Determination of interstellar pickup ion distributions in the solar wind with SOHO and Cluster

Over the last 10 years, the experimental basis for the study of the local interstellar medium has been substantially enhanced by the direct detection of interstellar pickup ions and of interstellar neutral helium within the heliosphere. Pickup ions can be studied for a wide range of interstellar species. However, currently the accuracy of the method to determine the parameters of the interstellar medium, namely neutral density, temperature and relative velocity, is hampered by two problems: (1) In most cases the crucial ionization rates are not available from simultaneous measurements and (2) the transport of the pickup ions in the interplanetary medium substantially modifies the measured spatial distribution of the ions. In this study we will discuss how the enhanced capabilities of the instrumentation on SOHO and Cluster in combination with ongoing efforts to model the pickup ion distributions will lead to a significant improvement over the coming years.     

From Mars to M81

Martian dust, erosion on Callisto, interstellar gas and the solar wind all play their part in this issue's round-up of space science from Peter Bond .     

Large molecules in space?

Dust, atoms and a few, small, molecules are known to exist in interstellar space. David Williams wonders whether bigger molecules exist around or between the stars, and what their presence would imply.     

On the formation of meteoritic chondrules by aerodynamic drag heating in the solar nebula

During the formation of the solar nebula interstellar grains were fallling into the nebula with velocities of the order of 10 km/s at the radial distance where the meteorites were to form. This kinetic energy is 20 times the amount of thermal energy needed to melt the grains. The grains were decelerated by aerodynamic drag in the nebula. Where grain-rich parcels of interstellar material fell into the nebula, heat generated by drag could not be radiated away because of the opacity imparted to the system by the grains, and high temperatures were reached. In this situation presolar aggregations of grains would melt to form chondrules. Many of the properties of chondrules (and also CAI's) are consistent with their formation by this means. The infall heating concept provides a new framework in which the formation and significance of chondritic meteorites can be understood.     

Shielded by the wind: the influence of the interstellar medium on the environment of Earth

We report on the recent studies on the long-term influence of cosmic rays on the Earth's environment. While on short time-scales solar activity is the driver for atmospheric changes suspected to be due to cosmic rays, for long time-scales the heliosphere, i.e. the circumsolar region occupied by the expanding part of the Sun's atmosphere, has to be considered. The heliosphere is identified as an important shield against interstellar influences and hazards. It has been demonstrated by quantitative modelling that a change of the interstellar medium surrounding the heliosphere as a result of the Sun's quasi-Keplerian motion around the galactic center triggers significant changes of planetary environments caused by enhanced fluxes of neutral atoms as well as by the increased cosmic ray fluxes. We give a compilation of recent space science results of interest to the atmospheric science community.     

Isotopic composition of carbonaceous-chondrite kerogen: evidence for an interstellar origin of organic matter in meteorites

Stepwise combustion has revealed systematic patterns of isotopic heterogeneity for C, H and N in the insoluble organic fraction (m-kerogen) from the Orgueil and Murray carbonaceous chondrites. Those patterns are essentially identical for both meteorites, indicating a common source of m-kerogen. The data cannot be reconciled with a single mass-fractionation process acting upon a single precursor composition. This indicates either a multi-path history of mass-dependent processing or a significant nucleogenetic contribution, or both. If mass-fractionation were the dominant process, the magnitude of the observed isotopic variability strongly suggests that ion-molecule reactions at very low temperatures, probably in interstellar clouds, were responsible. In any case, an interstellar, rather than solar nebular, origin for at least some of the meteoritic organic matter is indicated. This has interesting implications for the origin of prebiotic molecules, temperatures in the early solar system, and the isotopic compositions of volatiles accreted by the terrestrial planets.     

Large-scale structure of the magnetic field in the outer heliosphere

Summary Magnetic field structures at great distances from the Sun have been analyzed qualitatively for a simple vacuum reconnection model of the interplanetary and interstellar magnetic field. In dependence on the mutual orientation of the main solar dipole _s and the local interstellar fieldB ₀ , either an open or closed configuration of the large-scale field is formed. For(_s B ₀ )>0, the field lines are represented by a system of magnetic lines open towards interstellar space. In the case of(_s B ₀ )<0 there exist two zero-points and a separating surface below the heliopause separating the open lines of the interstellar field from the closed lines of the interplanetary field. The magnetic field configuration is characterized by a certain asymmetry, which is considered for(_s B ₀ )=0.     

Driving Galactic Turbulence by Supernova Explosions

We investigate the general properties of supernova driven interstellar turbulence using local three-dimensional MHD simulations under Galactic conditions. Our model includes the effects of large-scale shear due to Galactic differential rotation, density stratification, compressibility, magnetic fields, heating via supernova explosions and parameterized radiative cooling of the interstellar medium. In addition to investigating isolated supernova explosions we allow for multiple supernovae distributed randomly in the Galactic disc and exponentially in the vertical direction. Single supernova explosions drive a strong shock, the lifetime of which is approximately 2 Myr in our model. This stage is found to be characterized by a kinetic energy spectrum in the diffuse gas with spectral index consistent with k = –2. Large-scale shear and Coriolis force act on the supernova remnant producing some vorticity inside it, but this process was found to be very weak. In the case of multiple supernova explosions, older remnants have an important role causing density fluctuations in the interstellar medium. In this clumpy medium, the propagation velocity of the shock fronts changes due to the changing density, and vorticity is generated. In the absence of these supernova interactions the kinetic energy spectrum shows a relatively wide shock spectrum with spectral index k = –2, but when the supernova interactions become dominant the classical k = –5/3 spectrum is observed.     

The solar Na/Ca and S/Ca ratios: A close comparison with carbonaceous chondrites

Solar abundances based on recent laboratory oscillator strengths confirm the relationship between solar matter and carbonaceous chondrites. Within spectroscopic uncertainties (typically±40%) these meteorites contain refractory and volatile elements in solar proportions. Significant improvement of accuracy at present seems restricted to a few abundant elements having reliable quantum-mechanical oscillator strengths, and necessitates strictly differential spectrum analysis. Taking this into account, the solar abundance ratios Na/Ca and S/Ca have been determined to an accuracy of±15%. The results are:Na/Ca= 0.91and S/Ca= 6.8. These volatile/refractory ratios just match type 1 carbonaceous chondrites, but contrast with other types.These and related interstellar abundance features place constraints on the condensation process and a potential heterogeneity of the solar nebula. There is evidence that no drastic pre-solar separation of interstellar gas and grains has occurred, but minor imbalance may be a common mechanism co-determining stellar metal content.     

Molecules, ices and astronomy

David A Williams, Wendy A Brown, Stephen D Price, Jonathan M C Rawlings and Serena Viti of University College London review how far experimental work on molecular synthesis can be applied to the interstellar medium – and how it is becoming a tool for the understanding of astrophysical processes.     

Origin of the solar wind: Astrophysical and plasma-physical aspects of the problem

At what evolutionary stage of the Sun as a star the accumulation of its matter from the interstellar medium ended and the dominant outflow of solar wind (SW) streams started is one of the main outstanding questions in the astrophysical aspect of the problem of SW origin. It is unknown when and how this happened in detail, although the onset of thermonuclear reactions in the solar interior undoubtedly played a crucial role in the energetics and dynamics of the star, which could lead to such a change of the regimes. Therefore, it is hypothesized that the accretion or plasma outflow from the star was determined by the preceding evolution, i.e., by the “memory“ and not just by the distribution of instantaneous density, temperature, magnetic field, and other macroscopic parameters of the system that consists of this star, its nearest stellar environment, and interstellar gas. Depending on this, neighboring stars can serve as donors or acceptors of interstellar gas. Some of them can simultaneously or alternately play both roles. The polytropic solution for centrally symmetric flows obtained by Bondi is degenerate in the sign of the radial velocity. It is suitable for describing the quasi-stationary regimes of both types. However, the theory of transient processes has not been developed. Hence, the question of whether there exist stars similar in their internal structure and parameters to the present-day Sun but without stellar wind emission or even with interstellar gas accretion can be answered only observationally. The possibility that such stars exist is not ruled out; it does not contradict any laws of nature. The plasma-physical aspect of the problem of SW origin concerns much shorter time intervals than the main evolutionary time scale measured in billions of years. Hence, this aspect of the problem has been studied much better, although it is also fairly complex and has not yet been solved in much detail due to the multi-scale character of the flow formation processes. The SW as a permanent supermagnetosonic plasma outflow in the radial direction appears against the background of much more powerful nonequilibrium and unsteady motions, which are ordered only partially in the upper solar atmosphere and corona (turbosphere). The instantaneous SW state is controlled by the fluxes of free energy, matter, and momentum that enter the corona from the convection zone and underlying solar atmospheric layers. Although the main physical mechanisms of the transport of free energy of the electromagnetic field and plasma are generally well known, they need a quantitative analysis as applied to specific realizations in frequent and rare particle collisions in the solar corona to ascertain the nonlocal processes of the formation of fields and plasma flows, including the SW.     

An annual and a semiannual variation of the upper air density

Summary The fluctuation of the acceleration are analysed for several satellites. For the density variations three predominant influences can be distinguished: 1) The varying solar short ultra-violet radiation; 2) The day-night-effect; 3) An annual variation. The latter suggests a significant interaction of the terrestrial upper atmosphere with the interplanetary matter. The annual variation gives some evidence for an interstellar wind due to the solar motion in the local stellar system.     

A numerical modelling study on the processes of uplift and planation of the Tibetan Plateau

The Tibetan Plateau has experienced a number of processes of uplift and planation alternately since about 45 Ma B. P. when it began to raise. A differential equation model for describing the Plateau altitude variation with time is formulated on the basis of previous field studies and a theoretical hypothesis: if palaeomagnetic polarity is positive, the convective activity in the earth is strong; orogenic movement is violent; and the raising velocity of the Plateau is high andvice versa. The analytical solution of the equation is obtained. The altitude variation from the beginning of the Plateau uplift to present is computed through using the geomagnetic polarity reversals timing series and interstellar atomic hydrogen concentration data. A comparison between the model results and the field studies indicates that the former is quite similar to the latter. The model results are able to basically reproduce the alternating processes of uplift and planation of the plate geological history. In the present model, the influences of the denudation and the geomagnetic polarity and interstellar atomic hydrogen concentration on the raising velocity of the Plateau altitude are mainly considered. Project supported by the National Climbing Project “A Study on the Uplift, Evolution and Ecosystem of the Tibetan Plateau” and the CAS’ (Chincse Academy of Sciences) Foundations for Returned Scholars.     

The molecular universe

Helen J Fraser , Martin R S McCoustra and David A Williams present a simple guide to astrochemistry.
Molecules play a fundamental role in many regions of our universe. The science where chemistry and astronomy overlap is known as astrochemistry, a branch of astronomy that has risen in importance over recent years. In this article we review the significance of chemistry in several astronomical years. IN this article we review the significance of chemistry in several astronomical environments including the early universe, interstellar clouds, starforming regions and protoplanetary disks. We discuss theoretical models, laboratory experiments and observational data, and present several recent and exciting results that challenge our perception of the "molecular universe".     

Galactic Spiral Arms and Dynamo Control Parameters

We discuss the effects of galactic spiral arms on the -coefficient, turbulent diffusivity and turbulent energy density of the interstellar turbulence. We argue that the -coefficient and the dynamo number are larger in the interarm regions, whereas the kinetic energy density of turbulence is larger in the arms; the turbulent magnetic diffusivity can be only weakly affected by the spiral pattern.     

Ion implantation effects in “cosmic” dust grains

Cosmic dust grains, whatever their origin may be, have probably suffered a complex sequence of events including exposure to high doses of low-energy nuclear particles and cycles of turbulent motions. High-voltage electron microscope observations of micron-sized grains either naturally exposed to space environmental parameters on the lunar surface or artificially subjected to space simulated conditions strongly suggest that such events could drastically modify the mineralogical composition of the grains and considerably ease their aggregation during collisions at low speeds. Furthermore, combined mass spectrometer and ionic analyzer studies show that small carbon compounds can be both synthesized during the implantation of a mixture of low-energy D, C, N ions in various solids and released in space by ion sputtering. The present results have implications concerning the origin of small molecules in interstellar or circumstellar clouds, the “aging” of cosmic dust grains in space, and the “sticking” process in the solar nebula.     

Possible impact of interplanetary and interstellar dust fluxes on the Earth’s climate

This article considers the process of entry of cosmic substance into the Earth’s atmosphere and the further evolution of the formed extraterrestrial aerosol. It is shown that meteorite-derived aerosol generated in the atmosphere may affect the Earth’s climate in two ways: (a) particles of meteoric haze may serve as condensation nuclei in the troposphere and stratosphere; (b) charged meteor particles residing in the mesosphere may markedly change (by a few percent) the total atmospheric resistance and, thereby, affect the global current circuit. Changes in the global electric circuit, in turn, may influence cloud formation processes. The obtained results argue for the fact that the meteoric dust in the Earth’s atmosphere is potentially one of the important climate-forming agents. It is shown that the amount of interstellar dust in the Earth’s atmosphere is too small to have a considerable affect on atmospheric processes.     

The statistics of a passive scalar in field-guided magnetohydrodynamic turbulence

A variety of studies of magnetised plasma turbulence invoke theories for the advection of a passive scalar by turbulent fluctuations. Examples include modelling the electron density fluctuations in the interstellar medium, understanding the chemical composition of galaxy clusters and the intergalactic medium, and testing the prevailing phenomenological theories of magnetohydrodynamic turbulence. While passive scalar turbulence has been extensively studied in the hydrodynamic case, its counterpart in MHD turbulence is significantly less well understood. Herein we conduct a series of high-resolution direct numerical simulations of incompressible, field-guided, MHD turbulence in order to establish the fundamental properties of passive scalar evolution. We study the scalar anisotropy, establish the scaling relation analogous to Yaglom’s law, and measure the intermittency of the passive scalar statistics. We also assess to what extent the pseudo Alfvén fluctuations in strong MHD turbulence can be modelled as a passive scalar. The results suggest that the dynamics of a passive scalar in MHD turbulence is considerably more complicated than in the hydrodynamic case.     

Extraterrestrial Organic Compounds in Meteorites

Many organic compounds or their precursorsfound in meteorites originated in the interstellar or circumstellarmedium and were later incorporated intoplanetesimals during the formation of thesolar system. There they either survivedintact or underwent further processing tosynthesize secondary products on themeteorite parent body.The most distinct feature of CI and CM carbonaceouschondrites, two typesof stony meteorites, is their high carbon content(up to 3% of weight), either in theform of carbonates or of organic compounds. The bulkof the organic carbon consistsof an insoluble macromolecular material with a complexstructure. Also present is asoluble organic fraction, which has been analyzedby several separation and analyticalprocedures. Low detection limits can be achievedby derivatization of the organicmolecules with reagents that allow for analysisby gas chromatography/massspectroscopy and high performance liquidchromatography. The CM meteoriteMurchison has been found to contain more than70 extraterrestrial amino acids andseveral other classes of compounds includingcarboxylic acids, hydroxy carboxylicacids, sulphonic and phosphonic acids, aliphatic,aromatic and polar hydrocarbons,fullerenes, heterocycles as well as carbonylcompounds, alcohols, amines and amides.The organic matter was found to be enriched indeuterium, and distinct organiccompounds show isotopic enrichments of carbon andnitrogen relative to terrestrialmatter.