Showing papers by "International Space Science Institute published in 2009"

Fundamentals of collisionless shocks for astrophysical application, 2. Relativistic shocks

Abstract: A comprehensive review is given of the theory and properties of nonrelativistic shocks in hot collisionless plasmas—in view of their possible application in astrophysics. Understanding non-relativistic collisionless shocks is an indispensable step towards a general account of collisionless astrophysical shocks of high Mach number and of their effects in dissipating flow-energy, in heating matter, in accelerating particles to high—presumably cosmic-ray—energies, and in generating detectable radiation from radio to X-rays. Non-relativistic shocks have Alfvenic Mach numbers $${{\fancyscript{M}}_A\ll \sqrt{m_i/m_e}(\omega_{pe}/\omega_{ce})}$$ , where m i /m e is the ion-to-electron mass ratio, and ω pe , ω ce are the electron plasma and cyclotron frequencies, respectively. Though high, the temperatures of such shocks are limited (in energy units) to T < m e c 2. This means that particle creation is inhibited, classical theory is applicable, and reaction of radiation on the dynamics of the shock can be neglected. The majority of such shocks are supercritical, meaning that non-relativistic shocks are unable to self-consistently produce sufficient dissipation and, thus, to sustain a stationary shock transition. As a consequence, supercritical shocks act as efficient particle reflectors. All these shocks are microscopically thin, with shock-transition width of the order of the ion inertial length λ i = c/ω pi (with ω pi the ion plasma frequency). The full theory of such shocks is developed, and the different possible types of shocks are defined. Since all collisionless shocks are magnetised, the most important distinction is between quasi-perpendicular and quasi-parallel shocks. The former propagate about perpendicularly, the latter roughly parallel to the upstream magnetic field. Their manifestly different behaviours are described in detail. In particular, although both types of shocks are non-stationary, they have completely different reformation cycles. From numerical full-particle simulations it becomes evident that, on ion-inertial scales close to the shock transition, all quasi-parallel collisionless supercritical shocks are locally quasi-perpendicular. This property is of vital importance for the particle dynamics near the quasi-parallel shock front. Considerable interest focusses on particle acceleration and the generation of radiation. Radiation from non-relativistic shocks results mainly in wave–wave interactions among various plasma waves. Non-thermal charged particles can be further accelerated to high energies by a Fermi-like mechanism. The important question is whether the shock can pre-accelerate shock-reflected particles to sufficiently high energies in order to create the seed-population of the non-thermal particles required by the Fermi mechanism. Based on preliminary full-particle numerical simulations, this question is answered affirmatively. Such simulations provide ample evidence that collisionless shocks with high-Mach numbers—even when non-relativistic—could probably by themselves produce the energetic seed-particle population for the Fermi-process.

Ion Superdiffusion at the Solar Wind Termination Shock

Abstract: We investigate the propagation of 0.54-3.5 MeV ions accelerated at the termination shock of the solar wind. Data are from Voyager 2 and refer to a time interval about one year long, just before the Voyager 2 termination shock crossing at the end of 2007 August, at roughly 83.7 AU. A recently developed technique, which allows to unravel the transport properties from an analysis of the energetic particle time profiles, is used. The ion time profiles exhibit a power-law decay from a few days to 200 days before the shock front, so that transport is found to be superdiffusive, with a mean square deviation growing like Δx 2 ∝ t α, with α ~ 1.3. This means that ion propagation in the heliosphere can be intermediate between normal diffusion and ballistic motion. The implication of ion superdiffusion on particle acceleration mechanisms at the termination shock is discussed, as well as some observational evidence coming from both Voyager 1 and Voyager 2, which questions diffusive shock acceleration.

Quiescent current sheets in the solar wind and origins of slow wind

Abstract: [1] Solar wind near the heliospheric current sheet is investigated using Ulysses and ACE data in a superposed epoch analysis for several days on either side of the current sheets. Only data near sunspot minima are used, minimizing the influence of transients. New results are shown for composition and ionization state. Existing results showing a ∼2 day wide depletion in He/H (He++/H+) at the current sheet are confirmed, although the depletion is generally more narrow. A recent finding of a broad 5–10 day wide reduction in He/H around the current sheet is also confirmed. An important result is that the narrow depletion is not a real phenomenon but is instead a statistical consequence of the superposition of transient depletions that also create the broad reduction in the averages. These transient depletions last from a few hours up to several days, come from the core of streamers, and are embedded in a quasi-steady flow from streamers' legs. Most depletions contain a current sheet just inside one edge, leading to the apparent narrow depletion at the current sheet in the superposed epoch analysis. These results lead us to a hypothesis for how the He/H depletions form with a current sheet just inside one edge. Fe/O fluctuations associated with the He/H fluctuations further show that mixing of plasma from coronal holes adjacent to streamer brightness boundaries into outflow inside the brightness boundary is not an important process.

Turbulence and intermittency in the heliospheric magnetic field in fast and slow solar wind

Abstract: [1] We study the nonuniform solar wind turbulence using high-resolution Ulysses magnetic field data measured at different solar activity level, heliospheric latitudes, and distance. We define several types of solar wind dependent of the coronal region of origin and also of the dynamical behavior of the different streams, namely, “pure” fast wind, fast streams, “pure” slow wind, and slow streams. The turbulent properties of the solar wind types were investigated in terms of their scaling properties and spatial inhomogeneity. A clear trend in the power spectrum of the solar wind magnetic field magnitude is observed: the “pure” fast wind has a slope ∼−1.33 (1/f-like), the fast streams ∼−1.48 (Kraichnan-like), the “pure” slow wind ∼−1.67 (Kolmogorov-like), and the slow streams ∼−1.72. We find that the “pure” fast wind in the polar heliolatitudes is less intermittent than the other types: “pure” slow wind and both slow and fast streams, which is because of the absence of dynamical interactions between streams with different speeds. On the other hand, fast streams are more intermittent than the “pure” fast wind, and slow streams are less intermittent than the “pure” slow winds. A clear radial and latitudinal evolution of the intermittency is observed only for the “pure” fast wind, while in the equatorial plane, the fast streams, the “pure” slow wind, and the slow streams do not show evolution either in heliolatitude or in heliocentric distance.

Magnetic turbulence in space plasmas: Scale‐dependent effects of anisotropy

Abstract: [1] The presence of a background magnetic field induces anisotropy in magnetic turbulence. Understanding properties of anisotropy is important to characterize turbulence power spectrum. This paper presents a case study of anisotropy by using a minimum variance analysis, in three different regions of the heliosphere, namely in the solar wind, and in the Earth's foreshock and magnetosheath behind a quasiparallel bow shock. A strong anisotropy is found in all cases, with very interesting cross-scale effects at the ion cyclotron frequency. In particular, (1) the eigenvalues of the variance matrix have a strong intermittent behavior, with very high localized fluctuations below the ion cyclotron scale. As a consequence the probability distribution functions are almost Gaussian5 above the ion cyclotron scale and become power laws at smaller scales; (2) the minimum variance direction is almost parallel to the background magnetic field at scales larger than the ion cyclotron scale in the solar wind and in the foreshock, while their probability density functions become broader at smaller scales. In the magnetosheath the minimum variance direction exhibits a tendency to become nearly perpendicular to the large-scale magnetic field below the ion cyclotron scale.

Composition of Interstellar Neutrals and the Origin of Anomalous Cosmic Rays

Abstract: Knowledge of the elemental composition of the interstellar gas is of fundamental importance for understanding galactic chemical evolution. In addition to spectroscopic determinations of certain element abundance ratios, measurements of the composition of interstellar pickup ions and Anomalous Cosmic Rays (ACRs) have provided the principal means to obtain this critical information. Recent advances in our understanding of particle acceleration processes in the heliosphere and measurements by the Voyagers of the energy spectra and composition of energetic particles in the heliosheath provide us with another means of determining the abundance of the neutral components of the local interstellar gas. Here we compare the composition at the termination shock of six elements obtained from measurements of (a) pickup ions at ∼5 AU, (b) ACRs in the heliosphere at ∼70 AU, and (c) energetic particles as well as (d) ACRs in the heliosheath at ∼100 AU. We find consistency among these four sets of derived neutral abundances. The average interstellar neutral densities at the termination shock for H, N, O, Ne and Ar are found to be 0.055±0.021 cm−3, (1.44±0.45)×10−5 cm−3, (6.46±1.89)×10−5 cm−3, (8.5±3.3)×10−6 cm−3, and (1.08±0.49)×10−7 cm−3, respectively, assuming the He density is 0.0148±0.002 cm−3.

Swift observations of IGR J16479-4514 in outburst

Abstract: Aims. The supergiant fast X-ray transient source IGR J16479-4514 was observed in outburst two times with Swift. Its quiescent state was investigated in-depth only once in 2008 through a relatively long pointed observation with XMM-Newton. The latter observation was taken about 1.7 days after the outburst in 2008, and showed an X-ray eclipse-like event, likely caused by the supergiant companion. At present, this is the only supergiant fast X-ray transient that displayed an evidence for an X-ray eclipse. Methods. Here we carry out a comparison between the most recent outburst of IGR J16479-4514, caught by Swift on 29 January 2009 and those detected previously from this source. Results. The decay from the outbursts in 2005, 2008 and 2009 presents many similarities, and suggests a common mechanism that modulates the mass accretion rate onto the neutron star in IGR J16479-4514.

Triple F—a comet nucleus sample return mission

Abstract: The Triple F ( Fresh From the Fridge) mission, a Comet Nucleus Sample Return, has been proposed to ESA's Cosmic Vision program. A sample return from a comet enables us to reach the ultimate goal of cometary research. Since comets are the least processed bodies in the solar system, the proposal goes far beyond cometary science topics (like the explanation of cometary activity) and delivers invaluable information about the formation of the solar system and the interstellar molecular cloud from which it formed. The proposed mission would extract three sample cores of the upper 50 cm from three locations on a cometary nucleus and return them cooled to Earth for analysis in the laboratory. The simple mission concept with a touch-and-go sampling by a single spacecraft was proposed as an M-class mission in collaboration with the Russian space agency ROSCOSMOS.

The Heliospheric Magnetic Field and Its Extension to the Inner Heliosheath

Abstract: The general structure of the heliospheric magnetic field is well known and has been extensively studied, mostly in the inner heliosphere, out to the orbit of Saturn. Beyond 10 AU, the Pioneer and now the Voyager spacecraft have provided a view of the outer heliosphere. Its structure is strongly affected by large-scale phenomena originating in the Sun’s activity, such as the pattern of fast and slow solar wind streams around solar minimum that lead to Corotating Interaction Regions, and the increased frequency and strength of Coronal Mass Ejections around solar maximum. The large current sheet that separates the dominant magnetic polarities in the heliospheric medium, the Heliospheric Current Sheet, provides a variable structure that evolves from a relatively simple geometry close to the solar equatorial plane to what is likely to be a highly complex and dynamic surface reaching to high heliolatitudes at high levels of solar activity. The magnetic field observed in a fluctuating, dynamical heliosheath differs considerably from that in a static heliosheath. In particular, the time between current-sheet crossings (sectors) is quite sensitive to the radial speed of the solar-wind termination shock. If an inwardly moving termination shock moves past an observer on a slowly moving spacecraft, the time between current-sheet crossings in the heliosheath becomes larger, and can become very large, for reasonably expected inward shock speeds. This effect may help to explain recent observations of the magnetic field from the Voyager 1 spacecraft, where, in the heliosheath, the magnetic field remained directed outward from the Sun for several months without a current-sheet crossing. The crossings finally resumed and now occur somewhat regularly. In addition, the magnetic fluctuations in the heliosheath are observed to be quite different from those in the supersonic upstream solar wind.

ESSC-ESF Position Paper--Science-Driven Scenario for Space Exploration: Report from the European Space Sciences Committee (ESSC)

Abstract: In 2005 the then ESA Directorate for Human Spaceflight, Microgravity and Exploration (D-HME) commissioned a study from the European Science Foundation's (ESF) European Space Sciences Committee (ESSC) to examine the science aspects of the Aurora Programme in preparation for the December 2005 Ministerial Conference of ESA Member States, held in Berlin. A first interim report was presented to ESA at the second stakeholders meeting on 30 and 31 May 2005. A second draft report was made available at the time of the final science stakeholders meeting on 16 September 2005 in order for ESA to use its recommendations to prepare the Executive proposal to the Ministerial Conference. The final ESSC report on that activity came a few months after the Ministerial Conference (June 2006) and attempted to capture some elements of the new situation after Berlin, and in the context of the reduction in NASA's budget that was taking place at that time; e.g., the postponement sine die of the Mars Sample Return mission. At the time of this study, ESSC made it clear to ESA that the timeline imposed prior to the Berlin Conference had not allowed for a proper consultation of the relevant science community and that this should be corrected in the near future. In response to that recommendation, ESSC was asked again in the summer of 2006 to initiate a broad consultation to define a science-driven scenario for the Aurora Programme. This exercise ran between October 2006 and May 2007. ESA provided the funding for staff support, publication costs, and costs related to meetings of a Steering Group, two meetings of a larger ad hoc group (7 and 8 December 2006 and 8 February 2007), and a final scientific workshop on 15 and 16 May 2007 in Athens. As a result of these meetings a draft report was produced and examined by the Ad Hoc Group. Following their endorsement of the report and its approval by the plenary meeting of the ESSC, the draft report was externally refereed, as is now normal practice with all ESSC-ESF reports, and amended accordingly. The Ad Hoc Group defined overarching scientific goals for Europe's exploration programme, dubbed "Emergence and co-evolution of life with its planetary environments," focusing on those targets that can ultimately be reached by humans, i.e., Mars, the Moon, and Near Earth Objects. Mars was further recognized as the focus of that programme, with Mars sample return as the recognized primary goal; furthermore the report clearly states that Europe should position itself as a major actor in defining and leading Mars sample return missions. The report is reproduced in this article. On 26 November 2008 the Ministers of ESA Member States decided to give a high strategic priority to the robotic exploration programme of Mars by funding the enhanced ExoMars mission component, in line therefore with the recommendations from this ESSC-ESF report.

Bursting behavior of the Galactic center faint X-ray transient GRS 1741.9–2853

Abstract: Aims. The neutron star low-mass X-ray binary GRS 1741.9‐2853 is a known type-I burster of the Galactic Center. It is transient, faint, and located in a very crowded region, only 10 0 from the supermassive black hole Sgr A ? . Therefore, its bursting behavior has been poorly studied so far. In particular, its persistent emission has rarely been detected between consecutive bursts, due to lack of sensitivity or confusion. This is what made GRS 1741.9‐2853 one of the nine ”burst-only sources” identified by BeppoSAX a few years ago. The physical properties of GRS 1741.9‐2853 bursts are yet of great interest since we know very little about the nuclear regimes at stake in low accretion rate bursters. We examine here for the first time several bursts in relation with the persistent emission of the source, using INTEGRAL, XMM-Newton, and Swift observations. Methods. We investigate the source flux variability and bursting behavior during its 2005 and 2007 long outbursts. These events were almost entirely covered by INTEGRAL, both in the soft (3‐20 keV) and hard (20‐100 keV) X-ray bands, and also partly by XMM-Newton and Swift (2‐10 keV) in 2007. Results. In its last activity periods, 2005 and 2007, the persistent luminosity of GRS 1741.9‐2853 varied between 1.7 and 10:5 10 36 erg s 1 , i.e. 0:9 5:3% of the Eddington luminosity. The shape of the spectrum as described by an absorbed power-law remained with a photon index 2 and a column density NH 12 10 22 cm 2 throughout the outbursts. We discovered 11 type-I bursts with INTEGRAL, and inspected four additional bursts: two recorded by XMM-Newton and two by Swift. From the brigthest burst, we derive an upper limit on the source distance of 7 kpc. The observed bursts characteristics and source accretion rate suggest pure helium explosions igniting at column depths yign 0:8 4:8 10 8 g cm 2 , for typical energy releases of 1:2 7:4 10 39 erg.

Bursting behavior of the Galactic Center faint X-ray transient GRS 1741.9-2853

Abstract: The neutron star low-mass X-ray binary GRS 1741.9-2853 is a known type-I burster of the Galactic Center. It is transient, faint, and located in a very crowded region, only 10 arcmin from the supermassive black hole Sgr A*. Therefore, its bursting behavior has been poorly studied so far. In particular, its persistent emission has rarely been detected between consecutive bursts, due to lack of sensitivity or confusion. This is what made GRS 1741.9-2853 one of the nine "burst-only sources" identified by BeppoSAX a few years ago. The physical properties of GRS 1741.9-2853 bursts are yet of great interest since we know very little about the nuclear regimes at stake in low accretion rate bursters. We examine here for the first time several bursts in relation with the persistent emission of the source, using INTEGRAL, XMM-Newton, and Swift observations. We investigate the source flux variability and bursting behavior during its 2005 and 2007 long outbursts. The persistent luminosity of GRS 1741.9-2853 varied between ~1.7 and 10.5 10^36 erg s^-1, i.e. 0.9-5.3% of the Eddington luminosity. The shape of the spectrum as described by an absorbed power-law remained with a photon index Gamma ~ 2 and a column density $N_{\rm H} ~ 12 10^22 cm^-2 throughout the outbursts. We discovered 11 type-I bursts with INTEGRAL, and inspected 4 additional bursts: 2 recorded by XMM-Newton and 2 by Swift. From the brigthest burst, we derive an upper limit on the source distance of ~7 kpc. The observed bursts characteristics and source accretion rate suggest pure helium explosions igniting at column depths y_{ign} ~ 0.8-4.8 10^8 g cm^-1, for typical energy releases of ~1.2-7.4 10^39 erg.

The Dynamic Heliosphere: Outstanding Issues

Abstract: Properties of the heliospheric interface, a complex product of an interaction between charged and neutral particles and magnetic fields in the heliosphere and surrounding Circumheliospheric Medium, are far from being fully understood. Recent Voyager spacecraft encounters with the termination shock and their observations in the heliosheath revealed multiple energetic particle populations and noticeable spatial asymmetries not accounted for by the classic theories. Some of the challenges still facing space physicists include the origin of anomalous cosmic rays, particle acceleration downstream of the termination shock, the role of interstellar magnetic fields in producing the global asymmetry of the interface, the influence of charge exchange and interstellar neutral atoms on heliospheric plasma flows, and the signatures of solar magnetic cycle in the heliosheath. These and other outstanding issues are reviewed in this joint report of working groups 4 and 6.

The dynamic heliosphere: Outstanding issues: Report of working groups 4 and 6

Abstract: Properties of the heliospheric interface, a complex product of an interaction between charged and neutral particles and magnetic fields in the heliosphere and surrounding Circumheliospheric Medium, are far from being fully understood. Recent Voyager spacecraft encounters with the termination shock and their observations in the heliosheath revealed multiple energetic particle populations and noticeable spatial asymmetries not accounted for by the classic theories. Some of the challenges still facing space physicists include the origin of anomalous cosmic rays, particle acceleration downstream of the termination shock, the role of interstellar magnetic fields in producing the global asymmetry of the interface, the influence of charge exchange and interstellar neutral atoms on heliospheric plasma flows, and the signatures of solar magnetic cycle in the heliosheath. These and other outstanding issues are reviewed in this joint report of working groups 4 and 6.

Introduction to Solar Magnetism: The Early Years

Abstract: The year 2008 marked the one hundredth anniversary of the observational discovery by George Ellery Hale of magnetic field in sunspots (Hale in Astrophys. J. 28:315–343, 1908). This observation, the first to suggest a direct link between the best-known variable features on the Sun and magnetism, started a line of research that has widened considerably over the last 100 years and is continuing today. Knowledge about all aspects of the Sun has increased in a remarkable way over the past few decades. Variations in the appearance of the Sun and its corona, as well as deeper sources of quasi-regular and chaotic changes that make up solar variability have been extensively documented by both ground-based and space-based solar observatories. It has been recognized that solar magnetism is the key phenomenon that drives solar variability. The workshop devoted to the origin and dynamics of solar magnetism held in the International Space Science Institute in Bern, Switzerland, from 21 to 25 January 2008 reviewed the status of the field and has led to this volume that brings together the best available knowledge and understanding of solar magnetism 100 years after Hale’s pioneering paper. This introductory paper gives an outline of the history of research into solar variability up to the work of Hale and his colleagues. The achievements of the past decades are discussed extensively in the other contributions to this volume.

Abundances of hydrogen and helium isotopes in the Protosolar Cloud

Abstract: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan2455 Hayward St., Ann Arbor, MI 48109, USAemail: gglo@umich.eduAbstract.For our understanding of the origin and evolution of baryonic matter in the Universe,the Protosolar Cloud (PSC) is of unique importance in two ways: 1) Up to now, many of thenaturally occurring nuclides have only been detected in the solar system. 2) Since the timeof solar system formation, the Sun and planets have been virtually isolated from the galacticnuclear evolution, and thus the PSC is a galactic sample with a degree of evolution intermediatebetween the Big Bang and the present.The abundances of the isotopes of hydrogen andhelium in the Protosolar Cloud are primarilyderivedfromcompositionmeasurementsinthesolarwind,theJovianatmosphereand“planetarynoble gases” in meteorites, and also from observations of density profiles inside the Sun. Afterapplying the changes in isotopic and elemental composition resulting from processes in the solarwind, the Sun and Jupiter, PSC abundances of the four lightest stable nuclides are given.Keywords.Sun: solar wind, corona, abundances

What ignites on the neutron star of 4U 0614+091?

Abstract: [abridged] The LMXB 4U 0614+091 is a source of sporadic thermonuclear (type I) X-ray bursts. We find bursts with a wide variety of characteristics in serendipitous wide-field X-ray observations by EURECA/WATCH, RXTE/ASM, BeppoSAX/WFC, HETE-2/FREGATE, INTEGRAL/IBIS/ISGRI, and Swift/BAT, as well as pointed observations by RXTE/PCA and HEXTE. Most of them reach a peak flux of ~15 Crab, but a few only reach a peak flux below a Crab. One of the bursts shows a very strong photospheric radius-expansion phase. This allows us to evaluate the distance to the source: 3.2 kpc. The burst durations are between 10 sec to 5 min. However, after one of the intermediate-duration bursts, a faint tail is seen to at least ~2.4 hours after the start of the burst. One very long burst lasted for several hours. This superburst candidate was followed by a normal type-I burst only 19 days later. This is, to our knowledge, the shortest burst-quench time among the superbursters. A superburst in this system is difficult to reconcile if it accretes at ~1% L_Edd. The intermediate-duration bursts occurred when 4U 0614+091's persistent emission was lowest and calm, and when bursts were infrequent (on average one every month to 3 months). The average burst rate increased significantly after this period. The maximum average burst recurrence rate is once every week to 2 weeks. The burst behaviour may be partly understood if there is at least an appreciable amount of helium present in the accreted material from the donor star. If the system is an ultra-compact X-ray binary with a CO white-dwarf donor, as has been suggested, this is unexpected. If the bursts are powered by helium, we find that the energy production per accumulated mass is about 2.5 times less than expected for pure helium matter.