Io's Motion Research Articles

Location and Gaze-Dependent Shift of Inferior Oblique Muscle Position: Anatomic Contributors to Vertical Strabismus Following Lower Lid Blepharoplasty?

This study investigated, using high-resolution magnetic resonance imaging (MRI), inferior oblique muscle (IO) position relative to the adnexa in normal controls, subjects with and without vertical strabismus following lower lid blepharoplasty, and subjects with other hypertropia. Sagittal plane MRI was obtained in central gaze, infraduction, and supraduction in 19 controls, 11 subjects with and 2 without hypertropia following bilateral lower lid blepharoplasty, and 13 subjects with hypertropia unrelated to blepharoplasty. In the plane passing through the center of the inferior rectus muscle (IR), we analyzed IO position relative to the globe, as well as the distance from IO to the skin or orbital floor. The IO was located approximately 1 mm more anteriorly and 1.2 mm more inferiorly in hypertropic than hypotropic fellow orbits of the blepharoplasty group and controls (P < 0.05). From central gaze to infraduction, IO shift in subjects with blepharoplasty was redirected inferiorly, rather than posteriorly as in all other groups. However, from central gaze to supraduction, IO motion was similar in all groups. There was scarring between the IO-IR pulley and orbital floor in the hypertropic eye after lower lid blepharoplasty. Subjects with strabismus following lower lid blepharoplasty exhibit anterior and inferior IO pulley displacement in central gaze, as well as hindrance to normal posterior shift in infraduction. Proximity of IO to the orbital rim and lower eyelid skin is associated with strabismus following blepharoplasty, possibly because lower lid blepharoplasty may change lid forces on the IO-IR pulley system via scar tissue.

Power transmission and particle acceleration along the Io flux tube

Io's motion relative to the Jovian magnetic field generates a power of about 1012 W, which is thought to propagate as an Alfvén wave along the magnetic field line. This power is transmitted to the electrons, which will then precipitate and generate the observed auroral phenomena from UV to radio wavelengths. A more detailed look at this hypothesis shows some difficulties: Can the Alfvén waves escape the torus or are they trapped inside? Where and how are the particles accelerated? In which direction? Is there enough power transmitted to the particles to explain the strong brightness of the auroral emissions in UV, IR, visible, and radio? In other words, can we make a global, consistent model of the Io‐Jupiter interaction that matches all the observations? To answer these questions, we review the models and studies that have been proposed so far. We show that the Alfvén waves need to be filamented by a turbulent cascade process and accelerate the electrons at high latitude in order to explain the observations and to form a consistent scheme of the Io‐Jupiter interaction.

New constraints on Io's and Jupiter's tidal dissipation

Although the quest for tidal accelerations among the Galilean satellites has been ongoing since the beginning of the last century, no real agreement has been found so far. Using a numerical approach, we simulate the effect of tidal interactions on the evolution of Io's motion during the last century. We show how these tidal effects can vanish during the fit process to observational data. By testing different values of dissipation within Io and Jupiter, we show that a non-detection of significant Io's orbital acceleration does not imply a large dissipation within Jupiter, and we suggest an upper bound value ( ϕ surf ⩽ 7 W m −2 ) for the dissipation rate within Io.

Electron acceleration in the ionosphere of Io

A mechanism for the acceleration of electrons in the ionosphere of Io due to the moon's motion through the Jovian magnetic field and the presence of Io's ionosphere is considered. Attention is drawn to the important role of the anisotropic conductivity of the ionosphere, which results in the formation of a longitudinal (with respect to the planetary magnetic field) component of the charge-separation electric field. Owing to this anisotropy, the electric field induced by the motion of Io, Ei, produces in Io's ionosphere not only a Pedersen electrical current along Ei but also a Hall current that is approximately perpendicular to the moon's surface in the “upstream” and “downstream” parts of the ionosphere. However, this current cannot be closed through the surface, leading to the formation of a powerful charge-separation field in Io's ionosphere. This field has a component parallel to the magnetic field, with an amplitude comparable to that of the induced electric field. Electron runaway along the magnetic field is also considered, and the occurrence of “active longitudes” and preferred locations for the sources of decametric radio emission in the northern hemisphere of Jupiter are interpreted. The characteristic energies and fluxes of the accelerated electrons injected into Io's flux tube are estimated. The energy of these electron fluxes is sufficient to produce the electromagnetic radiation observed from Io's magnetic tube.

Galileo's close‐up view of the Io sodium jet

The Galileo spacecraft has imaged a remarkable atmospheric escape process occurring in the atmosphere of Io, Jupiter's volcanic moon. Electrodynamic consequences of Io's motion through Jupiter's magnetosphere drive mega‐amp currents through Io's ionosphere; some of the sodium ions carrying this current are neutralized as they leave the atmosphere. The Galileo images show that the resulting fast sodium jet removes ∼5×1025 atoms sec−1 from Io's atmosphere. The spatial profile of the jet shows that the source region is much smaller than Io itself, perhaps confined to volcanically active regions, or to an ionosphere restricted to the denser atmosphere near Io's equator.

Galileo radio occultation measurements of Io's ionosphere and plasma wake

Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Io's surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s−1 at a distance of 3 Io radii from the center of Io to 57 km s−1 at 7 Io radii. The latter corresponds to corotation with Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Io's terminator. The ionosphere is substantial, with the peak density exceeding 50,000 cm−3 at 9 out of 10 locations and reaching a maximum of 277,000 cm−3. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0°W) and away from (180°W) Jupiter and minima near the center of the downstream (90°W) and upstream (270°W) hemispheres. This pattern may be related to the Alfvénic current system induced by Io's motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from ∼200 km near the center of the upstream hemisphere to ∼400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202±14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location.

Rapid energy dissipation and variability of the lo–Jupiter electrodynamic circuit

THE electrodynamic interaction between Jupiter and the closest of its large moons, Io, is unique in the Solar system. Io's volcanoes eject a considerable amount of material into the inner jovian system (>1 tonne per second), much of it in the form of ions1; the motion of Io through Jupiter's powerful magnetic field in turn generates a million-ampere current2 between the charged near-Io environment and the planet's ionosphere. This current is presumably carried by Alfven waves3, the electromagnetic equivalent of sound waves. Here we present far-ultraviolet observations of the atmospheric footprint of this current, which demonstrate that most of the energy is dissipated rapidly when the waves first encounter Jupiter's ionosphere; the position of the footprint varies with time. We see no evidence for the multiple ionospheric interactions that have been proposed to explain the structure of the radio emissions associated with these waves4.

Orbital acceleration and the energy budget in the galilean satellite system

Prodigious amounts of heat are generated in Jupiter's satellite Io, as evidenced by its spectacular volcanism1 and infrared heat flux2–6 (∼7.6×l013 W (ref. 7)). Such heating had been predicted from tidal flexing of Io due to the eccentricity of its orbit8, which is maintained by its resonant gravitational interaction with the satellites Europa and Ganymede. The energy for tidal heating must come from Io's orbit, but due to the resonance the energy loss is shared among the three satellites' orbits9,10. This effect modifies the rate at which Io's semi-major axis would be expected to decrease, and also tends to drive the system out of resonance. Yoder9 suggested that the resonance is maintained by tides raised on Jupiter by Io, which act to transfer planetary rotational energy and angular momentum into Io's orbit, but the jovian torque may be insufficient for this purpose. Greenberg10 proposed that with smaller jovian torque the system may be evolving away from resonance, and that measurement of Io's orbital acceleration could show whether the evolution is in fact dominated by heating in Io. An acceleration value ṅ1/n1=4.6×10−l0 yr−1±20%, where n1 is Io's mean motion, derived11 from historical records of Io's motion, is in accord with the expected value ṅ1/n1=3.2×10−10±28% based on the measured thermal radiation and the assumption of negligible jovian torque. A much smaller negative acceleration has been determined by Lieske12,13, which would imply significant jovian torque, but not enough to maintain the equilibrium proposed by Yoder, unless the average tidal disruption in Io is much less than is indicated by the thermal flux.

Predictions of mutual phenomena of the Galilean satellites in 1985–1986

A thorough search covering more than 3 years shows that nearly 300 observable mutual eclipses and occultations of the Galilean satellites occur between May 1985 and April 1986, marking this apparition as a very favorable one. This paper tabulates quantities needed to obtain light curves of all events, excluding only those taking place either too close to Jupiter or with the planet too near the Sun. Since observations are relatively short and easy to incorporate into photometric programs, we urge an active campaign so as to provide the accurate astrometric data required to improve ephemerides (which are of immediate interest to the Galileo mission to Jupiter) and to look for tidal effects in the motion of Io.

Nonlinear standing Alfvén wave current system at Io: Theory

We present a nonlinear analytical model of the Alfvén current tubes continuing the currents through Io (or rather its ionosphere) generated by the unipolar inductor effect due to Io's motion relative to the magnetospheric plasma. We thereby extend the linear work by Drell et al. (1965) to the fully nonlinear, sub‐Alfvénic situation also including flow which is not perpendicular to the background magnetic field. The following principal results have been obtained: (1) The portion of the currents feeding Io is aligned with the Alfvén characteristics at an angle θA = tan−1 MA to the magnetic field for the special case of perpendicular flow where MA is the Alfvén Mach number. (2) The Alfvén tubes act like an external conductance ΣA = 1/(µ0VA(1 + MA² + 2MA sin θ)1/2) where VA is the Alfvén speed and θ the angular deviation from perpendicular flow towards the direction of Alfvén wave propagation. Hence the Jovian ionospheric conductivity is not necessary for current closure. (3) In addition, the Alfvén tubes may be reflected from either the torus boundary or the Jovian ionosphere. The efficiency of the resulting interaction with these boundaries varies with Io position. The interaction is particularly strong at extreme magnetic latitudes, thereby suggesting a mechanism for the Io control of decametric emissions. (4) The reflected Alfvén waves may heat both the torus plasma and the Jovian ionosphere as well as produce increased diffusion of high‐energy particles in the torus. (5) From the point of view of the electrodynamic interaction, Io is unique among the Jovian satellites for several reasons: these include its ionosphere arising from ionized volcanic gases, a high external Alfvénic conductance ΣA, and a high corotational voltage in addition to the interaction phenomenon with a boundary. (6) We find that Amalthea is probably strongly coupled to Jupiter's ionosphere while the outer Galilean satellites may occasionally experience super‐Alfvénic conditions.

The source of the Jupiter S-bursts

The transient radio S-bursts from Jupiter are strongly correlated with the orbital position of the satellite Io for all wave frequencies between 8 MHz and 30 MHz, and with system III longitude above about 15 MHz. The observations are most easily explained if the electrons generating the S-bursts are confined to the planetary magnetic flux tube intersecting Io (the IFT)1. The primary source of the electron energy is thought to be the electromotive force (EMF) generated by the relative motion of Io across the Jupiter magnetic field, the S-bursts being the result of convective instabilities in the consequent current flow in the IFT. The electrons are assumed to emit coherent electromagnetic radiation in the Doppler cyclotron mode as they travel up the IFT after being accelerated near its feet in the Jupiter ionosphere to energies of a few keV (refs 2–4).

Io's wobbling flux tube and nonuniform surface conductivity: Longitude control of decametric emission and other magnetospheric interactions

A model is proposed relating Io's modulation of Jupiter's decametric radio emission (DAM) and other magnetospheric interactions to periodic changes in flux tube orientation with Io’s surface of nonuniform surface electrical conductivity. Spectral reflectance and albedo data suggest that visually bright surface areas on Io may be highly conducting due to the abundance there of irradiation‐altered salts, while relatively dark areas may be extremely insulating due to the abundance of free sulfur. Geometric correlations show that Io's orbital motion through Jupiter's tilted and offset magnetic field results in a 13‐hour periodic variation in the Io‐graphic surface position of Io's flux tube contact junctions. The resulting variation in effective electrical conductivity of Io results in modulation (switching) of Io's motional emf currents and corresponding flux tube currents. This causes the observed magnetic‐longitude dependence of Io‐dependent DAM, while emission beaming accounts for the observed Io‐phase effect. This switching model suggests that Io is a variable unipolar inductor and that its flux tube varies in electrodynamic character as a function of magnetic longitude; the predicted behavior is consistent with pre‐Voyager observed particle‐injection spikes at Io's L shell, auroral emissions from Jupiter, and may reconcile inconsistencies between theoretical and observed particle sweeping effects of Io. The model predicts a relatively inactive flux tube current system and strong particle sweeping during the Voyager 1 flux tube crossing.