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.     

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.     

Charged dust in the outer planetary magnetospheres

Interplanetary dust grains entering the Jovian plasmasphere become charged, and those in a certain size range get magneto-gravitationally trapped in the corotating plasmasphere. The trajectories of such dust grains intersect the orbits of one or more of the Galilean satellites. Orbital calculations of micron sized dust grains show that they impact the outermost satellite Callisto predominantly on its leading face, while they impact the inner three — Io, Europa and Ganymede — predominantly on the trailing face. These results are offered as an explanation of the observed brightness asymmetry between the leading and trailing faces of the outer three Galilean satellites. The albedo of Io is likely to be determined by its volcanism.     

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.     

Magnetic field modulated dust streams from Jupiter in interplanetary space

High speed dust streams emanating from near Jupiter were first discovered by the Ulysses spacecraft in 1992. Since then the phenomenon has been re-observed by Galileo in 1995, Cassini in 2000, and Ulysses in 2004. The dust grains are expected to be charged to a potential of , which is sufficient to allow the planet's magnetic field to accelerate them away from the planet, where they are subsequently influenced by the interplanetary magnetic field (IMF). A similar phenomenon was observed near Saturn by Cassini. Here, we report and analyze simultaneous dust, IMF and solar wind data for all dust streams from the two Ulysses Jupiter flybys. We find that compression regions (CRs) in the IMF – regions of enhanced magnetic field – precede most dust streams. Furthermore, the duration of a dust stream is roughly comparable with that of the precedent CR, and the occurrence of a dust stream and the occurrence of the previous CR are separated by a time interval that depends on the distance to Jupiter. The intensity of the dust streams and their precedent CRs are also correlated, but this correlation is only evident at distances from the planet no greater than 2 AU. Combining these observations, we argue that CRs strongly affect dust streams, probably by deflecting dust grain trajectories, so that they can reach the spacecraft and be detected by its dust sensor.     

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.     

Characterization of Jovian plasma-embedded dust particles

As the data from space missions and laboratories improve, a research domain combining plasmas and charged dust is gaining in prominence. Our solar system provides many natural laboratories such as planetary rings, comet comae and tails, ejecta clouds around moons and asteroids, and Earth's noctilucent clouds for which to closely study plasma-embedded cosmic dust. One natural laboratory to study electromagnetically controlled cosmic dust has been provided by the Jovian dust streams and the data from the instruments which were on board the Galileo spacecraft. Given the prodigious quantity of dust poured into the Jovian magnetosphere by Io and its volcanoes resulting in the dust streams, the possibility of dusty plasma conditions exist. This paper characterizes the main parameters for those interested in studying dust embedded in a plasma with a focus on the Jupiter environment. I show how to distinguish between dust-in-plasma and dusty-plasma and how the Havnes parameter P can be used to support or negate the possibility of collective behavior of the dusty plasma. The result of applying these tools to the Jovian dust streams reveals mostly dust-in-plasma behavior. In the orbits displaying the highest dust stream fluxes, portions of orbits E4, G7, G8, C21 satisfy the minimum requirements for a dusty plasma. However, the P parameter demonstrates that these mild dusty plasma conditions do not lead to collective behavior of the dust stream particles.     

Radial diffusion in Jovian inner belt

Radial diffusion of equatorially mirroring particles of solar wind origin in Jovian inner magnetosphere is reviewed. Using the Pioneer 10 and 11 data on plasma and magnetic field parameters of Jupiter, phase-space density profile of the inner belt (i.e., 1 = L 5) has been derived.     

The role of cohesive forces in particle launching on the Moon and asteroids

The movement of small dust particles due to electrostatic forces, seismic activity and micrometeoroid bombardment has been hypothesized to occur on the Moon and asteroids. There currently exists significant uncertainty in the method of launching these small dust particles, which in turn makes the selection of accurate initial conditions for numerical simulations difficult. We evaluate the electric field strength required to launch small particles given surface gravitation, cohesion and seismic shaking. We find that the electric field strength required for dust particle launching is dominated by the cohesive force for micron-sized dust particles. There exists an intermediate dust particle size that requires the least electric field strength to launch. We see that the inclusion of the cohesive force significantly influences our understanding of dust lofting.     

Physical processes in Jupiter's ring: Clues to its origin by Jove!

The particles making up the Jovian ring may be debris which has been excavated by micrometeoroids from the surfaces of many unseen (R ? 1 km) parent bodies (or “mooms” as we will occasionally call them) residing in the ring. A distribution of particle sizes exists: large objects are sources for the small visible ring particles and also account for the absorption of charged particles noted by Pioneer; the small grains are generated by micrometeoroid impacts, by jostling collisions among different-sized particles, and by self-fracturing due to electrostatic stresses. The latter are most effective in removing surface asperities to thereby produce smooth and crudely equidimensional grains. The presence of intermediate-sized (radius of several to several hundred microns) objects is also expected; these particles will have a total area comparable to the area of the visible ring particles. The nominal size (?2 μm) of the visible particles derived from their forward-scattering characteristics is caused, at least in part, by a selection effect but may also reflect a fundamental grain size or the preferential generation of certain sizes along with the destruction of others. The tiny ring particles have short lifetimes (?10²?10³ years) limited by erosion due to sputtering and meteoroid impacts. Plasma drag significantly modifies orbits in ～10² years but Poynting-Robertson drag is not effective (T_PR ～ 10⁵ years) in removing debris. The ring width is influenced by the distribution of source satellites, by the initial ejection velocity off them, by electromagnetic scattering, and by solar radiation forces. In the absence of electromagnetic forces, debris will reimpact a mother satellite or collide with another particle in about 10 years. A relative drift between different-sized particles, caused by a lessened effective gravity due to the Lorentz force, will substantially shorten these times to less than a month. The ring thickness is determined by a balance between initial conditions (abetted perhaps by electromagnetic scattering) and collisional damping; existence of the “halo” over the diffuse disk compared to its relative absence over the bright ring indicates the presence of mooms in the bright ring but not in the faint disk. Small satellites (R ? 1 km) will not reaccumulate colliding dust grains whereas satellites having the size of J14 or J16 may be able to do so, depending upon their precise shape, size, density, and location. Visible ring structure could indicate separate source satellites. The particles in the faint inner disk are delivered from the bright ring by orbital evolution principally under plasma drag. The halo is comprised of small particles (～0.1 μm) partially drawn out of the faint disk by interactions with the tilted Jovian magnetic field.     

Lunar Dust: Properties and Investigation Techniques

Physical conditions in the near-surface layer of the Moon are overviewed. This medium is formed in the course of the permanent micrometeoroid bombardment of the lunar regolith and due to the exposure of the regolith to solar radiation and high-energy charged particles of solar and galactic origin. During a considerable part of a lunar day (more than 20%), the Moon is passing through the Earth’s magnetosphere, where the conditions strongly differ from those in the interplanetary space. The external effects on the lunar regolith form the plasma-dusty medium above the lunar surface, the so-called lunar exosphere, whose characteristic altitude may reach several tens of kilometers. Observations of the near-surface dusty exosphere were carried out with the TV cameras onboard the landers Surveyor 5, 6, and 7 (1967–1968) and with the astrophotometer of Lunokhod-2 (1973). Their results showed that the near-surface layer glows above the sunlit surface of the Moon. This was interpreted as the scattering of solar light by dust particles. Direct detection of particles on the lunar surface was made by the Lunar Ejects and Meteorite (LEAM) instrument deployed by the Apollo 17 astronauts. Recently, the investigations of dust particles were performed by the Lunar Atmosphere and Dust Environment Explorer (LADEE) instrument at an altitude of several tens of kilometers. These observations urged forward the development of theoretical models for the lunar exosphere formation, and these models are being continuously improved. However, to date, many issues related to the dynamics of dust and the near-surface electric fields remain unresolved. Further investigations of the lunar exosphere are planned to be performed onboard the Russian landers Luna-Glob and Luna-Resurs.     

Radio emission observed by Galileo in the inner Jovian magnetosphere during orbit A-34

The Galileo spacecraft encountered the inner magnetosphere of Jupiter on its way to a flyby of Amalthea on November 5, 2002. During this encounter, the spacecraft observed distinct spin modulation of plasma wave emissions. The modulations occurred in the frequency range from a few hundred hertz to a few hundred kilohertz and probably include at least two distinct wave modes. Assuming transverse EM radiation, we have used the swept-frequency receivers of the electric dipole antenna to determine the direction to the source of these emissions. Additionally, with knowledge of the magnetic field some constraints are placed on the wave mode of the emission based on a comparative analysis of the wave power versus spin phase of the different emissions. The emission appears in several bands separated by attenuation lanes. The analysis indicates that the lanes are probably due to blockage of the freely propagating emission by high density regions of the Io torus near the magnetic equator. Radio emission at lower frequencies (<40 kHz) appears to emanate from sources at high latitude and is not attenuated. Emission at is consistent with O-mode and Z-mode. Lower frequency emissions could be a mixture of O-mode, Z-mode and whistler mode. Emission for shows bands that are similar to upper hybrid resonance bands observed near the terrestrial plasmapause, and also elsewhere in Jovian magnetosphere. Based on the observations and knowledge of similar terrestrial emissions, we hypothesize that radio emission results from mode conversion near the strong density gradient of the inner radius of the cold plasma torus, similar to the generation of nKOM and continuum emission observed in the outer Jovian magnetosphere and in the terrestrial magnetosphere from source regions near the plasmapause.     

An explanation for time dependent variability of the solar dust ring

There are several observations showing an enhancement of infrared emission and optical polarization at a distance of 4R , (R is the angular radius of the Sun) implying a ring of dust in near-ecliptic orbit about the Sun; but there is an almost equal number of observations, which do not show any such enhancement. We plotted the observational results for the detection and the non-detection of the circum-solar dust on a diagram for the variation of the sun-spot number with time, and found that its detection and non-detection occurred near the solar minimum and maximum phases, respectively. We present the possibility that this phenomena is caused by an additional process, i.e., the Lorentz force acting on a charged dust particle. Since, at the maximum phase a dust particle in a near-solar region acquires a higher positive potential, and the solar magnetic field is very strong, the Lorentz force becomes dominant and can affect the orbit of the dust particle.     

Energetic Particles and Coronal Mass Ejections: a Case Study From ace and Ulysses

In 2001 the Ulysses spacecraft crossed the ecliptic plane near perihelion. The heliographic longitude with respect to the Earth was within ±20° of the west solar limb while it was ±15° of the ecliptic plane, which meant that coronal mass ejections seen off the solar west limb were likely to pass over Ulysses. On 10 May the largest >38 keV electron intensity of the mission, since the Jovian encounter in 1992, was observed, which was accompanied by a fast perpendicular shock. This event was preceded by a fast coronal mass ejection some two and a half days earlier which is the probable source of the shock. However, both the ACE spacecraft and Ulysses observed, simultaneously, an intense, prompt electron event on 7 May from a solar flare associated with earlier coronal mass ejections also observed off the west limb; Ulysses was magnetically connected to a longitude well behind the west limb. ACE did not observe any (at the 0.1% level) energetic electrons which were associated with the 10 May event seen at Ulysses. We discuss in detail the energetic particles seen at the two spacecraft during 7–11 May, with the objective of understanding the origin of the intense electron event seen on 10 May and the manner in which particles escaping from the shock populate the inner heliosphere. The energy spectrum of the ions at both ACE and Ulysses exhibits a maximum at around 400 keV; this form of the spectrum was seen at the shock itself. It appears that the strong shock driven by the fast coronal mass ejection is able to populate a large fraction of the inner heliosphere with accelerated ions. The shock-accelerated electrons do not pervade the inner heliosphere in the same manner as the ions. We suggest that the electron acceleration was enhanced by the presence of multiple coronal mass ejections.     

A model of the origin of the Jovian ring

Starting with the assumption that the micron-sized particles which make up the bright Jovian ring are fragments of erosive collisions between micrometeoroid projectiles and large parent bodies, a physical model of the ring is calculated. The physics of high-velocity impacts leads to a well-defined size distribution for the ejecta, the optical properties of which can be compared with observation. This gives information on the ejecta material (very likely silicates) and on the maximum size of the projectiles, which turns out to be about 0.1 μm. The origin of these projectiles is discussed, and it is concluded that dust particles ejected in volcanic activity from Io are the most likely source. The impact model leads quite naturally to a distribution in ejecta sizes, which in turn determines the structure of the ring. The largest ejecta form the bright ring, medium-sized ejecta form a disk extending all the way to the Jovian atmosphere, and the small ejecta form a faint halo, the structure of which is dominated by electromagnetic forces. In addition to the Io particles, interaction with interplanetary micrometeoroids is also considered. It is concluded that μm-sized ejecta from this source have ejection velocities which are several orders of magnitude too large, and thus cannot contribute significantly to the observed bright ring. However, the total mass ejection rate is significant. Destruction of these ejecta by the Io particles may provide additional particles for the halo.     

Observations of Electromagnetically Coupled Dust in the Jovian Magnetosphere

We report on dust measurements obtained during the seventh orbit of the Galileo spacecraft about Jupiter. The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by more than an order of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. In addition to the 5 and 10 h periodicities, a longer period which is compatible with Io's orbital period is evident in the dust impact rate. This feature indicates that Io most likely is the source of the dust streams. During a close (3,095 km altitude) flyby at Ganymede on 5 April 1997 an enhanced rate of dust impacts has been observed, which suggests that Ganymede is a source of ejecta particles. Within a distance of about 25 R_J(Jupiter radius, R_J= 71,492 km) from Jupiter impacts of micrometer-sized particles have been recorded which could be particles on bound orbits about Jupiter. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dust blobs in the solar nebula — primary distended atmosphere

When planetary accretion proceeds in the gas disk-solar nebula, a protoplanet attracts surrounding gas to form a distended H₂-He atmosphere. The blanketing effect of the atmosphere, hampering the escape of accretional energy, enhances the surface temperature of planets. Furthermore, evaporation of ice or reduction of surface silicate and metallic oxide can supply a huge amount of water vapor into the atmosphere, which would raise the temperature and promote evaporation. Evaporated materials can be efficiently conveyed outward by vigorous convection, and condensed dust particles should keep the atmosphere opaque during accretion. The size of this opaque atmosphere dust blob is defined by the gravitational radius, which exceeds 3 × 10⁸ m when the planetary mass is the Earth's mass (5.97 × 10²⁴ kg). This is larger than the radii of present Jovian planets and so-called brown dwarfs. The expected lifetime of dust blobs is 10⁶–10⁷ yr, which is longer than that of the later gas accreting and cooling stages of Jovian planets. The number of dust blobs could exceed that of Jovian planets. If the gas disk is rather transparent, the possibility of observing such objects with a distended atmosphere may be higher than that of detecting Jovian planets. Contamination of the gas disk by the dust from primary atmospheres is negligible.Paper presented at the Conference on Planetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Observations of electron plasma waves upstream of the Jovian bow shock

The Ulysses spacecraft encountered the planet Jupiter in February 1992, on its journey towards high heliospheric latitude. During the approach to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of the Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, first observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshock, upstream of the Earth's bow shock. These bursty emissions, with a peak frequency very close to the local electron plasma frequency F_pe, have a typical electric field amplitude in the range 0.01–0.1 mV m⁻¹, with some bursts above 1 mV m⁻¹. The frequency bandwidth over which significant power can be found above the instrument background noise ranges from below 0.2 F_pc to about 2 F_pc. On the basis of our present knowledge of similar emissions observed at Earth, we suggest that the broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.     

Gravitational sources of purely electromagnetic origin

The Einstein-Maxwell field equations for charged dust corresponding to static axially-symmetric metric of Levi-Civita have been studied. It has been shown that when the metric potentialsg _ij are functions of only one of the coordinates, viz.,r, the interior charged dust becomes purely of electromagnetic origin, in the sense that the physical quantities like the energy density, the effective gravitational mass, etc., are dependent only on the charge density and vanish when this charge density vanishes. Such models are known as electromagnetic mass models in the classical electrodynamics. An interior charged dust solution corresponding to this case has been obtained which, in a sense, represents an infinite dust distribution of electromagnetic origin. In the second case, viz., when the metric potentials are functions of the coordinatesr andz both, it has been shown that some of the situations correspond to electromagnetic mass models. An example to illustrate this case has been obtained. This represents the source of the Reissner-Nordström-Curzon field (an analogue of the Reissner-Nordström solution obtained by Curzon) which according to Curzon describes the exterior field of an electron.     

Dust transport in photoelectron layers and the formation of dust ponds on Eros

We investigate the electrostatic transport of charged dust in the photoelectron layer over the dayside surface of an asteroid. Micron-sized dust particles may be levitated above the surface in the photoelectron layer. Horizontal transport within the layer can then lead to net deposition of dust into shadowed regions where the electric field due to the photoelectron layer disappears. We apply a 2D numerical model simulating charged dust dynamics in the near-surface daytime plasma environment of Asteroid 433 Eros to the formation of dust deposits in craters. We find that dust tends to collect in craters and regions of shadow. This electrostatic dust transport mechanism may contribute to the formation of smooth dust ponds observed by the NEAR-Shoemaker spacecraft at Eros. The size distribution of transported dust depends on the particle density and work function, and the work function of the surface and solar wind electron temperature and density. With reasonable values for these parameters, μm-sized and smaller particles are levitated at Eros. Micrometeoroid bombardment is not a sufficient source mechanism for electrostatic transport to create the Eros dust ponds. Laboratory measurements of dust in a plasma sheath show that dust launched off the surface by direct electrostatic levitation can provide a sufficient source for transport to produce the observed Eros ponds.