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.     

Charged dust in the outer planetary magnetospheres

We assume that typical interplanetary grains are fragile, aggregates of the Brownlee type, and discuss the physical and dynamical processes associated with their entry into the Jovian magnetosphere. Limiting ourselves to the equatorial plane of the planet, we show that grains traversing the outer edge of the co-rotating magnetodisc (r35R _J ) are rapidly charged up to large negative potentials on both the day and the night sides. A parent grain of radiusR _g 20 is electrostatically disrupted when it attains a potential of about –220 V. While the eventual potential achieved by the smallest fragments (R _g 0.1 ) are controlled by the rapid field emission of electrons, those of the larger fragments (R _g 1 ) are set by the plasma and photoemission currents.All the negatively charged fragments are strongly attracted towards the planet by the (radial) corotational electric field and some are stably trapped. We suggest that the sudden enhancement by about 2 orders of magnitude of the interplanetary dust flux measured by Pioneer 10, at about 30R _J from Jupiter result from the combination of these two effects.The different brightness asymmetries between the leading and the trailing sides of the outer and inner Galilean satellites appear to be a natural consequence of the way the trajectories of these charged dust grains intersect these satellite surfaces. Finally, the similarity in the brightness asymmetries between the Jovian and Saturnian satellites, and our belief that they have a similar cause, leads us to the expectation that Saturn's magnetic momentM and spin , are parallel as in the case of Jupiter, with the limit of plasma co-rotation lying between the satellites Rhea and Iapetus.     

Sodium remote from Io

We find that faint sodium emission originating in the middle Jupiter magnetosphere has two distinct kinematical components. The “normal” signature of atoms on bound orbits with large apojoves seems always to be present, and we suggest these atoms are an extension of the bright, near-Io sodium cloud. The “fast” signature, with speeds up to at least 100 km sec^?1, is seen only occasionally, and we suggest it is due to an interaction of the near-Io sodium cloud with the corotating, heavy-ion plasma. Both elastic and charge-exchange collisions seem consistent with the observed kinematical and temporal signatures. Elastic collisions seem marginally more capable of producing the high observed sodium atom speeds. We predict observable occurences of the fast component in the hours following passage of the Io sodium cloud through the plasma centrifugal symmetry surface if Io is at a favorable orbital longitude. Between 10 and 20 R_J we find an atomic sodium density ～10^?2 cm^?3. If the photoionization lifetime applies, an Io source of at least 10²⁶ sodium atoms sec^? is required to maintain this remote sodium population.     

Instability near proton-cyclotron harmonics due to anti-loss cone proton distributions

Waves with frequencies near the harmonics of the proton-cyclotron frequency, and propagating almost transverse to the ambient magnetic field, can become unstable by hot protons having an anti-loss cone (ALC) distribution function. The maximum growth rates increase with an increase in anti-loss cone index, ratio of the temperatures of trapped to missing protons, and with a decrease in _H ( _H being the ratio of transverse thermal pressure of protons to magnetic field pressure). The growth rates are typically in the range 0.01–1.0 , where is the proton-cyclotron frequency. This instability may be relevant to the observations of EHC waves on auroral field lines (Kintner, 1979), ULF waves in the day-side magnetosphere (Perrautet al., 1978) and the lowfrequency part of the electric field spectrum (from 5 Hz to 20 Hz) in the region upstream of the bow-shock (Gurnettet al., 1979).     

The nature of the orbits of charged dust injected into the Jovian magnetosphere during the tidal break-up of comet Shoemaker-Levy 9

Assuming that the spin and magnetic axis of Jupiter are strictly parallel and that the grain charge remains constant we have derived two integrals of the 3D equations of motion of charged dust grains moving within the co-rotating regions of the Jovian magnetosphere taking into account both planetary gravitation and magnetospheric rotation. We then apply this model to study the fate of fine dust injected into the Jovian magnetosphere as a result of the tidal disruption of comet Shoemaker-Levy 9 during its first encounter with Jupiter in July 1992. This analysis, which uses the integrals of the equation of motion rather than the equation of motion itself as was done by Horanyi (1994), does not allow us to calculate the orbits or the orbital evolution of the grains. But it does allow us to construct the spatial regions to which the grains are confined, at least initially before evolutionary effects take over. We have chosen three points along the path of the disintegrating comet for the injection of dust and used two values for the uncertain floating potential of the dust in the inner Jovian magnetosphere. Grains can have three different fates, depending on their size, their acquired potential and their point of injection. While the smallest grains are quickly lost by collision with the planet at high latitudes independent of the sign of their charge, those in an intermediate but narrow size range, injected near the equatorial plane can be trapped in a region close to it, this being true for both positive and negative grains. While somewhat larger positive grains may be initially ejected outward by the co-rotational electric force, similar negative grains, pulled inward by this force collide with the planet at low latitudes. In all cases the largest grains, which are dominated by planetary gravity, initially escape from the inner magnetosphere by following in the path of the comet.Using a detailed time dependent numerical calculation of the jovicentric orbits of the charged dust debris of the disintegrating comet, that allows for variation in the grain potential, while also allowing for perturbations of the grain orbits due to solar radiation pressure and solar gravity Horanyi (1994) found that grains in the size range (1.5m <a < 2.5m) which initially make large excursions from the planet, will eventually form a ring in the radial range 4.5R _J <r < 6R _J . Our present analytical calculation cannot make such a prediction about the evolutionary fate of the dust debris. It can, however, estimate the size of the grains that are initially confined to regions near the points of injection, before evolutionary effects become important.     

The atmosphere of Io

Observations of Io suggest that it may have an atmosphere in which sodium vapor, ammonia, and nitrogen are important constituents. Several atmospheric models consisting of these gases are treated here. These are tested as a function of total content against the Pioneer 10 observations and for stability against escape. The results suggest that the atmosphere is very tenuous and that the interpretation of the ionosphere detected by Pioneer 10 by a static model may be inconsistent with the sodium cloud observations. It is postulated that ionization may also be escaping and that sodium may be comparable in content in the atmosphere with some molecular constituent such as NH₃ or N₂. Sodium and this molecular component then dominate the atmosphere. It is also suggested that particle precipitation contributes to heating of the atmosphere and to the production of ionization; furthermore, the difference between day- and nighttime ionospheres and possible trailing and leading side effects may relate to the nature of the particle energy distributions. These distributions may be the result of the peculiar interaction of Io with the Jovian magnetosphere.     

Interaction of the Galilean Moons with their plasma environments

The Galilean moons, especially Io, affect not just their local environment but also the Jovian ionosphere at the ends of the flux tubes connected to the moons. Moreover, the mass added to the magnetosphere by Io affects much of the rest of the magnetosphere. The magnetosphere is energized by this mass-loading, powering the aurora, accelerating radiation belt particles, and generating radio emissions. This review examines how the mass-loading affects the magnetosphere and ionosphere; the differences in the interactions of Io, Europa, Ganymede and Callisto; and some of the kinetic phenomena associated with the interaction.     

Very low frequency (VLF) hiss in Jupiter's magnetosphere

The particle energy required to generate the observed VLF hiss in the Jovian magnetosphere has been computed under longitudinal and transverse resonance condition. It is shown that the minimum energy required by electrons to generate VLF hiss under the longitudinal resonance condition lies in the range of 100eV–1keV for the wave frequencies of 2–10 kHz, while the corresponding energy range for the transverse resonance condition for the same frequency range comes out to be 8 keV–40 keV. Further, the average radiated power by the erenkov process in the Jupiter's magnetosphere atL=5.6 Rj by electrons of energy 10 eV, 100 eV, and 1 keV for the wave frequency of 5 kHz has also been computed.     

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 10³ 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%.     

Proton-cyclotron instabilities in non-uniform loss-cone magnetospheric plasma

The waves, propagating nearly transverse to the ambient magnetic field, with frequencies near the harmonics of the proton-cyclotron frequency are studied in an inhomogeneous plasma with protons having loss-cone distributions. Three types of drift cyclotron instabilities have been studied: (i) non-flute instability; (ii) B-resonant instability; and (iii) non-resonant instability. Increases of loss-cone and density gradient increase the growth rates of all three instabilities. Increases in the positive temperature gradient and _t (ratio of thermal pressure of trapped protons to magnetic field pressure) have a stabilizing effect on the non-flute and non-resonant instabilities and a destabilizing effect on the B-resonant instability. The non-resonant instability has an interesting feature: a particular harmonic can be excited in two separate bands of unstable wave numbers. These instabilities can play an important role in the dynamics of the ring current and the inner edge of the plasma sheet region of the magnetosphere. The discrete turbulence generated by them would give rise to precipitation of protons on the auroral field lines, which may contribute to the excitation of diffuse aurora. These instabilities may be relevant to the observation of harmonic waves at 6R _E by Perrautet al. (1978).     

Determination of the electromagnetic field produced by a magnetic oblique-rotator

The inertial effect on the structure of the magnetosphere of a rotating star is investigated, in the corotation approximation for a surrounding quasi-neutral plasma. The equation of motion reduces to a usual static balance equation between the electromagnetic and the centrifugal forces, in the rotating frame. However the MHD condition, which can be regarded as a special form of the generalized Ohm's law, is modified by the inclusion of inertial effect, with a violation of the frozen-in condition in case of a general (i.e., not restricted to corotation) plasma motion. The inertial effect on the electromagnetic field is summarized in a partial scalar potential named the non-Backus potential, which is proportional to the centrifugal potential in the corotation approximation.An approximate solution of this corotation problem is given, in which another characteristic radiusr _M appears besides the light radiusr _L . This radius defines a distance beyond which the inertial effect becomes dominant over the electromagnetic one, and is useful in estimating the magnitude of the terminal velocity of a centrifugal wind. A few examples of the modification of dipole magnetic field due to the inertial effect are visualized. In an oblique-rotation case, it can be seen that such a warp of the neutral sheet (the surface ofB _r =0) is reproduced as observed in the Jovian magnetosphere.     

Charged dust in the outer planetary magnetospheres

In this paper we consider the dynamics of the electrostatic disruption products of fragile interplanetary dust aggregates which are initially electrically charged on entering the Jovian plasmasphere. On account of their large specific charges, these small dust fragments are strongly effected both by the Lorentz electric force as well as by the polarization electric force resulting from the corotation of the Jovian plasmasphere. The detailed orbits of these charged dust fragments, which are shown to be confined to the equatorial plane, are computed for various launch angles. It is established that the fragments with radii typically around 1 are magneto-gravitationally trapped within the plasma sphere due to the velocity induced oscillation of their surface potentials. The spatial distribution of these fragments are evaluated and the time evolution of the distributions followed. On this basis it is argued that the distribution of micrometeoroid dust within the Jovian magnetospheres, observed by the Pioneer 10 and the recent Voyager spacecraft, is a result of this magneto-gravitational trapping and subsequent orbital evolution of these charged dust fragments. Our discussion includes both the sudden increase, by over an order of magnitude, of the micrometeoroid dust flux at about 30R _J observed by Pioneer 10, and the thin inner dust ring recently observed by the Voyaer spacecraft. The observed brightness asymmetries between the leading and trailing sides of the Galilean satellites appears to be a natural consequence of the impact geometries of these charged dust grains with the satellite surfaces.     

Sodium emission from Comet Shoemaker-Levy 9 in the magnetosphere of Jupiter

On July 20, 1994, before the Q fragments of Comet Shoemaker-Levy 9 fell to Jupiter, more than 200 spectra of the Jovian features were obtained at the Crimean Astrophysical Observatory in the wavelength range 5700–7600 Å with a 26 s exposure time and a spectral resolution of 20 Å. We found a time-varying Na D line emission in the form of two components with Doppler shifts of about 30 Å. The brightest and most frequent sodium flares were detected when the Q fragments passed through the Jovian inner magnetosphere at a distance of about three the Jovian radii (3R_J) from its center, where they crossed the Io-Jupiter current tube. A frequency analysis of our data revealed a flare recurrence time scale of 1 min. We conclude that sodium was released from the cometary dust and from the surfaces of numerous cometary debris and that its amount was enough to produce the observed emission. The observed high-speed clouds of sodium atoms are assumed to have been formed through ionization, ion acceleration by the bidirectional electric fields of Alfvén waves in the Io-Jupiter current tube, and their neutralization.     

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.     

The propagation of Alfven waves along Io's flux tube

The transmission of Alfvén waves from Io to the Jovian ionosphere is discussed. Various simplified laws for the variation of plasma density are analyzed and juxtaposed to simulate a realistic density variation along the Io-Jupiter flux line. Apparently the ionosphere is previously only to frequencies in excess of 1 Hz. The ionosphere participates in tilting the flux tube trapped by Io leading to the formation of a neutral point in the vicinity of the satellite.     

Magnetotail response to sudden changes in the interplanetary magnetic field

The effect of southward or northward changes in the interplanetary magnetic field is examined statistically in the nightside magnetosphere over the range of 6.6 to 80R _E from the Earth. After southward changes, the deformation of the magnetosphere toward a greater antisunward extension of field lines occurs at 6.6R _E with 10 min delay and spreads down the tail to 80R _E in 30 min. Around the onset of the field-line collapse that occurs 1–2 hr later, the southwarddirected field is observed briefly in the distant tail. The effect of northward changes could not be recognized in the lobe region of the tail.     

The satellites of Jupiter as a source of very energetic magnetospheric particles

The Jovian satellites and ring are continuously bombarded by high-energy galacic cosmic rays and magnetospheric ions. Nuclear interactions will create very energetic neutrons and pions. The decay of some of these unstable particles within the Jovian magnetosphere wil result in trapped protons and ultrarelativistic electrons and positrons. Although this source is weak compared to those that yield lower-energy magnetospheric particles, it is expected to generate the most energetic Jovian particles. These processes are briefly described.     

Hydrogen from Jupiter's atmosphere in the Jovian magnetosphere

The passage of Ulysses through Jupiter's magnetosphere presents a new opportunity to investigate the contribution to the Jovian magnetosphere of ions of atmospheric origin. A determination of the magnetospheric H⁺/He²⁺ flux ratio allows an estimate of the relative abundance of ionospheric material in the Jovian magnetosphere. We find that the H⁺/He²⁺ flux ratio, measured in the energy/charge range between 0.65 and 60 keV/e, steadily increases from a solar wind level of 25 at the magnetopause to a value of 700 at the point of closest approach, and then steadily decreases whilst approaching the magnetopause on the outbound path. We conclude from this that: (1) there is a significant solar wind component throughout the outer and middle magnetosphere; and (2) a significant fraction of the protons in the middle magnetosphere are of nonsolar origin.     

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.     

Theory of decametric radio emissions from Jupiter

It is suggested that the Jovian decametric emissions (DAM) originate in a cyclotron instability of weakly relativistic electrons trapped in the Jovian magnetic field. The resulting radiation has a group velocity in the magnetosphheric plasma which may be of order 10²km/sec, and thus takes much more time to escape the magnetosphere than if the group velocity were at or near the speed of light. Therefore, the asymmetry of the Io phase with respect to sources east and west of the Earth-Jupiter line does not imply an asymmetric beaming of DAM; it is caused by the delay the waves experience in traversing the magnetosphere. The frequency drifts of milli- and decasecond bursts are also explained. It is found that the rotation of the magnetosphere can play an important role, since the observer views the propagation velocity of the waves as the sum of their group velocity and the velocity of the medium itself. The rotation velocity is in opposite directions, relative to the observer, for sources east and west of the Earth-Jupiter line; the resultant vector addition gives positive frequency drifts for decasecond bursts from the early and fourth sources, and negative drifts for bursts from the main and third sources. The negative drifts of millisecond bursts may be the result of large density gradients of plasma in a temporarily compressed magnetosphere.