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.     

Perspectives on the Comet-Asteroid-Meteorite Link

We discuss the possibility that CI and CM carbonaceous chondrites are fragments of extinct cometary nuclei. Theoretical and observational work suggests that comets evolve into asteroids, and several extinct cometary nuclei are now suspected to be among the near Earth object population. This population is the most likely source of meteorites and consequently, we may expect that some meteorites are from extinct comets in this population. The mineralogy and chemistry of CI and CM chondrites is consistent with the view that they originate from asteroidal objects of carbonaceous spectral classes, and these objects in turn may have a cometary origin. We do not suggest that CI or CM chondrites are directly delivered by active comets during perihelion passage or that these chondrites come from cometary debris in meteor streams. Instead, we summarize arguments suggesting that CI and CM chondrites represent fragments of cometary nuclei which evolved into near Earth asteroids after losing their volatiles. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar Origin of Anisotropic Electron Events observed by ULYSSES and ACE

We investigate the solar origin of energetic collimated electron events observed in situ by the EPAM and/or HISCALE particle experiments aboard the ACE and ULYSSES spacecraft, respectively. This study has been performed for two months, September and October 1999 when Ulysses was magnetically connected to the solar disk at heliolongitudes visible at the Earth. The two spacecraft were separated by about 35° in latitude and up to 60° in longitude. In this paper, results are described for one period only, October 24–November 1, 1999. It is found that not all the anisotropic events are observed by both spacecrafts; there exists a strong dependence on the spacecraft's magnetic connection back to the Sun. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the Slow Solar Wind

A theory for the origin of the slow solar wind is described. Recent papers have demonstrated that magnetic flux moves across coronal holes as a result of the interplay between the differential rotation of the photosphere and the non-radial expansion of the solar wind in more rigidly rotating coronal holes. This flux will be deposited at low latitudes and should reconnect with closed magnetic loops, thereby releasing material from the loops to form the slow solar wind. It is pointed out that this mechanism provides a natural explanation for the charge states of elements observed in the slow solar wind, and for the presence of the First-Ionization Potential, or FIP, effect in the slow wind and its absence in fast wind. Comments are also provided on the role that the ACE mission should have in understanding the slow solar wind. This revised version was published online in June 2006 with corrections to the Cover Date.     

Gas-rich meteorites: Probes for particle environment and dynamical processes in the inner solar system

The discovery in the early sixties of precompaction solar wind irradiation records in the gas-rich meteorites opened up the possibility of studying the solar activity at different epochs in the distant past. Subsequent studies in several meteorites have led to the discovery of the precompaction records of irradiation of constituent grains by solar wind, solar flare and galactic cosmic ray particles. There are also microcraters resulting from their collisions with interplanetary dust grains. Analyses of these records and their observed similarity with those found in the lunar samples led to the hypothesis that the precompaction records in individual components of these meteorites were imprinted while they were residing in the near surface region of their parent bodies, most probably the asteroids. Although the asteroids are the most plausible candidates for the parent bodies of gas-rich meteorites, there exist certain dynamical arguments which tend to favor a cometary origin in certain cases. Also, recent studies indicate that in the case of gas-rich carbonaceous chondrites solar flare irradiation of grains may have occurred prior to formation of the parent bodies.In this review we summarize the significant advances that have taken place in the multi-disciplinary studies (petrography, chemistry, and radiation effects) of the gas-rich meteorites and critically evaluate the present state of our knowledge regarding the origin and evolution of the gas-rich meteorites. The information on the spatial and temporal variations in the interplanetary radiation and particle fluxes, obtained from the analysis of precompaction irradiation records in these meteorites is presented and further studies in certain specific topics are suggested for resolving some of the unsolved problems.     

Swan Observations of the Solar Wind Latitude Distribution and its Evolution Since Launch

SWAN is the first space instrument dedicated to the monitoring of the latitude distribution of the solar wind by the Lyman alpha method. The distribution of interstellar H atoms in the solar system is determined by their destruction during ionization charge-exchange with solar wind protons. Maps of sky Ly-α emission have been recorded regularly since launch. The upwind maximum emission region deviates strongly from the pattern that would be expected from a solar wind that is constant with latitude. It is divided in two lobes by a depression aligned with the solar equatorial plane, called the Lyman-alpha groove, due to enhanced ionization along the neutral sheet where the slow and dense solar wind is concentrated. The groove (or the anisotropy) is more pronounced in 1997 than in 1996, but it then decreases between 1997 and 1998. This revised version was published online in June 2006 with corrections to the Cover Date.     

Isotopes in comets: Implications from Cr in carbonaceous chondrites

According to their chemical composition, rich in volatile compounds, comets are thought to be primitive materials. They may provide prime samples for the study of nucleosynthetic components of the solar system and of the processes occurring during the formation of the outer planets. Their origin is largely a matter of conjecture. Chromium isotopic measurements in carbonaceous chondrites illustrate how the non-volatile part of cometary material can be investigated both for isotopic heterogeneity and for the extinct nuclide ⁵³Mn. Questions like the possible presence of ²⁶Al as a heat source can also be addressed by these measurements.     

Variations of the magnetic fields in large solar flares

We present preliminary results from high resolution observations obtained with the Michelson Doppler Imager (MDI) instrument on the SOHO of two large solar flares of 14 July 2000 and 24 November 2000. We show that rapid variations of the line-of-sight magnetic field occured on a time scale of a few minutes during the flare explosions. The reversibility/irreversibility of the magnetic field of both active regions is a very good tool for understanding how the magnetic energy is released in these flares. The observed sharp increase of the magnetic energy density at the time of maximum of the solar flare could involve an unknown component which deposited supplementary energy into the system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Large Scale Flows in the Solar Convection Zone

We discuss the current theoretical understanding of the large scale flows observed in the solar convection zone, namely the differential rotation and meridional circulation. Based on multi-D numerical simulations we describe which physical processes are at the origin of these large scale flows, how they are maintained and what sets their unique profiles. We also discuss how dynamo generated magnetic field may influence such a delicate dynamical balance and lead to a temporal modulation of the amplitude and profiles of the solar large scale flows.     

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.     

Isotopic Composition of H,HE and NE in the Protosolar Cloud

Observations and measurements in the solar wind, the Jovian atmosphere and the gases trapped in lunar surface material provide the main evidence from which the isotopic composition of H, He and Ne in the Protosolar Cloud (PSC) is derived. These measurements and observations are reviewed and the corrections are discussed that are needed for obtaining from them the PSC isotopic ratios. The D/H, ³He/⁴He (D+³He)/H, ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios adopted for the PSC are presented. Protosolar abundances provide the basis for the interpretation of isotopic ratios measured in the various solar system objects. In this article we discuss constraints derived from the PSC abundances on solar mixing, the origin of atmospheric neon, and the nature of the “SEP” component of neon trapped at the lunar surface. We also discuss constraints on the galactic evolution provided by the isotopic abundances of H and He in the PSC. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interstellar and Inner Source Pickup Ions Observed with Swics on Ulysses

Many species of pickup ions, both of interstellar origin and from an inner, distributed source have been discovered using data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses. Velocity distribution functions of these ions were measured for the first time over heliocentric distances between 1.35 and 5.4 AU, both at high and low latitudes, and in the disturbed slow solar wind as well as the steady fast wind of the polar coronal holes. This has given us the first glance at plasma properties of suprathermal ions in various solar wind flows, and is enabling us to study the chemical and, in the case of He, the isotopic composition of the local interstellar cloud. Among the new findings are (a) the surprisingly weak pitch-angle scattering of low rigidity, suprathermal ions leading to strongly anisotropic velocity distributions in radial magnetic fields, (b) the efficient injection and consequent acceleration of pickup ions, especially He+ and H+, in the turbulent solar wind, and (c) the discovery of a new extended source releasing carbon, oxygen, nitrogen and possibly other atoms and molecules in the inner solar system. Pickup ion measurements are now used to study the characteristics of the local interstellar cloud (LIC) and, in particular, to determine accurately the abundance of atomic H, He, N, O, and Ne, the isotopes of He and Ne, as well as the ionization fractions of H and He in the LIC. Pickup ion observations allow us to infer the location of the termination shock and, in combination with measurements of anomalous cosmic rays, to investigate termination shock acceleration mechanisms. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Solar Noble Gas Record in Lunar Samples and Meteorites

Lunar soil and certain meteorites contain noble gases trapped from the solar wind at various times in the past. The progress in the last decade to decipher these precious archives of solar history is reviewed. The samples appear to contain two solar noble gas components with different isotopic composition. The solar wind component resides very close to grain surfaces and its isotopic composition is identical to that of present-day solar wind. Experimental evidence seems by now overwhelming that somewhat deeper inside the grains there exists a second, isotopically heavier component. To explain the origin of this component remains a challenge, because it is much too abundant to be readily reconciled with the known present day flux of solar particles with energies above those of the solar wind. The isotopic composition of solar wind noble gases may have changed slightly over the past few Ga, but such a change is not firmly established. The upper limit of ~5% per Ga for a secular increase of the 3He/4He ratio sets stringent limits on the amount of He that may have been brought from the solar interior to the surface (cf. Bochsler, 1992). Relative abundances of He, Ne, and Ar in present-day solar wind are the same as the long term average recorded in metallic Fe grains in meteorites within error limits of some 15-20%. Xe, and to a lesser extent Kr, are enriched in the solar wind similar to elements with a first ionisation potential < 10 eV, although Kr and Xe have higher FIPs. This can be explained if the ionisation time governs the FIP effect (Geiss and Bochsler, 1986). This revised version was published online in June 2006 with corrections to the Cover Date.     

Origin, Injection, and Acceleration of CIR Particles: Theory Report of Working Group 7

On the basis of the observational picture established in the report of Mason, von Steiger et al. (1999) the status of theoretical models on origin, injection, and acceleration of particles associated with Corotating Interaction Regions (CIRs) is reviewed. This includes diffusive or first-order Fermi acceleration at oblique shocks, adiabatic deceleration in the solar wind, stochastic acceleration in Alfvén waves and oblique propagating magnetosonic waves, and shock surfing as possible injection mechanism to discriminate pickup ions from solar wind ions. This revised version was published online in August 2006 with corrections to the Cover Date.     

The carbonaceous chondrites

The carbonaceous chondrites are a group of stony meteorites characterized by the presence of an appreciable amount of carbonaceous material other than free carbon (diamond and graphite). They have been divided into three subgroups known respectively as Type I, Type II, and Type III. Analyses of Type I meteorites show about 3–5% of carbon and 20% of combined water; they consist largely of hydrated magnesium-iron silicate, magnetite, and magnesium sulfate, contain no chondrules, and have a density about 2.2. Analyses of Type II meteorites show about 2–3% of carbon and 10–15% of combined water; they consist of a groundmass of hydrated magnesium-iron silicate enclosing chondrules of olivine and pyroxene which are almost iron-free, and have a density of 2.6–2.9. Analyses of Type III meteorites show about 0.5–2% of carbon and 2% combined water; they consist largely of olivine (often variable in composition, but averaging 30–40 mole per cent Fe₂SiO₄), with accessory pigeonite and sulfide minerals, and have a density about 3.4.The carbonaceous material and combined water in these meteorites are clearly of extraterrestrial origin, but their significance is not well understood. A biological origin has been claimed for some of the organic compounds on the basis of their composition, but this claim is the subject of considerable dispute. Microscopic objects with regular outline (organized elements) have been recognized in some of these meteorites; some investigators have claimed these to be extraterrestrial fossils, others have ascribed them to terrestrial contamination or considered them to be crystals or crystal aggregates of non-biological origin.     

Isotopic Fractionation by Plasma Processes

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gy ago from the protosolar nebula. The solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. Data from space missions and theoretical models indicate that the isotopes of the volatile elements N, O, Ne, and Ar are fractionated by at most a few percent per atomic mass unit in different solar wind regimes. In contrast, isotopic abundances of solar and heliospheric energetic particles at energies larger than about 100 keV/amu are observed to strongly vary relative to solar abundances. Processes such as resonant acceleration or pre-acceleration by plasma waves, first-order Fermi acceleration, or propagation in the interplanetary plasma are discussed as causes for charge-to-mass dependent fractionation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Velocity Distribution of Interstellar H Atoms in the Heliospheric Interface

In this paper we present first results of a numerical computation of the velocity distribution function of interstellar H atoms in the heliospheric interface, the region of the solar and interstellar wind interaction. The velocity distribution is a key tool to evaluate uncertainties introduced by various simplified models of the interface. We numerically solve the kinetic equation for gas of H-atoms self-consistently with the hydrodynamic equations for plasma. Neutral and plasma components are efficiently coupled by charge exchange. The interaction disturbs the atom velocity distribution, which is assumed to be Maxwellian in the circumsolar local interstellar medium. It is shown that besides ‘original’ interstellar atoms, there are three other important atom populations originating in the heliospheric interface. Velocity distribution functions of these populations at the heliopause are presented and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Acceleration of the High Speed Solar Wind in Coronal Holes

We outline a theory for the origin and acceleration of the fast solar wind as a consequence of network microflares releasing a spectrum of high frequency Alfvén waves which heat (by cyclotron absorption) the corona close to the Sun. The significant features of our model of the fast wind are that the acceleration is rapid with the sonic point at around two solar radii, the proton temperatures are high (~ 5 million degrees) and the minor ions are correspondingly hotter, roughly in proportion to their mass. Moreover we argue that since the energy flux needed to power the quiet corona in closed field regions is about the same as that needed to drive the fast solar wind, and also because at deeper levels (< 2 × 10⁵ K) there is no great difference in the properties of supergranules and network in closed and open field regions, the heating process (i.e., dissipation of high frequency waves) must be the same in both cases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sun and Protosolar Nebula - Working Group Report

Volatile isotope abundances are tracers for the evolutionary processes of the solar system. At the same time they carry information on the galactic nucleosynthetic sources, from which solar matter originates. This working group report summarizes the present knowledge and addresses unresolved issues regarding fractionation of isotopes of volatile elements in the solar system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Isotopic Criteria for Identification of Organic Carbon on Earth and Meteorites

Contrary to the often stated view, an enrichment of organic material in the light isotope is not a conclusive evidence of its life-related origin. The β¹³C - σ¹³C correlation is a special feature of biological systems. Therefore it can be used as a criterion for identification of organic carbon. A survey of the available isotopic data for organic compounds in meteorites shows that they do not comply with the β¹³C - σ¹³C correlation. The prevalence of amino and hydroxy acids in purines and sugars found in carbonaceous meteorites indicates that condensation of HCN and HCHO passed through cyanohydrin reaction, while biological evolution proceeds through formation of adenosine triphosphate (ATP). This, in addition to the isotope criterion, indicates that the organic compounds in carbonaceous chondrites are not life-related substances. This revised version was published online in August 2006 with corrections to the Cover Date.