Research

We study the plasma environments of small bodies in the outer solar system, such as the moons of Jupiter and Saturn or the dwarf planet Pluto. We apply the following methods:

Current research projects:

  • The interaction between Callisto and Jupiter’s magnetosphere: We study the interaction between the Jovian moon Callisto and its magnetospheric environment by applying a hybrid simulation code (kinetic ions, fluid electrons). So far, no three-dimensional, self-consistent modelling studies of Callisto’s interaction with its plasma environment are available in the peer-reviewed literature. Therefore, the first part of the project will systematically analyze the morphology of Callisto’s plasma interaction at various orbital positions and at various distances to the Jovian current sheet. Emphasis will be placed on the identification of asymmetries  associated with the large gyroradii of the pick-up ions and the non-negligible twist of the plasma flow pattern by the ionospheric Hall effect. By considering different density and compositional profiles for Callisto’s atmosphere, the model will also allow to impose constraints on the ionospheric electron density. The calculated ionospheric profiles will be compared against the  available radio occultation measurements of ionospheric electron densities. In the second part of the project, the magnetic field generated by induction in a subsurface water ocean will be incorporated into the simulation model, thereby allowing a systematic disentanglement of induction and plasma interaction signatures in Callisto’s magnetospheric environment. After analyzing the large-scale modification of the plasma flow pattern due to induction, comparisons between model results and magnetic field data from all available Galileo flybys will be applied to impose constraints on the properties (conductivity, thickness, depth) of a possible subsurface ocean shell at Callisto. Therefore, the project will not only make substantial contributions to the interpretation of Galileo data, but it can also support the planning of synergistic measurements during the upcoming JUICE mission. Besides, the project will investigate the strong similarities between the plasma environments of Callisto and Saturn’s largest moon Titan.
 

Three dimensional picture of Callisto’s magnetic environment (Liuzzo et al., 2016)

Three dimensional picture of magnetospheric plasma diverted around Callisto (Liuzzo et al., 2016)

(a) Schematic illustration and (b-e) AIKEF hybrid simulation results of the magnetic environment near Callisto in the moon’s equatorial plane (Liuzzo et al., 2016)

  • Plasma interaction signatures of plumes at Europa: Although observations from the Hubble Space Telescope (HST) in December 2012 suggested the presence of water vapor plumes at the south pole of the Jovian moon Europa, this finding could not be confirmed during several follow-up campaigns. Only very recently additional evidence of plumes at Europa was found in HST data. However, the mechanism that governs the activity variations of these vents is yet to be revealed. To better understand the nature of plumes at Europa and their impact on the moon’s magnetospheric environment, this project will explore the possibility to independently constrain the properties of these plumes through in-situ plasma and magnetic field observations from past (Galileo) and future (JUICE, Europa Clipper) spacecraft missions. By applying a hybrid simulation model (kinetic ions, fluid electrons), we will systematically assess how different plume source strengths and locations would modify the plasma and magnetic field perturbations generated by the interaction of Europa’s asymmetric, global atmosphere and induced dipole with the ambient magnetospheric plasma. Especially, we will model the impact of different plume configurations on the ambient plasma for various distances between Europa and Jupiter’s plasma sheet. When Europa is located away from the center of the sheet, the moon’s immediate magnetic environment is dominated by the dipole field induced in its conducting subsurface ocean. We will determine whether this induced field can obscure the visibility of possible plumes in magnetic field data. In addition, we will analyze to what degree large-scale asymmetries in Europa’s global atmosphere can complicate the identification of the magnetic and plasma signatures generated by plumes. Based on the model, we will also assess the impact of possible plumes on the current systems and magnetic field signatures in Europa’s distant Alfven wings, thereby exploring the possibility to use data from distant flybys for a remote characterization of plumes.

Image credit: National Aeronautics and Space Administration

  • Ion cyclotron waves in Saturn’s magnetosphere: We investigate the properties of ion cyclotron waves in Saturn’s equatorial magnetosphere by applying a combination of Cassini magnetic field (MAG) data analysis and hybrid plasma simulations (kinetic ions, fluid electrons). In the first part of the project, we will systematically search MAG data from all available low-inclination orbits for signatures of ion cyclotron waves and determine the dependency of the wave amplitudes on (i) the L shell value and (ii) the azimuthal distance to Saturn’s inner icy satellites. Especially, we will compare the wave amplitudes observed during close flybys of Enceladus and during crossings of the Enceladus $L$ shell  that occurred at large distances to the moon. In this way, we will quantitatively constrain the asymmetries that the Enceladus plume imposes on the neutral gas and plasma populations in Saturn’s inner magnetosphere. This investigation will also reveal whether the recently observed correlation between Enceladus’ orbital position and the activity of the moon’s south-polar plume  leaves a clear imprint in the measured wave amplitudes. Besides, we will investigate whether pick-up from the dilute atmospheres observed around Dione and Rhea generates measurable enhancements in the wave amplitudes. In the second part of the project, we will use a hybrid simulation code to convert the observed wave amplitudes into local ion production rates. For the plasma conditions at various L shell values and for various ion production rates, we will determine the amplitudes of the resulting waves from the simulations. For the first time, our study will consider the influence of the electron-absorbing dust grains in the Enceladus torus on the wave amplitudes. These grains were already found to have a tremendous impact on Enceladus’ local electromagnetic environment. Therefore, this project will not only provide a comprehensive catalogue of the ion production rates in Saturn’s equatorial  magnetosphere, but the study will also give valuable insights into the physics of ion cyclotron wave generation in a dust-enriched plasma.

 

Region of ion cyclotron wave occurrence in the inner magnetosphere of Saturn (Meeks and Simon, 2017)

Identification of ion cyclotron waves in Cassini MAG data. Similar spectra for the entire Cassini MAG dataset are available from zachary.meeks@gatech.edu upon request.

  • The magnetic environment of Saturn’s moon Titan: We develop a comprehensive picture of the ambient magnetic field conditions along Titan’s orbit and the field perturbations within the moon’s induced magnetosphere by applying a combination of Cassini magnetic field (MAG) data analysis and hybrid (kinetic ions, fluid electrons) plasma simulations. In the first part of the project, we will apply the classification technique of Simon et al. (2010) to all available crossings of Titan’s orbit and determine whether these crossings took place in a current sheet or a lobe-type magnetospheric environment. These results will then be combined to an empirical model of Titan’s magnetic environment, providing the probability for the occurrence of each environment category as a function of Saturnian Local Time and season in the Saturnian year. This model will be compared to an analogous survey of the magnetospheric electron opulations along Titan’s orbit. The second part of the project deals with a systematic characterization of the extensions and magnitudes of the field perturbations in Titan’s ramside magnetic pile-up region and wakeside magnetotail. By transforming MAG data into a Draping Coordinate System that takes into account the local magnetic field orientation, the dependency of these tructures on Titan’s orbital position, on the distance to the moon and on the type of the magnetospheric environment (current sheet, magnetodisk lobe) will be investigated. Cassini MAG data from all Titan flybys will then be superimposed to generate  averaged profiles of the moon’s induced magnetosphere when exposed to a certain type of plasma environment. In the third part of the project, these profiles of Titan’s induced magnetosphere will be compared against the output of a hybrid simulation of Titan’s plasma interaction. In this way, we shall infer whether the superposition of MAG data from multiple flybys allows to construct a quantitatively consistent three-dimensional picture of the average state of Titan’s induced magnetosphere.

Three-dimensional numerical model of the magnetic field near Titan (Feyerabend et al., GRL, 2016)

  • Emission of energetic neutral atoms from Titan’s atmosphere: Based on a combination of numerical modelling and Cassini observations, we study the emission morphology of energetic neutral atoms (ENAs) from Titan’s atmosphere. These emission patterns contain an admixture of information on the particle distribution functions and the draped magnetic field configuration near Titan, i.e., the ENA images taken by Cassini can be regarded  as snapshots of Titan’s plasma interaction region on a global scale. In the first part of the project, we will systematically investigate the impact of the draped magnetic field near Titan on the ENA emission pattern. The structure of the electromagnetic fields near Titan is determined by symmetries due to large pick-up ion gyroradii as well as the moon’s changing distance to the center of Saturn’s magnetodisk current sheet and its orbital position. The electromagnetic field configuration near Titan will be computed with an established hybrid (kinetic ions, fluid electrons) model of the moon’s interaction with Saturn’s magnetospheric plasma. For various sets of upstream plasma/magnetic field conditions, we will then calculate the ENA emission flux through a concentric sphere around Titan. To isolate the impact of the draped and asymmetric field configuration near Titan, we will compare the results to the emission patterns calculated for spatially homogeneous electromagnetic fields. We will also model the impact of short-scale time variations in Titan’s electromagnetic environment on the trajectories of the energetic parent ions. The second part of the project will focus on the generation of synthetic ENA images for specific Titan flybys and their comparison against images taken by the Cassini Ion and Neutral Camera (INCA). In this way, we shall constrain the potential of ENA observations to characterize the various asymmetries of Titan’s induced magnetosphere on a global scale. The magnetic field output from the hybrid model will also be compared against Cassini agnetometer data, thereby simultaneously constraining the asymmetries of Titan’s induced magnetosphere on a local scale.

Modeled emission morphology of energetic neutral atoms from Titan’s atmosphere (Kabanovic et al., 2017)

  • The plasma environment of the dwarf planet Pluto: We analyze the interaction between Pluto and the solar wind at the time of the New Horizons (NH) flyby by applying a hybrid (kinetic ions, fluid electrons) simulation model. The use of a hybrid model is necessary since the gyroradii of the involved ion species are more than an order of magnitude larger than the obstacle to the solar wind. Thus, Pluto’s interaction region is expected to display considerable asymmetries. In the first part of the project, we shall investigate the three-dimensional structure and extension of the various plasma signatures seen along the NH trajectory. Especially, we will constrain possible asymmetries in the shape of Pluto’s bow shock, plasma tail and Plutopause (i.e., the boundary between the solar wind and the population of plutogenic ions) which may arise from the large ion gyroradii. Starting from the upstream solar wind parameters measured by NH,  we will also investigate the dependency of the observed plasma signatures on the density of Pluto’s ionosphere and on the solar wind ram pressure. In this way, we will quantify the contribution of the unusually strong solar wind pressure during the NH encounter to the observed plasma signatures. The second part of the project will focus on the inclusion of Pluto’s largest moon Charon in the simulation model and on a study of the simultaneous interaction between both bodies and the solar wind. Data from NH suggest that Charon mainly acts as a plasma absorber without an appreciable atmosphere. For various relative positions of Pluto and Charon, we will investigate the deformation of Charon’s wake when exposed to the inhomogeneous plasma flow in the Pluto interaction region. We will also search for a possible feedback of Charon on the structure of Pluto’s induced magnetosphere.

Deflection of the solar wind (black) and of ionospheric heavy ions (white) near Pluto (Feyerabend et al., 2017)