold ion energization near Ganymede's magnetopause was observed during the Galileo G1 flyby (e.g., Collinson et al., 2018) and is often seen in the low‐latitude boundary layer of Earth's magnetopause (e.g., Gosling et al., 1990; Vines, Fuselier, Trattner, et al., 2017), typically in association with recently reconnected magnetic field lines.…”

Juno's Close Encounter With Ganymede—An Overview

“…The presence of H 2 + and H 3 + is a strong indicator that the composition is dominated by water products from Ganymede's surface (Allegrini et al, 2022). Observation of heated, streaming electrons near Ganymede's magnetopause was consistent with magnetic reconnection (Ebert et al, 2022;Valek et al, 2022).…”