In Situ Ion Composition Observations of Ganymede's Outflowing Ionosphere

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.


Introduction
Ganymede, the largest moon in the solar system, was first identified as a satellite of Jupiter by Galileo in 1,610 and may have been observed by Chinese astronomer Gan De as early as 364 BCE.Despite being an object of study for so long, major discoveries are still being made of Ganymede from recent in situ observations by the Galileo and Juno spacecraft.For example, the first Ganymede flybys with the Galileo spacecraft led to the discovery that Ganymede has its own magnetic field (Kivelson et al., 1996) and magnetosphere (Gurnett et al., 1996).
During the Galileo flybys, observations of the low energy ions were made by the Galileo plasma analyzers instrument (Frank et al., 1997).However, Frank et al. (1997) reported that the orientation of the instrument prevented the use of the mass spectrometer for the identification of the outflowing ions.The lack of mass resolved measurements of the outflowing ions has led to debate and uncertainty on the composition of this population.Frank et al. (1997) interpreted the observed outflow as consisting of hydrogen ions.The data was re-analyzed by Vasyliunas and Eviatar (2000) who interpreted the data to be singly ionized oxygen (O + ).Currently, there is no consensus on the composition of the outflowing ions from the observations or models of the Ganymede ionosphere (see Carnielli et al., 2019;Vorburger et al., 2022 and references within).See Garland et al. (2022) and Kivelson et al. (2022) for a review of the magnetosphere and ionosphere of Ganymede.
The 7 June 2021 flyby of Ganymede by the Juno spacecraft provided direct measurements of the outflowing composition by the Jovian Auroral Distributions Experiment-Ion (JADE-I) sensor (McComas et al., 2017).A discussion of the JADE ion and electron observations for the flyby is given by Allegrini et al. (2022) .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.H 3 + ions are an exception to this, with the ratio of H 3 + /H 2 + 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.
Plain Language Summary On 7 June 2021 the Juno spacecraft passed through the Ganymede magnetosphere, with a closest approach altitude of 1,046 km.During this flyby, the Jovian Auroral Distributions Experiment-Ion (JADE-I) sensor observed ions flowing out from the ionosphere.The JADE-I observations are the first direct measurement of the composition of the ionospheric ions.This is an important measurement since there is currently not consensus of the composition.2022) reported observations from a radio occultation experiment with the Juno spacecraft that during ingress an ionosphere with peak density 2,000 ± 500 (1 − σ) cm −3 but no statistically significant signature was detected on egress.Observations of the Ganymede aurora were also captured during this flyby (Greathouse et al., 2022), where they report the emissions originate near the surface and extend to 25-50 km in altitude.See Hansen et al. (2022) for an overview of the Juno observations.Here we present the first composition separated observations of the Ganymede outflowing ionospheric ions.

Observations
During the 7 June 2021 flyby of Ganymede, the Juno spacecraft (Bolton et al., 2017) came to within 1,046 km of the surface while traveling with a relative velocity of ∼18.6 km/s.At the time of this flyby, Ganymede was in the pre-midnight sector of the Jovian magnetosphere (Jovian local time of 2100).Juno approached Ganymede from downstream on the night side, and exited on the dayside.We do not intend to imply that the transition from dayside to night side is the cause of the changes in the distributions presented here instead of, for example, upstream versus downstream.We use dayside/nightside to simply identify different phases of the flyby.During the flyby Ganymede was near the center of Jupiter's plasmadisk (Jovian magnetic latitude of −2.8°).The spacecraft was at high Ganymede magnetic latitudes during closest approach.
During the flyby, low energy ions were observed by JADE-I.JADE-I is a spherical top hat Electrostatic Analyzer (ESA) that uses a Time of Flight (TOF) section to measure both the energy per charge and mass per charge of the ion plasma population.JADE-I has an instantaneous field of view of 270° × 8°, with the 270° plane parallel to the spin axis.The full 4π field of view is swept out as the spacecraft spins, once every ∼30 s.
The energy per charge range in the spacecraft frame is 13 eV/q-46 keV/q, with a resolution of ΔE/E ≈ 28%.A full logarithmic energy sweep is performed once every 2 s.The large relative spacecraft velocity adds an equivalent energy of 1.8 eV/amu when looking in the ram direction.This permits the observations of low velocity ions, especially for heavier species.
After passing through the ESA, ions are given a 10 keV/q post acceleration into the TOF section.The ions pass through an ultrathin carbon foil (Allegrini et al., 2016), where secondary electrons are liberated.An eight kV potential inside the TOF section accelerates the electrons to the MicroChannelPlate detector where they provide a start timing pulse.The ions that pass through the carbon foil continue through the TOF section to the detector and provide the stop timing pulse.The TOF section can determine the mass per charge over a range from 1 to >64 amu/q, with a mass resolution of M/ΔM ≥ 5.
The ions incident on the carbon foil exit with a distribution of charge states (Gonin et al., 1994;Kallenbach et al., 1995), with the most probable being a neutral charge for ions in the JADE-I energy range.Since the TOF section is not a field free region, secondary peaks are observed in the TOF spectra.These secondary peaks are used to improve the mass determination of JADE-I.See Kim et al., 2019 for a complete description of this effect, and its usage for mass determination.
The JADE-I ion energy spectra for the flyby is shown below in Figure 1.This combines all species and look directions of JADE-I.The periodic, 30-s signal in the data comes from the spacecraft spin.The energies shown in Figure 1 are measured in the spacecraft frame.At times before ∼16:43 and after ∼17:01, the Juno spacecraft is in the Jovian magnetosphere.Jovian plasma is seen at energies of ∼1-20 keV/q.Between ∼16:45 and 16:50 we observe an interaction between the Jovian and Ganymede magnetospheres.The period between 16:50 and 17:00 the Juno spacecraft is in the Ganymede magnetosphere, as indicated by the presence of the low energy ion population.This low energy population is interpreted as coming from Ganymede ionosphere after being accelerated to the 10's-100's of eV energies observed at JADE.The rest of this paper will focus on this period inside the Ganymede magnetosphere.
When inside the Ganymede magnetosphere, Jovian ions are still present, though at a much lower intensity.The Jovian ions have energies >1 keV, and the mass spectrum of these higher energy ions (not shown) matches that from times when the Juno spacecraft is outside the Ganymede magnetosphere.When observed in the spacecraft frame, Ganymede ionospheric ions are at energies below a few hundred eV/q.The light ions (m/q < 4 amu) have energies below ∼50 eV/q.Heavier ions have energies between ∼60 and 200 eV/q. 10.1029/2022GL100281 3 of 7 The TOF spectra indicates that there are five major species observed flowing out of the Ganymede ionosphere.
The TOF spectra used to determine the composition of the ionospheric ions is shown in Figure 2, with the primary peaks labeled. .The H + peak, which extends to energies above 50 eV, includes the low energy tail of the Jovian plasma.JADE-I's mass resolution of M/ΔM of ∼5 implies an uncertainty of ∼3 AMU for the ions labeled O + in Figure 2.This signal may include ions with m/q between 16 and 19 amu, however the peak of the spectra is where we would expect to find O + .The data is from a minute (two  The JADE-I high resolution TOF data shown in Figure 2 comes from a data product where counts are summed over look direction to reduce the data volume.The full spatial information is in a different data product that has counts collapsed into three mass ranges: protons, light ions (∼2-7.5 amu/q), and heavy ions (>7.5 amu/q).Both the TOF and the full spatial data sets have the same energy and time resolution (64 energy steps per 2-s sweep).The TOF data is used to separate the high spatial resolution data into the five species identified above (Figure 2).For each step of an energy sweep, the TOF spectra is fit to a linear combination of H + , H 2 + , H 3 + , O + , and O 2 + basis functions.The individual mass basis functions were determined from a combination of laboratory calibration and modeling (Basis functions included in Supporting Information S1).
The fit of the TOF spectra allows for the separation of the high-spatial resolution, low-mass resolution data products into their constituents.Since the TOF spectra has lower spatial resolution, this technique assumes that for each step of the 2-s energy sweep, there is a similar spatial distribution for H 2 + and H 3 + and for O + and O 2 + .
The velocity distributions in the Ganymede frame for two species, H 2 + and O 2 + , at two times are shown in Figure 3.It is shifted by the spacecraft velocity to account for the spacecraft motion.The low energy cutoff is more significant for the lighter ions, but the peak of the distribution does appear to be captured for all ions.Plots of the velocity distributions for all five masses and for the time between 16:56:00 and 16:59:30 are included in Supporting Information S1.
During the flyby, Ganymede outflowing (-v para ) ions are observed for each of the five species.Near closest approach (left column, Figure 3), the outflow velocity was <40 km/s for the hydrogen ions, and ∼10 km/s for the heavier ions.Farther away from closest approach the velocity is seen to increase.The lack of bi-directional flow indicates that the spacecraft is on open magnetic field lines (i.e., one foot point attached to Ganymede and the other to Jupiter).Near the magnetopause boundary (bottom right hand panel of Figure 3) some heavy ions are also observed flowing downward, possibly indicating closed magnetic field lines.
Numerical moments of the five main species were performed using standard techniques as described in Paschmann and Daly (2000) and are shown in Figure 4. Table of the values plotted in Figure 4 are included in Supporting Information S1.The low energy cutoff for the light ions is near the peak of the flux, so much of the low energy tail may be missing.The result of not including this low energy tail would be lower densities, and higher bulk velocities, from the numerical moments.The heavy ions (O + and O 2 + ) are energetic enough to be fully resolved.Since a full spin of data is required to calculate the moments, the values have a time resolution of 30-s.
The ion densities from JADE-I are shown in Figure 4.The dominant ion, by density, for most of the pass is O 2 + .Total electron densities for this time period where determined by Kurth et al. (2022) using the upper hybrid resonance measured by the Waves instrument.The Waves determined electron densities have a higher time resolution of 1 s, so we averaged these values over 30-s time bins to compare them to the JADE-I densities.The total ion densities from JADE-I (Figure 4) has the same temporal profile as the averaged Waves electron densities, but are a factor of 2.5 larger.We consider the Waves derived densities to be a more direct and reliable measure of the total plasma density.
The light ions H + and H 2 + follow a common scale height that is most probably associated with the scale height of the primary neutral H 2 that is responsible for their production since the chemical lifetimes (inverse of the chemical loss rates due to electron recombination are longer than the transport time scale at these altitudes).Some of the H 2 + reacts chemically with H 2 to form H 3 + .Thus, the steeper slope of the H 3 + density fall off due to its dependence on the square of the H 2 density.This effect is more pronounced after closest approach when there is a clear indication of ion acceleration taking place, which may further inhibit the production of H 3 + due to its shorter residence time at altitudes where ion chemistry can take place.This acceleration boundary happens to coincide with the day/night terminator and the boundary identified in the Waves data where the thermal electron characteristics change near 16:57 UTC (see Kurth et al., 2022).Whether this change in ion composition and electron structure is due to day/night variations or due to the changing plasma conditions on the Jupiter facing flank of the Ganymede magnetosphere cannot be determined from this single data set.
The parallel and perpendicular velocities are also shown in Figure 4.The night side parallel velocities generally increase with distance away from Ganymede, and the night side perpendicular velocity is more variable.On the dayside, the parallel (perpendicular) velocity has a local minimum (maximum) near the same time the densities peak.Further away for Ganymede, the ions velocity tends to be more parallel.
On the dayside the ion convection velocity (v_perp) for O 2 + goes from about 20 km s −1 to a value near 4 or 5 km s −1 at 3,000 km, which corresponds to a dependence of the inverse of the radial distance from the center of Ganymede squared.However, on the night side the variance of the O 2 + and the lighter ion perpendicular velocities is likely due to missing part of the low energy distribution of the light ions and/or overall variance of the numerical methodology (∼50%) and probably reflects the uncertainty in the outward ion outflow determination as well.
The outward flux of ions show a similar trend to that of the perpendicular velocities.The light ions tend to have larger outward fluxes on the inbound (nightside), peaked near the closest approach.Again, the H 3 + differs from the other light ions by having a significant less outward flux on the outbound (dayside).The heavy ions have larger outward fluxes on the outbound.The outward flux of O 2 + is peaked at 16:57:30.The O 2 + ion outflow of about 5 × 10 11 ions m −2 s −1 is only a few percent of the ion production estimated to take place in the atmospheric column below the point of measurement that results from the precipitation of energetic electrons with energy fluxes between 0.8 and 1.8 mW m −2 (see Allegrini et al., 2022).

Summary
During the Ganymede flyby Juno on 7 June 2021, outflowing ionospheric ions were observed by the JADE-I sensor.The ion species were O 2 + , O + , H 2 + , H + , and H 3 + .JADE-I's mass resolution has an uncertainty of ∼3 amu for the ions identified as O + , so we cannot be definitive about the ratios between O + , OH + , H 2 O + and H 3 O + .The TOF distribution peak for these ions are most consistent with O + ions.We unfortunately do not have a calibration spectra for H 2 O + ions for the JADE-I flight instrument.However, an estimate of the upper bound on water was made by approximating these ions with a similar spectral shape as O + , but shifted to longer Times of Flight due to the larger mass of 18 amu.Repeating the analysis describe above with a 6th (water) species puts an upper bound on the H 2 O + contribution to being less than 15% of the density of the m/q = 16 signal, and less than a few percent of the total density observed by JADE-I.
Velocity distributions show the ionospheric ions are outflowing, with no bi-directional flow except possibly near the magnetopause.Numerically calculated densities agree with the electron density determined by the Waves instrument, except the JADE-I densities are a factor of ∼2.5 larger.The light ions (H + , H 2 + , and H 3 + ) have a different character than the heavy ions (O + and O 2 + ).While the differences are most striking on the dayside leg, there are also differences between the light and heavy ions both inbound and outbound.For example, the O 2 + ions density inbound are nearly constant with altitude.The light ions appear to be associated with the scale height due to molecular hydrogen, and the altitude profile is generally symmetric across the day-night boundary.H 3 + ions are an exception to this, on the dayside the H 3 + ions fall off much faster on the than the H 2 + ions due to the dependence on the square of the molecular hydrogen density.Whether changes in ion composition and electron density structure are due to day/night variations or due to the changing plasma conditions on the Jupiter facing flank of the Ganymede magnetosphere cannot be determined from this single data set.The outflowing flux of light ions peak near closest approach, but the heavy ions outflowing flux peak on the dayside, similar to the density and appears to be due to local ion acceleration.

Figure 1 .
Figure 1.Panels (a, b) show the Juno trajectory during the Ganymede flyby in GPhiO coordinates, where X is in the direction of local co-rotational flow, Z is parallel the spin axis of Jupiter, and Y completes the right handed system and points toward Jupiter.Panel (c) is an energy-time spectrogram of the differential energy flux observed in the Jovian Auroral Distributions Experiment-Ion, Time of Flight data (i.e., all species included).Energy per charge measured is in the spacecraft frame.The ticks in panels (a, b) are spaced every 5 min.The thick green segments in (a, b), and above (c), highlight the portion of the flyby discussed in this paper.

Figure 2 .
Figure 2. Energy-Time of flight spectrogram with major ionospheric species are identified.

Figure 3 .
Figure 3. Velocity distributions for H 2 + (top row) and O 2 + (bottom row).The left hand column is near closest approach, and the right hand column is later on the dayside.The velocity is mapped to the Ganymede rest frame.Ion outflow is in the negative V parallel direction.The gray circle near the center of the images indicates velocity space below the Jovian Auroral Distributions Experiment-Ion measurement range.

Figure 4 .
Figure 4. Ion density (top row), parallel velocity, magnitude of the perpendicular velocity, and number flux (bottom row) of outflowing Ganymede ionospheric ions.The same quantities are plotted versus time (left column) and altitude (right column).The same colors are used in each panel.For the time period presented here, outflowing ions have negative parallel velocities.
. Buccino 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 O 2