T. W. Hill

The interaction of the atmosphere of Enceladus with Saturn's plasma.

During the 14 July 2005 encounter of Cassini with Enceladus, the Cassini Plasma Spectrometer measured strong deflections in the corotating ion flow, commencing at least 27 Enceladus radii (27 × 252.1 kilometers) from Enceladus. The Cassini Radio and Plasma Wave Science instrument inferred little plasma density increase near Enceladus. These data are consistent with ion formation via charge exchange and pickup by Saturns magnetic field. The charge exchange occurs between neutrals in the Enceladus atmosphere and corotating ions in Saturns inner magnetosphere. Pickup ions are observed near Enceladus, and a total mass loading rate of about 100 kilograms per second (3 × 1027 H2O molecules per second) is inferred.

Cassini plasma spectrometer thermal ion measurements in Saturn's inner magnetosphere

represented by two anisotropic Maxwellian distributed species, H + and a water group ion, W + . Saturn’s magnetospheric plasma is shown to subcorotate by 15–30% below rigid corotation within this region, with a minimum in fractional lag between 7 and 9 RS. There is a suggestion of a small radial outflow, but the selection of data for this study precluded the inclusion of interchange injection events. Ion densities are in excellent agreement with the Cassini plasma wave instrument, giving confidence in the forward modeling technique. Plasma moments including density, temperatures, and velocities are presented, along with empirical models for density and azimuthal velocity. Water group temperature anisotropies T?/Tk have values between 3 and 8 near 5.5 RS, becoming less anisotropic as distance increases, but are still not isotropic by 10 RS. The implications of these results for mass loading in the Saturnian magnetosphere are discussed, with the conclusion that an important fraction of the plasma source is located inside of the 5.5 RS boundary of this study.

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturns magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturns auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturns aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturns auroral emissions behave fundamentally differently from those at the other planets.

Corotation Lag in Jupiter's Magnetosphere: Comparison of Observation and Theory.

Voyager 1 plasma flow data are compared with a recent theory that predicted measurable departures from rigid corotation in Jupiters magnetosphere as a consequence of rapid plasma production and weak atmosphere-magnetosphere coupling. The comparison indicates that the theory can account for the observed corotation lag, provided that the plasma mass production rate during the Voyager 1 encounter was rather larger than expected, namely ∼ 1030 atomic mass units per second.

A Bx-interconnected magnetosphere model for Mercury

Abstract The access of solar wind plasma to the surface of Mercury depends on the magnetic fields in the vicinity of the planet. We present the structure of the Hermean magnetosphere obtained by the Toffoletto–Hill (J. Geophys. Res. 98 (1993)) model of a magnetically interconnected (“open”) magnetosphere modified for the size of Mercury and the strength of its magnetic field. We calculate open regions for the access of incident particles to the surface as a function of the interplanetary magnetic field (IMF) direction and magnitude. These results are compared with existing sodium data obtained during a week-long period of observations in November 1997. Although quantitatively crude, the model gives a qualitative approach on how to interpret a good part of the sodium emissions. We conclude that increased ion-sputtering due to solar wind–magnetosphere interactions may explain the temporal and spatial variations of the sodium exosphere seen at Mercury. We predict that emissions should be stronger in the southern hemisphere for a positive Bx component, and in the northern hemisphere for a negative Bx. The Bz component regulates the size and position of the open field line region. More negative IMF Bz corresponds to more equatorial open flux regions.

Numerical simulation of torus‐driven plasma transport in the Jovian magnetosphere

The Rice convection model has been modified for application to the transport of Io-generated plasma through the Jovian magnetosphere. The new code, called the RCM-J, has been used for several ideal-MHD numerical simulations to study how interchange instability causes an initially assumed torus configuration to break up. In simulations that start from a realistic torus configuration but include no energetic particles, the torus disintegrates too quickly (∼50 hours). By adding an impounding distribution of energetic particles to suppress the interchange instability, reasonable lifetimes were obtained. For cases in which impoundment is insufficient to produce ideal-MHD stability, the torus breaks up predominantly into long fingers, unless the initial condition strongly favors some other geometrical form. If the initial torus has more mass on one side of the planet than the other, fingers form predominantly on the heavy side (which we associate with the active sector). Coriolis force bends the fingers to lag corotation. The simulation results are consistent with the idea that the fingers are formed with a longitudinal thickness that is roughly equal to the latitudinal distance over which the invariant density declines at the outer edges of the initial torus. Our calculations give an average longitudinal distance between plasma fingers of about 15°, which corresponds to 20 to 30 minutes of rotation of the torus. We point to some Voyager and Ulysses data that are consistent with this scale of torus longitudinal irregularity.

Interchange stability of a rapidly rotating magnetosphere

Abstract A rotation-dominated magnetosphere is unstable to magnetic flux-tube interchange motions if and only if the plasma content of a unit magnetic flux tube is a decreasing function of distance from the spin axis. For a spin-aligned dipole field the marginally stable distribution is approximately ρr 9/2 = constant, where ρ is the plasma mass density at the radial distance r in the equatorial plane. Plasma filling the Jovian magnetosphere from internal sources would initially violate this stability criterion so that interchange motions would act to establish the marginally stable distribution.

Fundamental Plasma Processes in Saturn's Magnetosphere

In this chapter, we review selected fundamental plasma processes that control the extensive space environment, or magnetosphere, of Saturn (see Chapter 9, for the global context). This writing occurs at a point in time when some measure of maturity has been achieved in our understanding of the operations of Saturns magnetosphere and its relationship to those of Earth and Jupiter. Our understanding of planetary magnetospheres has exploded in the past decade or so partly because of the presence of orbiting spacecraft (Galileo and Cassini) as well as remote sensing assets (e.g., Hubble Space Telescope). This book and chapter are intended to take stock of where we are in our understanding of Saturns magnetosphere following the successful return and analysis of extensive sets of Cassini data. The end of the prime mission provides us with an opportunity to consolidate older and newer work to provide guidance for continuing investigations.

A nonsingular model of the open magnetosphere

We present a modified version of the Toffoletto and Hill (1989) open magnetosphere model that incorporates a tail-like interconnection field with a discontinuity to represent the slow-mode expansion fan that defines the high-latitude tail magnetopause. (The interconnection field is defined as the perturbation on an initially closed magnetosphere model to make it open.) The expansion fan controls the open field line region in the tail, and the intersection of the fan with the tail current sheet is, by design, the x line. The new interconnection field allows greater control of the tail field structure; in particular, it enables us to eliminate the nightside mapping singularity that occurs in previous models when the interplanetary magnetic field is nonsouthward. Also, in contrast to earlier models, the far tail x line extends farther downstream on the flanks than in the center of the tail, consistent with observations.

The Dynamics of Saturn's Magnetosphere

The dynamics of Saturns magnetosphere differs considerably from that at the Earth. Saturns magnetosphere responds to both external and internal drivers. The solar wind ram pressure, rather than the solar wind speed and interplanetary field orientation, provides the primary external driver at Saturn, while the planets rotation provides the main internal driver. Saturns magnetosphere generally moves in the corotation sense all the way to the magnetopause, although at speeds less than rigid corotation. Little evidence for classic substorm phenomena exists, although substorm-like processes such as plasmoid formation have been detected. Brief, narrow injections of hot plasma from the outer to inner magnetosphere play an important role in the dynamics at Saturn, as do energetic ion acceleration events in the outer magnetosphere as revealed by energetic neutral atom bursts resulting from charge exchange. Internal variations of the magnetosphere exhibit strong modulations at ~10.8 hours and ~10.6 hours: this periodicity is manifest in Saturn kilometric radiation, energetic ions and electrons, low energy plasma, magnetic fields, energetic neutral atoms, and the motions of the plasma sheet and magnetopause. Slower, long term variations (~year) in the periodicities occur, and faster (~weeks) variations are linked to changes in the solar wind speed. The mechanisms driving the periodicities are an active subject of inquiry at this writing.