Fran Bagenal

Empirical model of the Io plasma torus: Voyager measurements

We present a description of plasma conditions in the Io plasma torus, between 5 and 10 RJ, based on Voyager 1 observations obtained in March 1979. The model includes updated analyses of Plasma Science (PLS) data obtained along the spacecraft trajectory as well as Ultraviolet Spectrometer (UVS) observations of composition made remotely from Jupiter. The plasma characteristics observed along the spacecraft trajectory have been extrapolated along magnetic field lines by numerically solving the equations of diffusive equilibrium to produce radial profiles of plasma properties at the centrifugal equator as well as maps of the densities of the major ionic species in a meridian plane. The diffusive equilibrium distribution of plasma along magnetic field lines depends mainly on T∥. Unfortunately, we only have measurements of T⊥ and must make assumptions about the thermal anisotropy of the plasma. We assume the thermal populations and the suprathermal electrons to be isotropic. The suprathermal ions have probably been recently picked-up and are expected to be highly anisotropic. Varying the thermal anisotropy of the hot ions between A=T⊥/T∥=1 to 5 has a minor effect on the plasma maps but makes a significant difference to the fraction of hot ions in the plasma when integrated over a complete shell of magnetic flux. We have found that the vertical extrapolation of plasma density is insensitive to the geometry of different magnetic field models except inside 5 RJ (where the plasma scale height is comparable to uncertainties in the location of the centrifugal equator) and outside 8 RJ (where the magnetospheric current sheet significantly perturbs the magnetic field). The radial profile of flux tube content (N L²) exhibits the same “precipice”, “ledge,” and “ramp” features as previous studies as well as confirming small - scale features which indicate local sources of plasma in the cold torus and near the orbit of Europa. The observations of O++ and molecular (SO2+ or S2+) ions inside 5.4 RJ, far from Io, in a region of cold dense plasma, remain difficult to explain, indicating either strong temporal variability in the Io plasma source or a strong source of plasma, possibly from the dissociation of dust, inside Ios orbit. Further evidence of a Europa source are the decrease in the ratio of sulfur to oxygen ions and the increase in plasma temperature outside 8 RJ.

Plasma Observations Near Uranus: Initial Results from Voyager 2

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.

Plasma Observations Near Jupiter: Initial Results from Voyager 1

Extensive measurements of low-energy positive ions and electrons were made throughout the Jupiter encounter of Voyager 1. The bow shock and magneto-pause were crossed several times at distances consistent with variations in the upstream solar wind pressure measured on Voyager 2. During the inbound pass, the number density increased by six orders of magnitude between the innermost magnetopause crossing at ∼47 Jupiter radii and near closest approach at ∼5 Jupiter radii; the plasma flow during this period was predominately in the direction of corotation. Marked increases in number density were observed twice per planetary rotation, near the magnetic equator. Jupiterward of the Io plasma torus, a cold, corotating plasma was observed and the energylcharge spectra show well-resolved, heavy-ion peaks at mass-to-charge ratios A/Z* = 8, 16, 32, and 64.

Plasma Observations Near Neptune: Initial Results from Voyager 2

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptunes magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptunes magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Tritons atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptunes magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

The ionization source near Io from Galileo wake data

From measurements of plasma fluxes obtained along the Galileo trajectory through the wake of Io we estimate the source of ions within 5 RIo of the satellite to be 0.5 to 1.7 × 1028 s−1 (18 to 58% of the canonical 3 × 1028 s−1 torus supply rate). This source of ionization close to Io can supply 0.3 to 0.8 × 1012 watts to the torus plasma (15 to 28% of the canonical 3 × 1012 watts of EUV emissions from the torus). Modeling the energy flux at the spacecraft with local ionization matches the observations reasonably well, which we interpret as evidence against extensive additional charge exchange reactions occurring near Io.

ROSAT observations of the Jupiter aurora

Rontgen satellite (ROSAT) high-resolution imager (HRI) and position sensitive proportional counter (PSPC) observations of Jupiter obtained in April 1991 and May 1992 reveal soft X ray emissions apparently associated with Jupiters aurora and similar to X ray emissions observed earlier by the Einstein Observatory. The HRI images show emission mainly from Jupiters northern hemisphere at all Jovian longitudes observed, and there is some indication of a longitudinal modulation of the emission in phase with the well-known ultraviolet modulation of the northern aurora. The PSPC data reveal a very soft spectrum. Comparison of the observed spectrum with models for both electron bremsstrahlung radiation and line emission from S and O ions indicates that the line spectrum gives a much better statistical fit to the observed spectrum. The X ray observations presented here therefore support the hypothesis that ion precipitation is the most likely cause of the Jovian X ray emissions, a result first suggested by the Einstein results [Metzger et al., 1983].

Solar minimum streamer densities and temperatures using Whole Sun Month coordinated data sets

We model electron densities of the simplest, most symmetric solar minimum streamer structure observed during the Whole Sun Month (WSM) campaign, using coronal observations of both visible white light and extreme ultraviolet (EUV) emission. Using white light data from the SOHO/LASCO/C2 and HAO/Mauna Loa Mark 3 coronagraphs, we determine electron densities by way of a Van de Hulst inversion. We compare the white light densities to those determined from the density sensitive EUV line ratios of Si IX 350/342 A observed by the SOHO/coronal diagnostic spectrometer (CDS). Moreover, from the white light density profiles we calculate hydrostatic temperature profiles and compare to temperatures derived from the Si XII/Mg X line ratio. We find the white light and spectral analysis produce consistent density and temperature information.

Ion cyclotron waves in the Io torus during the Galileo encounter: Warm plasma dispersion analysis

During the Galileo-Io flyby, nearly field-aligned, left-hand circularly polarized ion cyclotron waves were observed in a band near the sulfur-dioxide ion gyrofrequency. We have performed a warm plasma dispersion analysis using nominal Io torus composition ratios, pickup ion “ring”-type velocity-space distributions, and a thermal background plasma of typical torus temperature. Analysis shows that the SO2+ wave is dominant, particularly as the ring begins to broaden. The observed spectral peak indicates that the ring distribution of pickup SO2+ is highly unstable and generates waves, but there is unlikely to be sufficient time for ions to fully thermalize before dissociation occurs. Assuming wave-particle scattering of ions towards a “bispherical” shell-type distribution, a free energy analysis and comparison with observed wave amplitudes suggests that the ions are not scattered far from the ring and/or that the SO2+ composition ratio in the Io torus falls off considerably from Io wake values.

Cassini UVIS observations of the Io plasma torus: IV. Modeling temporal and azimuthal variability

In this fourth paper in a series, we present a model of the remarkable temporal and azimuthal variability of the Io plasma torus observed during the Cassini encounter with Jupiter. Over a period of three months, the Cassini Ultraviolet Imaging Spectrograph (UVIS) observed a dramatic variation in the average torus composition. Superimposed on this long-term variation, is a 10.07-h periodicity caused by an azimuthal variation in plasma composition subcorotating relative to System III longitude. Quite surprisingly, the amplitude of the azimuthal variation appears to be modulated at the beat frequency between the System III period and the observed 10.07-h period. Previously, we have successfully modeled the months-long compositional change by supposing a factor of three increase in the amount of material supplied to Ios extended neutral clouds. Here, we extend our torus chemistry model to include an azimuthal dimension. We postulate the existence of two azimuthal variations in the number of superthermal electrons in the torus: a primary variation that subcorotates with a period of 10.07 h and a secondary variation that remains fixed in System III longitude. Using these two hot electron variations, our model can reproduce the observed temporal and azimuthal variations observed by Cassini UVIS.

Cassini UVIS observations of the Io plasma torus: I. Initial results

Abstract During the Cassini spacecrafts flyby of Jupiter (October, 2000–March, 2001), the Ultraviolet Imaging Spectrograph (UVIS) produced an extensive dataset consisting of 3349 spectrally dispersed images of the Io plasma torus. Here we present an example of the raw data and representative EUV spectra (561–1181 A) of the torus, obtained on October 1, 2000 and November 14, 2000. For most of the flyby period, the entire Io torus fit within the UVIS field-of-view, enabling the measurement of the total power radiated from the torus in the extreme ultraviolet. A typical value for the total power radiated in the wavelength range of 580–1181 A is 1.7×1012 W, with observed variations of up to 25%. Several brightening events were observed. These events lasted for roughly 20 hours, during which time the emitted power increased rapidly by ∼20% before slowly returning to the pre-event level. Observed variations in the relative intensities of torus spectral features provide strong evidence for compositional changes in the torus plasma with time. Spatial profiles of the EUV emission show no evidence for a sharply peaked “ribbon” feature. The ratio of the brightness of the dusk ansa to the brightness of the dawn ansa is observed to be highly variable, with an average value of 1.30. Weak longitudinal variations in the brightness of the torus ansae were observed at the 2% level.