Greater Heliocentric Distances Research Articles

Constraining the origin of Mars with simulations of multi-stage core formation

It remains an elusive goal to simultaneously model the astrophysics of Solar System accretion while reproducing the mantle chemistry of more than one inner terrestrial planet. Here, we used a multistage core-mantle differentiation model based on Rubie et al. (2011, 2015) to track the formation and composition of Earth and Mars in various Grand Tack formation simulations. Prior studies showed that in order to recreate Earth’s mantle composition, it must grow first from reduced (Fe-metal rich and O-poor) building blocks and then from increasingly oxidized (FeO rich) material. This accretion chemistry occurs when an oxidation gradient exists across the disk so that the innermost solids are reduced and increasingly oxidized material is found at greater heliocentric distances. For a suite of Grand Tack simulations, we investigated whether Earth and Mars can be simultaneously produced by the same oxidation gradient. Our model did not find an oxidation gradient that simultaneously reproduces the mantle compositions of Earth and Mars. Due to its small mass and rapid formation, the formation history of Mars-like planets is very stochastic which decreases the likelihood of compatibility with an Earth-producing oxidation gradient in any given realization. To reconcile the accretion history and ideal chemistry of the Mars-like planet with the oxidation gradient of an Earth-producing disk, we determined where in the Earth-producing disk Mars must have formed. We find that the FeO-rich composition of the Martian mantle requires that Mars’ building blocks must originate exterior to 1.0 astronomical units (AU).

Water-susceptible primordial noble gas components in less-altered CR chondrites: A possible link to cometary materials

We report detailed characterizations of the petrology, mineralogy, and noble gas signatures of Renazzo-type (CR) chondrites EET 92048, MIL 090657, NWA 801, and hydrothermally treated MIL 090657 samples to investigate primordial noble gases hosted by water-susceptible materials in aqueously less-altered CR chondrites. The noble gases were extracted by a stepwise heating method. The hydrothermal treatment on MIL 090657 resulted in the alteration of amorphous silicates in the matrix and the removal of large amounts of He and Ne that were released at low temperature from the untreated sample. The 36Ar/132Xe ratios in EET 92048 and MIL 090657 are ~5 times higher than that of the Q noble gas component, although both meteorites do not show any evidence for solar wind noble gases. The analyses revealed that aqueously less-altered CR chondrites contain two distinct primordial noble gas components hosted by water-susceptible materials: an isotopically Q-like Ne-rich component and an Ar-rich component. The characteristics of the Q-like Ne-rich component are consistent with those of Q-like Ne in cometary materials. We propose that the CR chondrites formed at a greater heliocentric distance and share similar materials hosting the Q-like Ne-rich component with comets. The presence of the Ar-rich component in the CR chondrites, as well as in ordinary chondrites and other anhydrous carbonaceous chondrites, provides evidence of the global distribution of the Ar-rich carriers in the ordinary and carbonaceous chondrite forming regions.

Random Walk and Trapping of Interplanetary Magnetic Field Lines: Global Simulation, Magnetic Connectivity, and Implications for Solar Energetic Particles

Abstract The random walk of magnetic field lines is an important ingredient in understanding how the connectivity of the magnetic field affects the spatial transport and diffusion of charged particles. As solar energetic particles propagate away from near-solar sources, they interact with the fluctuating magnetic field, which modifies their distributions. We develop a formalism in which the differential equation describing the field line random walk contains both effects due to localized magnetic displacements and a non-stochastic contribution from the large-scale expansion. We use this formalism together with a global magnetohydrodynamic simulation of the inner-heliospheric solar wind, which includes a turbulence transport model, to estimate the diffusive spreading of magnetic field lines that originate in different regions of the solar atmosphere. We first use this model to quantify field line spreading at 1 au, starting from a localized solar source region, and find rms angular spreads of about 20°–60°. In the second instance, we use the model to estimate the size of the source regions from which field lines observed at 1 au may have originated, thus quantifying the uncertainty in calculations of magnetic connectivity; the angular uncertainty is estimated to be about 20°. Finally, we estimate the filamentation distance, i.e., the heliocentric distance up to which field lines originating in magnetic islands can remain strongly trapped in filamentary structures. We emphasize the key role of slab-like fluctuations in the transition from filamentary to more diffusive transport at greater heliocentric distances.

Origin of volatile element depletion among carbonaceous chondrites

Compared to the composition of CI chondrites and the Sun, all other carbonaceous chondrites are variably depleted in volatile elements. However, the origin of these depletions, and how they are related to volatile loss during high-temperature processes within the solar nebula, are unclear. To better understand the processes that caused volatile element fractionations among carbonaceous chondrites, we obtained mass-dependent Te isotopic compositions and Te concentrations for a comprehensive set of samples from the major carbonaceous chondrite groups. The chondrites exhibit well-resolved inter-group Te isotope variations towards lighter isotopic compositions for increasingly volatile-depleted samples. The Te isotopic compositions and concentrations are also correlated with the mass fraction of matrix and with nucleosynthetic ε54Cr anomalies. Combined, these correlations indicate mixing between volatile-rich, isotopically heavy, and 54Cr-rich CI-like matrix with volatile-poor, isotopically light, and 54Cr-poorer chondrules or chondrule precursors. The Te-Cr isotopic correlation suggests that all carbonaceous chondrites contain CI-like matrix, and that chondrules and this CI-like matrix formed from isotopically distinct material originating from different regions of the disk. The only samples plotting off the Te-Cr correlation are CR chondrites, indicating that CR chondrules formed from different precursor material than chondrules from other carbonaceous chondrites, either because they formed at greater heliocentric distance and/or at a later time. Plots of volatile element abundances versus matrix mass fraction reveal that chondrules/chondrule precursors display CI-chondritic ratios for volatile elements with 50% condensation temperatures below ∼750 K, with an overall abundance of ∼0.13 × CI. Mixing between these two components, therefore, naturally results in CI-like ratios for these elements in all carbonaceous chondrites, in spite of different degrees of volatile depletion. A corollary of this observation is that the CI-like ratios of volatile elements in the bulk silicate Earth may result from the accretion of volatile-depleted materials and do not require accretion of CI chondrites themselves.

Electrons in the Young Solar Wind: First Results from the Parker Solar Probe

Abstract The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. We derive the parameters of the core, halo, and strahl populations utilizing a combination of fitting to model distributions and numerical integration for ∼100,000 electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as ∼0.17 au. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure (β e ) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, β e , and/or collisional age. Near the Sun, some trends not clearly seen at 1 au become apparent, including anticorrelations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.

An Empirical Model of Energetic Neutral Atom Imaging of the Heliosphere and Its Implications for Future Heliospheric Missions at Great Heliocentric Distances

Abstract Several concepts for heliospheric missions operating at heliocentric distances far beyond Earth orbit are currently investigated by the scientific community. The mission concept of the Interstellar Probe, e.g., aims at reaching a distance of 1000 au away from the Sun within this century. This would allow the coming generation to obtain a global view of our heliosphere from an outside vantage point by measuring the energetic neutral atoms (ENAs) originating from the various plasma regions. It would also allow for direct sampling of the unperturbed interstellar medium, as well as for many observation opportunities beyond heliospheric science, such as visits to Kuiper Belt objects, a comprehensive view on the interplanetary dust populations, and infrared astronomy free from the foreground emission of the zodiacal cloud. In this study, we present a simple empirical model of ENAs from the heliosphere and derive basic requirements for ENA instrumentation on board a spacecraft at great heliocentric distances. We consider the full energy range of heliospheric ENAs from 10 eV to 100 keV because each part of the energy spectrum has its own merits for heliospheric science. To cover the full ENA energy range, two or three different ENA instruments are needed. Thanks to parallax observations, some insights about the nature of the IBEX ribbon and the dimensions of the heliosphere can already be gained by ENA imaging from a few au heliocentric distance. To directly reveal the global shape of the heliosphere, measurements from outside the heliosphere are, of course, the best option.

Nucleosynthetic molybdenum isotope anomalies in iron meteorites – new evidence for thermal processing of solar nebula material

We have investigated nucleosynthetic Mo isotope anomalies in 38 different bulk iron meteorites from 11 groups, to produce by far the largest and most precise dataset available to date for such samples. All magmatic iron groups were found to display deficits in s-process Mo isotopes, with essentially constant anomalies within but significant variations between groups. Only meteorites of the non-magmatic IAB/IIICD complex revealed terrestrial Mo isotopic compositions.The improved analytical precision achieved in this study enables two isotopically distinct suites of iron meteorites to be identified. Of these, the r=p suite encompasses the IC, IIAB, IIE, IIIAB, IIIE and IVA groups and exhibits relatively modest but ‘pure’ s-process deficits, relative to Earth. The second r>p suite includes groups IIC, IIIF and IVB. These iron meteorites show larger s-process deficits than the r=p suite, coupled with an excess of r-process relative to p-process components.Comparison of the results with data for other elements (e.g., Cr, Ni, Ru, Ti, Zr) suggests that the Mo isotope variability is most likely produced by thermal processing and selective destruction of unstable presolar phases. An updated model is proposed, which relates the iron meteorite suites to different extents of thermal processing in the solar nebula, as governed by heliocentric distance. In detail, the r=p suite of iron meteorite parent bodies is inferred to have formed closer to the Sun, where the extent of thermal processing was similar to that experienced by terrestrial material, so that the meteorites exhibit only small s-process deficits relative to Earth. In contrast, the r>p suite formed at greater heliocentric distance, where more subtle thermal processing removed a smaller proportion of r- and p-process host phases, thereby generating larger s-process deficits relative to the terrestrial composition. In addition, the thermal conditions enabled selective destruction of p- versus r-isotope carrier phases, to produce the observed divergence of r- and p-process Mo isotope abundances.

Ruthenium isotopic evidence for an inner Solar System origin of the late veneer.

The excess of highly siderophile elements in the Earth's mantle is thought to reflect the addition of primitive meteoritic material after core formation ceased. This 'late veneer' either comprises material remaining in the terrestrial planet region after the main stages of the Earth's accretion, or derives from more distant asteroidal or cometary sources. Distinguishing between these disparate origins is important because a late veneer consisting of carbonaceous chondrite-like asteroids or comets could be the principal source of the Earth's volatiles and water. Until now, however, a 'genetic' link between the late veneer and such volatile-rich materials has not been established or ruled out. Such genetic links can be determined using ruthenium (Ru) isotopes, because the Ru in the Earth's mantle predominantly derives from the late veneer, and because meteorites exhibit Ru isotope variations arising from the heterogeneous distribution of stellar-derived dust. Although Ru isotopic data and the correlation of Ru and molybdenum (Mo) isotope anomalies in meteorites were previously used to argue that the late veneer derives from the same type of inner Solar System material as do Earth's main building blocks, the Ru isotopic composition of carbonaceous chondrites has not been determined sufficiently well to rule them out as a source of the late veneer. Here we show that all chondrites, including carbonaceous chondrites, have Ru isotopic compositions distinct from that of the Earth's mantle. The Ru isotope anomalies increase from enstatite to ordinary to carbonaceous chondrites, demonstrating that material formed at greater heliocentric distance contains larger Ru isotope anomalies. Therefore, these data refute an outer Solar System origin for the late veneer and imply that the late veneer was not the primary source of volatiles and water on the Earth.

QUASI-BIENNIAL MODULATION OF GALACTIC COSMIC RAYS

The time variability of the cosmic-ray (CR) intensity at three different rigidities has been analyzed through the empirical mode decomposition technique for the period 1964-2004. Apart from the {approx}11 yr cycle, quasi-biennial oscillations (QBOs) have been detected as a prominent scale of variability in CR data, as well as in the heliomagnetic field magnitude at 1 AU and in the sunspot area. The superposition of the {approx}11 yr and QBO contributions reproduces the general features of the CR modulation, such as most of the step-like decreases and the Gnevyshev Gap phenomenon. A significant correlation has also been found between QBOs of the heliospheric magnetic field and the CR intensity during even solar activity cycles, suggesting that the former are responsible for step-like decreases in CR modulation, probably dominated by the particle diffusion/convection in such periods. In contrast, during odd-numbered cycles, no significant correlation is found. This could be explained with an enhanced drift effect also during the solar maximum or a greater influence of merged interaction regions at great heliocentric distances during odd cycles. Moreover, the QBOs of CR data are delayed with respect to sunspot activity, the lag being shorter for A > 0 periods of even cycles ({approx}1-4more » months) than for A < 0 periods of odd cycles ({approx}7-9 months); we suggest that solar QBOs also affect the recovery of the CR intensity after the solar activity maximum.« less

Origin of the differences in refractory-lithophile-element abundances among chondrite groups

Chondrite groups can be distinguished on the basis of their abundances of refractory lithophile elements (RLE). These abundances are, in part, functions of the mass fraction of Ca–Al-rich inclusions (CAIs) within the chondrites. Carbonaceous chondrites contain the most CAIs and the highest RLE abundances; they also contain modally abundant fine-grained matrix material that consists largely of modified nebular dust. The amount of dust varied throughout the solar nebula: enstatite and ordinary chondrites formed in low-dust regions in the inner part of the nebula, R chondrites formed in higher-dust zones at somewhat greater heliocentric distances, and carbonaceous chondrites formed in even dustier regions farther from the Sun. The amount of ambient dust peaked in the region where CV and CK chondrites accreted; these chondrites have abundant matrix, the highest modal abundances of CAIs, and the highest bulk RLE contents. Substantial amounts of nebular dust occurred in highly porous multi-millimeter-to-centimeter-size dustballs that were on the order of 100 times more massive than CAIs. Radial drift processes in the nebula affected these dustballs to approximately the same extent as the CAIs; both types of objects were aerodynamically concentrated in the same nebular regions. These regions maintained approximately the same relative amounts of dust through the periods of chondrule formation and chondrite accretion.

Radio remote sensing of the corona and the solar wind

AbstractModern radio telescopes are extremely sensitive to plasma on the line of sight from a radio source to the antenna. Plasmas in the corona and solar wind produce measurable changes in the radio wave amplitude and phase, and the phase difference between wave fields of opposite circular polarization. Such measurements can be made of radio waves from spacecraft transmitters and extragalactic radio sources, using radio telescopes and spacecraft tracking antennas. Data have been taken at frequencies from about 80 MHz to 8000 MHz. Lower frequencies probe plasma at greater heliocentric distances. Analysis of these data yields information on the plasma density, density fluctuations, and plasma flow speeds in the corona and solar wind, and on the magnetic field in the solar corona. This paper will concentrate on the information that can be obtained from measurements of Faraday rotation through the corona and inner solar wind. The magnitude of Faraday rotation is proportional to the line of sight integral of the plasma density and the line-of-sight component of the magnetic field. Faraday rotation provides an almost unique means of estimating the magnetic field in this part of space. This technique has contributed to measurement of the large scale coronal magnetic field, the properties of electromagnetic turbulence in the corona, possible detection of electrical currents in the corona, and probing of the internal structure of coronal mass ejections (CMEs). This paper concentrates on the search for small-scale coronal turbulence and remote sensing of the structure of CMEs. Future investigations with the Expanded Very Large Array (EVLA) or Murchison Widefield Array (MWA) could provide unique observational input on the astrophysics of CMEs.

Pluto's light curve in 1933–1934

The Pluto–Charon system has complex photometric variations on all time scales; due to rotational modulations of dark markings across the surface, the changing orientation of the system as viewed from Earth, occultations and eclipses between Pluto and Charon, as well as the sublimation and condensation of frosts on the surface. The earliest useable light curve for Pluto is from 1953 to 1955 when Pluto was 35 AU from the Sun. Earlier data on Pluto has the potential to reveal properties of the surface at a greater heliocentric distance with nearly identical illumination and viewing geometry. We are reporting on a new accurate photographic light curve of Pluto for 1933–1934 when the heliocentric distance was 40 AU. We used 43 B-band and V-band images of Pluto on 32 plates taken on 15 nights from 19 March 1933 to 10 March 1934. Most of these plates were taken with the Mount Wilson 60″ and 100″ telescopes, but 7 of the plates (now at the Harvard College Observatory) were taken with the 12″ and 16″ Metcalf doublets at Oak Ridge. The plates were measured with an iris diaphragm photometer, which has an average one-sigma photometric error on these plates of 0.08 mag as measured by the repeatability of constant comparison stars. The modern B and V magnitudes for the comparison stars were measured with the Lowell Observatory Hall 1.1-m telescope. The magnitudes in the plate's photographic system were converted to the Johnson B- and V-system after correction with color terms, even though they are small in size. We find that the average B-band mean opposition magnitude of Pluto in 1933–1934 was 15.73 ± 0.01 , and we see a roughly sinusoidal modulation on the rotational period (6.38 days) with a peak-to-peak amplitude of 0.11 ± 0.03 mag . With this, we show that Pluto darkened by 5% from 1933–1934 to 1953–1955. This darkening from 1933–1934 to 1953–1955 cannot be due to changing viewing geometry (as both epochs had identical sub-Earth latitudes), so our observations must record a real albedo change over the southern hemisphere. The later darkening trend from 1954 to the 1980's has been explained by changing viewing geometry (as more of the darker northern hemisphere comes into view). Thus, we now have strong evidence for albedo changes on the surface of Pluto, and these are most easily explained by the systematic sublimation of frosts from the sunward pole that led to a drop in the mean surface albedo.

Petschek‐type magnetic reconnection exhausts in the solar wind well inside 1 AU: Helios

Petschek‐type reconnection exhausts can be recognized in solar wind plasma and magnetic field data as accelerated or decelerated plasma flows confined to magnetic field reversal regions. Using that characteristic signature, we have identified 28 reconnection exhausts in the Helios 1 and 2 data, thus extending observations of exhausts associated with local, quasi‐stationary reconnection in the solar wind inward to heliocentric distances of 0.31 AU. Most of the exhaust jets identified in the Helios data had the same general physical character as solar wind exhausts observed at greater heliocentric distances and latitudes by ACE, Wind, and Ulysses. The magnitude of the velocity changes from outside to inside the exhausts was generally comparable to, but somewhat less than (by a factor of 0.75 on average), the inflow Alfvén speeds. In a few of the Helios events, plasma number densities within the exhausts were intermediate to densities observed immediately outside, indicating that transitions from outside to inside the exhausts were not always slow‐mode‐like on both sides. We have identified pairs of closely spaced, but independent, reconnection exhausts bounding regions where the heliospheric magnetic field folded back toward the Sun. We find that plasma and magnetic field conditions in the high‐speed wind from coronal holes are not generally favorable for sustained magnetic reconnection and for the formation and propagation of Petschek‐type exhausts. Finally, we have not yet identified reconnection events common to both spacecraft, partially because of a relative lack of times when high data rate observations were available from both spacecraft.

Rotational properties of asteroids, comets and TNOs

Over the past 25 years the number of reliably determined rotation rates of asteroids has increased by an order of magnitude, from 157 in 1979 to 1686 in 2005. As the numbers have increased, various special classes and features have emerged. Asteroids larger than diameter have a dispersion of spin rates that is well represented by a single Maxwellian distribution. Smaller asteroids have a more dispersed distribution, with both slow and fast spinning populations. We see a “spin rate barrier” in the size range of 1–10 km diameter that suggests that even rather small asteroids are “rubble piles”. Among the very slow rotators are some (but not all) that are “tumbling” in non-principal axis rotation states. Among the smallest asteroids (less than a few hundred m diameter) are some that spin dramatically faster than the “spin barrier”, indicating that they must have some tensile strength rather than consisting of loose regolith. In the last few years it has been recognized that the spins of asteroids smaller than a few tens of km diameter are affected by radiation pressure torques that tend to either speed up or slow down asteroid spin rates, thus providing an explanation for the dispersion of small asteroid spins, and also their non-random axis orientations. Lightcurves have also revealed the presence of binary asteroids among both Near-Earth and Main-Belt populations. Automated robotic observatories and next-generation survey instruments promise to increase the rate of production of asteroid lightcurves so that we may soon have tens of thousands of lightcurve results, extending down to even smaller sizes. In contrast, there are only about 20 rotation rates known for comets, and 15 for TNOs. Very little can be said from such meager statistics; the mean spin rate of TNOs appears to be comparable to that of asteroids, without extremes of fast or slow rotation; the mean spin rate of comets appears to be a bit slower than asteroids, perhaps due to lower mean density, and there may be an excess of slow rotators, probably due to gas jetting effects. The future is promising for studies of these objects as larger telescopes become available to do photometry to fainter magnitudes, so that comet nuclei can be studied at greater heliocentric distance with less coma interference, and more TNOs can be observed.

Impact Seeding and Reseeding in the Inner Solar System

Assuming that asteroidal and cometary impacts onto Earth can liberate material containing viable microorganisms, we studied the subsequent distribution of the escaping impact ejecta throughout the inner Solar System on time scales of 30,000 years. Our calculations of the delivery rates of this terrestrial material to Mars and Venus, as well as back to Earth, indicate that transport to great heliocentric distances may occur in just a few years and that the departure speed is significant. This material would have been efficiently and quickly dispersed throughout the Solar System. Our study considers the fate of all the ejected mass (not just the slowly moving material), and tabulates impact rates onto Venus and Mars in addition to Earth itself. Expressed as a fraction of the ejected particles, roughly 0.1% and 0.001% of the ejecta particles would have reached Venus and Mars, respectively, in 30,000 years, making the biological seeding of those planets viable if the target planet supported a receptive environment at the time. In terms of possibly safeguarding terrestrial life by allowing its survival in space while our planet cools after a major killing thermal pulse, we show via our 30,000- year integrations that efficient return to Earth continues for this duration. Our calculations indicate that roughly 1% of the launched mass returns to Earth after a major impact regardless of the impactor speed; although a larger mass is ejected following impacts at higher speeds, a smaller fraction of these ejecta is returned. Early bacterial life on Earth could have been safeguarded from any purported impact-induced extinction by temporary refuge in space.

Origin and dynamics of the heliospheric streamer belt and current sheet

The broad high‐density, low‐temperature region around the thin heliospheric current sheet at solar minimum forms a relatively stable “streamer belt” associated with the slow wind flow. This region contains highly structured magnetic fields, with large rotations and discontinuities being common. This observational study examines the likely origins and dynamics of the interplanetary plasma and current sheets primarily using Helios data. The striking differences sometimes observed between the plasma on the two sides of the current sheet support the common interpretation that the plasma above and below the sheet comes from often very different regions located either side of the helmet streamer at the base of the slow wind flow. The entropy per proton in the streamer belt often shows sharply defined regions of strongly different plasma; this implies that the origin of the filamentary structure is in initial conditions near the Sun because dynamical evolution can only equalize entropy. The observed large relative density and other fluctuations may thus represent the conditions on flow tubes with different boundary conditions. The average entropy in the streamer belt increases by about the same factor as in the surrounding high‐speed streams, indicating that this region is heated substantially, consistent with studies of the temperature evolution and with turbulence modeling. Compressive stream interaction regions are not preferentially heated (in the sense of an entropy increase) in the inner heliosphere. Strong anticorrelations between density and both temperature and magnetic field magnitude are observed within the streamer belt but not in the surrounding regions, and these become weaker as the flow moves outward. Both the smoother appearance of the entropy at greater heliocentric distance and simulation evidence support the view that the streamer belt region undergoes significant dynamical evolution. This evolution seems to also affect the current sheet because sector boundary crossings are observed to become more complex (more multiple crossings) with increasing heliocentric distance. The crossings are more complex in general near solar maximum, although the complexity still increases with radial distance.

A study of density fluctuations in the solar wind acceleration region

Interplanetary scintillation (IPS) measurements of the density fluctuations in the fast and slow solar wind were made at 2 GHz and 8 GHz in 1994 at the Kashima Space Research Center, Communications Research Laboratory. Using the observations which cover the distance range from 5 to 76 solar radii (Rs), we investigate the radial evolution of the dissipation scale length of the density fluctuations, the so‐called “inner scale.” Our IPS observations reveal that the inner scale shows different radial profiles between the inside and outside of the acceleration region. The size of the inner scale outside 25 Rs increases linearly with radial distance, showing good agreement with previous observations made at r ≥ 50 Rs. The inner scale inside 25 Rs, on the other hand, deviates from the linear relation. We simulate the radial variation of the inner scale using a solar wind acceleration model and then compare the results to that of the observed inner scale. We find that, in the low‐speed solar wind, the radial profile of the computed inner scale is in good agreement with that of the observed inner scale and that the solar wind acceleration causes the deviation of the inner scale from a linear relation. However, in the high‐speed wind, we cannot reproduce the radial profile of the observed inner scale with the acceleration model, even though the line‐of‐sight integration effect on IPS measurements is taken into account. We propose that this disagreement is due to the density and magnetic field fluctuations not being correlated in the high‐speed solar wind, as shown at greater heliocentric distances by Helios measurements.

The compositional classification of chondrites: VII. The R chondrite group

Bulk compositional and petrographic data clearly define the new R (Rumuruti) group of chondrites consisting of Rumuruti (the only fall), ALH85151, Acfer 217, Carlisle Lakes, Dar al Gani 013, PCA91002, PCA91241, Y-75302, Y-793575, and Y-82002. Compositional, petrographic, rare-gas, and 0-isotopic data strongly suggest that PCA91002 and PCA91241 are paired. The Yamato specimens are probably not paired. The matrices of the known R chondrites have experienced similar, minor degrees of metamorphism; petrographic types are 3.8-3.9 with the exception of ALH85151, 3.6. All except Carlisle Lakes contain equilibrated (R5-R6) clasts. Petrographically, the R chondrites are characterized by a low chondrule/matrix modal abundance ratio, high states of oxidation (reflected by abundant NiO-bearing olivine with Fa37-40), relatively small chondrules (mean apparent diameters of ∼400 μ) abundant (up to ∼11 wt%) sulfides (mainly pyrrhotite and pentlandite), and negligible amounts of metallic Fe-Ni. Refractory lithophile abundances are ∼0.95 X Cl, intermediate between those in ordinary chondrites (OC) and CI chondrites. Abundances of the volatile elements Se and Zn are greatly enhanced relative to OC. The R chondrites are clearly distinguished from other chondrite groups on the basis of Al/Mn and Zn/Mn abundance ratios. The oxygen isotopic data plot roughly along a slope- 1 2 line, with whole-rock Δ 17O values higher than for any other chondrite group. Rumuruti, Acfer 217, ALH85151, PCA91002, and PCA91241 have light/ dark structures and solar-wind-implanted rare gases indicating that they are regolith breccias. The Yamato specimens also have light/dark structures and are inferred to be regolith breccias. Carlisle Lakes lacks solar rare gases and is unbrecciated. Based on similarities in refractory lithophile abundances 00.95 X CI), oxygen isotope compositions ( Δ 17O ≥ 0), and refractory inclusion abundances (<0.1 vol%;none have been reported), the R chondrites probably belong to a noncarbonaceous superclan of chondrites that also includes ordinary and enstatite chondrites. The high oxidation state, high matrix /chondrule modal abundance ratio, relatively low abundance of droplet chondrules, and high Δ 17O composition suggest that the R chondrites formed at greater heliocentric distances than the OC.

Global, quasi‐steady dynamics of the distant solar wind, 1. Origin of north‐south flows in the outer heliosphere

Regular, large amplitude north‐south flow deflections were observed near the heliographic equator by Voyager 2 in the outer heliosphere in 1986. We present results of numerical simulations supporting the idea that these motions are driven by interplanetary dynamical interactions occurring at quasi‐steady stream fronts associated with the tilted‐dipole coronal magnetic configuration at solar minimum. It is shown that under these conditions the coronal geometry is deeply impressed upon the large scale flow and it determines the disposition and orientation of solar wind structures to great heliocentric distances. The observed predominance of the north‐south flow deflections (relative to the east‐west deflections) and their observed quasiperiodicity are seen to be a natural consequence of the two‐sector tilted‐dipole stream structure in which the fronts of successive corotating interaction regions have opposing north‐south tilts.

Helios 1+helios 2: a summary of IMF observations performed in the inner solar system during 1975–1981

The experimental observations performed by Helios 1 and Helios 2 spacecraft allow one to monitor the IMF conditions in the inner solars ystem (0.3≦r≦0.6A.U.) over many years. The field orientation closely corresponds to the theoretical predictions and the histograms of the distributions of the angular elements are possibly more peaked around the expected values than those ones obtained at greater heliocentric distances. The field behaviour, as examined over many years, does not show any clear long-term variation and only supports some evidence for a field modulation in the rising part of the new solar cycle.