Satellite Io Research Articles

Structure of the Magnetic Field of the Jovian Magnetosphere

We exactly solved the problem of the interaction between the rotating magnetic field of Jupiter and the equatorial plasma disk formed by the gases flowing from the Jovian satellite Io. The disk is shown to expel the Jovian magnetic field in both directions, inward, toward Jupiter, compressing its dipole magnetic field, and outward. Jupiter spins up the disk up to velocities that correspond to nearly constant angular rotation, but with an angular frequency lower than the angular frequency of Jupiter itself. The radial velocity of the plasma in the disk approaches its azimuthal velocity. We determined the power of Jupiter’s rotational energy losses. Part of this energy is transferred to the disk, and the other part goes into heating the Jovian ionosphere. We show that the Pedersen surface conductivity of the Jovian ionosphere must have a lower limit to maintain the electric current that arises in the disk-rotating magnetic field system. This current in the Jovian magnetosphere flows only along the preferential magnetic surfaces that connect the inner and outer edges of the disk to the ionosphere.

Fast radio imaging of Jupiter's magnetosphere at low-frequencies with LOFAR

Jupiter emits intense decameter (DAM) radio waves, detectable from the ground in the range ∼ 10–40 MHz. They are produced by energetic electron precipitations in its auroral regions (auroral-DAM), as well as near the magnetic footprints of the Galilean satellite Io (Io-DAM). Radio imaging of these decameter emissions with arcsecond angular resolution and millisecond time resolution should provide: (1) an improved mapping of the surface planetary magnetic field, via imaging of instantaneous cyclotron sources of highest frequency; (2) measurements of the beaming angle of the radiation relative to the local magnetic field, as a function of frequency; (3) detailed information on the Io–Jupiter electrodynamic interaction, in particular the lead angle between the Io flux tube and the radio emitting field line; (4) direct information on the origin of the sporadic drifting decameter S-bursts, thought to be electron bunches propagating along magnetic field lines, and possibly revealing electric potential drops along these field lines; (5) direct observation of DAM emission possibly related to the Ganymede–Jupiter, Europa–Jupiter and/or Callisto–Jupiter interactions, and their energetics; (6) information on the magnetospheric dynamics, via correlation of radio images with ultraviolet and infrared images of the aurora as well as of the Galilean satellite footprints, and study of their temporal variations; (7) an improved mapping of the Jovian plasma environment (especially the Io torus) via the propagation effects that it induces on the radio waves propagating through it (Faraday rotation, diffraction fringes, etc.); (8) possibly on the long-term a better accuracy on the determination of Jupiter's rotation period. Fast imaging should be permitted by the very high intensity of Jovian decameter bursts. LOFAR's capability to measure the full polarization of the incoming waves will be exploited. The main limitation will come from the maximum angular resolution reachable. We discuss several approaches for bringing it close to the value of ∼ 1 ″ at 30–40 MHz, as required for the above studies.

New accurate ephemerides for the Galilean satellites of Jupiter

We present a new model of the four Galilean satellites Io, Europa, Ganymede and Callisto, able to deliver accurate ephemerides over a very long time span (several centuries). In the first paper (Lainey et al. 2004, A&A, 420, 1171) we gave the equations of the dynamical model. Here we present the fit of this model to the observations, covering more than one century starting from 1891. Our ephemerides, based on this first fit called L1, are available on the web page of the IMCCE at the URL http://www.imcce.fr/ephemeride_eng.html.

A theory of the equilibrium figure and gravitational field of the Galilean satellite Io: The second approximation

We construct a theory of the equilibrium figure and gravitational field of the Galilean satellite Io to within terms of the second order in the small parameter α. We show that to describe all effects of the second approximation, the equation for the figure of the satellite must contain not only the components of the second spherical function, but also the components of the third and fourth spherical functions. The contribution of the third spherical function is determined by the Love number of the third order h3, whose model value is 1.6582. Measurements of the third-order gravitational moments could reveal the extent to which the hydrostatic equilibrium conditions are satisfied for Io. These conditions are J3=C32=0 and C31/C33=−6. We have calculated the corrections of the second order of smallness to the gravitational moments J2 and C22. We conclude that when modeling the internal structure of Io, it is better to use the observed value of k2 than the moment of inertia derived from k2. The corrections to the lengths of the semiaxes of the equilibrium figure of Io are all positive and equal to ∼64.5, ∼26, and ∼14 m for the a, b, and c axes, respectively. Our theory allows the parameters of the figure and the fourth-order gravitational moments that differ from zero to be calculated. For the homogeneous model, their values are:\(s_4 = \frac{{885}}{{224}}\alpha ^2 ,s_{42} = - \frac{{75}}{{224}}\alpha ^2 ,s_{44} = \frac{{15}}{{896}}\alpha ^2 ,J_4 = - \frac{{885}}{{224}}\alpha ^2 ,C_{42} = \frac{{75}}{{224}}\alpha ^2 ,C_{44} = \frac{{15}}{{896}}\alpha ^2 \).

New accurate ephemerides for the Galilean satellites of Jupiter

We present a new theory of the four Galilean satellites Io, Europa, Ganymede and Callisto. This theory aims to deliver highly accurate ephemerides able to represent the Galilean satellites' motion over several centuries. It is based on the numerical integration of elaborated equations of motion. This first paper describes and tests many usually neglected perturbations. We are then able to retain some of them in the dynamical model for the Galilean system. A numerical method developed to adjust the model to the observations is given. We used a general formalism so it can be extended to systems other than the Galilean one. As an example of this method, we compare our model to the current E5 ephemerides of the four Galileans.

Io as the trigger of energetic electron disturbances in the inner Jovian magnetosphere

Io adds approximately a ton of mass per second to the Jovian magnetosphere, mass that is consequently energized to circle Jupiter at the corotation velocity with a commensurate thermal velocity. The plasma moves radially outward and eventually is lost down the tail. The mass addition process and its subsequent transport are far from uniform and are thought to be ultimately responsible for driving much of the dynamics of the Jovian magnetosphere. One of the manifestations of this dynamic behavior is a disturbance of the energetic electron fluxes measured by the Galileo spacecraft. When we examine disturbances in the dipolar portion of the magnetosphere, that we conservatively take to be within 12 Jovian radii ( R J ), we find that energetic electron events are more often seen on passes in which Io is close to Galileo's perijove longitude. The majority of these events occur when Galileo is within ±25° of Io. For those events for which injection longitudes can be calculated, the injection point is consistent with the longitude of Io at the time of injection. This correlation suggests that disturbances initiated by Io are affecting energetic particles as far out as jovicentric distances of 12 R J , 6 R J outside the Io orbit.

Longitude variation of ion temperature in the Io plasma torus

The Io plasma torus shows an unanticipated variation of ion temperature as a function of System III (magnetic) longitude λIII. The simple expectation for the perpendicular temperature T⊥, based on ion pick‐up in a tilted dipole magnetic field, is a double‐peaked function with two maxima and two minima per 360° of longitude, having an average value of ∼480 eV for S+ and a relatively small variation amplitude of ∼±7 eV. Instead, the observed T⊥(S+) near Io's orbit has only one maximum and one minimum, with a much smaller average value of ∼30 eV and a much larger relative variation amplitude of ∼±50%. To explain the observed temperature variation, we propose that the longitude variation of the Pedersen conductance of Jupiter's ionosphere causes the pickup speed of ions injected near Io to vary with longitude. We obtain a satisfactory fit to the observed temperature variation if we make the straightforward assumption that the ionospheric conductance varies inversely with the strength of the ionospheric magnetic field. Both the magnitude and phase of the observed T⊥ variation are consistent with this simple model. The observed parallel temperature T∥(S+) has a somewhat smaller average magnitude and a similar longitude variation, but shifted about 70° in phase toward larger λIII. The T∥ behavior is consistent with the idea that T∥ derives mainly from T⊥ through pitch‐angle scattering in the presence of the observed corotation lag of torus plasma. The 70° phase shift requires a scattering timescale of ∼50 hours, consistent with the classical timescale for ion‐ion Coulomb scattering.

Physical Electron Belt Model from Jupiter's surface to the orbit of Europa

The three‐dimensional model, Salammbô‐3D, which was initially developed to model the Earth's radiation belts, has been adapted to Jupiter's radiation environment. As a first step, this model has been validated between L = 1 and L = 6 (L: MacIlwain parameter), just inside Io's obit. In order to extend our three‐dimensional (3‐D) code up to L = 9.5, just inside Europa's orbit, a more realistic magnetic field than the dipole field used before has been introduced in Salammbô. Two magnetic field models are available: The model of Connerney [1981] and the one of Khurana [1997]. Both of them are composed of two parts: An internal magnetic field, intrinsic to the planet, and an external magnetic field, due to the current sheet. Results deduced from Salammbô‐3D, using these two different models, will be shown and compared. Then, to validate our 3‐D code, from Jupiter up to Europa's orbit, comparisons between simulations and two kinds of observations will be done. Firstly, Salammbô results will be compared with spacecraft data (Pioneer 10 and 11) and secondly with radio observations (Very Large Array: VLA). Indeed, with the help of Salammbô‐3D and a synchrotron model, two‐dimensional images of Jupiter's synchrotron emission can be deduced. It is then possible to investigate the global radiation belt shape by comparing simulations and VLA observations. Two important results emerge from this study. First, the extension of our model outside Io's orbit aims to show that Io does not play any role on relativistic electron dynamics, i.e., it does not create losses of particles like the inner moons (Metis, Adrastea, Amalthea, and Thebe). The second important result is that contrary to Io, Europa seems to play a significant role in determining the electron distribution in the Jovian radiation belts.

Io as the trigger of energetic electron disturbances in the inner Jovian magnetosphere

Io adds approximately a ton of mass per second to the Jovian magnetosphere, mass that is consequently energized to circle Jupiter at the corotation velocity with a commensurate thermal velocity. The plasma moves radially outward and eventually is lost down the tail. The mass addition process and its subsequent transport are far from uniform and are thought to be ultimately responsible for driving much of the dynamics of the Jovian magnetosphere. One of the manifestations of this dynamic behavior is a disturbance of the energetic electron fluxes measured by the Galileo spacecraft. When we examine disturbances in the dipolar portion of the magnetosphere, that we conservatively take to be within 12 Jovian radii ( R J), we find that energetic electron events are more often seen on passes in which Io is close to Galileo's perijove longitude. The majority of these events occur when Galileo is within ±25° of Io. For those events for which injection longitudes can be calculated, the injection point is consistent with the longitude of Io at the time of injection. This correlation suggests that disturbances initiated by Io are affecting energetic particles as far out as jovicentric distances of 12 R J, 6 R J outside the Io orbit.

Nature of the iogenic plasma source in Jupiter's magnetosphere: I. Circumplanetary distribution

Three-dimensional calculations are presented for the circumplanetary nature of the iogenic plasma source (pickup ions produced by electron and charge exchange processes in the plasma torus) created by O and S gases located above Io's exobase in its corona and escaping extended neutral clouds (designated as the “Outer Region”). These calculations are undertaken using neutral cloud models for O and S with realistic incomplete collisional cascade source velocity distributions and rates at Io's exobase and realistic spacetime loss processes in the plasma torus. The resulting spatial distributions for O and S about Jupiter are highly peaked at Io but extend at much lower density levels all about the planet, particularly within Io's orbit where they may play a role in the pitch angle scattering and energy loss of radially inward diffusing energetic electrons for the synchrotron radiation belts of Jupiter, in producing bite-outs in the energy distribution of energetic heavy ions near Io's orbit, and in providing a charge exchange source for energetic neutral atoms (ENAs) detected both near and far from Jupiter. For the iogenic plasma source created by these neutrals, two-dimensional distributions produced by integrating the three-dimensional information along the magnetic field lines are presented for the instantaneous values of the pickup ion rates, the total- and net-mass loading rates, the mass-per-unit-magnetic-flux source rate, the pickup conductivity, the pickup radial current, and the pickup ion power (or energy rate). On the circumplanetary spatial scale, the instantaneous iogenic plasma source is highly peaked about Io's position on its orbit around Jupiter. The degree of orbital asymmetry and its physical origin are discussed, and overall spatially integrated rates are presented. The spatially integrated net-mass loading rate is 154 kg s −1 and the total (electron impact and charge exchange) mass loading rate is 275 kg s −1. Rough minimum estimates are made for the spatially integrated total-mass loading rate created by the “Inner Region” (spatial region below Io's exobase) and are at least ∼1 to 2.5 times larger than that for the Outer Region. Implications of the iogenic plasma source created by the Outer Region and the Inner Region are discussed.

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.

Volcanically emitted sodium chloride as a source for Io's neutral clouds and plasma torus.

The atmosphere of Jupiter's satellite Io is extremely tenuous, time variable and spatially heterogeneous. Only a few molecules--SO2, SO and S2--have previously been identified as constituents of this atmosphere, and possible sources include frost sublimation, surface sputtering and active volcanism. Io has been known for almost 30 years to be surrounded by a cloud of Na, which requires an as yet unidentified atmospheric source of sodium. Sodium chloride has been recently proposed as an important atmospheric constituent, based on the detection of chlorine in Io's plasma torus and models of Io's volcanic gases. Here we report the detection of NaCl in Io's atmosphere; it constitutes only approximately 0.3% when averaged over the entire disk, but is probably restricted to smaller regions than SO2 because of its rapid photolysis and surface condensation. Although the inferred abundance of NaCl in volcanic gases is lower than predicted, those volcanic emissions provide an important source of Na and Cl in Io's neutral clouds and plasma torus.

Observations of planetary satellites with ISO

Several observational programmes were conducted with ISO (Kessler et al., 1996) aiming at the investigation of the near- and far- infrared spectrum of the satellites of the giant planets. Thus, Jupiter's satellites Callisto, Io and Ganymede were explored mainly with the spectrometers, while the spectrum of Titan, Saturn's largest satellite, was investigated thoroughly by all the instruments. The analysis of the data has provided original and precious information on the satellites' surfaces and Titan's atmosphere in particular.

The Io current wing

Jupiter's satellite Io produces a standing magnetospheric disturbance, variously called the Alfvén wing or the Io flux tube, which carries current away from the satellite along the magnetic field and downstream. The physics governing this phenomenon remains controversial. This paper generalizes previous analytic treatments to include effects of plasma compressibility. The nonlinear MHD equations are shown to be equivalent to a nonlinear second‐order partial differential equation for a single scalar function of position in the plane transverse to the wing. This equation bears a strong resemblance to the velocity potential equation for two‐dimensional compressible flow in an ideal fluid. In certain limits the resemblance is exact, which permits approximate analytic solutions of the fluid case to be used to explore the properties of the compressible magnetofluid. The derivation allows translational symmetry in any direction, unlike the Alfvén‐wing model which requires symmetry along an Alfvén characteristic. The wing current is governed by an effective conductance that generalizes the Alfvén wing conductance but is flexible enough to maintain a constant value in regions with different plasma densities. Hence this formalism suggests how the local plasma can adjust to provide a continuous current along the wing, all the way from the dense torus, through the rarified high‐latitude regions, to Jupiter's ionosphere.

Discovery of Soft X‐Ray Emission from Io, Europa, and the Io Plasma Torus

We report the discovery of soft (0.25-2 keV) X-ray emission from the Galilean satellites Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O, and S ions from the region of the IPT seems to be the likely source of the X-ray emission from the Galilean satellites. According to our estimates, fluorescent X-ray emission excited by solar X-rays, even during flares from the active Sun, charge-exchange processes, previously invoked to explain Jupiter's X-ray aurora and cometary X-ray emission, and ion stripping by dust grains fail to account for the observed emission. On the other hand, bremsstrahlung emission of soft X-rays from nonthermal electrons in the few hundred to few thousand eV range may account for a substantial fraction of the observed X-ray flux from the IPT.

Effects of MHD slow shocks propagating along magnetic flux tubes in a dipole magnetic field

Abstract. Variations of the plasma pressure in a magnetic flux tube can produce MHD waves evolving into shocks. In the case of a low plasma beta, plasma pressure pulses in the magnetic flux tube generate MHD slow shocks propagating along the tube. For converging magnetic field lines, such as in a dipole magnetic field, the cross section of the magnetic flux tube decreases enormously with increasing magnetic field strength. In such a case, the propagation of MHD waves along magnetic flux tubes is rather different from that in the case of uniform magnetic fields. In this paper, the propagation of MHD slow shocks is studied numerically using the ideal MHD equations in an approximation suitable for a thin magnetic flux tube with a low plasma beta. The results obtained in the numerical study show that the jumps in the plasma parameters at the MHD slow shock increase greatly while the shock is propagating in the narrowing magnetic flux tube. The results are applied to the case of the interaction between Jupiter and its satellite Io, the latter being considered as a source of plasma pressure pulses.

Dark ring in southwestern Orientale Basin: Origin as a single pyroclastic eruption

A large, 154 km diameter dark annular ring, located in the southwestern part of the Orientale Basin along the Montes Rook ring, was first discovered and documented in Soviet Zond 8 images. Clementine UV‐visible multispectral data indicate that the dark ring consists of material similar to the pyroclastic glasses collected by the Apollo astronauts, with the glasses being more closely related to the orange glass beads that comprise the Aristarchus Plateau than to the crystallized black beads typical of Taurus‐Littrow. This implies relatively rapid cooling times for the eruption products. We propose that the dark ring is the manifestation of a pyroclastic eruption originating at a fissure vent, an elongate 7.5 km by 16 km depression, located near the center of the ring. The event producing the eruption began with a dike rapidly emplaced from subcrustal depths to within ∼3–4 km of the surface. The dike stabilized and degassed over ∼1.7 years to form an upper foam layer which then penetrated to the surface to cause an eruption, lasting ∼1–2 weeks. The eruption produced a ∼38 km high symmetrical spray of pyroclasts into the lunar vacuum at velocities of ∼350 to ∼420 m/s, and the pyroclastic material accumulated in a symmetrical ring around the vent. The geometry of eruption caused the deposits to accumulate preferentially in a ring representing the material ejected at 45°. The paucity of pyroclastic rings of this type on the Moon can be attributed to the low probability of a dike stalling at just the right depth (∼3–4 km) to create these eruption conditions. The detailed characteristics of this ring provide important new insight into the emplacement of pyroclastics in the more regionally continuous lunar dark mantle deposits, suggesting that their sources are dominated by effusive and Hawaiian‐style eruptions. The Orientale dark ring deposit has similarities to the pyroclastic rings on the Galilean satellite Io. Both the Orientale dark ring and Ionian eruptions involve acceleration of small particles to a high velocity in an expanding gas stream, silicate pyroclasts derived from the disrupted foam layer in the lunar case, and a mixture of silicate pyroclasts and “snowflakes” of condensing SO2 solids on Io. In both cases the silicate pyroclasts ejected from the vent are accelerated by the gas until they become decoupled and continue on ballistic trajectories controlled only by gravity. Differences in mass flux and cloud opacity between the Moon and Io are the direct result of the origin and mass fraction of the volatile phase: the accumulation of a magmatic foam layer on top of a silicate intrusion in the lunar case and the intimate mixing on Io of a steadily erupting magma and liquid SO2. The interpretation of the Orientale dark mantle ring as a pyroclastic eruption provides an alternative to the hypothesis that the dark ring represents the presence of an ancient pre‐Orientale impact structure and that the Orientale Basin cavity of excavation must therefore lie within the Outer Rook Mountain ring.

Galileo NIMS Observations of Io

AbstractThe Near Infrared Mapping Spectometer (NIMS) on the Galileo spacecraft has been observing the volcanic Jovian satellite Io at regular intervals since June 1996. These infrared observations have allowed detailed mapping of the distribution and grain size of sulfur dioxide on Io’s surface, identification of volcanic centers, mapping of the distribution of hot spots, and investigations into the style and evolution of individual volcanic eruptions.

Foreword [to Special Section on Plasma Interaction With Io's Atmosphere

The Jovian moon Io resides in one of the harshest space environments yet visited by spacecraft. Indeed, the energetic particles that make Jupiter's inner magnetosphere interesting to the space physicist give space engineers nightmares. After the initial orbit‐insertion passage in December 1995, the Galileo spacecraft was deliberately kept well outside the orbit of Io and the plasma torus. After 20 plus successful orbits exploring the outer 3 Galilean moons, the spacecraft's perijove was reduced to allow 3 close flybys of Io in late 1999 to early 2000.

The Io mass‐loading disk: Model calculations

The observations of ion cyclotron waves up to 0.5 RJ beyond the orbit of Io are best explained by the presence of a thin disk of fast neutrals whose ionization and pickup provide the free energy for the waves. We extend the model of Wilson and Schneider [1999] in order to explain the observed properties of this mass‐loading region, especially the most recent Galileo observations near Io. In the extended model, some of the molecules of sulfur compounds in Io's exobase are first ionized by photoionization, impact ionization, and charge exchange. These charged particles are accelerated in the corotation electric field associated with the motion of the magnetized Io torus plasma that is moving through its exosphere. After a period of acceleration the heavy ions are neutralized by charge exchange with other exospheric neutral particles or combined with local electrons. These newly neutralized particles continue with high velocities similar to those of their former charged state, moving only under the influence of the gravity fields of Jupiter and Io, not affected by the electric and magnetic field. If they do not impact Io or its atmosphere, these neutral particles can propagate large distances across the magnetic field before they are reionized. Eventually, the reionized particles are lost by dissociation. Characteristic Io mass‐loading particle distributions, such as high torus plasma density outside Io's orbit and the lower density inside, and the directional feature of the mass‐loading neutral cloud, are qualitatively reproduced in the model. Meanwhile, the configuration of the mass‐loading region, in which the ion cyclotron waves are observed, is obtained, and the results are consistent with Galileo wave observations. Three parameters are found to control the structure of the neutral and ion loading disks: the characteristic lifetimes of the initially created ions, of the neutral molecules, and of the ions generated by neutral particle reionization. In addition, particle source geometry, gravity, and the near Io field configurations play important roles in the Io mass‐loading process.