Hybrid Magnetospheric Modelling at the Outer Planets using Python

Modelling planetary magnetospheres is essential to develop understanding of how these dynamic regions of space respond to forcing from both internal and external sources of mass, momentum and energy. Obtaining an exact solution for the governing equations describing these complex systems is very difficult. Therefore, simplified models are required for investigation. The size of planetary magnetospheres presents additional complications when creating models of them as important dynamics occur on spatial scales ranging from planetary radii down to the kinetic ion and electron levels. Such challenges are present in simulating bulk plasma transport in Jupiter's inner and middle magnetosphere, where plasma flows from Io's plasma torus radially outwards. The process of radial transport is attributed to the centrifugal-interchange instability. A hybrid kinetic-ion/fluid-electron approach is taken to modelling these magnetospheric plasma flows. Hybrid techniques are able to capture large-scale flow dynamics as well as interactions between particles. Whilst most models of this type are written in C/C++ or Fortran, the aim in this project is to provide a Python codebase that allows for prototype physical effects to be examined before incorporation into an optimised implementation. Writing a version in a modern accessible language also has pedagogical value

Trapped Particle Motion in Magnetodisk Fields

The spatial and temporal characterization of trapped charged particle trajectories in magnetospheres has been extensively studied in dipole magnetic field structures. Such studies have allowed the calculation of spatial quantities, such as equatorial loss cone size as a function of radial distance, the location of the mirror points along particular field lines (L‐shells) as a function of the particle's equatorial pitch angle, and temporal quantities such as the bounce period and drift period as a function of the radial distance and the particle's pitch angle at the equator. In this study, we present analogous calculations for the disk‐like field structure associated with the giant rotation‐dominated magnetospheres of Jupiter and Saturn as described by the University College London/Achilleos‐Guio‐Arridge (UCL/AGA) magnetodisk model. We discuss the effect of the magnetodisk field on various particle parameters and make a comparison with the analogous motion in a dipole field. The bounce period in a magnetodisk field is in general smaller the larger the equatorial distance and pitch angle, by a factor as large as ∼8 for Jupiter and ∼2.5 for Saturn. Similarly, the drift period is generally smaller, by a factor as large as ∼2.2 for equatorial distances ∼20–24 RJ at Jupiter and ∼1.5 for equatorial distances ∼7–11 RS at Saturn

Hybrid Ion-Kinetic, Fluid-Electron Modelling of Radial Plasma Flows in the Magnetospheres of the Outer Planets

The magnetospheres of the gas giants, Jupiter & Saturn, are both loaded internally with plasma. The source of this material in the Jovian system is the volcanic moon of Io and the icy moon of Enceladus in the Saturnian, creating the Io plasma torus and the Enceladus neutral torus. In both systems plasma is removed from these tori mainly via ejection as energetic neutrals and by bulk transport into the outer magnetosphere. The physical mechanism responsible for the bulk transport process is the radial-interchange instability (reviewed in §3). In order to improve understanding of the bulk transport process a new hybrid kinetic ion, fluid-electron plasma model is constructed in 2.5-dimensions. The Jovian magnEtospheRIC kinetic-ion, fluid-electron Hybrid plasma mOdel, JERICHO, is detailed in §4 & 5. The technique of hybrid modelling allows for the probing of plasma motions from the scale of planetary-radii down to the ion-inertial length scale, considering constituent ion species kinetically, as charged particles, and forming the electrons into a single magnetised fluid continuum. Simulation results permit the examination of bulk transport on spatial scales not currently accessible with state-of-the-art models. To ensure JERICHO is physically accurate a series of physical benchmarks are examined in §6 and the parameter space within which it must be operated is identified. Application to Saturnian magnetospheric system is presented in §7. Plasma injections are introduced and develop radial-interchange instabilities on spatial scales of 10−1 RS. These motions create fingers of dense plasma interspersed with narrow tenuous plasma channels. A parameter survey is performed, varying the magnetic field strength & density of plasma injections. A potential link between the temporal scales of the instability and magnetic field strength is identified, however no correlation is found between either of these parameters and the spatial scales of the instabilities

Electromagnetic induction in the icy satellites of Uranus

The discovery of subsurface oceans in the outer solar system has transformed our perspective of ice worlds and has led to consideration of their potential habitability. The detection and detailed characterisation of induced magnetic fields due to these subsurface oceans provides a unique ability to passively sound the conducting interior of such planetary bodies. In this paper we consider the potential detectability of subsurface oceans via induced magnetic fields at the main satellites of Uranus. We construct a simple model for Uranus’ magnetospheric magnetic field and use it to generate synthetic time series which are analysed to determine the significant amplitudes and periods of the inducing field. The spectra not only contain main driving periods at the synodic and orbital periods of the satellites, but also a rich spectrum from the mixing of signals due to asymmetries in the uranian planetary system. We use an induction model to determine the amplitude of the response from subsurface oceans and find weak but potentially-detectable ocean responses at Miranda, Oberon and Titania, but did not explore this in detail for Ariel and Umbriel. Detection of an ocean at Oberon is complicated by intervals that Oberon will spend outside the magnetosphere at equinox but we find that flybys of Titania with a closest approach altitude of 200 km would enable the detection of subsurface oceans. We comment on the implications for future mission and instrument design

JERICHO: a Kinetic-Ion, Fluid-Electron Hybrid Plasma Model for the Outer Planets

Plasma in the Jovian magnetosphere is removed from Io's torus mainly via bulk transport into sink regions in the outer magnetosphere and by ejection as energetic neutrals. The physical process generally considered to be responsible for bulk transport is the centrifugal-interchange instability. This mechanism allows magnetic flux tubes containing hot, tenuous plasma to exchange places with tubes containing cool, dense plasma, moving material from the inner to outer magnetosphere whilst returning magnetic flux to the inner magnetosphere. In order to examine the transport we have developed a full hybrid kinetic-ion, fluid-electron plasma model in 2.5-dimensions, JERICHO. The technique of hybrid modelling allows for probing of plasma motions from the scale of planetary-radii down to the ion-inertial length scale, considering constituent ion species kinetically as charged particles and forming the electrons into a single magnetised fluid continuum. Results from this model will allow for the examination of bulk transport on spatial scales not currently accessible with state-of-the-art models, improving understanding of mechanisms responsible for moving particles between flux tubes and from the inner to the outer magnetosphere. In this presentation we will examine the structure of the model logic and numerics before analysing the latest results from the model

Local Time Asymmetries in Jupiter's Magnetodisc Currents

We present an investigation into the currents within the Jovian magnetodisc using all available spacecraft magnetometer data up until 28th July, 2018. Using automated data analysis processes as well as the most recent intrinsic field and current disk geometry models, a full local time coverage of the magnetodisc currents using 7382 lobe traversals over 39 years is constructed. Our study demonstrates clear local time asymmetries in both the radial and azimuthal height integrated current densities throughout the current disk. Asymmetries persist within 30 R$_\mathrm{J}$ where most models assume axisymmetry. Inward radial currents are found in the previously unmapped dusk and noon sectors. Azimuthal currents are found to be weaker in the dayside magnetosphere than the nightside, in agreement with global magnetohydrodynamic simulations. The divergence of the azimuthal and radial currents indicates that downward field aligned currents exist within the outer dayside magnetosphere. The presence of azimuthal currents is shown to highly influence the location of the field aligned currents which emphasizes the importance of the azimuthal currents in future Magnetosphere-Ionosphere coupling models. Integrating the divergence of the height integrated current densities we find that 1.87 MA R$_\mathrm{J}^{-2}$ of return current density required for system closure is absent

Modeling Non-Force-Free and Deformed Flux Ropes in Titan’s Ionosphere

Previous work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes

Vertical Current Density Structure of Saturn's Equatorial Current Sheet

Routine spacecraft encounters with the Saturn current sheet due to the passage of aperiodic waves provide the opportunity to analyze the current sheet structure. The current density is expected to peak where the field strength reaches a minimum if approximated as a Harris current sheet. However, in Earth's magnetotail this is not always the case as the sheet is sometimes bifurcated (having two or more maxima in the current density). We utilize measurements of Saturn's magnetic field to estimate the current density during crossings of the current sheet by time differentiating the B a component of the field in a current sheet coordinate system, where B a is perpendicular to both the current and current sheet normal. This is then averaged and organized by the magnitude of B a. Using this method, we can identify a classical Harris-style or bifurcated current sheet as a peak at the center or two distinct maxima on either side of B a=0, respectively. We find that around 10% of current sheet profiles exhibit a bifurcated current sheet signature, which is substantially lower than an ∼25% occurrence rate at Earth

Photoelectrons in the Enceladus plume

The plume of Enceladus is a remarkable plasma environment containing several charged particle species. These include cold magnetospheric electrons, negative and positive water clusters, charged nanograins, and “magnetospheric photoelectrons” produced from ionization of neutrals throughout the magnetosphere near Enceladus. Here we discuss observations of a population newly identified by the Cassini Plasma Spectrometer (CAPS) electron spectrometer instrument—photoelectrons produced in the plume ionosphere itself. These were found during the E19 encounter, in the energetic particle shadow where penetrating particles are absent. Throughout E19, CAPS was oriented away from the ram direction where the clusters and nanograins are observed during other encounters. Plume photoelectrons are also clearly observed during the E9 encounter and are also seen at all other Enceladus encounters where electron spectra are available. This new population, warmer than the ambient plasma population, is distinct from, but adds to, the magnetospheric photoelectrons. Here we discuss the observations and examine the implications, including the ionization source these electrons provide

1-D Hybrid Kinetic/Fluid Modelling of the Jovian Magnetosphere

Based on early measurements from the Juno spacecraft, magnetosphere-ionosphere-thermosphere (MIT) coupling studies of the Jovian system are thought to have under-estimated the densities of plasma species in the high-latitude regions of the magnetosphere. As the main auroral oval of Jupiter is driven by particles precipitating into the planetary atmosphere in these regions, characterising the density and potential structure along high-latitude magnetic field lines is of particular interest. To that end, a 1-D spatial, 2-D velocity hybrid kinetic/fluid model is under development. This model will allow the middle magnetosphere regions (~20-50 RJ) responsible for the main oval to be investigated numerically. Previous 1-D kinetic models of the Jovian system have been constrained to the Io flux tube. Through the use of code parallelisation, non-uniform spatial mesh and fluid treatment of species, the computational challenge of modelling of the Jovian middle magnetosphere can be reduced. The model allows density profiles, potential structures, current flow and precipitating particle fluxes to be found. Comparison of these outputs to data from Juno will help to validate the model, along with providing refinements to MIT theory