Dust on the Outskirts of the Jovian System

The outer region of the jovian system between ∼50 and 300 jovian radii from the planet is found to be the host of a previously unknown dust population. We used the data from the dust detector aboard the Galileo spacecraft collected from December 1995 to April 2001 during Galileo's numerous traverses of the outer jovian system. Analyzing the ion amplitudes, calibrated masses and speeds of grains, and impact directions, we found about 100 individual events fully compatible with impacts of grains moving around Jupiter in bound orbits. These grains have moderate eccentricities and a wide range of inclinations—from prograde to retrograde ones. The radial number density profile of the micrometer-sized dust is nearly flat between about 50 and 300 jovian radii. The absolute number density level (∼10 km⁻³ with a factor of 2 or 3 uncertainty) surpasses by an order of magnitude that of the interplanetary background. We identify the sources of the bound grains with outer irregular satellites of Jupiter. Six outer tiny moons are orbiting the planet in prograde and fourteen in retrograde orbits. These moons are subject to continuous bombardment by interplanetary micrometeoroids. Hypervelocity impacts create ejecta, nearly all of which get injected into circumjovian space. Our analytic and numerical study of the ejecta dynamics shows that micrometer-sized particles from both satellite families, although strongly perturbed by solar tidal gravity and radiation pressure, would stay in bound orbits for hundreds of thousands of years as do a fraction of smaller grains, several tenths of a micrometer in radius, ejected from the prograde moons. Different-sized ejecta remain confined to spheroidal clouds embracing the orbits of the parent moons, with appreciable asymmetries created by the radiation pressure and solar gravity perturbations. Spatial location of the impacts, mass distribution, speeds, orbital inclinations, and number density of dust derived from the data are all consistent with the dynamical model.     

Two-dimensional Non-linear Alfvén Wings Generated by the Electrodynamic Interaction between Callisto and the Jovian Magnetosphere

When a highly conducting magnetized plasma passes an object with lower conductivity, or a body with inhomogeneous conductivity, 2-D structures are formed, the so-called `Alfvén wings'. These structures may arise, for example, at a Jovian moon without an intrinsic magnetic field (Callisto). In this case, Alfvén wings could be generated in the magnetized Jovian magnetospheric plasma flow owing to the in homogeneity of the moon's ionosphere/atmosphere conductivity. Such Alfvén wings may be considered as a satellite magnetosphere; the satellite magnetospheric magnetic field is a disturbed field of the Jovian magnetospheric plasma flow. An analytical solution is obtained in a simple proposed model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Developments in Jovian Radio Emissions Tomography and Observations Techniques

Jupiter radio emission is known to be the most powerful nonthermal planetary radiation. In recent years specifically space-based observations allow us to permanently cover a large frequency band(from 100 kHz up to 40 MHz combined with ground-based telescopes)of the Jovian spectrum. The Plasma and Wave Science experiment onboard Galileo enables the observation of Jovian kilometric and hectometric emissions; Wind/WAVES and ground-based telescopes (mainly Decametric Array in Nancay, France, and UTR-2 in Kharkov, Ukraine) cover also hectometric and mainly decametric emissions. Specific geometrical configurations between Cassini approaching Jupiter and Wind spacecraft orbiting Earth, with Galileo orbiting Jupiter and Wind, in combination with ground-based observations provide a new approach to perform Jovian radio tomography. The tomography technique is used to analyze ray paths of Jovian radio emission observed in different directions (e.g. solar and anti-solar direction) and for different declination of Earth. The developments of Jovian radio emission tomography in recent years treated refraction effects and its connection to the local magnetic field in the radio source as well as the radio wave propagation through the Io torus and the terrestrial ionosphere. Most recently ground-based multi-site and simultaneous Jupiter decametric radio observations by means of digital spectropolarimeter and waveform receiver provide the basis of a new data analysis treatment. The above addressed topics are without exemption deeply connected to the plasma structures the radio waves are generated in and propagating through. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Observations of the Dust Around Evolved Stars

ISOPHOT has been used to obtain low resolution spectra from 2.5μm to 5μm and 5.8μm to 11.6μm and multi-aperture photometry at 60μm of several evolved stars; oxygen-rich and carbon-rich (including the peculiar carbon-rich stars R CrB and RY Sgr). R CrB was observed early in the ISO mission, 3 weeks after it had been at minimum light. Another spectrum was obtained several months later. The second spectrum shows that the broad plateau (from around 6μm to 8μm) is still present but the flux density has declined from 60Jy to 50Jy. The spectrum for RY Sgr shows the same type of plateau. The multi-aperture data suggest that the dust shells are resolved around R CrB, RY Sgr, Y CVn and RS Lib. This revised version was published online in August 2006 with corrections to the Cover Date.     

ISO Observations of Wolf-Rayet Dust Shells

Observations of circumstellar dust clouds of Wolf-Rayet stars made with the Short Wavelength Spectrometer on ISO reveal a subtle variety of spectral energy distributions not evident from ground-based observations. The modelling of these using the radiative transfer code "Dusty" by Ivezić, Nenkova & Elitzur is reported. The results are used to examine the contributions by different possible grain types to the emission and of circumstellar and interstellar components to the reddening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observations at the inner edge of the Jovian current sheet: evidence for a dynamic magnetosphere

It is widely recognized that Io, the innermost of the Galilean satellites, releases matter into the rapidly-rotating Jovian magnetosphere at rates that may be as high as a ton per second. Following ionization, this iogenic, heavy-ion plasma dominates the dynamics of the Jovian magnetosphere. On average this plasma must be lost at a rate that balances its generation but we do not know whether this process is steady or intermittent. Measurements by the Galileo magnetometer suggest that this process is unsteady. By estimating the magnetic and particle stresses from these observations, we further can derive a mass density profile that is consistent with earlier measurements of the current sheet density and that is consistent with estimates of the radial transport of mass in the middle Jovian magnetosphere.     

Callisto in the Magnetosphere of Jupiter

Solar System Research - The study of the Galilean satellites can help clarify the structure of the magnetospheric magnetic field from observations of their projections along magnetic field lines in...     

Sodium in the Jovian magnetosphere

Observations of sodium D-line emission from Io and the magnetosphere of Jupiter are reported. A disk-shaped cloud of sodium is found to exist in the Jovian magnetosphere with an inner edge at about 4R and an outer edge at about 10R . The gravitational scale height above the equatorial plane is a few Jovian radii. The data are interpreted in terms of a sputtering model, in which the sodium required to maintain the cloud is sputtered off the surface of Io by trapped energetic radiation-belt protons. Conditions on the atmospheric density are obtained. The Keplerian orbits attainable by such escaping sputtered atoms can provide the observed spatial distribution. The required 500-keV proton flux required to provide the 1–10 keV protons which will sputter the sodium at the surface of Io is consistent with the limiting trapped flux determined by ion-cyclotron turbulence.Publication No. 1410, Institute of Geophysics and Planetary Physics, University of California, Los Angeles 90024, Cal., U.S.A.     

Infrared Observations Of Dust Emission From Comet Hale-Bopp

We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate that varies with heliocentric distance as ∶ r^−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow speed of ∶ 0.4 km s⁻¹. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30% carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell et al., 1991), an overall dust production rate of 2 × 10⁵ kg s⁻¹, a dust-to-gas ratio of ∶5, and an albedo of 39%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dust in Hydrogen-Poor Stellar Winds, from Theory to Observations

We review the nature of dust in hydrogen-deficient stellar winds, in particular cool, carbon-rich Wolf-Rayet (WC) stars, and present new observations of WC objects taken with the Short Wavelength Spectrometer on board of the ISO satellite. Predictions from theoretical models of grain precursor formation are also presented and future directions in both observational and theoretical studies are outlined. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Jovian stratosphere in the ultraviolet

The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150A?has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978–1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800A?, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (5–15 ppb) of propadiene/allene (C₃H₄). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600A?can be reproduced best with the presence of cyclopropane (C₃H₆, <15ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-attitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000A?.     

Magnetosphere of barionic stars

Conclusions Our chief result is the proof that pulsars can possess a quasi-steady-state magnetosphere with temperature T10⁴–10⁶. The magnetosphere can be maintained in this state in its part nearest the star if the plasma is heated by radiation from the star (except for P 0531, for which such radiation is nearlyinsignificant). Plasma in the main part of the magnetosphere is maintained in such a hot state as a result of Joule heat due to drift currents. Radiation from the magnetosphere of P 0531 is found basically in the optical spectrum, though the intensity is several orders of magnitude less than the observed value, so that it does not correspond to the observed optical emission from the pulsar in the Crab nebula.Erevan State University. Translated from Astrofizika, Vol. 12, No. 2, pp. 339–349, April–June, 1976.     

Multiwavelength Observations of the Infrared Dust Bubble N75 and its Surroundings

Infrared dust bubbles play an important role in the study of star formation and the evolution of the interstellar medium.In this work,we study the infrared dust bubble N75 and the infrared dark cloud G38.93 mainly using the tracers C¹⁸O,HCO⁺,HNC and N₂H⁺observed by the 30 m IRAM telescope.We also study the targets using data from large-scale surveys:GLIMPSE,MIPSGAL,GRS,NRAO VLA Sky Survey and Bolocam Galactic Plane Survey.We found that the C¹⁸     

Observations of the Dust Cloud in Comet Hale-Bopp on 9–11 May 1997

During our monitoring observations of comet Hale-Bopp, we found several sporadic ejections of dust from the nucleus. The most prominent ejection was observed on May 6–9, 1997, in the post-perihelion phase of the apparition. In this paper, we report preliminary analysis of this event, in which the total mass of the dust cloud is estimated to be 1.6 × 10¹¹ g. This revised version was published online in July 2006 with corrections to the Cover Date.     

Location of the Jovian magnetic dipole

Measurements of the position of Jupiter's radio centroid at 11 cm were made using the Parkes telescope. They indicate that any systematic displacement of the Jovian magnetic dipole along the rotation axis is to the north of the center of the planet, contrary to a model proposed by Warwick. Any offset of the dipole normal to the rotation axis does not exceed 0.1 of a planetary radius.     

Magnetorotational and Tayler Instabilities in the Pulsar Magnetosphere

The magnetospheres around neutron stars should be very particular because of their strong magnetic field and rapid rotation. A study of the pulsar magnetospheres is of crucial importance since it is the key issue to understand how energy outflow to the exterior is produced. In this paper, we discuss magnetohydrodynamic processes in the pulsar magnetosphere. We consider in detail the properties of magnetohydrodynamic waves that can exist in the magnetosphere and their instabilities. These instabilities lead to formation of magnetic structures and can be responsible for short-term variability of the pulsar emission.     

Thermal structure of the Jovian plasmasphere

An equation of heat transport in the Jovian magnetosphere is formulated and solved in the L range between 2 and 7. Sources of thermal energy include the heating associated with inward radial diffusion and a hypothetical heat supply originating from Io's dynamo action. The principal sink of the thermal energy is charge exchange in Io's hydrogen torus. In order to explain the density and temperature profile reported by Frank et al. (1976), the presence of the heat source at Io is essential and the density of the torus hydrogen has to be considerably lower than the value inferred from L_α observations by Carlson and Judge (1975). Radial diffusion represents the principal heating mechanism for plasma at very low L values.