Environments in the Outer Solar System

The outer planets of our solar system Jupiter, Saturn, Uranus, and Neptune are fascinating objects on their own. Their intrinsic magnetic fields form magnetic environments (so called magnetospheres) in which charged and neutral particles and dust are produced, lost or being transported through the system. These magnetic environments of the gas giants can be envisaged as huge plasma laboratories in space in which electromagnetic waves, current systems, particle transport mechanisms, acceleration processes and other phenomena act and interact with the large number of moons in orbit around those massive planets. In general it is necessary to describe and study the global environments (magnetospheres) of the gas giants, its global configuration with its large-scale transport processes; and, in combination, to study the local environments of the moons as well, e.g. the interaction processes between the magnetospheric plasma and the exosphere/atmosphere/magnetosphere of the moon acting on time scales of seconds to days. These local exchange processes include also the gravity, shape, rotation, astrometric observations and orbital parameters of the icy moons in those huge systems. It is the purpose of this chapter of the book to describe the variety of the magnetic environments of the outer planets in a broad overview, globally and locally, and to show that those exchange processes can dramatically influence the surfaces and exospheres/atmospheres of the moons and they can also be used as a tool to study the overall physics of systems as a whole.     

Separation of the Magnetic Field into External and Internal Parts

The magnetic field of a planet or a planetary moon contains contributions from a variety of sources in the environment of the body (external sources) and its interior (internal sources). This chapter describes different methods that have been developed for the separation of external and internal source contributions, and their application to selected planets and one of Jupiter’s moons, Ganymede.     

The Galileo magnetic field investigation

The Galileo Orbiter carries a complement of fields and particles instruments designed to provide data needed to shed light on the structure and dynamical variations of the Jovian magnetosphere. Many questions remain regarding the temporal and spatial properties of the magnetospheric magnetic field, how the magnetic field maintains corotation of the embedded plasma and the circumstances under which corotation breaks down, the nature of magnetic perturbations that transport plasma across magnetic shells in different parts of the system, and the electromagnetic properties of the Jovian moons and how they interact with the magnetospheric plasma. Critical to answering these closely related questions are measurements of the dc and low-frequency magnetic field. The Galileo Orbiter carries a fluxgate magnetometer designed to provide the sensitive measurements required for this purpose. In this paper, the magnetometer is described. The instrument has two boom-mounted, three-axis sensor assemblies. Flipper mechanisms are included in each sensor assembly for the purpose of offset calibration. The microprocessor controlled data handling system produces calibrated despun data that can be used directly without further processing. A memory system stores data for those periods when the spacecraft telemetry is not active. This memory system can also be used for storing high time-resolution snapshots of data.     

Dynamo Models for Planets Other Than Earth

Observations from planetary spacecraft missions have demonstrated a spectrum of dynamo behaviour in planets. From currently active dynamos, to remanent crustal fields from past dynamo action, to no observed magnetization, the planets and moons in our solar system offer magnetic clues to their interior structure and evolution. Here we review numerical dynamo simulations for planets other than Earth. For the terrestrial planets and satellites, we discuss specific magnetic field oddities that dynamo models attempt to explain. For the giant planets, we discuss both non-magnetic and magnetic convection models and their ability to reproduce observations of surface zonal flows and magnetic field morphology. Future improvements to numerical models and new missions to collect planetary magnetic data will continue to improve our understanding of the magnetic field generation process inside planets.     

1. Transport of Mass,Momentum and Energy in Planetary Magnetodisc Regions

The rapid rotation of the gas giant planets, Jupiter and Saturn, leads to the formation of magnetodisc regions in their magnetospheric environments. In these regions, relatively cold plasma is confined towards the equatorial regions, and the magnetic field generated by the azimuthal (ring) current adds to the planetary dipole, forming radially distended field lines near the equatorial plane. The ensuing force balance in the equatorial magnetodisc is strongly influenced by centrifugal stress and by the thermal pressure of hot ion populations, whose thermal energy is large compared to the magnitude of their centrifugal potential energy. The sources of plasma for the Jovian and Kronian magnetospheres are the respective satellites Io (a volcanic moon) and Enceladus (an icy moon). The plasma produced by these sources is globally transported outwards through the respective magnetosphere, and ultimately lost from the system. One of the most studied mechanisms for this transport is flux tube interchange, a plasma instability which displaces mass but does not displace magnetic flux—an important observational constraint for any transport process. Pressure anisotropy is likely to play a role in the loss of plasma from these magnetospheres. This is especially the case for the Jovian system, which can harbour strong parallel pressures at the equatorial segments of rotating, expanding flux tubes, leading to these regions becoming unstable, blowing open and releasing their plasma. Plasma mass loss is also associated with magnetic reconnection events in the magnetotail regions. In this overview, we summarise some important observational and theoretical concepts associated with the production and transport of plasma in giant planet magnetodiscs. We begin by considering aspects of force balance in these systems, and their coupling with the ionospheres of their parent planets. We then describe the role of the interaction between neutral and ionized species, and how it determines the rate at which plasma mass and momentum are added to the magnetodisc. Following this, we describe the observational properties of plasma injections, and the consequent implications for the nature of global plasma transport and magnetodisc stability. The theory of the flux tube interchange instability is reviewed, and the influences of gravity and magnetic curvature on the instability are described. The interaction between simulated interchange plasma structures and Saturn’s moon Titan is discussed, and its relationship to observed periodic phenomena at Saturn is described. Finally, the observation, generation and evolution of plasma waves associated with mass loading in the magnetodisc regions is reviewed.     

Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts

It is only now, with low-frequency radio telescopes, long exposures with high-resolution X-ray satellites and γ-ray telescopes, that we are beginning to learn about the physics in the periphery of galaxy clusters. In the coming years, Sunyaev-Zel’dovich telescopes are going to deliver further great insights into the plasma physics of these special regions in the Universe. The last years have already shown tremendous progress with detections of shocks, estimates of magnetic field strengths and constraints on the particle acceleration efficiency. X-ray observations have revealed shock fronts in cluster outskirts which have allowed inferences about the microphysical structure of shocks fronts in such extreme environments. The best indications for magnetic fields and relativistic particles in cluster outskirts come from observations of so-called radio relics, which are megaparsec-sized regions of radio emission from the edges of galaxy clusters. As these are difficult to detect due to their low surface brightness, only few of these objects are known. But they have provided unprecedented evidence for the acceleration of relativistic particles at shock fronts and the existence of μG strength fields as far out as the virial radius of clusters. In this review we summarise the observational and theoretical state of our knowledge of magnetic fields, relativistic particles and shocks in cluster outskirts.     

Surface, Subsurface and Atmosphere Exchanges on the Satellites of the Outer Solar System

The surface morphology of icy moons is affected by several processes implicating exchanges between their subsurfaces and atmospheres (if any). The possible exchange of material between the subsurface and the surface is mainly determined by the mechanical properties of the lithosphere, which isolates the deep, warm and ductile ice material from the cold surface conditions. Exchanges through this layer occur only if it is sufficiently thin and/or if it is fractured owing to tectonic stresses, melt intrusion or impact cratering. If such conditions are met, cryomagma can be released, erupting fresh volatile-rich materials onto the surface. For a very few icy moons (Titan, Triton, Enceladus), the emission of gas associated with cryovolcanic activity is sufficiently large to generate an atmosphere, either long-lived or transient. For those moons, atmosphere-driven processes such as cryovolcanic plume deposition, phase transitions of condensable materials and wind interactions continuously re-shape their surfaces, and are able to transport cryovolcanically generated materials on a global scale. In this chapter, we discuss the physics of these different exchange processes and how they affect the evolution of the satellites’ surfaces.     

Cassini-Huygens Investigations of Satellite Surfaces and Interiors

The Saturnian system contains 18 known satellites ranging from 10 km to 2575 km in radius. In bulk properties and surface appearance these objects show less regularity than the sparser Jupiter system. The Galilean-sized moon Titan sports a dense atmosphere of nitrogen and methane which renders surface observations difficult, but also makes this moon intriguing from the standpoints of climate change and exobiology. The Cassini-Huygens mission will make extensive observations of the satellites over a range of wavelengths, as well as using in-situ sampling of satellite environments (and in the case of Titan, sampling of atmosphere and surface). The goals of these extensive investigations are to understand the bulk properties of the satellites, their surface compositions and evolution through time, as well as interactions with the magnetosphere and rings of Saturn. This knowledge in turn should provide a deeper understanding of the origin of the Saturnian system as a whole and underlying causes for the distinctive differences from the Jovian satellite system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Towards a Global Unified Model of Europa’s Tenuous Atmosphere

Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.     

Magnetic field of the earth and planets

Knowledge of geomagnetism and its effects was recently much augmented during the IGY; when added to what is known of stellar fields, interplanetary fields, and meteoric magnetism, and combined with magnetic theory, this knowledge not only suggests that much may be learned of our neighbors in space by making an early determination of their magnetic fields, but also provides a foundation for many inferences and rough estimates on the magnetic fields of other solar-system bodies. Present estimates range from 660 cgs for Jupiter to 0.004 for the smaller satellites and the planetoids.     

The internal constitutions of the inner planets and the moon

The internal structures of the moon, Mars, Venus, and Mercury are examined in the light of what is known about the constitution of the earth. The gravitational figure of the earth as obtained from orbits of artificial satellites is used to estimate the possible deviations from hydrostatic equilibrium on other planets. Observations of the orbital and rotational motion of the moon are consistent with the hypothesis that the interior of the moon supports density inhomogeneities of the same order as those supported by the earth. The available data on the moon are insufficient to determine whether or not the moon is differentiated. The orbits of Phobos and Deimos yield an adequate value for the moment of inertia of Mars. The moment of inertia and the mass are consistent with a metallic core containing about 10 per cent of the mass of Mars. The observations of the possible magnetic field of Mars would be of importance both to the understanding of planetary magnetic fields and elucidating the internal structure of that planet. Seismic investigations on the earth yield an equation of state for silicates to pressures of about 1 × 10⁶ bars. This equation of state is used in determining density variation within Mars.The surface heat flow for the earth is consistent with the hypothesis that the concentration of radioactive elements is the same as that in chondritic meteorites. The observed ratio of potassium to uranium in surface and near-surface rocks is not consonant with the chondritic hypothesis. The moon can be of chondritic composition only if it is differentiated with the radioactivity concentrated in the upper few hundred kilometers. A chondritic composition for Mars would require a differentiation in excess of that consistent with its mass and moment of inertia. It is concluded that a chondritic composition is not a satisfactory chemical model for the inner planets.     

Induced Magnetic Fields in Solar System Bodies

Electromagnetic induction is a powerful technique to study the electrical conductivity of the interior of the Earth and other solar system bodies. Information about the electrical conductivity structure can provide strong constraints on the associated internal composition of planetary bodies. Here we give a review of the basic principles of the electromagnetic induction technique and discuss its application to various bodies of our solar system. We also show that the plasma environment, in which the bodies are embedded, generates in addition to the induced magnetic fields competing plasma magnetic fields. These fields need to be treated appropriately to reliably interpret magnetic field measurements in the vicinity of solar system bodies. Induction measurements are particularly important in the search for liquid water outside of Earth. Magnetic field measurements by the Galileo spacecraft provide strong evidence for a subsurface ocean on Europa and Callisto. The induction technique will provide additional important constraints on the possible subsurface water, when used on future Europa and Ganymede orbiters. It can also be applied to probe Enceladus and Titan with Cassini and future spacecraft.     

Quasistatic electric field measurements with spherical double probes on the GEOS and ISEE satellites

Spherical double probes for measurements of electric fields on the GEOS-1, GEOS-2, and ISEE-1 satellites are described. An essential feature of these satellites is their conductive surfaces which eliminate errors due to differential charging and enable meaningful diagnostic experiments to be carried out. The result of these experiments is a good understanding of interactions between the plasma, the satellite and the probes, including photo-electron emission on satellite and probes. Electric field measurements are compared with measurements of plasma drift perpendicular to the magnetic field in the solar wind and the magnetosphere and the error bar for the absolute values of the electric field is found to be in the range ±(0.5–1.0) mV m^-1 whereas relative variations can be determined with much better accuracy. A useful by-product from a spherical double probe system is the determination of satellite floating potential which is related to the plasma electron flux. This measurement allows high time resolution studies of boundary crossings. Examples of electric field measurements, which reflect the recent scientific results, are given for different regions of the magnetosphere from the bow shock, the inner magnetosphere and the tail. Several examples of simultaneous GEOS-ISEE observations are described.     

Numerical analysis of deflection control of a gas plasma jet based on magnetohydrodynamic staggered electrode configuration

To control the deflection of the gas plasma jet, a new analytical method is proposed based on the Magnetohydrodynamic (MHD) technique. Based on the typical MHD power generation model, the applied voltage is applied to the staggered electrodes, that is, a pair of electrodes on the same side wall are connected to generate an axial current in the channel. Under the action of the magnetic field perpendicular to the direction of the flow, the plasma is subjected to electromagnetic forces perpendicular to these two directions, and the jet is deflected. The computational model including the Navier-Stokes equations coupled with electromagnetic source terms, the electric potential equation and Ohm’s law is solved. The deflection of the gas jet under the action of an electromagnetic field is observed, and the maximum deflection angle is about 14.8°. The influences of the electric field, magnetic field, and conductivity on the jet deflection are studied. Results show that although the influences of these three factors on the deflection are similar, and the effect of increasing the electric field strength is slightly greater, priority should be given to increasing the magnetic field strength from the perspective of reducing energy consumption. The Stuart number is introduced to assess the ability of electromagnetic force to control jet deflection. When the electromagnetic parameters are constant, this solution provides better control of low-density and low-speed fluid flows. The calculation results show that using the staggered electrode method configuration is feasible in terms of controlling the deflection of a plasma jet deflection.     

Sources of Ion Outflow in the High Latitude Ionosphere

Ion composition observations from polar-orbiting satellites in the past three decades have revealed and confirmed the occurrence of a variety of ion outflow processes in the high-latitude ionosphere. These processes constitute a dominant source of ionospheric plasma to the Earth's magnetosphere. We review the current state of our observational knowledge on their occurrence, energy, composition, variability, interrelationships, and quantitative contributions to the overall mass input to the magnetosphere. In addition, we identify the prevalent sources and the gaps of our current understanding of these sources.     

Mapping Magnetospheric Equatorial Regions at Saturn from Cassini Prime Mission Observations

Saturn??s rich magnetospheric environment is unique in the solar system, with a large number of active magnetospheric processes and phenomena. Observations of this environment from the Cassini spacecraft has enabled the study of a magnetospheric system which strongly interacts with other components of the saturnian system: the planet, its rings, numerous satellites (icy moons and Titan) and various dust, neutral and plasma populations. Understanding these regions, their dynamics and equilibria, and how they interact with the rest of the system via the exchange of mass, momentum and energy is important in understanding the system as a whole. Such an understanding represents a challenge to theorists, modellers and observers. Studies of Saturn??s magnetosphere based on Cassini data have revealed a system which is highly variable which has made understanding the physics of Saturn??s magnetosphere all the more difficult. Cassini??s combination of a comprehensive suite of magnetospheric fields and particles instruments with excellent orbital coverage of the saturnian system offers a unique opportunity for an in-depth study of the saturnian plasma and fields environment. In this paper knowledge of Saturn??s equatorial magnetosphere will be presented and synthesised into a global picture. Data from the Cassini magnetometer, low-energy plasma spectrometers, energetic particle detectors, radio and plasma wave instrumentation, cosmic dust detectors, and the results of theory and modelling are combined to provide a multi-instrumental identification and characterisation of equatorial magnetospheric regions at Saturn. This work emphasises the physical processes at work in each region and at their boundaries. The result of this study is a map of Saturn??s near equatorial magnetosphere, which represents a synthesis of our current understanding at the end of the Cassini Prime Mission of the global configuration of the equatorial magnetosphere.     

Electromagnetic Induction Effects and Dynamo Action in the Hermean System

Embedded in a large mass density and strong interplanetary magnetic field solar wind environment and equipped with a magnetic field of minor strength, planet Mercury exhibits a small magnetosphere vulnerable to severe solar wind buffeting. This causes large variations in the size of the magnetosphere and its associated currents. External fields are of far more importance than in the terrestrial case and of a size comparable to any internal, dynamo-generated field. Induction effects in the planetary interior, dominated by its huge core, are thought to play a much more prominent role in the Hermean magnetosphere compared to any of its companions. Furthermore, the external fields may cause planetary dynamo amplification much as discussed for the Galilean moons Io and Ganymede, but with the ambient field generated by the dynamo and its magnetic field-solar wind interaction.     

Modeling of Venus, Mars, and Titan

Increased computer capacity has made it possible to model the global plasma and neutral dynamics near Venus, Mars and Saturn??s moon Titan. The plasma interactions at Venus, Mars, and Titan are similar because each possess a substantial atmosphere but lacks a global internally generated magnetic field. In this article three self-consistent plasma models are described: the magnetohydrodynamic (MHD) model, the hybrid model and the fully kinetic plasma model. Chamberlain and Monte Carlo models of the Martian exosphere are also described. In particular, we describe the pros and cons of each model approach. Results from simulations are presented to demonstrate the ability of the models to capture the known plasma and neutral dynamics near the three objects.     

磁控热防护系统在天地往返运载器上的应用仿真

磁控热防护技术在高超声速领域显现出广泛的应用前景。考虑高超声速流动磁流体力学控制涉及的等离子体生成机制、多电离组分导电机理以及电磁流动能量/动量输运机制,通过耦合求解电磁场泊松方程和带电磁源项的高温热化学非平衡流动控制方程组,搭建了高超声速磁控热防护数值模拟平台。结合美国航天飞机"哥伦比亚"号（OV-102）近似外形和5种磁场配置方案,较为系统地开展了磁控热防护系统在高超声速"滑翔返回式"天地往返运载器上的应用仿真研究。结果表明：搭建的磁控热防护仿真平台具备偶极子磁场、均匀磁场、螺线管磁场及多个磁场组合条件下复杂外形飞行器气动热环境数值模拟能力,其校验结果与文献或飞行试验数据符合较好;采用合适的磁场配置能有效降低航天飞机的表面热流,显著改善了航天飞机的气动热环境,典型状态的表面热流下降25%以上;局部磁场方向与流动方向的夹角,在一定程度上决定了洛伦兹力的强度和方向,对磁控效果的影响明显。     

Evolution of Icy Satellites

Evolutionary scenarios for the major satellites of Jupiter, Saturn, Neptune, and Pluto-Charon are discussed. In the Jovian system the challenge is to understand how the present Laplace resonance of Io, Europa, and Ganymede was established and to determine whether the heat being radiated by Io is in balance with the present tidal dissipation in the moon. In the Saturnian system, Enceladus and Titan are the centers of attention. Tidal heating is the likely source of activity at the south pole of Enceladus, although the details of how the heating occurs are not understood. An evolutionary scenario based on accretion and internal differentiation is presented for Titan, whose present substantial orbital eccentricity is not associated with any dynamical resonance. The source and maintenance of methane in Titan’s present atmosphere remain uncertain. Though most attention on the Saturnian moons focuses on Titan and Enceladus, the mid-size satellites Iapetus, Rhea, Tethys, and the irregular satellite Phoebe also draw our interest. An evolutionary scenario for Iapetus is presented in which spin down from an early rapidly rotating state is called upon to explain the satellite’s present oblate shape. The prominent equatorial ridge on Iapetus is unexplained by the spin down scenario. A buckling instability provides another possible explanation for the oblateness and equatorial ridge of Iapetus. Rhea is the only medium-size Saturnian satellite for which there are gravity data at present. The interpretation of these data are uncertain, however, since it is not known if Rhea is in hydrostatic equilibrium. Pluto and Charon are representative of the icy dwarf planets of the Kuiper belt. Did they differentiate as they evolved, and do either of them have a subsurface liquid water ocean? New Horizons might provide some answers when it arrives at these bodies.