A cometary ionosphere model for Io

The ionosphere of Jupiter's satellite Io, discovered by the Pioneer 10 radio-occultation experiment, cannot easily be understood in terms of a model of a gravitationally bound, Earth-like ionosphere. Io's gravitational field is so weak that a gravitationally bound ionosphere would probably be blown away by the ram force of the Jovian magnetospheric wind — i.e., the plasma corotating in the Jovian magnetosphere. We propose here a model in which the material for Io's atmosphere and ionosphere is drawn from the ionosphere of Jupiter through a Birkeland current system that is driven by the potential induced across Io by the Jovian corotation electric field. We argue that the ionization near Io is caused by a comet-like interaction between the corotating plasma and Io's atmosphere. The initial interaction employs the critical velocity phenomenon proposed many years ago by Alfvén. Further ionization is produced by the impact of Jovian trapped energetic electrons, and the ionization thus created is swept out ahead of Io in its orbit. Thus, we suggest that what has been reported as a day-night ionospheric asymmetry is in fact an upstream-downstream asymmetry caused by the Jovian magnetospheric wind.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30th May, 1978.     

Lead angles and emitting electron energies of Io-controlled decameter radio arcs

The Io-controlled radio arcs are emissions in the decametric radio range which appear arc shaped in the time-frequency plane. Their occurrence is controlled by Io's position, so it has been for long inferred that they are powered by the Io-Jupiter electrodynamic interaction. Their frequency ranges correspond to the electron cyclotron frequencies along the Io Flux tube, so they are expected to be generated by cyclotron maser instability (CMI). The arc shape was proposed to be a consequence of the strong anisotropy of the decametric radio emissions beaming, combined with the topology of the magnetic field in the source and the observation geometry. Recent papers succeeded at reproducing the morphologies of a few typical radio arcs by modeling in three dimensions the observation geometry, using the best available magnetic field model and a beaming angle variation consistent with a loss-cone driven CMI. In the continuation of these studies, we present here the systematic modeling of a larger number of observations of the radio arcs emitted in Jupiter's southern hemisphere (including multiple arcs or arcs exhibiting abrupt changes of shape), which permits to obtain a statistical determination of the emitting field line localization (lead angle) relative to the instantaneous Io field line, and of the emitting particle velocities or energies. Variations of these parameters relative to Io's longitude are also measured and compared to the location of the UV footprints of the Io-Jupiter interaction. It is shown that the data are better organized in a reference frame attached to the UV spot resulting from the main Alfvén wing resulting from the Io-Jupiter interaction. It is proposed that the radio arcs are related to the first reflected Alfvén wing rather than to the main one.     

Electric potential jumps in the Io-Jupiter flux tube

The Io flux tube (IFT), along which Io interacts with the Jovian magnetosphere, is the place of plasma acceleration processes resulting in auroral like emissions, in UV, IR and Radio emissions in the decameter range. At Earth, the study of the acceleration processes is mainly made by in situ measurements. Acceleration processes at Jupiter were first deduced from the observation of a particular kind of decameter radio emissions from the IFT: the short (S-)bursts. These radio bursts present a negative drift in the time-frequency domain, which is related to the motion of the energetic electrons which produce them. The measure of their drift thus permits the kinetic energy of the electrons to be obtained, as well as its variations along the IFT which have been interpreted as electric potential jumps. Using an enhanced S-burst detection and drift measurement method, more than 1 h of quasi-continuous decametric emissions recorded at the Kharkov UTR-2 radiotelescope have been analyzed. We observe the evolution of the electron kinetic energy with the longitude of Io with a resolution of , and detect the presence of acceleration structures with characteristics being consistent with electric potential jumps of few hundred volts, and moving along the IFT in the upward direction (toward Io) at the local sound velocity.     

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.     

Cassini observations of Io's visible aurorae

More than 500 images of Io in eclipse were acquired by the Cassini spacecraft in late 2000 and early 2001 as it passed through the jovian system en route to Saturn (Porco et al., 2003, Science 299, 1541-1547). Io's bright equatorial glows were detected in Cassini's near-ultraviolet filters, supporting the interpretation that the visible emissions are predominantly due to molecular SO₂. Detailed comparisons of laboratory SO₂ spectra with the Cassini observations indicate that a mixture of gases contribute to the equatorial emissions. Potassium is suggested by new detections of the equatorial glows at near-infrared wavelengths from 730 to 800 nm. Neutral atomic oxygen and sodium are required to explain the brightness of the glows at visible wavelengths. The molecule S₂ is postulated to emit most of the glow intensity in the wavelength interval from 390 to 500 nm. The locations of the visible emissions vary in response to the changing orientation of the external magnetic field, tracking the tangent points of the jovian magnetic field lines. Limb glows distinct from the equatorial emissions were observed at visible to near-infrared wavelengths from 500 to 850 nm, indicating that atomic O, Na, and K are distributed across Io's surface. Stratification of the atmosphere is demonstrated by differences in the altitudes of emissions at various wavelengths: SO₂ emissions are confined to a region close to Io's surface, whereas neutral oxygen emissions are seen at altitudes that reach up to 900 km, or half the radius of the satellite. Pre-egress brightening demonstrates that light scattered into Jupiter's shadow by gases or aerosols in the giant planet's upper atmosphere contaminates images of Io taken within 13 minutes of entry into or emergence from Jupiter's umbra. Although partial atmospheric collapse is suggested by the longer timescale for post-ingress dimming than pre-egress brightening, Io's atmosphere must be substantially supported by volcanism to retain auroral emissions throughout the duration of eclipse.     

Formation of whistler ducts

A theory of whistler duct formation is presented. By means of order of magnitude calculations it is shown that, when the ring current overlaps the outer plasmasphere, irregularities will cause field-aligned currents to flow, which are below the threshold sensitivity of satellite-borne magnetometers. These currents must be continuous with horizontal ionospheric currents, which produce horizontal electric fields. These fields map up to the equatorial plane and are large enough to produce flux tube interchange and hence the formation of whistler ducts in the outer plasmasphere.     

On the structure of the Io torus

It is now recognized that a number of neutral-plasma interaction processes are of great importance in the formation of the Io torus. One effect not yet considered in detail is the charge exchange between fast torus ions and the atmospheric neutrals producing fast neutrals energetic enough to escape from Io. Since near Io the plasma flow is reduced, the neutrals of charge exchange origin are not energetic enough to leave the Jovian system; these neutrals are therefore distributed over an extensive region as indicated by the sodium cloud. It is estimated here that the total neutral injection rate can reach 10²⁷ s^?1 if not more. New ions subsequently created in the distributed neutral atomic cloud as a result of charge exchange or electron impact ionization are picked up by the corotating magnetic field. The pick-up ions are hot with initial gyration speed near the corotation speed. The radial current driven by the pickup process cannot close in the torus but must be connected to the planetary ionosphere by field-aligned currents. These field-aligned currents will flow away from the equator at the outer edge of the neutral cloud and towards it at the inner edge. We find that the Jovian ionospheric photoelectrons alone cannot supply the current flowing away from the equator, and torus ions accelerated by a parallel electric field could be involved. The parallel potential drop is estimated to be several kV which is large enough to push the torus ions into the Jovian atmosphere. This loss could explain the sharp discontinuous change of flux tube content and ion temperature at L = 5.6 as well as the generation of auroral type hiss there. Finally we show that the inner torus should be denser at system III longitudes near 240° as a result of the enhanced secondary electron flux in this region. This effect may be related to the longitudinal brightness variation observed in the SII optical emissions.     

Are Io's Alfvén wings filamented? Galileo observations

The interaction of Io with the Jovian magnetosphere generates auroral and radio emissions. The underlying electron acceleration process is not understood and few observations exist to constrain the theoretical models. The source of energy for the electron acceleration is in all likelihood supplied from the Alfvén wings that stretch out from both poles of Io into the two Jovian hemispheres. The form of the current system associated with the Alfvén wings has been disputed, some suggesting that the greatly slowed flow near Io implies that a steady current loop links Io to Jupiter's ionosphere, others arguing that the return waves appear only downstream of Io and others suggesting that both forms develop. Given the finite inclination of the Alfvén wings implied by the finite value of the Alfvén Mach number and the strong reflection that occurs at the boundary of the Io torus, we argue that no steady current loop can be invoked between Io and Jupiter's ionosphere. However, the energetics of the auroral and radio emissions imply that most of the energy in the Alfvén wings is transformed into electron acceleration at high-latitudes, that is, outside the Io torus. The dilemma then is to understand how a large fraction of the power penetrates the reflecting boundary. We present data from Galileo's multiple flybys of Io that suggest that the coupling with the Jovian ionosphere is mediated by filamentary Alfvén wings associated with electromagnetic waves propagating out of the torus. In particular, we report on the systematic observation, within the cross-section of Io's Alfvén wings and in their immediate vicinity, of intense electromagnetic waves at frequencies up to several times the proton gyrofrequency. We interpret these “high-frequency/small-scale” waves as the signature of a strong filamentation/fragmentation of the Alfvén wings before they reflect off of the sharp boundary gradient of the Io torus. As a consequence, we suggest that most of the primary energy is converted into “high-frequency/small-scale” electromagnetic waves that can propagate out from the torus toward Jupiter's ionosphere. Reaching high-latitudes, these waves are able to accelerate electrons to almost relativistic speeds.     

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.     

Diffuse auroral precipitation in the jovian upper atmosphere and magnetospheric electron flux variability

The deposition of energetic electrons in Jupiter's upper atmosphere provides a means, via auroral observations, of monitoring electron and plasma wave activity within the magnetosphere. Not only does particle precipitation indicate a potential change in atmospheric chemistry, it allows for the study of episodic, pronounced flux enhancements in the energetic electron population. A study has been made of the effects of such electron injections into the jovian magnetosphere and of their ability to provide the source population for variations in diffuse auroral emissions. To identify the source region of precipitating auroral electrons, we have investigated the pitch-angle distributions of high-resolution Galileo Energetic Particle Detector (EPD) data that indicate strong flux levels near the loss cone. The equatorial source region of precipitating electrons has been determined from the locations of Galileo's in situ measurements by tracing magnetic field lines using the KK97 model. The primary source region for Jupiter's diffuse aurora appears to lie in the magnetic equator at 15-40 RJ, with the predominant contribution to precipitation flux (tens of ergs cm⁻² s⁻¹ sr⁻¹) stemming from <30 RJ. Variability of flux for energetic electrons in this region is also important to the irradiation of surfaces and atmospheres for the Galilean moons: Europa, Ganymede, and Callisto. The average diffuse auroral precipitation flux has been shown to vary by as much as a factor of six at a given radial location. This variability appears to be associated with electron injection events that have been identified in high-resolution Galileo EPD data. These electron flux enhancements are also associated with increased whistler-mode wave activity and magnetic field perturbations, as detected by the Galileo Plasma Wave Subsystem (PWS) and Magnetometer (MAG), respectively. Resonant interactions with the whistler-mode waves cause electron pitch-angle scattering and lead to pitch-angle isotropization and precipitation.     

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.     

Io's volcanic control of Jupiter's extended neutral clouds

Dramatic changes in the brightness and shape of Jupiter's extended sodium nebula are found to be correlated with the infrared emission brightness of Io. Previous imaging and modeling studies have shown that varying appearances of the nebula correspond to changes in the rate and the type of loss mechanism for atmospheric escape from Io. Similarly, previous IR observational studies have assumed that enhancements in infrared emissions from Io correspond to increased levels of volcanic (lava flow) activity. In linking these processes observationally and statistically, we conclude that silicate volcanism on Io controls both the rate and the means by which sodium escapes from Io's atmosphere. During active periods, molecules containing sodium become an important transient in Io's upper atmosphere, and subsequent photochemistry and molecular-ion driven dynamics enhance the high speed sodium population, leading to the brightest nebulas observed. This is not the case during volcanically quiet times when omni-present atmospheric sputtering ejects sodium to form a modest, base-level nebula. Sodium's role as a “trace gas” of the more abundant species of sulfur (S) and oxygen (O) is less certain during volcanic episodes. While we suggest that volcanism must also affect the escape rates of S and O, and consequently their extended neutral clouds, the different roles played by lava and plume sources for non-sodium species are far too uncertain to make definitive comparisons at this time.     

Tidally Induced Volcanism

The dissipation of tidal energy causes the ongoing silicate volcanism on Jupiter's satellite, Io, and cryovolcanism almost certainly has resurfaced parts of Saturn's satellite, Enceladus, at various epochs distributed over the latter's history. The maintenance of tidal dissipation in Io and the occurrence of the same on Enceladus depends crucially on the maintenance of the respective orbital eccentricities by the existence of mean motion resonances with nearby satellites. A formation of the resonances among the Galilean satellites by differential expansion of the satellite orbits from tides raised on Jupiter by the satellites means the onset of the volcanism on Io could be relatively recent. If, on the other hand, the resonances formed by differential migration from resonant interactions of the satellites with the disk of gas and particles from which they formed, Io would have been at least intermittently volcanically active throughout its history. Either means of assembling the Galilean satellite resonances lead to the same constraint on the dissipation function of Jupiter Q _J 10⁶, where the currently high heat flux from Io seems to favor episodic heating as Io's eccentricity periodically increases and decreases. Either of the two models might account for sufficient tidal dissipation in the icy satellite Enceladus to cause at least occasional cryovolcanism over much of its history. However, both models are assumption-dependent and not secure, so uncertainty remains on how tidal dissipation resurfaced Enceladus.     

On limits to Jupiter's magnetospheric diffusion rates

X-ray fluxes at Earth estimated from hypothetical fluxes and spectra of energetic particles trapped in Jupiter's magnetic field are found to be 1/170000 times the upper limit X-ray flux from Jupiter based on published results from a rocket experiment. Detection of the calculated X-ray flux from Jupiter does not necessarily provide information on an energetic trapped proton component because the X-ray flux due to the hypothetical trapped energetic proton fluxes alone is comparable in magnitude to that due alone to trapped energetic electron fluxes at Jupiter.     

The dependence of Jupiter's decametric emissions on the Jovian magnetic latitude of Io

The present analysis confirms the contention that the majority of Jupiter's decametric radio emissions occur when Io is above the northern magnetic hemisphere of Jupiter. However, a substantial portion of the non-Io-related component of source A was found to occur when Io was above southern magnetic latitudes of Jupiter.     

Nature of the iogenic plasma source in Jupiter's magnetosphere: II. Near-Io distribution

Three-dimensional calculations are presented for the distribution of the iogenic plasma source that is created near Io (i.e., within ∼24 satellite radii about Io) by O and S gases located in the volume above Io's exobase (i.e., corona and escaping extended neutral clouds, designated as the “Outer Region”) and are complementary to a preceding paper for calculations on a circumplanetary scale. The instantaneous pickup ion production rates for both electron impact and charge exchange have significant radial, north-south, and orbital-plane asymmetries beginning just inside and/or beyond Io's Lagrange sphere (5.81 Io radii) and, within the Lagrange sphere, are distributed nearly symmetrically about Io and are highly peaked about Io's exobase. The spatial natures of the corresponding pickup ion density, mass loading rates, and the pickup ion conductivity, current, and magnetic field are investigated. Spatially integrated rates are calculated for the corona volume and compared to larger Outer Region circumplanetary volumes and are also compared to estimates drawn from the scientific literature (but not modeled here) of the spatially integrated rates for pickup processes in the strongly perturbed “Inner Region” below the exobase. Within the corona volume, the spatially-integrated net-mass loading rate and total (electron impact and charge exchange) mass loading rate are only a factor of ∼3 and ∼5, respectively, smaller than those estimated for the Inner Region, whereas for the whole plasma torus, the Outer Region rates are larger or comparable to those estimated for the Inner Region. The total pickup ion gyration power supplied to the whole plasma torus is estimated to be significantly less than the power radiated by the plasma torus, indicating that an additional power source, likely a circumplanetary distribution of nonthermal electrons, is present.     

Ion cyclotron waves at Io: implications for the temporal variation of Io's atmosphere

When the flowing torus plasma encounters the upper atmosphere of Jupiter's moon, Io, newly created ions are rapidly accelerated by the motional electric field. Many of these ions are reneutralized and form a spray of fast neutrals that travel far away from Io before being reionized by photoionization and impact. These ions, now far from Io, are unstable to the generation of ion cyclotron waves. These waves in turn act as a mass spectrometer allowing Galileo magnetic measurements to be used to probe the composition of the atmosphere of Io and how it varies in time and in space. We now have six Galileo passes by Io on which we have measurements with sufficient cadence to examine the ion cyclotron waves. One of these passes, on Galileo's 32nd orbit has not been discussed previously. These passes provide sufficient observations to begin to distinguish the sources of variability. We find that while the atmosphere of Io varies temporally throughout the mission, it also has a spatial variation in composition at any instant of time.     

Relative contributions of sublimation and volcanoes to Io's atmosphere inferred from its plasma interaction during solar eclipse

We present a model that describes Io's delayed electrodynamic response to a temporal change in Io's atmosphere. Our model incorporates the relevant physical processes involved in Io's atmosphere-ionosphere-magnetosphere electrodynamic interaction to predict the far-ultraviolet (FUV) radiation as Io enters Jupiter's shadow and re-emerges into sunlight. The predicted FUV brightnesses are highly nonlinear as the strength of the electrodynamic interaction depends on the ratios of ionospheric conductances to the torus Alfvén conductance, but the former are functions of electrodynamics and the atmospheric density, which decays rapidly upon entering eclipse. Key factors governing the time evolution are the column density due to sublimation and the column density due to volcanoes, which maintain the background atmosphere during eclipse. The plasma interaction does not react instantaneously, but lags to a temporarily changing atmosphere. We find three qualitatively different scenarios with two of them including a post-eclipse brightening. The brightness ratio of in-sunlight/in-eclipse coupled with the existence of a sub-jovian equatorial spot constrains the volcanic column density to several times 10¹⁸ m⁻², based on the currently available observations. Thus in sunlight, the sublimation driven part of Io's atmosphere dominates the volcanically driven contribution by roughly a factor of 10 or more.     

The condensation and vaporization behavior of ices containing SO2, H2S, and CO2: implications for Io

In an extension of previously reported work on ices containing CO, CO2, H2O, CH3OH, NH3, and H2, measurements of the physical and infrared spectral properties of ices containing molecules relevant to Jupiter's moon Io are presented. These include studies on ice systems containing SO2, H2S, and CO2. The condensation and sublimation behaviors of each ice system and surface binding energies of their components are discussed. The surface binding energies can be used to calculate the residence times of the molecules on a surface as a function of temperature and thus represent important parameters for any calculation that attempts to model the transport of these molecules on Io's surface. The derived values indicate that SO2 frosts on Io are likely to anneal rapidly, resulting in less fluffy, "glassy" ices and that H2S can be trapped in the SO2 ices of Io during night-time hours provided that SO2 deposition rates are on the order of 5 micrometers/hr or larger.