Ionosphere Of Jupiter Research Articles

A dynamical model of Jupiter's auroral electrojet

A global simulation for the auroral electrojet on Jupiter is presented. The required sequence of models was computed using JIM (the Jovian Ionospheric Model), a time-dependent, three-dimensional model for the thermosphere and ionosphere of Jupiter, and an a priori model for the planet's ionospheric electric field. We describe the plasma dynamics in the model by considering ion and electron motions at pressure levels less than 2 µbar, lying above Jupiter's dynamo region, and including the region of maximum energy deposition by auroral particles. By considering the motions of the neutral species being `dragged' by the electrojet, we quantify the electrodynamic coupling between the neutral thermosphere and the auroral ionosphere. Two distinct altitude regions evolve in the model simulations, distinguished by different thermospheric flow patterns. Higher-altitude regions are subject to gas dynamic flow, while lower-altitude regions are strongly influenced by electrodynamic flow, associated with the transfer of momentum from the electrojet to the neutral gas. The electrojet models provide a basis for physical interpretation of current observational detections of ion motions in the Jovian auroral regions; as well as a means of optimizing future observations, in order to make similar detections.

Meteoric Ions in the Ionosphere of Jupiter

A reanalysis of the Voyager 2 radio occultation data has recently revealed a low-altitude layer in the jovian ionosphere (Hinson et al. 1998, J. Geophys. Res. 103, 9505–9520). The peak electron density of the layer measured on egress, which was at 93° solar zenith angle near the morning terminator, was inferred to be of the order of 10 4 cm −3. A substantial low-altitude layer of hydrocarbon ions in the jovian ionosphere was predicted by Kim and Fox (1994), but the peak total ion density at predawn was about 10 2 cm −3, two orders of magnitude smaller than the noon values, due to the efficient recombination of molecular ions during the night. The existence of large electron densities in the jovian ionospheric E region at predawn suggests the presence of ions with long lifetimes and/or those produced by a source that exhibits little local time dependence, such as ions originating from meteoroid ablation in Jupiter's atmosphere. We have modeled the production rates and subsequent chemistry of seven meteoric ions, including O +, C +, Si +, Fe +, Mg +, Na +, and S +, their compounds with H, H 2, and hydrocarbons, and the corresponding neutral species. The models predict a layer of meteoric ions in the altitude region of 350–450 km above the 1-bar level, with peak total ion densities of several times 10 4 cm −3, which are comparable to the observed values. The peak of the meteoric atomic ion layer is most apparent at predawn and is located higher than that of the hydrocarbon ion layer during the daytime and higher than the altitude of peak production of ions by meteor ablation. At the altitude of peak ablation, about 350 km, meteoric ions are mainly removed by reactions with hydrocarbons in either two-body or three-body reactions, and the molecular ions produced are neutralized efficiently by dissociative recombination. Meteoric ions may also form adduct ions by termolecular reactions with hydrogen molecules, but metallic ions, such as Na +, Mg +, and Fe +, may be reformed from the adduct ions by a series of reactions with H atoms. Thus the net ion loss process at the metal ion peak may be dominated by rediative recombination, and the meteoric ion density profiles show little diurnal variation. The predicted peak electron density and altitude and the relative densities of the ions are dependent on the rate coefficients assumed for many of the reactions involved, and measurements of key rate coefficients are needed to further constrain the models.

The plasma wave environment of Europa

The Galileo spacecraft has executed nine close flybys of Jupiter's moon Europa for which plasma wave observations were obtained. This paper presents an analysis of the observations from these flybys taking into consideration the variable geometry of the trajectories in an attempt to characterize the general plasma-wave environment associated with the interaction of the Jovian magnetosphere with the moon. A wide variety of plasma-wave phenomena are found to be associated with this interaction. While there are apparently temporal variations which complicate the analysis, a crude model of the distribution of these phenomena around Europa is derived. Primarily on the upstream side of Europa, and working inward to the moon, electron–cyclotron harmonics are first observed, followed by a region within about two Europa radii of the moon with whistler-mode hiss or chorus, and culminating in a region closest to the moon where a band at the upper hybrid resonance frequency is sometimes enhanced over its ambient intensity. The wake region is approximately two Europa radii across and comprises a broadband, highly variable, and bursty electrostatic phenomenon. Upon closer inspection, these bursty emissions appear as solitary structures similar to those in Earth s auroral zone and plasma sheet boundary layer. In addition to the survey of wave phenomena in the vicinity of Europa, we provide density profiles derived primarily from the upper hybrid resonance frequency which is readily apparent throughout most of each of the flybys. Finally, we suggest that the whistler mode, electron cyclotron harmonic, and upper hybrid resonance emissions are driven by some combination of factors including variations in the magnetic field near Europa and the loss and production of plasma at Europa as a result of the interaction of the Jovian magnetosphere with the moon. By analogy with studies of the ion and electron holes and broadband electrostatic noise at Earth and Jupiter, we argue that the electrostatic solitary structures in the wake are associated with currents and beams coupling Europa to Jupiter's ionosphere.

Observations of plasmas in the Io torus with the Galileo spacecraft

On December 7, 1995, the Galileo spacecraft passed through the Io torus near the magnetic equatorial plane of Jupiter. High– resolution measurements of positive ions and electrons in the energy/charge (E/Q) range of 0.9 V to 52 kV were acquired over the jovicentric radial distances of 5.6 to 7.8 Rj. At radial distances beyond Io's orbit at 5.9 Rj the plasma instrument (PLS) observed intense magnetically field‐aligned beams of electrons with average energies of hundreds of eV to several keV. The directional energy fluxes at the spacecraft position ranged from ∼1.5 to 90 erg cm−2S−1sr−1 in directions parallel and antiparallel to the magnetic field. The corresponding range of energy fluxes into Jupiter's ionosphere was ∼9 to 560 erg Cm−2s−1, sufficient for the production of auroral emissions at far‐ultraviolet wavelengths with brightnesses in the range of 0.06 to 3 mega‐Rayleighs. The largest energy fluxes were observed at ∼7.8 Rj and were comparable to those observed during the close flyby of Io. The torus plasmas were observed to rigidly corotate with Jupiter out to a radial distance of ∼7 Rj and are slower at greater radial distances. Fluctuations of the bulk ion speeds and densities were found to be superposed upon the general pattern of corotation and are suggestive of interchange of magnetic flux tubes in the plasma torus. For an isolated field‐aligned electron beam, the radial component of plasma flow was directed toward Jupiter at a speed of ∼4 km s−1. The other electron beams also occurred coincidentally with the fluctuations in the radial component of flow. The observations of these low‐energy electron beams are consistent with the absence of magnetically field‐aligned acceleration of electrons into Jupiter's ionosphere. The electron acceleration appears to be due to intense heating of the ionospheric plasmas associated with large currents driven by the interchange motions in the torus plasmas.

Infrared spectroscopic studies of the jovian ionsophere and aurorae

Abstract We review recent spectroscopic studies of the ionosphere of Jupiter. These demonstrate the importance of the H3+ molecular ion in understanding not only the ion-molecule chemistry of the jovian upper atmosphere, but of its energetics and dynamics as well. Comparisons are made with a new three-dimensional, global circulation model of Jupiter's ionosphere and thermosphere, JIM.

Europa and Callisto: Induced or intrinsic fields in a periodically varying plasma environment

Magnetometer data from four Galileo passes by the Jovian moon Europa and three passes by Callisto are used to interpret the properties of the plasma surrounding these moons and to identify internal sources of magnetic perturbations. Near Europa the measurements are consistent with a plasma rich in pickup ions whose source is freshly ionized neutrals sputtered off of the moon's surface or atmosphere. The plasma effects vary with Europa's height above the center of Jupiter's extended plasma disk. Europa is comet‐like when near the center of the current sheet. It is therefore likely that the strength of the currents coupling Europa to Jupiter's ionosphere and the brightness of a Europa footprint will depend on System III longitude. Magnetic perturbations on the scale of Europa's radius can arise from a permanent dipole moment or from an induced dipole moment driven by the time‐varying part of Jupiter's magnetospheric field at Europa's orbit. Both models provide satisfactory fits. An induced dipole moment is favored because it requires no adjustable parameters. The inductive response of a conductive sphere also fits perturbations on two passes near Callisto. The implied dipole moment flips direction as is predicted for greatly differing orientations of Jupiter's magnetospheric field near Callisto in the two cases. For both moons the current carrying shells implied by induction must be located near the surface. An ionosphere cannot provide the current path, as its conductivity is too small, but a near surface ocean of ∼10 km or more in thickness would explain the observations.

Io‐controlled decameter arcs and Io‐Jupiter interaction

The motion of the satellite Io in Jupiter's magnetic field results in an electrodynamic circuit, approximately fixed in Io's frame, revealed by ultraviolet and infrared spots at the footprints of Io's flux tube (IFT) as well as prominent (so‐called “Io‐controlled”) decameter radio arcs. We analyze the frequency‐time shape of nine such arcs, corresponding to four observation geometries (A,B,C,D) and detected over their full frequency extent in joint Nançay and Wind data. We compute the radio beaming angle as a function of frequency and lag of the radio‐emitting flux tube(s) relative to the IFT. No a priori assumption is made regarding the radio source beaming, and its location is only constrained by the emission occurring near the local electron gyrofrequency. We find northern sources for A and B arcs and southern sources for C and D arcs. The shape of all Io arcs is consistent with an origin at a single flux tube (in Io's frame), shifted by an average of 10° (in the south) and 25° (in the north) with respect to the IFT. This lag must be accumulated before Io's magnetic perturbation reaches the radio emission region, at high latitudes, at altitudes ≤ 1 RJ above the Jovian surface. Radio emission is found to be beamed in a hollow cone of average half‐apex angle 70°–75° and thickness ≈1 °. Arc shapes are fully determined by the geometry of observation. Radio fringes preceding the main Io B arcs are well explained by multiple reflections of the magnetic perturbation between Jupiter's ionosphere and Io's torus. The weak trailing part of Io‐ B arcs may be accounted for through double beaming of the radio emission or through a frequency‐dependent lag of the corresponding radio source. The latter explanation suggests an emission scenario in which electron acceleration “leaks” from the magnetic perturbation on its way to Jupiter. Jovian magnetic field models are compared and evaluated in the analysis.

JIM: A time‐dependent, three‐dimensional model of Jupiter's thermosphere and ionosphere

We present the Jovian Ionospheric Model (JIM), a time‐dependent, three‐dimensional model for the thermosphere and ionosphere of Jupiter. We describe the physical inputs for the hydrodynamic, thermodynamic and chemical components of the model, which is based on the UCL Thermosphere Model of Fuller‐Rowell and Rees [1980]. We then present the results of an illustrative simulation in which an initially neutral homogeneous planet evolves for approximately 4 Jovian rotations, under the influence of solar illumination and auroral (electron) precipitation at high latitudes. The model shows that solar zenith angle, auroral activity, ion recombination chemistry and, to a lesser degree, magnetic field orientation, all play a role in forming the dayside and nightside global ionization patterns. We compare auroral and nonauroral/equatorial ionospheric compositions and find the signature of ion transport by fast winds. We also include a localized “spot” of precipitation in our model and comment on the associated ionization signatures which develop in response to this Io‐like aurora. The simulation also develops strong outflows with velocities up to ∼600 m s−1 from the auroral regions, driven mainly by pressure gradients. These pressure gradients, in turn, arise from the differences in chemical composition between the auroral and nonauroral upper thermospheres, as evolution proceeds. This preliminary study indicates a strong potential for JIM in analysis of two‐dimensional image data and simulation of time‐dependent global events.

New models of Jupiter's magnetic field constrained by the Io flux tube footprint

Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian radii). The IFT footprint is a valuable constraint on magnetic field models, providing “ground truth” information in a region close to the planet and thus far not sampled by spacecraft. The magnetic field is represented using a spherical harmonic expansion of degree and order 4 for the planetary (“internal”) field and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. Models fitting Voyager 1 and Pioneer 11 magnetometer observations and the IFT footprint are obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, octupole, and a subset of higher‐degree and higher‐order spherical harmonic coefficients are determined and compared with earlier models.

Jupiter's ionosphere: New results from Voyager 2 radio occultation measurements

High‐quality radio occultation data were acquired as the Voyager 2 spacecraft flew behind Jupiter on July 10, 1979. We have conducted a thorough analysis of dual‐frequency data from the ionosphere using a new method of data processing based on scalar diffraction theory. The method is capable of deciphering the effects of multipath propagation, which had not previously been possible for these data. This allows retrieved vertical profiles of electron number density to be extended downward throughout the lower ionosphere. The electron density profile at occultation entry (66°S, 258°W) has a topside scale height of 1020 km, consistent with previous results. New results include a peak density of 3.5 × 105 cm−3 at a height of 640 km above the 1‐bar pressure level. In addition, the lower side of the main peak contains quasi‐sinusoidal density oscillations with a vertical spacing of 25–40 km at altitudes of 450–600 km. These appear to signal the presence of upward propagating gravity waves. The electron density profile at occultation exit (51°S, 148°W) exhibits a peak density of 2.3 × 105 cm−3 at an altitude of 1900 km, and the topside has a uniform scale height of 850 km. These characteristics of the upper ionosphere, where the principal ion is believed to be H+, are consistent with earlier results. New results include two secondary peaks at lower altitudes. An intermediate layer, where the principal ion may be H3+, is centered near 1000 km with a peak density of 4.6 × 104 cm−3. A lower layer, possibly composed of metallic ions, is situated between 200 and 550 km with densities of 2–12 × 104 cm−3. The latter contains multiple, thin layers of ionization at altitudes of 300–450 km, which may be the result of vertical shear in the zonal wind or plasma instabilities. The height‐integrated Pedersen conductivity is about 0.5 and 0.4 mho at entry and exit, respectively.

Jupiter's ionosphere: Results from the First Galileo Radio Occultation Experiment

The Galileo spacecraft passed behind Jupiter on December 8, 1995, allowing the first radio occultation measurements of its ionospheric structure in 16 years. At ingress (24°S, 68°W), the principal peak of electron density is located at an altitude of 900 km above the 1‐bar pressure level, with a peak density of 105 cm−3 and a thickness of ∼200 km. At egress (43°S, 28°W), the main peak is centered near 2000 km altitude, with a peak density of 2×104 cm−3 and a thickness of ∼1000 km. Two thin layers, possibly forced by upwardly propagating gravity waves, appear at lower altitudes in the ingress profile. This is the first in a two‐year series of observations that should help to resolve long‐standing questions about Jupiter's ionosphere.

… And Io Yields Still More Surprises

Researchers have come to expect spectacular surprises from Io, the innermost of Jupiter's Galilean satellites. The Voyager missions revealed Io to be the most volcanically active object in the Solar System. Voyager also revealed the importance of Io's interactions with Jupiter's magneto‐sphere, which generate a potential of 400 kV across the moon's diameter and currents of over a million amperes between Io and Jupiter's ionosphere. As a result of this past performance, researchers eagerly waited for Galileo to pass within 900 km of Io, a factor of 20 closer than any of Voyager's flyby's. Initial analysis of the data indicates that, once again, Io has not disappointed.

The structure and dynamics of Jupiter's ring

JUPITER'S ring has posed a problem since it was first discovered 15 years ago. Its inner edge flares into a torus, the shape of which has not yet been accurately modelled, although it was recognized early on1–3 that electromagnetic effects might be important in determining the spatial and size distribution of the dust particles comprising the ring. These early models suggested that sulphur and oxygen ions dominate the plasma environment of the ring; as these ions diffuse inwards from Io, they produce negatively charged dust grains in the ring. Here we propose that Jupiter's ionosphere is the dominant source of plasma and that the low plasma density allows ultraviolet radiation from the Sun to photoionize the grains, giving them a positive charge. The resulting gradient in the equilibrium charge distribution transports the grains rapidly—on surprisingly short timescales of hours to days—towards Jupiter. The brightness distribution resulting from this model matches closely the observed brightness distribution, suggesting that our model captures the most important processes that shape this ring.

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.

Branching processes in the dissociative recombination of H3+

p3 over the energy range 0.001 to 20 eV using the electron cooler at the storage ring CRYRING. The H 1 3 are stored and cooled at an energy of 6.5 MeVyamu. The recombined products pass through a “translucent” perforated barrier and their total energy is recorded. Thus all particles are detected. Particles passing through holes (70 mm) deposited 6.5 MeV each in the detector, while those passing through the solid (stainless steel 50 mm thick) are not stopped but give pulses of only 4.7 MeV per atom. Analysis of the pulse height spectrum gives the branching fractions directly. PACS numbers: 34.80.Gs, 34.80.Kw The simplest of polyatomic molecules must surely be the two electron H 1 3 ion. Acting as a proton donor, it is an important intermediate in the formation of many interstellar molecules and it has been shown to be present in the ionospheres of Jupiter [1], Saturn [2], and Uranus [3]. Its principal mode of formation is via the homogeneous reaction [4] H 1 2 1 H2 ! H 1 3 1 H 1 1.7 eV . (1)

Electrodynamical interaction between comet Shoemaker‐Levy 9 and Jupiter

The electrodynamical interaction between Jupiter and comet Shoemaker‐Levy 9, which is expected to impact on Jupiter around July 20, 1994, is investigated. The comet consists of a sequence of 21 or so nuclei, each surrounded by a neutral cloud of outgassed material. A model is constructed of one single such cloud which is subject to electron impact ionization during the passage through Jupiter's magnetosphere. The cloud is assumed to couple electrically to the surroundings either by means of Alfvén wings, or through a dc circuit that closes in Jupiter's ionosphere. The magnetic‐field‐aligned currents resulting from the Jupiter‐Io interaction are strongly connected to the Jovian Decametric Radiation. The Shoemaker‐Levy comet can theoretically supply ions to Jupiter's magnetosphere at a rate not so far below Io's, and will have a higher velocity relative to Jupiter, and could therefore conceivably drive similar processes. However, we find that, due to the high latitude trajectory of the comet, the resulting currents become very weak. The field‐aligned currents obtained in this study are a factor 500 below those driven by Io, while the dissipated power is almost a factor 106 below. It is therefore proposed that very little or no detectable electromagnetic radiation will arise during the comet's passage through Jupiter's magnetosphere.

H3+: from first principles to Jupiter

Abstract H3 + is the simplest poly atomic molecule and as such has always been the subject of extensive theoretical study. It has also long been known to be an important constituent of hydrogen plasmas that are prevalent in many locations in the Universe. This article traces the study of the spectrum of H3 + from the first principles quantum mechanical calculation to its observation in the ionosphere of Jupiter and beyond.

Plasma Composition in Jupiter's Magnetosphere: Initial Results from the Solar Wind Ion Composition Spectrometer

The ion composition in the Jovian environment was investigated with the Solar Wind Ion Composition Spectrometer on board Ulysses. A hot tenuous plasma was observed throughout the outer and middle magnetosphere. In some regions two thermally different components were identified. Oxygen and sulfur ions with several different charge states, from the volcanic satellite lo, make the largest contribution to the mass density of the hot plasma, even at high latitude. Solar wind particles were observed in all regions investigated. Ions from Jupiter's ionosphere were abundant in the middle magnetosphere, particularly in the highlatitude region on the dusk side, which was traversed for the first time.

Ulysses at Jupiter: An Overview of the Encounter

In February 1992, the Ulysses spacecraft flew through the giant magnetosphere of Jupiter. The primary objective of the encounter was to use the gravity field of Jupiter to redirect the spacecraft to the sun's polar regions, which will now be traversed in 1994 and 1995. However, the Ulysses scientific investigations were well suited to observations of the Jovian magnetosphere, and the encounter has resulted in a major contribution to our understanding of this complex and dynamic plasma environment. Among the more exciting results are (i) possible entry into the polar cap, (ii) the identification of magnetospheric ions originating from Jupiter's ionosphere, lo, and the solar wind, (iii) observation of longitudinal asymmetries in density and discrete wave-emitting regions of the lo plasma torus, (iv) the presence of counter-streaming ions and electrons, field-aligned currents, and energetic electron and radio bursts in the dusk sector on high-latitude magnetic field lines, and (v) the identification of the direction of the magnetic field in the dusk sector, which is indicative of tailward convection. This overview serves as an introduction to the accompanying reports that present the preliminary scientific findings. Aspects of the encounter that are common to all of the investigations, such as spacecraft capabilities, the flight path past Jupiter, and unique aspects of the encounter, are presented herein.

Drift wave instability in the Io plasma torus

We present a linear normal‐mode analysis of low‐frequency electrostatic drift waves in the Io plasma torus, providing a kinetic description of the centrifugal interchange instability of the torus regulated by coupling to Jupiter's ionosphere. We assume a periodic potential perturbation with azimuthal wavenumber m≫1 and solve a boundary value problem to obtain the wave dispersion relation and radial eigenfunction. For given m the growing mode is a standing wave in the corotating reference frame. If the outer torus boundary is taken as a discontinuity, the growth rate is proportional to m times the torus flux tube content times the ion drift frequency, divided by the Pedersen conductivity of Jupiter's ionosphere. The inner torus boundary has a modest stabilizing effect for azimuthal wavelengths greater than the radial thickness of the torus. The finite slope of the outer torus density profile has a more pronounced stabilizing effect, reducing the growth rate by the factor 2β/m, where β∼2 is the exponent of the assumed power law decline of flux tube content with distance. Even so, the e‐folding time is of the order of 1 hour, much less than the inferred residence time of torus ions. The growth rate can be further reduced dramatically by the stabilizing effect of an as‐yet unobserved ring current distribution, the “impoundment” effect proposed by Siscoe et al. (1981). Various theoretical models of global radial transport in Jupiter's magnetosphere, including corotating convection, interchange diffusion, and transient flux tube convection, can be understood as plausible nonlinear evolutions of electrostatic drift waves. Further observations and/or numerical simulations are needed to ascertain the relative importance of these transport mechanisms.