We present a model of Saturn's global auroral response to the solar wind as observed by simultaneous Hubble Space Telescope (HST) auroral images and Cassini upstream measurements of the solar wind taken during the month of January 2004. These observations show a direct correlation between solar wind dynamic pressure and (1) auroral brightening toward dawn local time, (2) an increase of rotational movement of auroral features to as much as 75% of the corotation speed, (3) the movement of the auroral oval to higher latitudes, and (4) an increase in the intensity of Saturn Kilometric Radiation (SKR). Our model, referred to as the centrifugal instability model, provides an alternative to the reconnection model of Cowley et al. (2004a, 2004b, 2005); we suggest the above observations result from Saturn's magnetosphere being a fast rotator. Since the torques on Saturn's outer magnetosphere are relatively low, its outer magnetosphere will tend to conserve angular momentum. When compressed on the dayside, the outer magnetosphere spins up to higher angular velocities, and when it expands, the outer magnetosphere spins down to lower angular velocities. This response occurs since Saturn's ionosphere is unable to enforce corotation. The outer boundary of the plasma sheet at L ∼ 15 is identified as the primary source location for the auroral precipitating particles. Enhanced wave activity, which can precipitate the auroral producing particles, may be present at this boundary. If radial transport is dominated by centrifugally driven flux tube interchange motions, when the magnetosphere spins up, outward transport will increase, and the precipitating particles will move radially outward (since the radial gradient in electron energy flux is negative). This mechanism will cause the auroral oval to move to higher latitudes as observed. The Kelvin‐Helmholtz instability may contribute to the enhanced emission along the dawn meridian, as observed by HST, via enhanced wave activity and corresponding charged particle precipitation.
We have performed an evaluation to determine whether or not Neptune's magnetospheric electrons can provide the ionization of Triton's ionosphere as previously suggested or whether photoionization is the dominant ionization mechanism. Our approach has been to determine the accessibility of magnetospheric electrons to Triton's ionosphere. Using scaling relationships based on Venus and Titan observations, we have developed estimates of the centrifugal, gradient B and E × B drifts. We have computed trajectories of magnetospheric electrons and studied their accessibility to the Triton ionosphere. The following conclusions can be reached from this study: (1) Centrifugal drift delivers electrons to the ionopause. If centrifugal drift is impaired, then electron precipitation is severely limited. (2) Low‐energy electrons ( E < 5 keV) are lost through E×B drift around the ionopause. (3) At higher electron energy the probability of precipitation increases. If the electron gyroradius is small relative to the ionopause thickness, then at pitch angles ∼90° grad B drift dominates with trapping of electrons in the ionopause and subsequent exclusion from the ionosphere. At pitch angles 0° and 180° curvature drift dominates, and electrons will precipitate on entry into the ionopause. If the electron gyroradius is large compared to the ionopause thickness, then electrons will precipitate at any pitch angle. Mass loading is estimated to be unimportant at Triton, and this contributes to the importance of E × B drift and the exclusion of low‐energy electrons to Triton's ionosphere. Our calculations have intentionally overestimated the effects of centrifugal drift to present the best case for electron precipitation. Although collisions are more important for low‐energy electrons ( E < 5 keV), we estimate that cross‐field diffusion is small for ionopause heights greater than 725 km. At higher electron energies where collisions are less important, the threshold energy above which electrons become untrapped is only dependent upon the ionopause thickness and not collisions. Pressure balance arguments show that the ionopause is thick with Δ z > 200 km. A magnetized ionosphere would be equivalent to the high ram pressure case for the Venus interaction. A thick ionopause would contribute to prevention of precipitation of magnetospheric electrons into Triton's ionosphere when E < 50 keV. Although our calculations at the present level of development cannot rule out the importance of electron precipitation as the source of Triton's ionosphere, we suggest that photoionization be considered viable for the production of Triton's ionosphere.
Voyager images of the icy satellites of Saturn, Dione and Enceladus, suggest that they may have been geologically active and are not only composed of ice. Recent observations by the Hubble Space Telescope have shown the presence of ozone at both Dione and Rhea, which also implies the presence of molecular oxygen at these bodies. Observations of Ariel, Europa, Ganymede, and Callisto indicate the presence of CO 2 , so its presence on the Saturnian satellites is also expected. The Cassini Plasma Spectrometer (CAPS) will provide the capability to determine the global composition of these bodies by measuring the pickup ions produced by the ionization of their sputter‐produced atmospheres. We will present a model of these atmospheres and associated pickup ions and demonstrate CAPS ability to distinguish the freshly produced picked up ions from the ambient plasma. Such ions are expected to form a ring distribution that will have a uniquely different energy‐angle dependence than the ambient plasma ions. In the case of Dione we expect the potential for a moderate strength interaction for which both Voyager 1 and Pioneer 11 spacecraft measured ion cyclotron waves centered on the Dione L shell and near the equatorial plane. SKR radio emissions also displayed emissions occurring at the orbital period of Dione which could indicate some intrinsic activity due to Dione. So again, something interesting may be going on at Dione. Since Enceladus, or material in orbit near Enceladus, may be the source of the E‐ring, some surprises may be encountered during its close encounter with the Cassini spacecraft. In the case of Dione we will show that a wake pass at 500 km altitude is more than an order of magnitude better than an upstream pass at 500 km altitude. Pickup ion detection for minor ion species such as NH 3 + is possible for 500 km altitude wake pass but not for ≈500 km altitude upstream pass at closest approach. For navigation reasons a 100 km pass is not allowed. Therefore it is essential to have a wake pass to maximize the science return for a targeted flyby with Dione. The CAPS observations when combined with magnetometer, plasma wave and energetic particle observations will allow us to estimate the source of ions into Saturn's magnetosphere due to these two bodies and to characterize the nature of the interaction with Saturn's magnetosphere.
We present a comprehensive analysis of Voyager 1 and 2 electron observations within Saturn's magnetosphere. This analysis entails the merging of electron observations from the Plasma Science (PLS) experiment, the Low Energy Charged Particle (LECP) experiment and the Cosmic Ray System (CRS) experiment. For each encounter, the three instruments combined allow us to compute the electron energy spectra over a wide range of energies from 10 eV to ∼ 2MeV between the closest approaches and L = 18.5. The instruments use different technologies, different sensitivities, and different fields of view; however, we observe a surprisingly good matching of the data sets on a 15‐min timescale. The PLS‐LECP‐CRS spectra include the low‐energy thermal component of the magnetospheric plasma, the keV suprathermal electrons, and the high‐energy tail extending into the MeV energy range. From the combined spectra, we compute a comprehensive set of macroscopic parameters (electron density, pressure, beta factor, and electron current at the spacecraft): the analysis reveals a variety of radial gradients for these quantities and the corresponding electron populations. We also compute phase space densities over a wide range in energy and radial distances, analyzing local time symmetries, electron source distributions, and temporal variations of Saturn's magnetosphere. The ultimate goal of this study is to provide a comprehensive empirical model of the charged particle population within Saturn's magnetosphere. It will be used to support the development of the Cassini mission and to allow detailed planning of the tour design with regard to charged particle science and radiation hazards.
Voyager 2 observations have shown that Uranus possesses a well‐developed bipolar magnetotail similar in certain characteristics to that of Earth, in spite of an anomalously large tilt of the planetary magnetic dipole to the rotation axis at Uranus. The intensity of the magnetic field in the tail lobes decreases with increasing distance down the tail from the planet as | x SM | −0.59±0.03 . This gradient is similar to that found in the Earth's tail but significantly less steep than that observed in the tails of Jupiter and Saturn. The thickness of the plasma sheet is a minimum (∼10 R U ) near the tail center, increasing toward the flanks as at Earth. Pressure balance within the plasma sheet is maintained predominantly by protons and electrons with energies 10 eV to 6 keV. Except in transient events, the contribution of ≥28‐keV protons to pressure balance in the sheet is <5%. This is in contrast to the dominant role played by more energetic plasma ions in the Jovian magnetotail. An average value of β ∼ 7 was found in the plasma sheet at Uranus and ∼0.1 in the lobe plasma. The Uranian magnetic tail was observed to rotate 360° about its longitudinal axis, a result of the approximately sunward pointing planetary rotation axis at the time of encounter. This, together with the large tilt (60°) of the magnetic dipole, results in a small but measurable twist in the tail's magnetic lines of force, with a derived helical pitch of 5.5° ± 3.0°. The B yz (SM) component of the tail field can be modeled as the sum of the twist component, a radial (diverging or converging from the x SM axis) component, and a component parallel to the z SM axis that closes through the neutral sheet and is strongest there. The cross‐tail current density at the neutral sheet is estimated to be ≃3 × 10 −11 A m −2 . Large temporal variations observed in magnetic fields and plasmas during the Voyager 2 traverse of the magnetotail may have been produced by substorm activity.
Magnetic field and plasma data from five spacecraft (Voyager 1 and 2, Helios 1 and 2, and IMP 8) were used to analyze the flow behind an interplanetary shock. The shock was followed by a turbulent sheath in which there were large fluctuations in both the strength and the direction of the magnetic field. This in turn was followed by a region (magnetic cloud) in which the magnetic field vectors were observed to change by rotating nearly parallel to a plane, consistent with the passage of a magnetic loop. This loop extended at least 30° in longitude between 1 and 2 AU, and its radial dimension was approximately 0.5 AU. In the cloud the field strength was high, and the density and temperature were relatively low. Thus the dominant pressure in the cloud was that of the magnetic field. The total pressure inside the cloud was higher than outside, implying that the cloud was expanding as it moved outward, even at the distance of 2 AU. The momentum flux of the cloud at 2 AU was not higher than that of the preshock plasma, indicating that the cloud was not driving the shock at this distance. It is possible, however, that the shock was driven by the cloud closer to the sun where the cloud may have moved faster. An extraordinary filament was observed at the rear of the cloud. It was bounded by current sheets whose orientations were preserved over at least 0.12 AU and which were related to the plane of maximum variance of the magnetic field in the cloud.
Electron and ion measurements made by the Voyager 1 plasma science instrument revealed a plasma wake surrounding Titan in Saturn's rotating magnetosphere. This wake is characterized by a plasma that is more dense and cooler than the surrounding subsonic magnetospheric plasma. The density enhancement is produced by the deflection of magnetospheric plasma around Titan and the addition of exospheric ions picked up by the rotating magnetosphere. By using simple models for ion pickup in the ion exosphere outside Titan's magnetic tail and ion flow within the boundaries of the tail, the interaction between Saturn's rotating magnetosphere and Titan is shown to resemble the interaction between the solar wind and Venus. Outside the magnetic tail of Titan, pickup of H + formed by ionization of the H exosphere is indicated when synthetic and observed ion spectra are matched. Close to the boundary of the tail, a reduction in plasma flow speed is found, providing evidence for mass loading by the addition of N 2 + /H 2 CN + and N + to the flowing plasma. The boundary of the tail is indicated by a sharp reduction in the flux of high‐energy electrons, which are removed by inelastic scattering with the atmosphere and centrifugal drift produced when the electrons traverse the magnetic field draped around Titan. Within the tail the plasma is structured as the result of spatial and/or temporal variations. The ion mass cannot be determined uniquely in the tail; however, one measurement suggests the presence of a heavy ion with a mass of order 28 amu: One candidate is H 2 CN + , suggested as the dominant topside ion of the ionosphere, which may flow from the ionosphere into the tail.
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