Rotation Of Jupiter Research Articles

The impact of comet SL9 with Jupiter: IR observations at TIRGO

Infrared images of Jupiter have been obtained on 5 nights before, during and shortly after the period of the impacts of the fragments of comet Shoemaker-Levy 9 (1993e) with the giant planet. Long lived bright spots produced by the impacts have been measured and analyzed. By measuring the intensity variation of the spots as a function of Jupiter rotation we show that these spots are likely constituted by large and thin clouds of dust located above the methane layer. The IR relative albedos has been also measured for some of these spots.

Three-dimensional particle anisotropies in and near the plasma sheet of Jupiter observed by the EPAC experiment onboard the Ulysses spacecraft

During its inbound journey into Jupiter's magnetosphere, Ulysses had several encounters with the Jovian plasma sheet near the magnetic equator, which were related with intensity maxima in the energetic particles. We show for the first time anisotropies in three dimensions of three ion species (protons, helium and oxygen) in the energy range 0.24 < E < 0.77 [MeV/nucleon]. The data, obtained with the Energetic Particle Composition Experiment (EPAC) onboard Ulysses have been analysed by using spherical harmonics in three dimensions. We show that the first-order anisotropies of ions in or near the plasma sheet are strongest in a plane parallel to the ecliptic plane and more or less azimuthal with respect to the rotation of Jupiter. We show that the first-order anisotropy amplitude is larger for helium and oxygen ions than for protons in nearly the same energy per nucleon range. We find flow velocities for helium ions which are not consistent with corotation, but are larger by a factor of 2 in and near the Jovian plasma sheet on the dayside magnetosphere. In contrast for protons we observe nearly corotation. Far from the plasma sheet, at high magnetic latitudes, the flow velocities are less than corotation for protons, as well as for helium and oxygen. The azimuthal particle anisotropies are explained by intensity gradients perpendicular to the centre of the plasma sheet, by E × B particle drifts, and by nonadiabatic orbits of the particles near the Jovian plasma sheet. All of the three phenomena act in the same azimuthal direction, perpendicular to the mainly radial magnetic field direction. Each of them can be estimated, but their individual effects cannot be distinguished from each other. In addition, we find a radial component of the anisotropy which apparently is stronger for protons than for heavier ions. This radial anisotropy component is interpreted as a result of the radial outward displacement of ions in an azimuthally swept back magnetic field.

Measurement of direct current electric fields and plasma flow speeds in Jupiter's magnetosphere

During the encounter of Ulysses with Jupiter, we have measured two components of the dc electric field and deduced from them the flow speed in the Io torus, as well as the presence of a polar cap region and what we interpret as a cleft region. Within the torus the flow speed is approximately equal to the speed of a plasma corotating with Jupiter but has significant deviations. The dominant deviations have an apparent period of the order of Jupiter's rotation period, but this might be a latitudinal effect. Other important periods are about 40 min and less than 25 min.

Radial diffusion of iogenic plasma in a centrifugally-driven turbulence

A quantitative model of plasma diffusion from the Io torus, driven by the centrifugal force of Jupiter's rotation, is developed. It is argued that the unstable mode of perturbation, which would lead to a system of deterministic motion of plasma, should be dominated by a more turbulent mode of motion in order to effect a longer trapping time of the Iogenic plasma as implied by the observations. The energy of the turbulent motion is derived internally from the centrifugal potential of the planetary rotation. The length scale of the turbulence cells is determined by the interaction between the magnetospheric plasma and the dissipative ionosphere, and found to be of order 0.1 R J for the Jovian magnetosphere. Since the cells are smaller than the dimension of the system, the transport of the plasma can be studied as a problem of eddy diffusion, governed by the Fokker-Planck equation. A specific model is established based on the above considerations. Investigation of this model shows that the nominal L −5 dependence of plasma density can be reproduced in an appropriate limit. Other details of the density distribution (e.g. the density ramp located at L = 7.5) can be attributed to the spatial variation of the ionospheric Pedersen conductance. The density ramp implies a conductivity peak in its magnetically conjugate ionosphere. The conductivity profile consistent with the observed density distribution is solved. The inward expansion of the Io torus effected by fringing electric field of the outer torus is also briefly discussed.

The Jupiter-Io Connection: An Alfvén Engine in Space

Much has been learned about the electromagnetic interaction between Jupiter and its satellite Io from in situ observations. Io, in its motion through the Io plasma torus at Jupiter, continuously generates an Alfvén wing that carries two billion kilowatts of power into the jovian ionosphere. Concurrently, Io is acted upon by a J x B force tending to propel it out of the jovian system. The energy source for these processes is the rotation of Jupiter. This unusual planet-satellite coupling serves as an archetype for the interaction of a large moving conductor with a magnetized plasma, a problem of general space and astrophysical interest.

Jovian magnetospheric neutral wind and auroral precipitation flux

We propose a mechanism for the transport of energy from the rotation of Jupiter into the heavy ions whose precipitation into the atmosphere is responsible for the intense ultraviolet aurora. The process involves charge exchange of heavy ions in the Io torus, their flight to the outer magnetosphere as neutrals, and their reionization in a region where the pickup magnetic moment is large. The particles diffuse back inward adiabatically and are precipitated at 9 RJ with an energy exceeding 10 keV/nucleon. A field and flow model based on Voyager observations is used that defines a source region extending from 32 RJ to 80 RJ on the nightside. This region is capable of delivering 4×1013 W of power to the auroral region, well within the realm of the power required by observations. We also demonstrate that this mechanism provides a neutral planetary wind from Jupiter. The predicted wind will consist of two beams: one intense (5×1028 s−1) at 25 eV/nucleon and the other tenuous (1×1025 s−1) at 600 eV/nucleon and above. These neutral particles will appear as a suprathermal heavy ion component in the solar wind.

Corotating magnetospheric convection

The longitudinal asymmetry of the Io plasma torus, as predicted by the magnetic‐anomaly model and observed by Earth‐based optical astronomy, provides a driving mechanism for a corotating convection system in Jupiter's magnetosphere. Here we deduce some qualitative properties of this convection system from the general equations that govern a steady state corotating convection system (although we expect that time‐dependent effects may also have to be included for a complete description of Jovian convection). The corotating convection system appears capable of providing both the dominant radial transport mechanism (with a time scale possibly as short as a few rotation periods) and the dominant mechanism for extracting energy from Jupiter's rotation (at a rate ∼1015 W) for driving a wide variety of magnetospheric phenomena. A similar corotating convection system may occur in other rotation‐dominated magnetospheres, for example, those of pulsars and Saturn.

Electromagnetic heating of Io

Electrical‐energy deposition within Io is a possible mechanism for heating the interior, the source of the electrical current system being the relative motion of Io with respect to Jupiter's magnetic field. The transverse‐electric mode currents circulate entirely within Io, with the frequency of Jupiter's rotation rate as observed in the Io frame. Maximum heating for Io is obtained if the electrical conductivity is 0.02 mho m−1, a value spanned by estimates of rock conductivity for hot interiors. Under present conditions, the maximum heating available from the transverse‐electric mode is some 3 orders of magnitude less than would be necessary to maintain Io's estimated heat flow. Confinement of Io's field by the external plasma would raise this estimate only moderately. The transverse‐magnetic mode is effectively blocked by the cold outer surface layers, the current being bypassed by an alternative current conduction path through the ionosphere, leaving an insignificant amount in the interior. Although the transverse‐magnetic heat source can be concentrated at local surface hot spots, the temperature rise is not significant on a time scale appropriate to localized heat sources. It is possible that significant heating was generated in the past by a magnetic field that was larger and had a higher frequency. Heating could have been significant also if Io's ionosphere had not been present.

Limit on rotational energy available to excite Jovian aurora

There is a fundamental relationship between the power that is extracted from Jupiter's rotation to drive magnetospheric processes and the rate at which mass is injected into the Io plasma torus. Half of this power is consumed by bulk motion of the plasma and the other half represents an upper limit on the energy from rotation available for dissipation and in particular to excite the Jovian aurora. Since the rotation of the planet is the only plausible source of energy, the power inferred from the observed auroral intensities requires a plasma injection rate of 2.6 × 1029 AMU/sec or greater. This in turn leads to a residence time of a torus particle of 48 d. or less. These results raise doubts about the applicability of equilibrium thermodynamics to the determination of plasma parameters in the Io torus.

Mass-injection rate from Io into the Io plasma torus

Theoretical arguments have been presented to the effect that both plasma and energy are supplied to the Jovian magnetosphere primarily from internal sources. If we assume that Io is the source of plasma for the Jovian magnetosphere and that outward flow of plasma from the torus is the means of drawing from the kinetic energy of rotation of Jupiter to drive magnetospheric phenomena, we can obtain a new, independent estimate of the rate of mass injection from Io into the Io plasma torus. We explicitly assume the solar wind supplies neither plasma nor energy to the Jovian magnetosphere in significant amounts. The power expended by the Jovian magnetosphere is supplied by torus plasma falling outward through the corotational-centrifugal-potential field. A lower limit to the rate of mass injection into the torus, which on the average must equal the rate of mass loss from the torus, is therefore derivable if we adopt a value for the power expended to drive the various magnetospheric phenomena. This method yields an injection rate of at least 10 3 kg/sec, a value in agreement with the results obtained by two other independent methods of estimating mass injection rate. If this injection rate from Io and extraction of energy from Jupiter's kinetic energy of rotation has been maintained over geologic time, then approximately 0.1% of Io's mass (principally in the form of sulfur and oxygen) has been lost to the Jovian magnetosphere, and Jupiter's spin rate has been reduced by less than 0.1%.

Periodicities in the Jovian magnetosphere: Magnetodisc models after Voyager

The Voyager‐1 and 2 outbound observations of periodic double‐peak flux maxima (which mark encounters with the magnetodisc) are separated into two distinct branches ‐ a "leading" branch (N → S disc crossings) and a "trailing" branch (S → N disc crossings). On a plot of System III (1965.0) longitude vs. radial distance, the leading branch has a positive slope (∼ 1.2°/RJ) and the trailing branch has a relatively flat slope (∼ 0.3°/RJ). The two branches meet at ∼ 80‐100 RJ, beyond which periodic single peak or closely‐spaced multiple peaks are generally observed. This paper examines this structure using the 3 principal disc models which have emerged to explain periodicities in the Jovian magnetosphere: (1) the rigid magnetodisc model, (2) the bent or warped magnetodisc model, and (3) the wavy magnetodisc model. The rigid disc model does not allow for the merging of the double‐peak structure into a single‐peak structure, and the bent disc model cannot explain the phase lag of the leading peaks. Both the merging of the peaks and the phase asymmetry can be explained by a wavy‐disc model in which the wave amplitude is fixed. The disc surface is then described in Jovigraphic cylindrical coordinates by z = −ro tan α cos [ϕ + Ω (r − ro) /Vo] where ro ≈ 20 RJ, α ≈ 10.7° is the dipole axis tilt angle, Ω = 2π/10 hr−1 is the angular speed of Jupiter's rotation, and Vo ≈ 40 RJ/hr is the speed of wave propagation. Count rate profiles along the Voyager trajectories may then be reproduced using a simple model in which the rates vary exponentially in radial distance and δz, the distance from the disc surface.

Decametric radio measurement of Jupiter's rotation period

A total of 26 measurements of Jupiter's 12-year average rotation period were made at frequencies of 18, 20, and 22.2 MHz at observatories in Florida and Chile. An improved method was employed in which histograms of occurrence probability vs central meridian longitude obtained at the same frequency and observatory during apparitions about 12 years (one Jovian year) apart were cross correlated. The longitude shift giving maximum cross correlation was used to correct the initially assumed rotation period value. The mean of the measurements is 9 hr 55 min 29.689 sec, with a standard deviation of the mean of 0.005 sec. This is about 0.02 sec, or 4 standard deviations, less than the System III (1965) value. The measurements indicate that the rotation period was not changing (linearly) at a rate in excess of 0.03 sec/yr. If the synoptic monitoring program is continued through the next maximum of the jovicentric declination of the Earth ( D E), we will probably be able to detect a rate of change in rotation period as small as 0.002 sec/yr. This accuracy might be sufficient to reveal a secular drift in Jupiter's magnetic field.

Jupiter's Sulfur Ring

Seen from its satellite lo, Jupiter would nearly fill sky, huge, looming presence that would boggle mind accustomed to much smaller disk visible on earthly nights. Yet in another sense, lo also fills sky of Jupiter, by serving as source of various atoms, ions and perhaps molecules that spread in vast, diffuse nebulae throughout much of Jovian system. Though they are probably too rarified and faint to be visible to human observer at giant planet, they have been detected by instruments in both earth-based and spacecraft studies. The most conspicuous from earth has been sodium cloud, comprised of atoms sputtered up from lo's surface and visible by reflected sunlight as faint, golden haze. First detected in spectral measurements some years ago, it was finally photographed late in 1976 (SN: 3/5/77, p. 155), and until recently was only one of lo's various emissions to have actually had its picture taken. Now there is second. It is sulfur, again known for some time from earth-based data and lately measured directly by Voyager 1 and 2 spacecraft. Some may be sputtered up from lo's surface by particles crashing in along Jupiter's magnetic field line, and some may be erupted from lo's newly discovered volcanoes. (The eruption velocities, though apparently often higher than those of terrestrial volcanoes, are still less than lo's escape velocity. But once tossed free, sulfur atoms can be readily ionized by Jupiter-trapped particles, and swept away by fields.) But unlike sodium, whose reflected brightness essentially disappears when it becomes ionized, sulfur is rendered visible to some observations by ionization, although resulting emissions are perhaps thousand times fainter than sodium cloud. The Voyager instruments yielded spectra, not images, but during period between two spacecraft encounters with Jupiter, ionized sulfur was successfully photographed from earth. The images show sulfur in its singly ionized form, SI (the Voyagers also detected more highly ionized species, SI and S3+). They were made by Carl B. Pilcher of University of Hawaii, using 2.2-meter telescope at Mauna Kea Observatory and narrow-band filter tuned to 6731-angstrom forbidden emission line of S+. Pilcher took pictures on two successive evenings -April 9 and 10 of this year -and that fact is significant, for results suggest that St is highly variable phenomenon, changing markedly in short period of time. Because S+ is electrically charged, ions are trapped and held by Jupiter's magnetic field lines, which conduct them into ring-shape or torus surrounding planet. The Voyager 1 and 2 measurements, taken about four months apart, showed appreciable changes both in amount of sulfur and in proportions of different ionization levels. Pilcher's images, however, seem to indicate changes in very size and shape of torus -and over spans of 24 hours or less. The images show only partial arc of torus, so Pilcher drew in rest of ring in each view (estimating from visible portion) to indicate its shape and position. For three April 9 observations, he says, ring with radius of 5.3 times radius of Jupiter (5.3Rj) seems to give a good overall match with photos. In each view, torus seems to be tilted about 10.60 from planet's axis of rotation, which would mean, as expected, that torus was aligned with Jovian magnetic equator. However, he says, the ring had an entirely different appearance on following night. It was larger about 5.7RJ, Pilcher estimates and tilted only about 7.0?. A major part of difference, he suggests, could be matter of difference in kinetic temperature of observed ions between two nights. If ions on April 9 were suitably energetic, calculations have indicated, they would have been distributed symmetrically around Jupiter's magnetic equator. If, on other hand, they were cool and slow-moving enough on April 10, their motion might have been dominated by forces due to rotation of magnetic field in which they were trapped. This would have moved them into centrifugal symmetry surface, which would show observed 70 inclination. The first night's hotter S+ would presumably have been more recently ionized, while that from second night would have lost some of its energy from inter-particle collisions. But there was another difference. On April 9, torus was relatively thin in north-south direction, no thicker than about 0.3 RJ, while on second night its outer edge had flared out in fan (not shown) as thick as 1.1 RJ. And that raises problem in ascribing appearance of first night's torus to hotter, magnetically confined ions. If ions came from neutral sulfur atoms originating on lo, and were being created throughout Jupiter's rotation, April 9 ions should have been distributed over 200 of magnetic lati(Continued on p. 156) 0

Charged-particle absorption by Io

The electrostatic field associated with the rotation of Jupiter, relative to the rest frame of Io, would be distorted if the satellite were an electrical conductor. An idealized two-dimensional model of the distorted electric-field configuration, in the limit of a perfectly conducting satellite or satellite ionosphere, has been constructed and used to trace the adiabatic guiding-center trajectories of energetic protons and electrons across Jupiter's magnetic field lines, which are taken as rectilinear. The adiabatic trajectories of very low-energy particles (cold plasma) are found to avoid the satellite and escape absorption. In the limit of very high particle energies, the adiabatic trajectories are undistorted, and absorption proceeds as if Io were an insulator. The interpolation between these limits is monotonic for protons, such that Io sweeps out a drift shell half as wide as the satellite for first invariants of the order of 1 GeV per gauss. The situation for electrons is more complicated, and no absorption from adiabatic trajectories is found at first invariants not exceeding 46 GeV per gauss. Electrons having first invariants of at least 50 GeV per gauss are typically swept out of drift shells wider than the satellite itself. However, electrons can impact only a portion of Io's exposed hemisphere for first invariants of 50-200 GeV per gauss. Thus, the particle-absorbing characteristics of an electrically conducting Jovian satellite are found to depend on both the species and the energy of the incident particle, and the satellite's particle-absorbing cross section differs systematically from its geometric cross section.

Planetary spin period acceleration of particles in the Jovian magnetosphere

We describe a model for the acceleration of energetic protons and relativistic electrons in the Jovian magnetosphere, employing a four-step process in which ionospheric particles are (1) accelerated outward along field lines by the centrifugal force of corotation, (2) energized by magnetic field annihilation in the Jovian magnetospheric tail, (3) convected inward from the tail, thus experiencing adiabatic compression, and (4) diffused radially inward to form the Jovian trapped radiation belt. Steps 2–4 are analogous to the processes in the earth's magnetosphere that are thought to be responsible for the terrestrial radiation belts. There are three essential differences: the primary source of particles is assumed to be the ionosphere rather than the solar wind; steps 2 and 3 are not continuous but occur with the 10-hour period of Jupiter's rotation; and the energy source for the acceleration is the rotational energy of the planet rather than the solar wind. The periodicity is caused by a diurnal variation of the nightside distance to the last closed field line. The proposed model accounts for the 10-hour modulations in the energetic particle flux observed both outside and inside the Jovian magnetosphere by Pioneer 10 and Pioneer 11. We find that an additional mechanism is required in order to transfer some of the proton energy to the electrons (possibly through a wave instability of the two-stream type). Such an energy transfer mechanism appears also to be required in the terrestrial plasma sheet.

The Jupiter Radio Bursts

Like the strong non-thermal radio bursts from the solar corona and from the earth’s magnetosphere, the Jupiter radio bursts are characterized by their duration which may be from milliseconds to seconds, and by their complex structure on the frequency-time plane. In addition, they exhibit a variety of periodicities in their rate of occurrence, the primary one of which is associated with the rotation of Jupiter. The smaller physical scale of Jupiter compared with the Sun naturally leads to a much shorter time scale in the various radio phenomena and it is only recently that suitable equipment has become available to permit the detailed investigation of the dynamic spectra of the bursts with the necessary high time resolution of the order of 10–3 sec (Ellis 1973a,b,c). As in the case of the solar radio bursts, a number of distinct types of dynamic spectra are observed and they provide a convenient basis for classification.

Evidence from charged particle studies for the distortion of the Jovian magnetosphere

In this paper we consider the relationship between the rotation of Jupiter's magnetic field and time variations in the intensity of ∼6- to 30-MeV electrons observed by the University of Chicago experiment on Pioneer 10 in the outer regions of Jupiter's magnetosphere (R ⪞ 20 RJ). For R ⪝ 40 RJ we find our observations to be consistent with rigid corotation of the magnetosphere with Jupiter. For R ⪞ 40 RJ, significant deviations from rigid corotation appear with the observed phase of the intensity variations leading the phase expected for rigid corotation on the inbound pass and lagging on the outbound pass. From a different point of view we find that the time delay between the observed times of intensity minimums and the times expected on the basis of a rigid 9 hour 55 minute period for the intensity variations increased steadily while Pioneer 10 was within the magnetosphere and had reached approximately a 10-hour time difference when the spacecraft left the magnetosphere at R ≅ 98 RJ outbound. We discuss the alternative hypotheses that in the far outer regions of Jupiter's magnetosphere (R ≅ 90 RJ) these deviations are the result of magnetospheric distortions arising from forces exerted by the solar wind and from drag due to lack of corotation of the magnetospheric plasma at large radial distances from the planet or that the particle intensity in the outer magnetosphere is independent of magnetic latitude and system III longitude but is a function only of time. Although the latter hypothesis is more consistent with observations of Jovian electrons in interplanetary space reported by D. L. Chenette et al. (1974), the observations inside Jupiter's magnetosphere reported here seem to favor particle confinement to an extended magnetic equatorial region, significant distortions of the magnetosphere being invoked to explain the changes in phase of the intensity variations.

The planetary magnetic field and magnetosphere of Jupiter: Pioneer 10

Data obtained by the Pioneer 10 vector helium magnetometer are presented along with models of the intrinsic magnetic field of Jupiter and its magnetosphere. Data acquired between 2.84 and 6.0 RJ, where the intensity of the planetary field ranged between 1900 and 18,400γ, were used to develop a six-parameter eccentric dipole model of the field. The dipole so derived has a moment of 4.0 G RJ³ and a tilt angle with respect to Jupiter's rotation axis of 11°. The system III (epoch 1957) longitude of the magnetic pole in the northern hemisphere, which is a north-seeking pole, is 222°. The dipole is displaced from the center of Jupiter by 0.11 RJ in the direction of latitude 16° and system III longitude 176°. The dipole tilt and the longitude of the pole are in good agreement with values inferred from radio astronomy measurements. The magnetic moment and the offset derived from the Pioneer measurements represent a significant improvement in our knowledge of the planetary field. A model of the Jovian magnetosphere is presented in which the essential feature is an eastward current sheet that forms an annulus with Jupiter at the center. At large distances from the planet the current sheet is nearly parallel to Jupiter's equator but, in general, does not lie in it. The current sheet is warped, so that it is above the equator on one side and below it on the other. The current sheet rotates with the planet, more or less like a rigid body; this behavior causes an apparent up and down motion and periodic crossings of the current sheet by Pioneer. The origin of the current sheet appears to be the very large centrifugal force, associated with Jupiter's great size and rapid rotation, acting on trapped low-energy magnetospheric plasma. The density of this plasma is estimated to be approximately 1 particle cm−3. A retrograde spiraling of field lines out of meridian planes is also observed, presumably as a result of azimuthal drag forces exerted on the outer magnetosphere.

Bursts of relativistic electrons from Jupiter observed in interplanetary space with the time variation of the planetary rotation period

Bursts of Jovian electrons in the energy range 3 to 30 Mev have been detected to distances of the order of 1 AU from the planet. The duration of each burst was about two to three days, and the maximum intensity increased with increasing distance from the sun. These events could not be correlated with known solar activity. The conclusion that these electrons originated at Jupiter and were accelerated within the Jovian magnetosphere is based on observations of three of these bursts. The power spectrum of the temporal variations of the 6- to 30-MeV electron flux showed a persistent peak at Jupiter's rotation frequency (corresponding to the ten hour rotation period) out to distances greater than 100,000,000 km from the planet. The slope, or spectral index, of the differential energy spectrum measured as a function of time displayed the ten-hour cyclic variation in the value of the index, which was earlier found to be characteristic of the electrons within the Jovian magnetosphere.

Summary

Professor G. R. A. Ellis reviewed the wide range of radio emission from Jupiter. At centimetric wavelengths the thermal radiation corresponds to a blackbody at 130K. Between 2 m and 10 cm wavelength there is a powerful component of synchrotron radiation from the electrons trapped in the radiation belts. At longer wavelengths there is a great variety of impulsive radio emission from coherent plasma oscillations.The magnetic field of Jupiter is known from the polarisation of the synchrotron radiation to be situated centrally (within one tenth of the radius) and inclined at 10° to the rotation axis. The radiating electrons have energies of the order of 10 MeV, and a density of 10”-3 cm-3, much greater than in the case of the Earth’s radiation belts.The decametric radiation varies with the rotation of Jupiter, possibly analogously to pulsar radiation. Bursts at around 4 MHz reach very high brightness temperatures, exceeding 1017 K. The occurrence of these strong bursts is closely related to the position of the Jovian satellite Io, which must have an interaction with the main magnetic field.