A summary of whistlers observed by Voyager 1 at Jupiter

The Voyager 1 observations of whistlers at Jupiter are summarized in order to provide a basis for further analyses of the density profile of the Io plasma torus as well as to support studies of atmospheric lightning at Jupiter. All the whistlers detected by Voyager I fell into three general regions in the torus at radial distances ranging between 5 and 6R_J. An analysis of the broadband wave amplitudes measured by the Voyager 1 plasma wave instrument and estimates of the peak whistler amplitudes imply that the grouping of whistlers was due to variations in the sensitivity of the receiver to the whistlers and not to variations in the source or propagation paths of the whistlers. The whistler dispersions are presented in statistical form for each of the three groups of events and analyzed in view of the structure of the Io plasma torus as determined by plasma measurements. The results of these analyses give source locations for the whistlers at the foot of the magnetic field lines threading the torus in both hemispheres and over a range of longitudes.     

Photometry of Io and Europa at the Crimean Astrophysical Observatory and reasons for differences between ground-based and space observations

Photometric observations of Jupiter’s moons Io and Europa in the spectral band V have been made at the Crimean Astrophysical Observatory for four years in order to construct their light curves reduced to a Solar phase angle of 6°. Comparison of these data with other ground-based observations shows good agreement. This study confirms why the moons that are close to Jupiter have a brighter leading hemisphere. The trailing hemispheres of Io and Europa, which are located in the rapidly rotating magnetic field of Jupiter, are exposed to bombardment by charged particles of the magnetic field. Leaving out of consideration the differences in brightness between the two hemispheres results in serious discrepancies between the space and ground-based photometry data.     

Models, figures, and gravitational moments of Jupiter’s satellites Io and Europa

Two types of trial three-layer models have been constructed for the satellites Io and Europa. In the models of the first type (Io1 and E1), the cores are assumed to consist of eutectic Fe-FeS melt with the densities ρ ₁ = 5.15 g cm^?3 (Io1) and 5.2 g cm^?3 (E1). In the models of the second type (Io3 and E3), the cores consist of FeS with an admixture of nickel and have the density ρ ₁ = 4.6 g cm^?3. The approach used here differs from that used previously both in chosen model chemical composition of these satellites and in boundary conditions imposed on the models. The most important question to be answered by modeling the internal structure of the Galilean satellites is that of the condensate composition at the formation epoch of Jupiter’s system. Jupiter’s core and the Galilean satellites were formed from the condensate. Ganymede and Callisto were formed fairly far from Jupiter in zones with temperatures below the water condensation temperature, water was entirely incorporated into their bodies, and their modeling showed the mass ratio of the icy (I) component to the rock (R) component in them to be I/R ～ 1. The R composition must be clarified by modeling Io and Europa. The models of the second type (Io3 and E3), in which the satellite cores consist of FeS, yield 25.2 (Io3) and 22.8 (E3) for the core masses (in weight %). In discussing the R composition, we note that, theoretically, the material of which the FeS+Ni core can consist in the R accounts for ～25.4% of the satellite mass. In this case, such an important parameter as the mantle silicate iron saturation is Fe# = 0.265. The Io3 and E3 models agree well with this theoretical prediction. The models of the first and second types differ markedly in core radius; thus, in principle, the R composition in the formation zone of Jupiter’s system can be clarified by geophysical studies. Another problem studied here is that of the error made in modeling Io and Europa using the Radau-Darvin formula when passing from the Love number k ₂ to the nondimensional polar moment of inertia $\bar C$ . For Io, the Radau-Darvin formula underestimates the true value of $\bar C$ by one and a half units in the third decimal digit. For Europa, this effect is approximately a factor of 3 smaller, which roughly corresponds to a ratio of the small parameters for the satellites under consideration α _Io/α _Europa ～ 3.4. In modeling the internal structure of the satellites, the core radius depends strongly on both the mean moment of inertia I* and k ₂. Therefore, the above discrepancy in $\bar C$ for Io is appreciable.     

Fast neutral particles as probes of the extent of Io's exosphere

The spatial extent of ion cyclotron waves at Io has been interpreted as requiring a multistep acceleration and transport process: exospheric ions are accelerated outward (relative to Jupiter) due to the corotation electric field, neutralized due to charge exchange in the surrounding exosphere, and then reionized after traveling far across magnetic field lines, at which point they generate the waves. The trajectories of the particles away from Io are sensitive to the location of their initial ionization. This paper examines the spatial distributions of fast neutrals produced under varying conditions in order to provide constraints on the possible structure and nature of the Io exosphere. While a rapid onset of cyclotron waves at a specific location around Io can be modeled with a single, point-source region of ions, such as might occur over a volcano, the regional extent of the waves suggests multiple or distributed sources.     

The variation of Io's auroral footprint brightness with the location of Io in the plasma torus

Ultraviolet and near-infrared observations of auroral emissions from the footprint of Io's magnetic Flux Tube (IFT) mapping to Jupiter's ionosphere have been interpreted via a combination of the unipolar inductor model [Goldreich, P., Lynden-Bell, D., 1969. Astrophys. J. 156, 59-78] and the multiply-reflected Alfvén wave model [Belcher, J.W., 1987. Science 238, 170-176]. While both models successfully explain the general nature of the auroral footprint and corotational wake, and both predict the presence of multiple footprints, the details of the interaction near Io are complicated [Saur, J., Neubauer, F.M., Connerney, J.E.P., Zarka, P., Kivelson, M.G., 2004. In: Bagenal, F., Dowling, T.E., McKinnon, W.B. (Eds.), Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press, Cambridge, UK, pp. 537-560; Kivelson, M.G., Bagenal, F., Kurth, W.S., Neubauer, F.M., Paranicas, C., Saur, J., 2004. In: Bagenal, F., Dowling, T.E., McKinnon, W.B. (Eds.), Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press, Cambridge, UK, pp. 513-536]. The auroral footprint brightness is believed to be a good remote indicator of the strength of the interaction near Io, indicating the energy and current strength linking Io with Jupiter's ionosphere. The brightness may also depend in part on local auroral acceleration processes near Jupiter. The relative importance of different physical processes in this interaction can be tested as Jupiter's rotation and Io's orbital motion shift Jupiter's magnetic centrifugal equator past Io, leading to longitudinal variations in the plasma density near Io and functionally different variations in the local field strength near Jupiter where the auroral emissions are produced. Initial HST WFPC2 observations found a high degree of variability in the footprint brightness with time, and some evidence for systematic variations with longitude [Clarke, J.T., Ben Jaffel, L., Gérard, J.-C., 1998. J. Geophys. Res. 103, 20217-20236], however the data were not of sufficient quality to determine functional relationships. In this paper we report the results from a second, more thorough study, using a series of higher resolution and sensitivity HST STIS observations and a model for the center to limb dependence of the optically thin auroral emission brightness based on measurements of the auroral curtain emission distribution with altitude. A search for correlations between numerous parameters has revealed a strong dependence between Io's position in the plasma torus and the resulting footprint brightness that persists over several years of observations. The local magnetic field strength near Jupiter (i.e. the size of the loss cone) and the expected north/south asymmetry in auroral brightness related to the path of currents generated near Io through the plasma torus en route to Jupiter appear to be less important than the total plasma density near Io. This is consistent with the near-Io interaction being dominated by collisions of corotating plasma and mass pickup, a long-standing view which has been subject to considerable debate. The brightness of the auroral footprint emissions, however, does not appear to be proportional to the incident plasma density or energy, and the interpretation of this result will require detailed modeling of the interaction near Io.     

On the distribution of sodium in the vicinity of Io

We investigate the contribution of scattering in the telescope to our measurements of the size of Io's sodium cloud and to the distribution of emission intensity in the cloud. The brightest regions, within 30″ of Io near opposition and along the equatorial plane, are relatively undistorted but regions further than 45″ away and not close to the equatorial plane are very likely to consist of mainly scattered light. Portions of the cloud in the vicinity of the magnetic equator are also mostly scattered light when Io is near extreme magnetic latitude. The equatorial torus, however, extends up to 20 arcmin from Jupiter. The large size of the cloud is thus confirmed. High-resolution line profile shapes indicate that sodium streams from Io preferentially in the forward direction with velocities distributed up to 18 km sec^?1. The observed wavelength shifts of the peak intensities from Io's rest frame are compatible with a cloud streaming through a bound atmospheric component but they could also be caused by a velocity distribution peaked at very low velocities.     

Jupiter-Io electrodynamic interaction

Strong interaction between Jupiter and its satellite Io is revealed by the control of the decametric radiation, by the distributions of energetic particles, and perhaps by the location of the boundary of Jupiter's plasmasphere near Io's magnetic flux tube. Two opposed theories of this interaction depend on different relative motions of Io and its flux tube. In one case the flux tube is frozen into Io and moves with Io, while in the plasma-sheath model Io moves freely across magnetic field lines. It is shown that the plasma-sheath model is unacceptable, and that Io must drive its flux tube through the magnetosphere. The first error in the sheath theory is in the mechanism of sheath creation by thermal and photoelectric electrons. The second error is in the neglect of electric currents driven through the external plasma by powerful space-charge fields. The third error is in the neglect of hydromagnetic effects of electric currents in Io: the magnetic perturbations, Lorentz forces and power supplied from the kinetic energy of Io. These effects show that Io's force tube is dragged along with Io. This frozenin model is discussed briefly in connection with energetic electrons, the decametric emission, Io's ionosphere and Jupiter's plasmasphere.     

Explanation of dominant oblique radio emission at Jupiter and comparison to the terrestrial case

The Io-Jupiter S-bursts are series of quasi-periodic impulsive decameter radio emissions from the magnetic flux tube connecting Jupiter to its closest galilean satellite Io. This paper discusses the possibility, suggested by previous works by Hess et al., that the S-bursts are triggered by upgoing electrons accelerated (downward) by trapped Alfvén waves, that have mirrored above the Jupiter ionosphere. According to this theory, the S-bursts would correspond to wave modes that propagate at oblique angles with respect to the magnetic field. Oblique propagation is also inferred for the more slowly varying components of Io-Jupiter radio emissions. Previous works, mainly based on observations of the terrestrial AKR, whose generation process is closely related to those of S-bursts, showed that these waves are emitted on perpendicular wave modes. This discrepancy between the Jovian and Terrestrial cases has led to a controversy about the credibility of the S-bursts model by Hess et al. In the present paper, we show that indeed, the most unstable wave modes for Earth AKR, and Io-Jupiter S-bursts, as they are seen from ground based radio-telescopes, are not the same. Several causes are evaluated: observational bias, the different degree of plasma magnetization above Earth and Jupiter, the role of a cold plasma component and of plasma auroral cavities. Furthermore, we make predictions about what kind of radiation modes a probe crossing the low altitude Io-Jupiter flux tube will see.     

MHD simulation scenarios of the stellar wind interaction with Hot Jupiter magnetospheres

We present three-dimensional numerical simulations of the interaction between a Hot Jupiter and the stellar wind plasma of its host star in the framework of resistive magnetohydrodynamics (MHD). In a first step, we investigate the numerical realization of the plasma flow around the planet and the planetary magnetic field using a simplified model, before we simulate more realistic scenarios on the basis of the stellar wind model by Weber and Davis. A main goal is to understand the magnetic interaction between star and planet. In analogy to the well-known Jupiter Io scenario, we study the development of a magnetic field-aligned current system in different parameter regimes.     

The distribution of sodium in Io's cloud: Implications

Models for the distribution of sodium in Io's vicinity and in a disk in Io's orbital plane, compared with observational data, support arguments (1) that Io is the source of the sodium, (2) that sodium is ejected from the inside hemisphere and most of the high velocity sodium which is observed is ejected from the leading inside quadrant, (3) that most of the sodium leads Io in Io's vicinity but follows Io at distances of more than 7R_j from Jupiter, (4) that a significant fraction of the sodium flux is ejected at large angles with respect to Io's orbital plane, (5) that the source velocity distribution has a pronounced high-velocity tail, and (6) that impact ionization by electrons is significant at large distances from Io.     

Optical properties of solid and liquid sulfur at visible and infrared wavelengths

Knowledge of the optical constants of elemental sulfur has potential applications to Venus, Jupiter, Io, Amalthea, and the Earth. The real part, n, of the index of refraction of liquid sulfur (at 133°C) and of solid orthorhombic sulfur (at 25°C) for the wavelength range 0.4–2.0 μm were measured ellipsometrically. The imaginary part, k, of the refractive index of liquid sulfur was obtained by transmittance measurements at the same temperature and wavelength range. The reflectance of semi-infinite slabs of solid and liquid sulfur is calculated using the measured n and k values. We confirm that sulfur melts on Io would be classified as “black” by the Voyager imaging system.     

Io's sodium emission cloud

Strong evidence that Io's sodium emission is due to resonant scattering is given by our observations which show a monotonic increase of emission intensity with residual solar intensity. In addition we detected no emission during three eclipse observations of Io. We propose a resonant scattering model with two spacial components comprising an optically thick atmosphere extending 10³ km above Io's surface surrounded by an optically thin cloud which forms a partial torus around Jupiter. In this model a flux of 10⁷ cm^?2 sec^?1 sodium atoms are sputtered from Io's surface by heavy energetic ions which are accelerated in a plasma sheath around Io. The atoms sputtered from the surface collide with atoms in Io's atmosphere so the equipartition of kinetic energy is established. The total sodium abundance is about 3 × 10¹³ cm^?2. During Io's day, sodium and other atmospheric constituents are ionized, giving rise to the ionosphere observed by Pioneer 10. Atoms escape by means of Jeans escape from the critical level, which is at the top of the atmosphere and the base of the cloud. We have observed sodium emission 6arcsec (6 Io diameters) above and below Io's orbital plane and 23arcsec toward Jupiter in Io's orbital plane. No emission was detected at maximum elongation 180° from Io. We interpret these results to mean that atoms escaping from Io form a partial torus whose thickness is about 12 arcsec and whose length is at least one-fifth of Io's orbital circumference.     

Observations of Electromagnetically Coupled Dust in the Jovian Magnetosphere

We report on dust measurements obtained during the seventh orbit of the Galileo spacecraft about Jupiter. The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by more than an order of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. In addition to the 5 and 10 h periodicities, a longer period which is compatible with Io's orbital period is evident in the dust impact rate. This feature indicates that Io most likely is the source of the dust streams. During a close (3,095 km altitude) flyby at Ganymede on 5 April 1997 an enhanced rate of dust impacts has been observed, which suggests that Ganymede is a source of ejecta particles. Within a distance of about 25 R_J(Jupiter radius, R_J= 71,492 km) from Jupiter impacts of micrometer-sized particles have been recorded which could be particles on bound orbits about Jupiter. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Io-controlled decametric radiation

The (magnetic) amplitude of the Alfvén waves emitted by Io is related to the growth rate of coherent cyclotron radiation. The growth rate is large only in the dense parts of the Jovian ionosphere. The amplitude varies as a function of sub-Io longitude. This together with the beaming of the cyclotron radiation is used to explain the observed emission pattern of Io-controlled decametric radiation from Jupiter.     

On the surface composition of Io

The wavelength dependence of the reflectivity of Io indicates the presence of two materials on the surface of this satellite of Jupiter. These materials are sulfur and an unspecified material (R₁) which shows a wavelength dependence of its reflectivity for 0.3 μm < λ < 1.0 μm similar to the non-H₂O frost spectrum of the rings of Saturn. A 60/40 admixture of these two spectra matches the observed reflection spectrum of Io from 0.3μm–3 μm, if the spectrum of R₁ is featureless for λ > 1 μm. Sulfur will give rise to a posteclipse brightening. The variation with wavelength of the temperature dependence of the reflectivity of sulfur will allow an observational confirmation of the presence of sulfur on Io. The material R₁ should show a large geometrical albedo. The translucency of sulfur is consistent with the polarization-phase curve to Io. The material R₁ is also required to be translucent. The thermal conductivity of a cooled sulfur powder under vacuum was measured and found to agree with the value determined for the upper layer of Io from observations at 10 μm. It is shown that this agreement is not necessarily meaningful.     

Thermal history of the Jovian satellite Io

The discovery of large volcanic eruptions on Io suggests that Io is one of the most geologically active planetary bodies. The energy source of this geologic activity is believed to be tidal heating induced by Jupiter. A number of thermal history calculations were done to investigate the effect of tidal heating on the thermal history of Io taking into account solid state convection and advective heat transfer. These simulations show that the total tidal heating energy in Io is almost equal to the advectively transferred heat, indicating that the observed heat flow from Io is nearly equal to the total tidal heating energy. Since total tidal heating energy is dependent on the radius of the liquid mantle and the internal dissipation factor (Q), the radius of the liquid mantle can be estimated for a given value of Q. Some reasonable thermal history models of Io were obtained using a model with Q ≈ 25–50 in which the magma source of Ionian volcanism is at a depth of 100–300 km. The models satisfy the heat flow data and the existence of a thick lithosphere. Using a model with Q = 25 and L = 300 km (thickness of the advective region) as the standard model (model II), we then studied the effect of convective heat transfer and the initial temperature distribution on the Ionian thermal history. In these calculations, the other parameters are the same as in the standard model (model II). These calculations show that although the temperature distribution in the central region reflects the difference in the efficiency of convective heat transfer and initial temperature distribution, the temperature distribution in the outer region does not changes appreciably.     

Can late-type active stars be explained by a dipole magnetic trap?

In this paper we review four different types of X-ray and/or radio observations of active late-type stars. We then consider if a single magnetic source configuration – a toroidal dipole magnetic trap – can possibly explain these various different observations. We conclude that, indeed, dipole magnetic confinement (similar to the magnetic configurations of the Earth's radiation belts and the case of Jupiter and the Io torus) can explain all the diverse observational data. We take this to be very strong observational support for this type of magnetic confinement scheme. We also consider that this magnetic configuration is only likely to be established and maintained in the most active stars.     

Decametric modulation lanes as a probe for inner jovian magnetosphere

We use the specific scintillations of jovian decametric radio sources (modulation lanes), which are produced by plasma inhomogeneities in the vicinity of that planet, to probe the inner magnetosphere of Jupiter. The positions and frequency drift of 1762 lanes have been measured on the DAM spectra from archives. A special 3D algorithm is used for space localization of field-aligned magnetospheric inhomogeneities by the frequency drift of modulation lanes. As a result, the main regions of the lane formation are found: the Io plasma torus; the magnetic shell of the Gossamer Ring at Thebe and Amalthea orbits; and the region above the magnetic anomaly in the northern magnetosphere. It is shown that modulation lanes reveal the depleted magnetic tubes in practically unvisited, innermost regions of the jovian magnetosphere. The local and probably temporal plasma enhancement is found at the magnetic shell of Thebe satellite. Hence, the modulation lanes are a valuable instrument for remote sensing of those parts of jovian magnetosphere, which are not studied yet in situ.     

Cassini-VIMS at Jupiter: solar occultation measurements using Io

We report unusual and somewhat unexpected observations of the jovian satellite Io, showing strong methane absorption bands. These observations were made by the Cassini VIMS experiment during the Jupiter flyby of December/January 2000/2001. The explanation is straightforward: Entering or exiting from Jupiter's shadow during an eclipse, Io is illuminated by solar light which has transited the atmosphere of Jupiter. This light, therefore becomes imprinted with the spectral signature of Jupiter's upper atmosphere, which includes strong atmospheric methane absorption bands. Intercepting solar light refracted by the jovian atmosphere, Io essentially becomes a “mirror” for solar occultation events of Jupiter. The thickness of the layer where refracted solar light is observed is so large (more than 3000 km at Io's orbit), that we can foresee a nearly continuous multi-year period of similar events at Saturn, utilizing the large and bright ring system. During Cassini's 4-year nominal mission, this probing technique should reveal information of Saturn's atmosphere over a large range of southern latitudes and times.