OPS6

The study of Ganymede, the only known moon in the Solar System to possess an intrinsic magnetic field embedded within a planetary magnetosphere, is of significant importance in view of future missions to the Jovian system. Indeed, the dynamics of the entry and circulation inside Ganymede’s magnetosphere of the Jovian energetic ions, as well as the morphology of their precipitation on the moon’s surface determine the variability of the sputtered-water release and exosphere generation. The so-called planetary space weather conditions around Ganymede can also have a long-term impact on the weathering history of its icy surface.

In this talk, I will discuss some key characteristics of the circulation of the Jovian magnetospheric ions within the environment of Ganymede as derived from the application of a single-particle Monte Carlo model driven by the electromagnetic fields from a global MHD model. In particular, the Jovian energetic ion circulation and precipitation to Ganymede’ s surface was estimated for different relative configurations between the moon’s magnetic field and Jupiter’s plasma sheet, characterized by conditions similar to those encountered during the NASA Galileo G2, G8, and G28 flybys of Ganymede (i.e., when the moon was above, inside, and below the center of Jupiter’ s plasma sheet). The resulting differences between the various surface precipitation patterns and the implications in the water sputtering rate will be discussed. The results of this preliminary analysis are relevant to ESA’ s JUICE mission and in particular to the planning of future observation strategies for studying Ganymede’ s environment.

How to cite: Plainaki, C., Massetti, S., Jia, X., Mura, A., Milillo, A., Grassi, D., and Filacchione, G.: Circulation of energetic ions within Ganymede’s magnetosphere: a Monte-Carlo simulation approach, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-828, https://doi.org/10.5194/epsc2021-828, 2021.

Ganymede’s tenuous atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the OI1356 Å and OI1304 Å oxygen emissions were used to derive sputtered molecular oxygen, O_2, as an atmospheric constituent. We present a new analysis of high-sensitivity spectra and spectral images of Ganymede’s oxygen emissions acquired by the COS and STIS instruments on the Hubble Space Telescope. The COS eclipse observations constrain atomic oxygen, O, to be at least two orders of magnitude less abundant than O₂. We then show that dissociative excitation of water vapor, H₂O, is found to increase the OI1304 Å emissions relative to the OI1356 Å emissions around the sub-solar point, where H₂O is more abundant than O₂. Away from the sub-solar region, the emissions are more than two times brighter at OI1356 Å than at OI1304 Å, and O₂ prevails as found in previous analyses. A ~6-fold higher H₂O/O₂ mixing ratio on the warmer trailing hemisphere compared to the colder leading hemisphere, a spatial concentration at the sub-solar region, and the ratio-estimated H₂O densities identify icy surface sublimation as a local dayside atmospheric source.
Our analysis provides the first evidence for a sublimated atmosphere on an icy moon in the outer solar system.

How to cite: Roth, L., Ivchenko, N., Gladstone, R., Saur, J., Grodent, D., Bonfond, B., Molyneux, P., and Retherford, K.: Hubble Space Telescope detects sub-solar water atmosphere on Ganymede, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-176, https://doi.org/10.5194/epsc2021-176, 2021.

Abstract

During the Galileo mission, eight radio occultations of Ganymede were made with only one strong detection of an ionosphere. On June 7, 2021, the Juno spacecraft executed a close flyby of Ganymede, the third Galilean moon of Jupiter. During this close flyby, a detection attempt of Ganymede’s elusive ionosphere was made with an Earth radio occultation. The radio science instrumentation consisted of X-band and Ka-band downlink signals referenced to an X-band uplink signal from the Deep Space Network. Electrons encountered along the radio propagation path induce a dispersive phase delay on the radio link. Taking advantage of the dispersive nature, a linear combination of X-band and Ka-band signals provided a direct measurement of the electron content along the propagation path. The ingress occultation occurred at small ram angle and egress occultation occurred at a small solar zenith angle on the sun-lit side, providing a diverse geometry for detection.

Background

Ganymede’s ionosphere is tenuous. The atmosphere of Ganymede is predominately oxygen. It is thought the ionosphere is generated via photoionization of the neutral atmospheric oxygen from the Sun (McGrath et al 2004). The Galileo spacecraft executed a total of eight radio occultations of Ganymede throughout its mission, resulting in five non-detections, two weak detections, and one strong detection of an ionosphere. The strong ionosphere detection occurred during the Ganymede G8 ingress occultation (Kliore 1998) resulting in a peak electron density of 5,000 el/cm³ at an altitude of 16 km above the surface. Initially, the lack of detection was surprising, but it was hypothesized that positive detections occurred where the trailing hemisphere of the satellite was in sunlight; therefore, the atmosphere can be ionized by solar radiation to produce an observable ionosphere (Kliore et al 2001). Radio occultation measurements will help understand better the connection between Ganymede’s elusive ionosphere, its intrinsic magnetic field and Jupiter’s magnetosphere.

Observation

Juno Gravity Science Instrument (Asmar et al 2017) is a radio science instrument which utilizes dual-frequency X-band (8.4 GHz) and Ka-band (32 GHz) radio links between the Juno spacecraft and the Earth-based observing stations of NASA’s Deep Space Network (DSN).  On June 7, 2021 Juno’s extended mission trajectory will take the spacecraft on a close encounter with Ganymede at an altitude of 1000 km. An Earth occultation occurred during this flyby as shown in Figure 1. Measurement of Ganymede’s ionosphere is made via a radio occultation geometry, where the Juno spacecraft will set behind Ganymede as observed from Earth. In this way, the radio ray path propagates directly through the ionosphere of Ganymede twice, once on ingress and once on egress. During the radio occultation, Juno transmitted dual-frequency X-band and Ka-band to the DSS-43 and DSS-35 antennas at the Canberra DSN complex. Both downlink signals were referenced to a single X-band uplink signal sent from the DSS-35 antenna.

Figure 1. Occultation geometry of Juno's encounter of Ganymede on June 7, 2021, inbound to the Perijove-34 Jupiter flyby. As viewed from Earth, the spacecraft passed directly behind Ganymede for approximately 15 minutes.

Methods

The ionosphere perturbation is detectable in small changes in the received frequency. A refractivity profile is derived from the perturbations. The refractivity profile can then be inverted to determine electron density. In the link configuration used during the Ganymede flyby, two inversion techniques are available to compute the electron density from the signal frequency, with three independent analysis results:

a) X-band downlink referenced to X-band uplink (2-Way)

b) Ka-band downlink referenced to X-band uplink (2-Way)

c) Dual-Frequency X-band and Ka-band downlink (1-Way linear combination)

Methods (a) and (b) estimates the bending angle through geometric optics and the medium refractivity through an Abel transform (e.g. Withers and Moore 2020). In method (c), the integrated Total Electron Content (TEC) will be computed by combing the dual frequency measurements, given the dispersive nature of the plasma noise. In turn, this can be converted into electron density (el/cm³) via Abel Transform. The transponder configuration allows for the isolation of the plasma signature on the downlink only, analogously to a one-way measurement (e.g. Dalba and Withers 2019).

Acknowledgements

The work of DB, MP, RP, and SL was carried out at the Jet Propulsion Lab, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged. EG, LGC, PT and MZ are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2017-40-H.0, and its extension 2017-40-H.1-2020, for ESA’s BepiColombo and NASA’s Juno radio science experiments. PS and AH were supported by NASA Contract NNM06AA75C from the Marshall Space Flight Center under subcontract 699054X from Southwest Research Institute.

© 2021 California Institute of Technology. Government sponsorship acknowledged.

References

• McGrath, Melissa A., et al. "Satellite atmospheres." Jupiter: The Planet, Satellites and Magnetosphere(2004): 457-483.
• Kliore, A. J. (1998). Satellite atmospheres and magnetospheres. Highlights of Astronomy, 11(2), 1065-1069.
• Kliore, A. J., Anabtawi, A., & Nagy, A. F. (2001, December). The ionospheres of Europa, Ganymede, and Callisto. In AGU Fall Meeting Abstracts(Vol. 2001, pp. P12B-0506).
• Asmar, S. W., Bolton, S. J., Buccino, D. R., Cornish, T. P., Folkner, W. M., Formaro, R., ... & Simone, L. (2017). The Juno gravity science instrument. Space Science Reviews, 213(1), 205-218.
• Withers, P., & Moore, L. (2020). How to process radio occultation data: 2. From time series of two-way, single-frequency frequency residuals to vertical profiles of ionospheric properties. Radio Science, 55(8), 1-25.
• Dalba, P. A., & Withers, P. (2019). Cassini radio occultation observations of Titan's ionosphere: The complete set of electron density profiles. Journal of Geophysical Research: Space Physics, 124(1), 643-660.

How to cite: Buccino, D., Parisi, M., Park, R., Gramigna, E., Gomez-Casajus, L., Tortora, P., Zannoni, M., Steffes, P., Hodges, A., Levin, S., and Bolton, S.: A Dual-Frequency Radio Occultation of Ganymede’s Ionosphere with Juno, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-380, https://doi.org/10.5194/epsc2021-380, 2021.

The JUICE (JUpiter ICy moon Explorer) mission, selected by the European Space Agency in May 2012 to be the first large mission within the Cosmic Vision Program 2015–2025, will provide the most comprehensive exploration to date of the Jovian system in all its complexity, with particular emphasis on Ganymede as a planetary body and potential habitat (JUICE Red Book, 2014). The Galilean satellites are known to have thin atmospheres, technically exospheres (McGrath et al., 2004), produced by ion-induced sputtering and sublimation of the surface materials. These moons and tenuous atmosphere are embedded in the flowing plasma of the jovian. The interaction between the neutral environments of the Galilean satellites and the jovian plasma changes the plasma momentum, the temperature and generates strong electrical currents. In order to prepare the scientific return of the mission and the optimization of operation modes of plasma instruments, a modeling effort has been carried out at LATMOS (PhD R. Allioux, IRAP, 2012; L. Leclercq, LATMOS, 2015; O. Apurva, LATMOS, 2017). A 3D parallel multi-species hybrid model (Latmos Hybrid Simulation, LatHyS) has been developed to model and characterize the plasma environment of Ganymede (Leclercq et al, 2016; Modolo et al, 2016) and a 3D parallel multi-species exospheric model (Exospheric Global Model, EGM) to pattern the dynamic of the neutral envelopes of Ganymede (Turc et al, 2014; Leblanc et al, 2017). The presentation will examine the global structure of the interaction with the jovian plasma, to describe the formation of Alfvén wings, and to emphasize the phenomena related to the multi-species nature of the plasma. The simulation model supports the preparation of the JUICE mission and its Ganymede phase by characterizing boundary crossings.

How to cite: Modolo, R., Baskevitch, C., Leblanc, F., and Masters, A.: Ganymede’s interaction with the jovian plasma from hybrid simulation, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-683, https://doi.org/10.5194/epsc2021-683, 2021.

Ganymede is the only moon in our Solar System known to have its own global magnetic field, which generates a miniature moon magnetosphere inside the Jovian magnetosphere. Due to this unique characteristic of Ganymede, its auroral zone is also of particular scientific interest, as it is the only known example of this specific kind of interaction. The JUICE spacecraft will orbit Ganymede for almost a year, with a high inclination orbit with multiple auroral zone crossings. JUICE will study the auroral zone of Ganymede in more detail than ever before, providing both in-situ and remote sensing observations.

In this work, we use Spacecraft Plasma Interaction Software (SPIS) simulations to study the spacecraft charging of JUICE in the auroral zone. Hubble Space Telescope observations of the aurora of Ganymede show localized regions of bright spots superimposed on a continuous background emission (e.g. Feldman et al. 2000, Eviatar et al. 2001). In order to produce bright auroras, the electron population needs to be accelerated up to hundreds of eV (Eviatar et al. 2001). Preliminary simulation results, using an auroral electron population with temperature T_e = 200 eV and density n_e = 300 cm^-3, shows frame charging (i.e. spacecraft ground) of around 10 V and differential charging of around 30 V. High frame and differential potentials can cause disturbances in both particle and electric field measurements and prevent accurate characterization of the environment. Since the auroral zone of Ganymede is of particular scientific interest, it is important to study and prepare for this kind of disturbances.

References

D. Feldman et al., HST/STIS ultraviolet imaging of polar aurora on Ganymede, The Astrophysical Journal, 535(2), 2000

A. Eviatar et al., Excitation of the Ganymede ultraviolet aurora, The Astrophysical Journal, 555(2), 2001

How to cite: Holmberg, M., Cipriani, F., Déprez, G., Imhof, C., Witasse, O., Altobelli, N., and Huybrighs, H.: Spacecraft charging of JUICE in the auroral zone of Ganymede, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-661, https://doi.org/10.5194/epsc2021-661, 2021.

Jupiter’s aurora is complex and dynamic, with a large number of distinct auroral features and regions generated by multiple phenomena. Of these features, Io’s auroral signature is one of the most persistent and identifiable aurora, with a rich observational history spanning decades of remote observations. Since Juno arrived at Jupiter, providing in-situ transits through flux tubes directly connected to Io’s auroral emissions, its diverse set of instruments have revealed an even more complex and dynamic picture of Io’s auroral interaction. In this presentation, we report on Juno observations of precipitating electron fluxes connected to 18 crossings of Io’s footprint tail aurora, over altitudes of 0.15 to 1.1 Jovian radii (R_J). We will highlight how the strength of precipitating electron fluxes is dominantly organized by “Io-Alfvén tail distance”, the angle along Io’s orbit between Io and an Alfvén wave trajectory connected to the tail aurora. We will discuss how these fluxes were best fit with an exponential as a function of down-tail extent with an e-folding distance of 21˚, the acceleration region altitude likely increases down-tail, and most of the parallel electron acceleration sustaining the tail aurora occurs above 1 R_J in altitude. Finally, we will highlight how Juno has likely transited Io’s Main Alfvén Wing fluxtube, observing a characteristically distinct signature with precipitating electron fluxes ~600 mW/m² and an acceleration region extending as low as 0.4 R_J in altitude.

How to cite: Szalay, J., Allegrini, F., Bagenal, F., Bolton, S., Bonfond, B., Clark, G., Connerney, J., Ebert, R., Hue, V., McComas, D., Saur, J., Sulaiman, A., and Wilson, R.: A new framework to explain changes in Io's footprint tail electron fluxes in the Juno era, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-272, https://doi.org/10.5194/epsc2021-272, 2021.

Galilean moons are embedded in Jupiter’s giant magnetosphere. The jovian plasma particles interact with the atmosphere of the moons, exchanging momentum and energy, and generate different phenoma such as aurora, electric current, etc.

The exploration of the Galilean moons, and in particular Ganymede and Europa considered as potential habitats, are listed among the main objectives of the ESA JUpiter ICy moon Explorer mission. In preparation of future observations, a simulation effort is conducted to describe the Europa moon-magnetosphere system as well as a study of radio wave propagation in the environments of Ganymede and Europa using a ray tracing code.

LatHyS is a hybrid 3D, multi-species and parallel simulation model which is based on a kinetic description of ions and a fluid description of electrons. The model is based on the CAM-CL algorithm that Alan Matthews¹ outlined in 1994. It allows to describe the interaction between the jovian plasma and the moon environments. As Ganymede's environment has already been implemented, we propose to enrich the model by completing it with Europa's – jovian plasma interaction and to optimize it in order to improve the accuracy of the results.

Artemis-P, developed by Gautier² in 2013, is a ray tracing code that calculates the trajectory of waves through a given environment. Planetary environments are anisotropic and inhomogeneous, so that radio waves can undergo refraction, reflection, scattering, diffraction, interference, etc. between the source and the detector. The ray tracing methods allow to treat the refraction and reflection phenomena at large scales compared to the wavelength. The proposed work is to adjust this program to the environments of Ganymede and Europa using data from LatHyS simulations.

Références :

¹ Alan P. Matthews, Current Advance Method and Cyclic Leapfrog for 2D Multispecies Hybrid Plasma Simulations, Journal of Computational Physics, Volume 112, Issue 1, 1994, Pages 102-116, ISSN 0021-9991, https://doi.org/10.1006/jcph.1994.1084.

² Anne-Lise Gautier. Étude de la propagation des ondes radio dans les environnements planétaires. Planétologie et astrophysique de la terre [astro-ph.EP]. Observatoire de Paris, 2013. Français. tel-01145651v2

How to cite: Baskevitch, C.-A., Cecconi, B., and Modolo, R.: Europa’s interaction with the jovian plasma from hybrid simulation, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-617, https://doi.org/10.5194/epsc2021-617, 2021.

We investigate energetic proton depletions during Europa flybys E17 and E25A* by the Galileo mission. Energetic ion observations along trajectories like those of E17 & E25A are suitable for isolating the characteristics of the global configuration of the interaction region of Europa (or any Galilean moon) with the Jovian magnetosphere. Both of these flybys passed through Europa’s Alfvén wings further away from the moon, where ionospheric effects are small.

We simulate the measured flux with a Monte Carlo particle tracing code and investigate the effect of the following factors: inhomogeneous electromagnetic fields, Europa's induced dipole, atmospheric charge exchange and plumes.

We find that the homogeneous fields do not explain the Galileo data. We propose that the perturbed fields associated with the Alfvén wings affect the proton depletions. The inhomogeneous fields and induced dipole alter the pitch angle distribution of the depletion along the trajectory. The plumes that are investigated in this study have a minor effect on the proton depletions compared to the inhomogeneous fields and Alfvén wings. The contribution of atmospheric charge exchange to the depletion is negligible for these flybys. Finally, we compare the simulations to the measured proton flux and discuss the contribution of the effects we have considered.

* E25A is a segment of the Io flyby I25

How to cite: Huybrighs, H., Blöcker, A., Roussos, E., Van Buchem, C., Futaana, Y., Goetz, C., Holmberg, M., and Witasse, O.: The effect of Europa's perturbed electromagnetic fields and induced dipole on energetic proton depletions in the Alfvén wings, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-299, https://doi.org/10.5194/epsc2021-299, 2021.

How to cite: Winterhalder, T. and Huybrighs, H.: Assessing JUICE’s ability of in situ plume detection in Europa’s atmosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-350, https://doi.org/10.5194/epsc2021-350, 2021.

The parameter space for the very uncertain composition of sublimated H2O and its photochemical products H and H2 in Callisto's atmosphere is examined using the Direct Simulaton Monte Carlo (DSMC) method.

We focus on two significantly different versions of H2O production in which:

(1) the ice and dark, non-ice/ice-poor material are intimately mixed and H2O sublimates at Callisto's warm day-side temperatures (e.g., as in most atmospheric modeling efforts at Callisto to date [1-4]); and

(2) the ice and dark, non-ice/ice-poor material are segregated (e.g., consistent with interpretations of images of Callisto's surface taken by Voyager [5, 6] and Galileo [7]) and H2O sublimates at "ice" temperatures [8].

Our 2D molecular kinetic models track the motion H2O, whose sublimation yield varies several orders of magnitude depending on the description of Callisto's surface, its photochemical products H and H2, and a relatively dense O2 component. Whereas H is assumed to react in the regolith on return to the surface, H2 is assumed to thermalize and re-enter the atmosphere.

We compare the simulated LOS column densities of H to the detected H corona at Callisto [9], which was suggested to be produced primarily by photodissociation of sublimated H2O. Our goal is to use the corona observations to help constrain the source rate for H2O from Callisto’s complex surface.

References

[1] Liang et al., 2005. Atmosphere of Callisto. Journal of Geophysical Research: Planets.

[2] Vorburger et al., 2015. Monte-Carlo simulation of Callisto’s exosphere. Icarus.

[3] Hartkorn et al., 2017. Structure and density of Callisto’s atmosphere from a fluid-kinetic model of its ionosphere: Comparison with Hubble Space Telescope and Galileo observations. Icarus.

[4] Carberry Mogan et al., 2021 (under review). A tenuous, collisional atmosphere on Callisto. Icarus.

[5] Spencer and Maloney, 1984. Mobility of water ice on Callisto: Evidence and implications. Geophysical Research Letters.

[6] Spencer, 1987. Thermal segregation of water ice on the Galilean satellites. Icarus.

[7] Moore et al., 1999. Mass movement and landform degradation on the icy Galilean satellites: Results of the Galileo nominal mission. Icarus.

[8] Grundy et al., 1999. Near-infrared spectra of icy outer solar system surfaces: Remote determination of H2O ice temperatures. Icarus.

[9] Roth et al., 2017. Detection of a hydrogen corona at Callisto. Journal of Geophysical Research: Planets.

How to cite: Carberry Mogan, S., Tucker, O., Johnson, R., Vorburger, A., Galli, A., Roth, L., Tafuni, A., Kumar, S., Sahin, I., and Sreenivasan, K.: Atmospheres on Callisto composed of sublimated water vapor and its photochemical products, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-3, https://doi.org/10.5194/epsc2021-3, 2021.

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on board JUICE is designed to measure neutrals, ions, electrons, and energetic particles over an energy range from eV to MeV.

In the vicinity of Callisto, PEP will characterize the Jovian plasma environment and the outer parts of Callisto’s atmosphere and ionosphere. Roughly twenty Callisto flybys with closest approaches between 200 km and 5000 km altitude are planned over the course of the JUICE mission. This study aims at optimizing the scientific insight gained from the foreseen flybys by combining the input from the PEP science team and operation planning with recent model efforts for Callisto’s atmosphere, the plasma environment and the production of Energetic Neutral Atoms. The results of this study will inform both science operation planning of PEP and JUICE and they will guide future model development for Callisto’s atmosphere, ionosphere, and their interaction with the plasma environment.

How to cite: Galli, A., Vorburger, A., Carberry Mogan, S. R., Roussos, E., Stenberg-Wieser, G., Wurz, P., Föhn, M., Krupp, N., Fraenz, M., Barabash, S., Futaana, Y., Brandt, P. C., Kollmann, P., Haggerty, D., Jones, G. H., Johnson, R. E., Tucker, O. J., Simon, S., Tippens, T. F., and Liuzzo, L.: The Particle Environment Package on board JUICE: What Can We Learn about Callisto's Atmosphere and Space Environment?, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-169, https://doi.org/10.5194/epsc2021-169, 2021.

Abstract

Positive and negative ion velocities are derived in directions parallel and perpendicular to Cassini’s trajectory through Titan’s ionosphere, alongside spacecraft potentials, using in-situ measurements from sensors in the Cassini Plasma Spectrometer (CAPS) instrument.  Negative ions are detected by the Electron Spectrometer (ELS) while positive ions are detected by the Ion Beam Spectrometer (IBS). Preliminary analysis indicate that the magnitudes of these velocities are up to several hundred m/s, higher than previously reported in-situ measurements, but comparable to ALMA measurements of prograde neutral winds up to 390 m/s. These ion velocity measurements add to the understanding of high-altitude ion and neutral winds at Titan, aiding future investigation into this dynamic system.

Introduction

Pre-Cassini models of Titan predicted thermospheric winds of up to 60 m/s¹, however, from Cassini-Huygens and ALMA observations there have been indications of superrotation in the thermosphere/ionosphere, with neutral wind speeds up to 390 m/s². Previous in-situ measurements have measured positive ion velocities along Cassini’s trajectory(along-track), finding velocities up to 260 m/s. Positive and negative ion velocities perpendicular to Cassini’s trajectory (cross-track) have been previously estimated to be of similar magnitude to the along-track velocities³.

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS), CAPS Ion Beam Spectrometer (IBS)⁴ and the Radio & Plasma Wave Science (RPWS) Langmuir Probe (LP)⁵ instruments on Cassini can derive values for the spacecraft potential that arises from spacecraft charging. However, there are discrepancies in the magnitude of the derived spacecraft potential between the instruments. Derived spacecraft potentials of Cassini in Titan’s ionosphere are in the range between 0 and -3.5V³, with ELS-derived potentials typically being more negative than the Langmuir probe⁶, while IBS-derived potentials are more positive³. Although differential spacecraft charging can explain the discrepancy between the LP and the CAPS sensors, it cannot explain the discrepancy measured between the CAPS sensors.

Here we derive spacecraft potentials and along-track ion velocities from the energies of the observed ions and attempt to derive cross-track ion velocities by utilising the rotation of the CAPS instrument.

Methodology

The CAPS instrument consists of three electrostatic analysers, which measure the energy/charge ratios of ions. In this study, we utilise data from CAPS ELS and IBS which observed negative and positive ions respectively. In Titan’s ionosphere, ions are observed by CAPS as a supersonic beam in the instrument frame, therefore the energies of the ions are related to the ions’ mass, the spacecraft velocity, the along-track ion velocity, and the spacecraft potential. Fits are applied to positive and negative ions, to derive values for the along-track ion velocity and spacecraft potential.

Cross-track velocities are perpendicular to Cassini’s trajectory. These velocities cause the ions to be detected from a direction which is a small angle away from Cassini’s trajectory. This angle can be measured due to the actuation of CAPS across the spacecraft’s velocity vector.    

There are several sources of uncertainty in this methodology, including instrumental, spacecraft-plasma interactions and Titan’s ionosphere itself. Examples of instrumental effects include uncertainty in the actuator position and the energy resolution of the CAPS sensors. Spacecraft interaction uncertainties caused by the spacecraft include differential spacecraft charging and particle trajectories being deflected by the spacecraft potential. Lastly, Titan’s ionosphere itself, or electric fields due to its plasma interaction, may impact the measurements. Electric fields up to 3 µV^-1 have been detected in the ionosphere⁸, which would separate positive and negative ion trajectories. The methodology was adapted to mitigate these effects in several ways.

Results

Positive and negative ion velocities are measured both along Cassini’s trajectory (along-track) and perpendicular to it (cross-track). Proportionality is observed between the positive and negative ions for the derived along-track and cross-track velocities, which agrees with the expectation of collisional coupling between the positive and negative ions and the neutrals.

The magnitudes of these velocities are up to several hundred m/s, higher than previously reported from in-situ measurements, but comparable to ALMA measurements of prograde neutral winds of up to 390 m/s². Early analysis has shown no longitudinal or altitude dependence for the derived winds, although this is constrained by the limited sampling available. A slight latitudinal asymmetry is observed, which would be consistent with neutral wind findings of a stronger zonal wind in the southern hemisphere⁹.

References

1. Rishbeth H., R.V. Yelle et al., Dynamics of Titan’s thermosphere, P&SS, Vol 48, Issue 1, 2000, doi:10.1016/S0032-0633(99)00076-8

2. Cordiner, M., E. Garcia et al., Temporal Variability of Titan's High-Altitude Zonal Winds Detected using ALMA, 14th Europlanet Science Congress 2020, doi:10.5194/epsc2020-424

3. Crary, F.J., B.A. Magee et al., Heavy ions, temperatures and winds in Titan's ionosphere: Combined Cassini CAPS and INMS observations, PSS, Vol 57, Issues 14–15, 2009, doi:10.1016/j.pss.2009.09.006

4. Young, D. T., J. J. Berthelier et al., Cassini Plasma Spectrometer Investigation, Space Sci. Rev., Vol 114, Issue 1-4, 2004, doi:10.1007/s11214-004-1406-4

5. Gurnett, D. A., W. S. Kurth et al., The Cassini Radio and Plasma Wave Investigation, Space Sci. Rev., Vol 114, Issue 1-4, 2004, doi:10.1007/s11214-004-1434-0

6. Desai, R. T., A. J. Coates et al., Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan’s Ionosphere, Ap. J Letts, Vol 844, Issue 2, 2017, doi:10.3847/2041-8213/aa7851

7. Cravens, T. E., M. Richard et al., Dynamical and magnetic field time constants for Titan's ionosphere: Empirical estimates and comparisons with Venus, JGR, Vol 115, Issue A8, 2010, doi:10.1029/2009JA015050

8. Ågren, K., D. J. Andrews et al., Detection of currents and associated electric fields in Titan's ionosphere from Cassini data, JGR, Vol 116, Issue A4, 2011, doi:10.1029/2010JA016100

9. Vinatier, S., C. Mathé et al., Temperature and chemical species distributions in the middle atmosphere observed during Titan's late northern spring to early summer, A&A, Vol 641, 2020, doi:10.1051/0004-6361/202038411

How to cite: Haythornthwaite, R., Coates, A., Jones, G., and Wellbrock, A.: Deriving Cassini spacecraft potentials, cross-track and along-track ion velocities in Titan’s ionosphere using measurements from CAPS ELS and IBS, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-301, https://doi.org/10.5194/epsc2021-301, 2021.

Recent studies have shown that negatively charged dust dramatically alters the electric properties of plasmas, in particular planetary ionospheres. Utilizing Titan flybys from the entire Cassini mission and full plasma content of the moon’s ionosphere (electrons, positive ions and negative ions/dust grains) we derive the electric conductivities and currents, updating and extending previous results which did not include the charged dust and focused on a limited range of flybys.

Compared to the previous estimates, using the full plasma content increases the Pedersen conductivities by a factor ~2 and Hall conductivities by a factor ~1.2. We identify dusty plasma as the reason for the sharp increase of Pedersen conductivity below 1000 km altitude reported previously. Using the full range of Titan flybys also reveals the conductivities on the dayside to be factor ~7-9 larger than on the nightside, owing to higher dayside plasma densities as well as generally heavier plasma species on the nightside.

How to cite: Shebanits, O., Wahlund, J.-E., Perryman, R., Waite, H., and Dougherty, M.: Electric properties of Titan’s dusty ionosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-698, https://doi.org/10.5194/epsc2021-698, 2021.