Energetic particles in the jovian magnetotail

When the solar wind hits Jupiter's magnetic field, it creates a long magnetotail trailing behind the planet that channels material out of the Jupiter system. The New Horizons spacecraft traversed the length of the jovian magnetotail to >2500 jovian radii (RJ; 1 RJ identical with 71,400 kilometers), observing a high-temperature, multispecies population of energetic particles. Velocity dispersions, anisotropies, and compositional variation seen in the deep-tail (greater, similar 500 RJ) with a approximately 3-day periodicity are similar to variations seen closer to Jupiter in Galileo data. The signatures suggest plasma streaming away from the planet and injection sites in the near-tail region (approximately 200 to 400 RJ) that could be related to magnetic reconnection events. The tail structure remains coherent at least until it reaches the magnetosheath at 1655 RJ.     

The source of Saturn's G ring

The origin of Saturn's narrow G ring has been unclear. We show that it contains a bright arc located 167,495.6 +/- 1.3 km from Saturn's center. This longitudinally localized material is trapped in a 7:6 corotation eccentricity resonance with the satellite Mimas. The cameras aboard the Cassini spacecraft mainly observe small (1 to 10 micrometers) dust grains in this region, but a sharp decrease in the flux of energetic electrons measured near this arc requires that it also contain larger (centimeter- to meter-sized) bodies whose total mass is equivalent to that of a approximately 100-meter-wide ice-rich moonlet. Collisions into these bodies may generate dust, which subsequently drifts outward to populate the rest of the G ring. Thus, the entire G ring could be derived from an arc of debris held in a resonance with Mimas.     

Saturnian trapped radiation and its absorption by satellites and rings: the first results from pioneer 11

Electrons and protons accelerated and trapped in a Saturnian magnetic field have been found by the University of Chicago experiments on Pioneer 11 within 20 Saturn radii (Rs) of the planet. In the innermost regions, strong absorption effects due to satellites and ring material were observed, and from approximately 4 Rs inwards to the outer edge of the A ring at 2.30 Rs (where the radiation is absorbed), the intensity distributions of protons (>/= 0.5 million electron volts) and electrons (2 to 20 million electron volts) were axially symmetric, consistent with a centered dipole aligned with the planetary rotation axis. The maximum fluxes observed for protons (> 35 million electron volts and for electrons < 3.4 million electron volts) were 3 x 10(4) and 3 x 10(6) per square centimeter per second, respectively. Absorption of radiation by Mimas provides a means of estimating the radial diffusion coefficient for charged particle transport. However, the rapid flux increases observed between absorption features raise new questions concerning the physics of charged particle transport and acceleration. An absorption feature near 2.5 Rs has led to the discovery of a previously unknown satellite with a diameter of approximately 200 kilometers, semimajor axis of 2.51 Rs, and eccentricity of 0.013. Radiation absorption features that suggest a nonuniform distribution of matter around Saturn have also been found from 2.34 to 2.36 Rs, near the position of the F ring discovered by the Pioneer imaging experiment. Beneath the A, B, and C rings we continued to observe a low flux of high-energy electrons. We conclude that the inner Saturn magnetosphere, because of its near-axial symmetry and the many discrete radiation absorption regions, offers a unique opportunity to study the acceleration and transport of charged particles in a planetary magnetic field.     

Origin of Saturn's E Ring: Self-Sustained, Naturally

Saturn's diffuse E ring spans the region between 3 and 8 saturnian radii (R(s)), has its peak brightness near the orbit of the satellite Enceladus (3.95 R(s)), and is thought to be composed primarily of icy particles 1.0 +/- 0.3 micrometers in radius. Such particles are shown to move periodically along highly elliptical paths that cross the orbits of several saturnian satellites; the resulting energetic collisions of E ring particles with embedded satellites are capable of sustaining the E ring at its current optical depth. With several reasonable assumptions, this model naturally selects Enceladus as the primary source of ring material and may also provide mechanisms that explain the generation of the unusual amount of submicrometer dust in the neighboring F and G rings, the excess of OH molecules observed within the E ring, and the orbital brightness variations of nearby satellites.     

Cassini dust measurements at Enceladus and implications for the origin of the E ring

During Cassini's close flyby of Enceladus on 14 July 2005, the High Rate Detector of the Cosmic Dust Analyzer registered micron-sized dust particles enveloping this satellite. The dust impact rate peaked about 1 minute before the closest approach of the spacecraft to the moon. This asymmetric signature is consistent with a locally enhanced dust production in the south polar region of Enceladus. Other Cassini experiments revealed evidence for geophysical activities near Enceladus' south pole: a high surface temperature and a release of water gas. Production or release of dust particles related to these processes may provide the dominant source of Saturn's E ring.     

First plasma wave observations at uranus

Radio emissions from Uranus were detected by the Voyager 2 plasma wave instrument about 5 days before closest approach at frequencies of 31.1 and 56.2 kilohertz. About 10 hours before closest approach the bow shock was identified by an abrupt broadband burst of electrostatic turbulence at a radial distance of 23.5 Uranus radii. Once Voyager was inside the magnetosphere, strong whistler-mode hiss and chorus emissions were observed at radial distances less than about 8 Uranus radii, in the same region where the energetic particle instruments detected intense fluxes of energetic electrons. Various other plasma waves were also observed in this same region. At the ring plane crossing, the plasma wave instrument detected a large number of impulsive events that are interpreted as impacts of micrometer-sized dust particles on the spacecraft. The maximum impact rate was about 30 to 50 impacts per second, and the north-south thickness of the impact region was about 4000 kilometers.     

Energetic charged particles in the magnetosphere of neptune

The Voyager 2 cosmic ray system (CRS) measured significant fluxes of energetic [>/=1 megaelectron volt (MeV)] trapped electrons and protons in the magnetosphere of Neptune. The intensities are maximum near a magnetic L shell of 7, decreasing closer to the planet because of absorption by satellites and rings. In the region of the inner satellites of Neptune, the radiation belts have a complicated structure, which provides some constraints on the magnetic field geometry of the inner magnetosphere. Electron phase-space densities have a positive radial gradient, indicating that they diffuse inward from a source in the outer magnetosphere. Electron spectra from 1 to 5 MeV are generally well represented by power laws with indices near 6, which harden in the region of peak flux to power law indices of 4 to 5. Protons have significantly lower fluxes than electrons throughout the magnetosphere, with large anisotropies due to radial intensity gradients. The radiation belts resemble those of Uranus to the extent allowed by the different locations of the satellites, which limit the flux at each planet.     

Absorption of sunlight in the atmosphere of venus

In this report the fluxes measured by the solar flux radiometer (LSFR) of the Pioneer Venus large probe are compared with calculations for model atmospheres. If the large particles of the middle and lower clouds are assumed to be sulfur, strong, short-wavelength absorption results in a net flux profile significantly different from the LSFR net flux measurements. Models in which the smallest particles are assumed to be sulfur gave flux profiles consistent with the measurements if an additional source of absorption is included in the upper cloud. The narrowband data from 0.590 to 0.665 micrometer indicate an absorption optical depth of about 0.05 below the cloud bottom. The broadband data imply that either this absorption extends over a considerable wavelength interval (as might be the case for dust) or that a very strong absorption band lies on one side of the narrowband filter (as suggested by early Venera 11 and Venera 12 reports). Thermal balance calculations based on the measured visible fluxes indicate high surface temperature for reasonable assumptions of cloud opacity and water vapor abundance. The lapse rate becomes convective within the middle cloud. For water mixing ratios of 2.0 x 10(-4) below the clouds we find a subadiabatic region extending from the cloud bottom to altitudes near 35 kilometers.     

An Overview of Energetic Particle Measurements in the Jovian Magnetosphere with the EPAC Sensor on Ulysses

Observations of ions and electrons of probable Jovian origin upstream of Jupiter were observed after a corotating interplanetary particle event. During the passage of Ulysses through the Jovian bow shock, magnetopause, and outer magnetosphere, the fluxes of energetic particles were surprisingly low. During the passage through the "middle magnetosphere," corotating fluxes were observed within the current sheet near the jovimagnetic equato. During the outbound pass, fluxes were variably directed; in the later part of the flyby, they were probably related to high-latitude phenomena.     

Energetic Charged-Particle Phenomena in the Jovian Magnetosphere: First Results from the Ulysses COSPIN Collaboration

The Ulysses spacecraft made the first exploration of the region of Jupiter's magnetosphere at high Jovigraphic latitudes ( approximately 37 degrees south) on the dusk side and reached higher magnetic latitudes ( approximately 49 degrees north) on the day side than any previous mission to Jupiter. The cosmic and solar particle investigations (COSPIN) instrumentation achieved a remarkably well integrated set of observations of energetic charged particles in the energy ranges of approximately 1 to 170 megaelectron volts for electrons and 0.3 to 20 megaelectron volts for protons and heavier nuclei. The new findings include (i) an apparent polar cap region in the northern hemisphere in which energetic charged particles following Jovian magnetic field lines may have direct access to the interplanetary medium, (ii) high-energy electron bursts (rise times approximately 17 megaelectron volts) on the dusk side that are apparently associated with field-aligned currents and radio burst emissions, (iii) persistence of the global 10-hour relativistic electron "clock" phenomenon throughout Jupiter's magnetosphere, (iv) on the basis of charged-particle measurements, apparent dragging of magnetic field lines at large radii in the dusk sector toward the tail, and (v) consistent outflow of megaelectron volt electrons and large-scale departures from corotation for nucleons.     

Trapped radiation belts of saturn: first look

Pioneer 11 has made the first exploration of the magnetosphere and trapped radiation belts of Saturn. Saturn's magnetosphere is intermediate in size between Earth's and Jupiter's, with trapped particle intensities comparable to Earth's. The outer region of Saturn's magnetosphere contains lower energy radiation and is variable with time; the inner region contains higher energy particles. The pitch angle distributions show a remarkable variety of field-aligned and locally mirroring configurations. The moons and especially the rings of Saturn are effective absorbers of trapped particles; underneath the rings, the trapped radiation is completely absorbed. We confirm the discovery of a new ring, called the F ring, a new division, the Pioneer division, and a moon, called 1979 S 2. The latter has probably been seen from Earth. There may be evidence for more bodies like 1979 S 2, but at this stage the interpretation of the data is ambiguous. Using particle diffusion rates, we estimate that the cross-sectional area of the F ring is > 7 x 10(13) square centimeters and that the opacity is > 10(-5). Cosmic-ray albedo neutron decay should be looked into as a source of energetic particles in the inner magnetosphere of Saturn.     

Radiation belts of jupiter: a second look

The outbound leg of the Pioneer 11 Jupiter flyby explored a region farther from the equator than that traversed by Pioneer 10, and the new data require modification or augmentation of the magnetodisk model based on the Pioneer 10 flyby. The inner moons of Jupiter are sinks of energetic particles and sometimes sources. A large spike of particles was found near lo. Multiple peaks occurred in the particle fluxes near closest approach to the planet; this structure may be accounted for by a complex magnetic field configuration. The decrease in proton flux observed near minimum altitude on the Pioneer 10 flyby appears attributable to particle absorption by Amalthea.     

Comet west and the scattering function of cometary dust

Observations of Comet West (1975n) at wavelengths from 0.5 to 18 microns and at a variety of scattering angles are used to infer the scattering phase function for the cometary dust. This function is strongly peaked in the forward direction. The form of the function indicates that the particles are dielectric grains with radii of approximately 1 micron. Abrupt increases in the intrinsic brightness of the coma (both in scattered sunlight and in thermal emission) are consistent with the projected times of comet fragmentation.     

Zodiacal dust and deep-sea sediments

The recent detection of radioactive Al(26) in marine sediments has led to the conclusion that it is brought into the earth's atmosphere by micrometeorites which have been exposed, in interplanetary space, to solar high-energy protons. The Al(26) method is not precise enough to yield directly a reliable value for the mass accretion rate to the earth to better than about 3 orders of magnitude, but is sufficiently accurate to allow a crucial decision between two widely differing of interplanetary dust which have been proposed to explain observations of the zodiacal light. The two models lead to Al(26) concentrations which would differ by about 5 orders of magnitude. Thus, the presence of Al(26) is consistent with the zodiacal dust model with particles of some tens of microns rather then with submicron particles. From this model a mass accretion to the earth then be calculated which is set at 1250 (upper limit, 2500; lower limit, 250) tons per day, or 2.8 x 10(-15) g/cm(2) sec, or 4.5 x 10(11) g over the earth per This value does not depend on the flux of the solar high-energy particles, which may be uncertain by an order of magnitude or more. The presence of Al(26) supports the idea that an important fraction of the dust is stony in composition material density, and thus eliminates some more exotic dust models, as such one consisting entirely of carbon grains. We may also conclude that the accreted dust particles have been in the solar system and exposed to protons from solar high-energy particles for a time interval which is greater than a significant of the Al(26) half-life (0.74 x 10(6) years).     

Photometry from voyager 2: initial results from the uranian atmosphere, satellites, and rings

The Voyager 2 photopolarimeter successfully completed the Uranus encounter, acquiring new data on the planet's atmosphere, its principal satellites, and its ring system. Spatially resolved photometry of the atmosphere at 0.27 micrometer shows no enhancement in absorption toward the pole, unlike the case for Jupiter and Saturn. Stellar occultation measurements indicate the temperature at the 1-millibar level over the north pole is near 90 kelvins. The geometric albedos of the five large satellites of Uranus were measured at 0.27 and 0.75 micrometer and indicate the presence of low albedo, spetrally flat absorbing material. Titania seems to have a fluffy surface, as indicated by its phase curve. The nine ground-based rings were detected, and their internal structure, optical depths, and positions were determined. The sharp edges of the in ring made it possible to measure its edge thickness (less than 150 meters) and particle sizes (less than 30 meters); little or no dust was detcted. New narrow rings and partial rings (arcs) were measured, and the narrow component of the eta ring was found to be discontinuous.     

Low-Energy Charged Particles in Saturn's Magnetosphere: Results from Voyager 1

The low-energy charged particle instrument on Voyager 1 measured low-energy electrons and ions (energies >/= 26 and >/= 40 kiloelectron volts, respectively) in Saturn's magnetosphere. The first-order ion anisotropies on the dayside are generally in the corotation direction with the amplitude decreasing with decreasing distance to the planet. The ion pitch-angle distributions generally peak at 90 degrees , whereas the electron distributions tend to have field-aligned bidirectional maxima outside the L shell of Rhea. A large decrease in particle fluxes is seen near the L shell of Titan, while selective particle absorption (least affecting the lowest energy ions) is observed at the L shells of Rhea, Dione, and Tethys. The phase space density of ions with values of the first invariant in the range approximately 300 to 1000 million electron volts per gauss is consistent with a source in the outer magnetosphere. The ion population at higher energies (>/= 200 kiloelectron volts per nucleon) consists primarily of protons, molecular hydrogen, and helium. Spectra of all ion species exhibit an energy cutoff at energies >/= 2 million electron volts. The proton-to-helium ratio at equal energy per nucleon is larger (up to approximately 5 x 10(3)) than seen in other magnetospheres and is consistent with a local (nonsolar wind) proton source. In contrast to the magnetospheres of Jupiter and Earth, there are no lobe regions essentially devoid of particles in Saturn's nighttime magnetosphere. Electron pitch-angle distributions are generally bidirectional andfield-aligned, indicating closed field lines at high latitudes. Ions in this region are generally moving toward Saturn, while in the magnetosheath they exhibit strong antisunward streaming which is inconsistent with purely convective flows. Fluxes of magnetospheric ions downstream from the bow shock are present over distances >/= 200 Saturn radii from the planet. Novel features identified in the Saturnian magnetosphere include a mantle of low-energy particles extending inward from the dayside magnetopause to approximately 17 Saturn radii, at least two intensity dropouts occurring approximately 11 hours apart in the nighttime magnetosphere, and a pervasive population of energetic molecular hydrogen.     

Low-Energy Hot Plasma and Particles in Saturn's Magnetosphere

The low-energy charged particle instrument on Voyager 2 measured low-energy electrons and ions (energies greater, similar 22 and greater, similar 28 kiloelectron volts, respectively) in Saturn's magnetosphere. The magnetosphere structure and particle population were modified from those observed during the Voyager 1 encounter in November 1980 but in a manner consistent with the same global morphology. Major results include the following. (i) A region containing an extremely hot ( approximately 30 to 50 kiloelectron volts) plasma was identified and extends from the orbit of Tethys outward past the orbit of Rhea. (ii) The low-energy ion mantle found by Voyager 1 to extend approximately 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( approximately 30 kiloelectron volts), and there was an indication of a cooler ( < 20 kiloelectron volts) ion mantle on the nightside. (iii) At energies greater, similar 200 kiloelectron volts per nucleon, H(1), H(2), and H(3) (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H(2) and H(3) suggests that the Saturnian ionosphere feeds plasma into the magnetosphere, but relative abundances of the energetic helium, carbon, and oxygen ions are consistent with a solar wind origin. (iv) Low-energy ( approximately 22 to approximately 60 kiloelectron volts) electron flux enhancements observed between the L shells of Rhea and Tethys by Voyager 2 on the dayside were absent during the Voyager 1 encounter. (v) Persistent asymmetric pitch-angle distributions of electrons of 60 to 200 kiloelectron volts occur in the outer magnetosphere in conjunction with the hot ion plasma torus. (vi) The spacecraft passed within approximately 1.1 degrees in longitude of the Tethys flux tube outbound and observed it to be empty of energetic ions and electrons; the microsignature of Enceladus inbound was also observed. (vii) There are large fluxes of electrons of approximately 1.5 million electron volts and smaller fluxes of electrons of approximately 10 million electron volts and of protons greater, similar 54 million electron volts inside the orbits of Enceladus and Mimas; all were sharply peaked perpendicular to the local magnetic field. (viii) In general, observed satellite absorption signatures were not located at positions predicted on the basis of dipole magnetic field models.     

Earth-Based Radio Tracking of the Galileo Probe for Jupiter Wind Estimation

Although the Galileo probe was designed to communicate only to the orbiter, the probe radio signal was detected at two Earth-based radio observatories where the signal was a billion times weaker. The measured signal frequency was used to derive a vertical profile of the jovian zonal wind speed. Due to the mission geometry, the Earth-based wind estimates are less sensitive to descent trajectory errors than estimates based on probe-orbiter Doppler measurements. The two estimates of wind profiles agree qualitatively; both show high wind speeds at all depths sampled.     

Composition of saturnian stream particles

During Cassini's approach to Saturn, the Cosmic Dust Analyser (CDA) discovered streams of tiny (less than 20 nanometers) high-velocity (approximately 100 kilometers per second) dust particles escaping from the saturnian system. A fraction of these impactors originated from the outskirts of Saturn's dense A ring. The CDA time-of-flight mass spectrometer recorded 584 mass spectra from the stream particles. The particles consist predominantly of oxygen, silicon, and iron, with some evidence of water ice, ammonium, and perhaps carbon. The stream particles primarily consist of silicate materials, and this implies that the particles are impurities from the icy ring material rather than the ice particles themselves.