Disks, Extrasolar Planets and Migration

We review results about protoplanetary disk models, protoplanet migration and formation of giant planets with migrating cores. We first model the protoplanetary nebula as an α–accretion disk and present steady state calculations for different values of α and gas accretion rate through the disk. We then review the current theories of protoplanet migration in the context of these models, focusing on the gaseous disk–protoplanet tidal interaction. According to these theories, the migration timescale may be shorter than the planetary formation timescale. Therefore we investigate planet formation in the context of a migrating core, considering both the growth of the core and the build–up of the envelope in the course of the migration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evolution of the Protosolar Nebula and Formation of the Giant Planets

The formation of planetary systems is intimately tied to the question of the evolution of the gas and solid material in the early nebula. Current models of evolution of circumstellar disks are reviewed here with emphasis on the so-called “alpha models” in which angular momentum is transported outward by turbulent viscosity, parameterized by an dimensionless parameter α. A simple 1D model of protoplanetary disks that includes gas and embedded particles is used to introduce key questions on planetesimal formation. This model includes the aerodynamic properties of solid ice and rock grains to calculate their migration and growth. We show that the evolution of the nebula and migration and growth of its solids proceed on timescales that are generally not much longer than the timescale necessary to fully form the star-disk system from the molecular cloud. Contrary to a widely used approach, planet formation therefore can neither be studied in a static nebula nor in a nebula evolving from an arbitrary initial condition. We propose a simple approach to both account for sedimentation from the molecular cloud onto the disk, disk evolution and migration of solids. Giant planets have key roles in the history of the forming Solar System: they formed relatively early, when a significant amount of hydrogen and helium were still present in the nebula, and have a mass that is a sizable fraction of the disk mass at any given time. Their composition is also of interest because when compared to the solar composition, their enrichment in elements other than hydrogen and helium is a witness of sorting processes that occured in the protosolar nebula. We review likely scenarios capable of explaining both the presence of central dense cores in Jupiter, Saturn, Uranus and Neptune and their global composition. This revised version was published online in August 2006 with corrections to the Cover Date.     

On Noble Gas Processing in the Solar Accretion Disk

Two fractionation models are applied to the problem of generating the widely distributed “Q-component” noble gases in meteorites from the solar-like isotopic and elemental compositions that presumably characterized the early solar accretion disk. Noble gas fractionation by mass-dependent dissipation of the solar nebula, as suggested by Ozima et al. (1998), is examined in the context of a model developed by Johnstone et al. (1998) for accretion disk photoevaporation driven by intense UV radiation from a neighboring giant star. Hydrodynamic escape of heavier species entrained in hydrogen outflow from the UV-heated outer regions of the disk can generate substantial noble gas fractionations, but they do not match the observed Q-component isotopic pattern and moreover require the physically unrealistic assumption that the fractionated gases are confined to the heated disk boundary zone, without mixing with the interior nebula, for long periods of time. It seems more likely that hydrodynamic outflow is actually established below this zone, in the body of the disk. In this case fractionations are governed by Rayleigh distillation of the entire remaining nebula, and are negligible at the time when disk erosion is halted by the gravitational potential of the young sun embedded in the disk. A “local” model of noble gas fractionation by hydrodynamic blowoff of transient, methane-rich atmospheres outgassed from the interiors of large primitive planetesimals (Pepin, 1991) is updated and assessed against current data. Degassed atmospheres are assumed to contain isotopically solar noble gases except for an additional nucleogenic Xe component that contributes primarily to the two heaviest isotopes; there is evidence that this same component is present at varying levels in other solar-system volatile reservoirs, possibly reflecting a compositional change with time in the solar nebula. Single fixed values for the two free parameters in the blowoff modeling equations can generate fractionated Xe, Kr, Ar and Ne compositions in the residual atmosphere that closely match observed meteoritic isotopic distributions, and Q-gas elemental ratios are approximated by adsorption of fractionated gases on planetesimal surface grains using plausible values of relative Henry Law constants. Additional requirements for adsorption of sufficient absolute amounts of Q-gases on carrier grains, and their subsequent ejection to space, mixing in the nebula, and dispersal into meteorite bodies, are examined in the context of current models for body sizes and dynamical evolution in an early mass-rich asteroid belt (Chambers and Wetherill, 2001). Despite its ability to replicate isotopic compositions, uncertainties about the environments in which the blowoff model can successfully operate suggest that there is, as yet, no entirely satisfactory understanding of how the Q-component noble gases might have evolved from solar-like precursor compositions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Formation and Composition of Planetesimals

The composition of planetesimals depends upon the epoch and the location of their formation in the solar nebula. Meteorites produced in the hot inner nebula contain refractory compounds. Volatiles were present in icy planetesimals and cometesimals produced in the cold outer nebula. However, the mechanism responsible for their trapping is still controversial. We argue for a general scenario valid in all regions of the turbulent nebula where water condensed as a crystalline ice (Hersant et al., 2004). Volatiles were trapped in the form of clathrate hydrates in the continuously cooling nebula. The epoch of clathration of a given species depends upon the temperature and the pressure required for the stability of the clathrate hydrate. The efficiency of the mechanism depends upon the local amount of ice available. This scenario is the only one so far which proposes a quantitative interpretation of the non detection of N₂ in several comets of the Oort cloud (Iro et al., 2003). It may explain the large variation of the CO abundance observed in comets and predicts an Ar/O ratio much less than the upper limit of 0.1 times the solar ratio estimated on C/2001 A2 (Weaver et al., 2002). Under the assumption that the amount of water ice present at 5 AU was higher than the value corresponding to the solar O/H ratio by a factor 2.2 at least, the clathration scenario reproduces the quasi uniform enrichment with respect to solar of the Ar, Kr, Xe, C, N and S elements measured in Jupiter by the Galileo probe. The interpretation of the non-uniform enrichment in C, N and S in Saturn requires that ice was less abundant at 10 AU than at 5 AU so that CO and N₂ were not clathrated in the feeding zone of the planet while CH₄, NH₃ and H₂S were. As a result, the 14N/15N ratio in Saturn should be intermediate between that in Jupiter and the terrestrial ratio. Ar and Kr should be solar while Xe should be enriched by a factor 17. The enrichments in C, N and S in Uranus and Neptune suggest that available ice was able to form clathrates of CH₄, CO and the NH₃ hydrate, but not the clathrate of N₂. The enrichment of oxygen by a factor 440 in Neptune inferred by Lodders and Fegley (1994) from the detection of CO in the troposphere of the planet is higher by at least a factor 2.5 than the lower limit of O/H required for the clathration of CO and CH4 and for the hydration of NH₃. If CO detected by Encrenaz et al. (2004) in Uranus originates from the interior of the planet, the O/H ratio in the envelope must be around of order of 260 times the solar ratio, then also consistent with the trapping of detected volatiles by clathration. It is predicted that Ar and Kr are solar in the two planets while Xe would be enriched by a factor 30 to 70. Observational tests of the validity of the clathration scenario are proposed.     

The Solar Origin of Corotating Interaction Regions and Their Formation in the Inner Heliosphere

Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heliospheric ENA fluxes: How Sensitive Are They to the Ionization State of LIC?

We explore the sensitivity of the fluxes of heliospheric energetic neutral atoms (ENA) at 1 AU to the ionization state of the local interstellar cloud (LIC). The solar wind plasma is compressed and heated in the termination shock transition. The shocked solar plasma is convected toward the heliospheric tail in the heliosheath, the region between the termination shock and the heliopause. The ENAs are produced in charge exchange of the plasma protons and background neutral gas and can be readily detected at 1 AU. The expected ENA fluxes depend on the shocked plasma density, temperature, and velocity in the heliosheath. The size and structure of the heliospheric interface region depend on the parameters of the interstellar medium. ENA fluxes would thus reveal the LIC parameters. We demonstrate the sensitivity of the heliospheric ENA fluxes to the ionization state of the LIC. The axi-symmetric model of the solar wind/LIC interaction includes the self-consistent treatment of the plasma-gas coupling and Monte Carlo simulations of the neutral gas distribution. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cometary dynamics

The modern theory of cometary dynamics is based on Oort's hypothesis that the solar system is surrounded by a spherically symmetric cloud of 10¹¹ to 10¹² comets extending out to interstellar distances. Dynamical modeling and analysis of cometary motion have confirmed the ability of the Oort hypothesis to explain the observed distribution of energies for the long-period comet orbits. The motion of comets in the Oort cloud is controlled by perturbations from random passing stars, interstellar clouds, and the galactic gravitational field. Additionally, comets which enter the planetary region are perturbed by the major planets and by nongravitational forces resulting from jetting of volatiles on the surfaces of the cometary nuclei. The current Oort cloud is estimated to have a radius of 6 to 8 × 10⁴ AU, and to contain some 2 × 10¹² comets with a total mass of 7 to 8 Earth masses. Evidence has begun to accumulate for the existence of a massive inner Oort cloud extending from just beyond the orbit of Neptune to 10⁴ AU or more, with a population up to 100 times that of the outer Oort cloud. This inner cloud may serve as a reservoir to replenish the outer cloud as comets are stripped away by the various perturbers, and may also provide a more efficient source for the short-period comets. Recent suggestions of an unseen solar companion star or a tenth planet orbiting in the inner cloud and causing periodic comet showers on the Earth are likely unfounded. The formation site of the comets in the Oort cloud was likely the extended nebula accretion disc reaching from about 15 to 500 AU from the forming protosun. Comets which escape from the Oort cloud contribute to the flux of interstellar comets, though capture of interstellar comets by the solar system is extremely unlikely. The existence of Oort clouds around other main sequence stars has been suggested by the detection by the IRAS spacecraft of cool dust shells around about 10% of nearby stars.     

Filtration of Interstellar Atoms through the Heliospheric Interface

Interstellar atoms penetrate deep into the heliosphere after passing through the heliospheric interface—the region of the interaction of the solar wind with the interstellar medium. The heliospheric interface serves as a filter for the interstellar atoms of hydrogen and oxygen, and, to a lesser extent, nitrogen, due to their coupling with interstellar and heliospheric plasmas by charge exchange and electron impact ionization. The filtration has great importance for the determination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends on the level of their coupling with the plasma in the interaction region. The recent studies of the filtration of the interstellar atoms in the heliospheric interface region is reviewed in this paper. The dependence of the filtration on the local interstellar proton and H atom number densities is discussed and the roles of the charge exchange and electron impact ionization on the filtration are evaluated. The influence of electron temperature in the inner heliosheath on the filtration process is discussed as well. Using the filtration coefficients obtained from the modeling and SWICS/Ulysses pickup ion measurements, the local interstellar abundances of the considered elements are determined.     

Observation of the interstellar helium cone by the NOZOMI spacecraft

‘The Japanese Mars probe, NOZOMI, is staying in the interplanetary space (1–1.5 AU) until its Mars’ orbit insertion scheduled in early 2004. Every 16 months on this interplanetary orbit the spacecraft crosses around 1 AU the ‘gravitational focusing cone’ of the interstellar helium, which are penetrating into the inner heliosphere under the solar gravity. During the first crossing of the cone in the season of March–May 2000, we observed these helium particles after the solar wind pickup process with an E/q type ion detector aboard NOZOMI. We have estimated the original temperature of the interstellar helium as 11 000 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) for the Mars Express Mission

The general scientific objective of the ASPERA-3 experiment is to study the solar wind – atmosphere interaction and to characterize the plasma and neutral gas environment with within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1–60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products (secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity (the hydrogen energy range is 0.1–10 keV) and mass (H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01–20 keV. These three sensors are located on a scanning platform which provides scanning through 180^∘ of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to the ASPERA-3 main unit. IMA provides ion measurements in the energy range 0.01–36 keV/charge for the main ion components H⁺, He⁺⁺, He⁺, O⁺, and the group of molecular ions 20–80 amu/q. ASPERA-3 also includes its own DC/DC converters and digital processing unit (DPU).     

Simultaneous Observations of Solar Energetic Particle Events by imp 8 and the Ulysses Cospin High Energy Telescope at High Solar Latitudes

With Ulysses approaching the south solar polar latitudes during a period of high solar activity, it is for the first time possible to study the distribution of solar energetic particles (SEPs) in solar latitude as well as in radius and longitude. From July 1997 to August 2000, Ulysses moved from near the solar equator at ∼5 AU to ∼67° S latitude at ∼3 AU. Using observations of >∼30 MeV protons from Ulysses and IMP-8 at Earth we find good correlation between large SEP increases observed at IMP and Ulysses, almost regardless of the relative locations of the spacecraft. The observations show that within a few days after injection of SEPs, the flux in the inner heliosphere is often almost uniform, depending only weakly on the position of the observer. No clear effect of the increasing solar latitude of Ulysses is evident. Since the typical latitudinal extent of CMEs, which most likely accelerate the SEPs, is only ∼30°, this suggests that the enhanced cross-field propagation for cosmic rays and CIR-accelerated particles deduced from Ulysses’ high latitude studies near solar minimum is also true for SEPs near solar maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

The ‘Standard’ Sun Modelling and Helioseismology

The ‘standard’ solar model is based on a number of simplifying assumptions and depends on knowledge of the physical properties of matter in the Sun. Given these assumptions, the constraint that the model have the observed surface luminosity provides an estimate of the initial solar helium abundance. From helioseismic analyses further information can be obtained about the present composition, including a fairly precise measure of the envelope helium abundance and an estimate of the hydrogen profile in the radiative interior. It must be emphasized, however, that these inferences may suffer from systematic error arising from incomplete knowledge about the equation of state and opacity of the solar interior. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cometary Refractory Grains: Interstellar and Nebular Sources

Comets are heterogeneous mixtures of interstellar and nebular materials. The degree of mixing of interstellar sources and nebular sources at different nuclear size scales holds the promise of revealing how cometary particles, cometesimals, and cometary nuclei accreted. We can ascribe cometary materials to interstellar and nebular sources and see how comets probe planet-forming process in our protoplanetary disk. Comets and cometary IDPs contain carbonaceous matter that appears to be either similar to poorly-graphitized (amorphous) carbon, a likely ISM source, or highly labile complex organics, with possible ISM or outer disk heritage. The oxygen fugacity of the solar nebula depends on the dynamical interplay between the inward migration of carbon-rich grains and of icy (water-rich) grains. Inside the water dissociation line, OH^? reacts with carbon to form CO or CO₂, consuming available oxygen and contributing to the canonical low oxygen fugacity. Alternatively, the influx of water vapor and/or oxygen rich dust grains from outer (cooler) disk regions can raise the oxygen fugacity. Low oxygen fugacity of the canonical solar nebula favors the condensation of Mg-rich crystalline silicates and Fe-metal, or the annealing of Fe-Mg amorphous silicates into Mg-rich crystals and Fe-metal via Fe-reduction. High oxygen fugacity nebular conditions favors the condensation of Fe-bearing to Fe-rich crystalline silicates. In the ISM, Fe-Mg amorphous silicates are prevalent, in stark contrast to Mg-rich crystalline silicates that are rare. Hence, cometary Mg-rich crystalline silicates formed in the hot, inner regions of the canonical solar nebula and they are the touchstone for models of the outward radial transport of nebular grains to the comet-forming zone. Stardust samples are dominated by Mg-rich crystalline silicates but also contain abundant Fe-bearing and Fe-rich crystalline silicates that are too large (?0.1 μm) to be annealed Fe-Mg amorphous silicates. By comparison with asteroids, the Stardust Fe-bearing and Fe-rich crystalline silicates suggests partial aqueous alteration in comet nuclei. However, aqueous alteration transforms Fe-rich olivine to phyllosilicates before Mg-rich olivine, and Stardust has Mg-rich and Fe-rich olivine and no phyllosilicates. Hence, we look to a nebular source for the moderately Fe-rich to nearly pure-Fe crystalline silicates. Primitive matrices have Mg-Fe silicates but no phyllosilicates, supporting the idea that Mg-Fe silicates but not phyllosilicates are products of water-rich shocks. Chondrule-formation is a late stage process in our protoplanetary disk. Stardust samples show comet 81P/Wild 2 formed at least as late to incorporate a few chondrules, requiring radial transport of chondrules out to perhaps >20 AU. By similar radial transport mechanisms, collisional fragments of aqueously altered asteroids, in particular achondrites that formed earlier than chondrules, might reach the comet-forming zones. However, Stardust samples do not have phyllosilicates and chondrules are rare. Hence, the nebular refractory grains in comet 81P/Wild 2, as well as other comets, appear to be pre-accretionary with respect to asteroid parent bodies. By discussing nebular pathways for the formation of Fe-rich crystalline silicates, and also phyllosilicates and carbonates, we put forth the view that comets contain both the interstellar ingredients for and the products of nebular transmutation.     

Interstellar Dust in the Solar System

The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse almost perpendicular to the ecliptic plane (inclination 79°, perihelion distance 1.3 AU, aphelion distance 5.4 AU) since it encountered Jupiter in 1992. The in situ dust detector on board continuously measured interstellar dust grains with masses up to 10⁻¹³ kg, penetrating deep into the solar system. The flow direction is close to the mean apex of the Sun’s motion through the solar system and the grains act as tracers of the physical conditions in the local interstellar cloud (LIC). While Ulysses monitored the interstellar dust stream at high ecliptic latitudes between 3 and 5 AU, interstellar impactors were also measured with the in situ dust detectors on board Cassini, Galileo and Helios, covering a heliocentric distance range between 0.3 and 3 AU in the ecliptic plane. The interstellar dust stream in the inner solar system is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the time varying interplanetary magnetic field. We review the results from in situ interstellar dust measurements in the solar system and present Ulysses’ latest interstellar dust data. These data indicate a 30° shift in the impact direction of interstellar grains w.r.t. the interstellar helium flow direction, the reason of which is presently unknown.     

CIR Associated Energetic Particles in the Inner and Middle Heliosphere

Energetic particles associated with Corotating Interaction Regions (CIRs) are observed throughout the inner and middle heliosphere, showing large positive (>100%/AU) radial intensity gradients. Their appearance at 1 AU is associated with the appearance of fast, recurrent solar wind streams. At several AU, CIR energetic particles are accelerated at shocks which propagate away from the interface of fast and slow solar wind streams. CIR energy spectra at 1 AU cover the range >35 keV to several MeV/amu; the spectra steepen above ∼1 MeV/amu, and show no turnover even at the lowest energies. The ion composition of CIRs is similar to solar material, but with significant differences that might be due to properties of the seed population and/or the acceleration process. This paper summarizes properties of energetic particles in CIRs as known through the early 1990s, prior to the launch of the Ulysses, and WIND spacecraft, whose new results are presented in Kunow, Lee et al. (1999) in this volume. This revised version was published online in August 2006 with corrections to the Cover Date.     

Protostellar Winds and Chondritic Meteorites

We discuss the interaction between the magnetosphere of a young star and its surrounding accretion disk. We consider how an X-wind can be driven magnetocentrifugally from the inner edge of the disk where accreting gas is diverted onto stellar field lines either to flow onto the Sun or to be flung outwards with the wind. The X-wind satisfies many observational tests concerning optical jets, Herbig-Haro objects, and molecular outflows. Connections may exist between primitive solar system materials and X-winds. Chondrules and calcium-aluminum-rich inclusions (CAIs) experienced short melting events uncharacteristic of the asteroid belt where meteorites originate. The inner edge of the solar nebula has the shortest orbital timescale available to the system, a few days. Protosolar flares introduce another timescale, tens of minutes to hours. CAIs may form when solids are lifted from shaded portions of the disk close to the Sun and are exposed to its intense light for a day or so before they are flung by the X-wind to much larger distances. Chondrules were melted, perhaps many times, by flares at larger distances from the Sun before being launched and annealed, but not remelted, in the X-wind. Aerodynamic sorting explains the narrow range of sizes with which CAIs and chondrules are found in chondritic meteorites. Flare-generated cosmic-rays may induce spallation reactions that produce some of the short-lived radioactivities associated with primitive solar system rocks. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tracing the magnetic topology of coronal mass ejection events by Ulysses/HI-SCALE energetic particle observations in and out of the ecliptic

In January 2000, the Ulysses spacecraft observed an ICME event at 43° S heliographic latitude and ∼ 4.1 AU. We use electron (E _e>38 keV) observations to trace the topology of the IMF embedded within the ICME. The still controversial issue of whether ICMEs have been detached from the solar corona or are still magnetically anchored to it when they arrive at the spacecraft is tackled. An in ecliptic ICME event is also presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

Interstellar Matter and the Boundary Conditions of the Heliosphere

The interstellar cloud surrounding the solar system regulates the galactic environment of the Sun, and determines the boundary conditions of the heliosphere. Both the Sun and interstellar clouds move through space, so these boundary conditions change with time. Data and theoretical models now support densities in the cloud surrounding the solar system of n(H0)=0.22±0.06 cm−3, and n(e−)∼0.1 cm−3, with larger values allowed for n(H0) by radiative transfer considerations. Ulysses and Extreme Ultraviolet Explorer satellite He0 data yield a cloud temperature of 6400 K. Nearby interstellar gas appears to be structured and inhomogeneous. The interstellar gas in the Local Fluff cloud complex exhibits elemental abundance patterns in which refractory elements are enhanced over the depleted abundances found in cold disk gas. Within a few parsecs of the Sun, inconclusive evidence for factors of 2–5 variation in Mg+ and Fe+ gas phase abundances is found, providing evidence for variable grain destruction. In principle, photoionization calculations for the surrounding cloud can be compared with elemental abundances found in the pickup ion and anomalous cosmic-ray populations to model cloud properties, including ionization, reference abundances, and radiation field. Observations of the hydrogen pile up at the nose of the heliosphere are consistent with a barely subsonic motion of the heliosphere with respect to the surrounding interstellar cloud. Uncertainties on the velocity vector of the cloud that surrounds the solar system indicate that it is uncertain as to whether the Sun and α Cen are or are not immersed in the same interstellar cloud. This revised version was published online in June 2006 with corrections to the Cover Date.     

Propagating Shocks

A survey of propagating interplanetary shocks reveals the following properties: (1) More shocks are observed around solar-activity maximum than minimum. (2) The maximum frequency of shock occurrence is between 2 and 5 AU. (3) Shocks slow down in the inner solar system, but in the outer solar system some may get a boost in speed when overtaken by a following shock. (4) The average shock strength (as measured by either the Mach number or the density ratio) also reaches peak values at distances of 2–5 AU. (5) Shocks are stronger at their noses than at their flanks. (6) At 1 AU, there are many more quasiperpendicular than quasiparallel shocks with the frequency of occurrence roughly constant with the cosine of the angle between the upstream field and the shock normal.