F. M. Neubauer

Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer

The Cassini magnetometer has detected the interaction of the magnetospheric plasma of Saturn with an atmospheric plume at the icy moon Enceladus. This unanticipated finding, made on a distant flyby, was subsequently confirmed during two follow-on flybys, one very close to Enceladus. The magnetometer data are consistent with local outgassing activity via a plume from the surface of the moon near its south pole, as confirmed by other Cassini instruments.

The sub-Alfvénic interaction of the Galilean satellites with the Jovian magnetosphere

Recent observations by the Galileo spacecraft and Earth-based techniques have motivated us to reconsider the sub-Alfvenic interaction between the Galilean satellites of Jupiter and the magnetosphere. (1) We show that the atomic processes causing the interaction between the magnetoplasma and a neutral atmosphere can be described by generalized collision frequencies with contributions from elastic collisions, ion pickup, etc. Thus there is no fundamental difference in the effect of these processes on the plasma dynamics claimed in the recent literature. For a magnetic field configuration including possible internal fields, we show that the sub-Alfvenic, low-beta interaction can be described by an anisotropically conducting atmosphere joined to an Alfven wing as one extreme case and the Jovian ionosphere as the other extreme case. (2) The addition of a small magnetic field of internal origin does not modify the general Alfven wing model qualitatively but only quantitatively. All magnetic moments discussed in the literature for In are small in this sense. For an aligned internal dipole and ambient Jovian magnetic field the interaction will be enhanced by focusing of the electric field. (3) A qualitative change occurs by the additional occurrence of closed magnetic field lines for larger internal magnetic fields as in the case of Ganymede. Here the focusing is even enhanced. (4) The first discussion of nonstationary plasma flows at the satellites shows that electromagnetically induced magnetic fields may play an important role if the satellite interiors are highly conducting. From the point of view of the external excitation, induction effects may be strong for Callisto, In, Europa, and Ganymede in order of decreasing importance. The magnetic field observations at the first Callisto encounter can be explained by these effects.

Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere

A three-dimensional plasma model was developed to understand the sources and sinks that maintain Europas neutral atmosphere and to study the interaction of the Jovian magnetosphere with this atmosphere and the formation of an ionosphere. The model includes self-consistently the feedback of the plasma action on the atmosphere through mass balance. Suprathermal torus ions with a contribution from thermal ions sputter O 2 from the water ice surface, and thermal torus ions remove the O 2 atmosphere by sputtering. For an oxygen column density of 5 × 10 18 m -2 the calculated intensities of the oxygen lines OI 130.4 nm and 135.6 nm produced by electron impact dissociation agree with observations by the Hubble Space Telescope [Hall et al., 1995]. Mass balance is also consistent with this column density, with a net atmospheric mass loss of 50 kg s -1 . For a given neutral atmosphere and magnetospheric conditions, the electrodynamic model computes self-consistently plasma density, plasma velocity, electron temperature of the thermal and the suprathermal population, electric current and electric field in the vicinity of Europa, with the assumption of a constant homogeneous Jovian magnetic field. Europas ionosphere is created by electron impact ionization where the coupling of the ionosphere with the energy reservoir of the plasma torus by electron heat conduction supplies the energy to maintain ionization. The calculated distribution of electron densities with a maximum value of nearly 10 4 cm -3 is in general agreement with densities derived by Kliore et al. [1997] from the Galileo spacecraft radio occultations. The Alfvenic current system closed by the ionospheric Hall and Pedersen conductivities carries a total current of 7 × 10 5 A in each Alfven wing.

Three‐dimensional plasma simulation of Io's interaction with the Io plasma torus: Asymmetric plasma flow

A three-dimensional, stationary, two-fluid plasma model for electrons and one ion species was developed to understand the local interaction of Ios atmosphere with the Io plasma torus and the formation of Ios ionosphere. Our model calculates, self-consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a given neutral atmosphere and imposed Io plasma torus conditions but assumes for the magnetic field the constant homogeneous Jovian field. With only photoionization in a pure SO2 atmosphere it is impossible to correctly model the plasma measurements by the Galileo spacecraft. With collisional ionization and photoionization the observations can be successfully modeled when the neutral atmospheric column density is Ncol = 6 × 1020 m−2 and the atmospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy for the maintenance of the collisional ionization process and thus the formation of Ios ionosphere. Anisotropic conductivity is shown numerically as well as analytically to be essential to understand the convection patterns and current systems across Io. The electric field is very greatly reduced, because the ionospheric conductances far exceed the Alfven conductance ΣA, and also strongly twisted owing to the Hall effect. We find that the electric field is twisted by an analytic angle tan Θtwist = Σ2/(Σ1 + 2ΣA) from the anti-Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Σ1 and Σ2 within a circle encompassing Ios ionosphere. Because the electron velocity is approximately equal to the E × B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, with E and B the electric and magnetic fields, respectively. Since Σ1 ∼ Σ2, the electron flow is strongly asymmetric during convection across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupiter when passing Io. Large electric currents flow in Ios ionosphere owing to these substantially different flow patterns for electrons and ions, and our calculations predict that a total electric current of 5 million A was carried in each Alfven wing during the Galileo flyby. We also find a total Joule heating rate dissipated in Ios ionosphere of P = 4.2 × 1011 W.

The magnetic memory of Titan's ionized atmosphere

After 3 years and 31 close flybys of Titan by the Cassini Orbiter, Titan was finally observed in the shocked solar wind, outside of Saturns magnetosphere. These observations revealed that Titans flow-induced magnetosphere was populated by “fossil” fields originating from Saturn, to which the satellite was exposed before its excursion through the magnetopause. In addition, strong magnetic shear observed at the edge of Titans induced magnetosphere suggests that reconnection may have been involved in the replacement of the fossil fields by the interplanetary magnetic field.

Alfvén wings and electromagnetic induction in the interiors: Europa and Callisto

The recent evidence for the importance of magnetic fields due to electromagnetic induction in the conducting interiors of the satellites Europa and Callisto has motivated this extension of the Alfven wing model to include induction effects. These are due to the temporal variations of the Jovian magnetospheric field at the satellite positions. We show that the maximum strength of the induction effects in a satellite during a Jovian synodic rotation increases as a function of distance from Jupiter because of the increasing significance of the current sheet. For the modified Alfven wings we obtain the following results which also order the recent Galileo results: (1) For very small Alfven Mach numbers the maximum Alfven wing currents are reduced by the induction effects. This behavior is favored by a small distance between the conducting satellite interior and outer ionospheric boundary. (2) During a Jovian synodic rotation past a satellite the Alfven wings are strongest at crossings of the central current sheet, whereas induction magnetic fields are most significant at maximum magnetic latitude. This result may be important for satellite footprint observations. (3) The maximum Alfven wing current in item 1 also applies for somewhat larger Alfven Mach numbers, if the ionospheric conductivity distribution is symmetric between the upstreamside and the downstreamside. For a dominating conductivity on the downstreamside the maximum Alfven wing current is enhanced and vice versa for the opposite conductivity distribution. (4) Magnetic field measurements from satellite orbiters can be used to probe their conducting interiors, for examples any salty oceans.

Velocity space diffusion and nongyrotropy of pickup water group ions at comet Grigg‐Skjellerup

The diffusion of water group cometary ions in velocity space at comet Grigg-Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three-dimensional distributions are examined. Transformation of these parameters into a field-aligned solar wind frame allows us to test the gyrotropy of the distributions. The observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach.

Low-frequency electromagnetic plasma waves at comet P/Grigg-Skjellerup: Overview and spectral characteristics

Large-amplitude electromagnetic plasma waves are one of the dominant features of the solar wind-comet interaction. Wave characteristics strongly depend on parameters such as the solar wind flow and Alfven velocities and the angle between flow and interplanetary magnetic field as well as the production rate. With respect to the latter the flyby of the spacecraft Giotto at comet P/Grigg-Skjellerup provides a unique possibility to study such waves in further detail. Pickup ion-related wave signatures have been observed up to a distance of 600,000 km from the nucleus. Peak spectral power in the spacecraft frame of reference occurs at frequencies mainly somewhat below the water group ion gyrofrequency. From this the waves are determined to be mainly left-hand polarized waves, causing one-sided pitch angle diffusion outbound. The wave activity strongly increases close to the comet; upstream it exhibits a quadratic dependence on the water group pickup ion free energy. Furthermore, a phenomenological study of the wave characteristics provides a unique description of the fine-structure of the interaction region. Indications of steepened magnetosonic waves have been found in the outbound magnetosheath region.

MAGNETOSPHERIC AND PLASMA SCIENCE WITH CASSINI-HUYGENS

Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an ‘induced’ magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturns ‘intrinsic’ magnetosphere, the magnetic cavity Saturns planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment.The flow of magnetospheric plasma around the obstacle, caused by Titans atmosphere/ionosphere, produces an elongated cavity and wake, which we call an ‘induced magnetosphere’. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titans ionosphere, which is the obstacle to the external plasma flow. We then study Titans induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan.Saturns magnetosphere, which is dynamically and chemically coupled to all other components of Saturns environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturns upper atmosphere, and the source of Saturns auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them.Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other.Saturns magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment — extended solid surfaces, dust and gas clouds, plasma and energetic particles — are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form.

Low‐frequency electromagnetic plasma waves at comet P/Grigg‐Skjellerup: Analysis and interpretation

The propagation and polarization characteristics of low-frequency electromagnetic wave fields near comet P/Grigg-Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted with the following new findings: (1) Broad pickup-generated wave fields exist in the magnetoplasma around comet P/GS. (2) Outside the bow wave/shock, wave fields can be pictured as plane waves with propagation angles of about 10-degrees with respect to +/-B0underbar, where B0underbar the background magnetic field. (3) An envelope of very regular wave fields with almost exclusively left-hand polarization in the spacecraft frame exists around the bow wave/shock. More complex polarization properties are found farther away. (4) Because of the unusual interplanetary magnetoplasma characteristics the ratio M(Ar) of the local ring beam velocity component parallel to the magnetic field in the plasma frame of reference and the Alfven velocity varies between zero and a little greater than 1. As a consequence, the waves excited in the plasma frame must be predominantly left-hand Alfven waves propagating away from the Sun in contrast to the right-hand waves propagating toward the Sun in the same frame of reference at comets P/Giacobini-Zinner and P/Halley. (5) it is argued that the regular character of the waves close to but upstream of the bow wave/shock is due to the effects of nongyrotropy and/or nonlinear dispersive effects. (6) Magnetosheath waves are characterized as compressive, nonplanar waves with mixed polarization.