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| Cluster observations of energetic electrons and electromagnetic fields within a reconnecting thin current sheet in the Earth's magnetotail |
| We study the acceleration of energetic electrons during magnetotail reconnection by using Cluster simultaneous measurements of three-dimensional electron distribution functions, electric and magnetic fields, and waves in a thin current sheet. We present observations of two consecutive current sheet crossings where the flux of electrons 35-127 keV peaks within an interval of tailward flows. The first crossing shows the signatures of a tailward moving flux rope. The observed magnetic field and density indicate that the flux rope was very dynamic, and a comparison with numerical simulation suggests a crossing right after coalescence of smaller flux ropes. The second crossing occurs within the ion diffusion region. The flux of electrons is largest within the flux rope where they are mainly directed perpendicular to the magnetic field. At the magnetic separatrices, the fluxes are smaller, but the energy spectra are harder and electrons are mainly field aligned. Reconnection electric fields EY ~ 7 mV/m are observed within the diffusion region, whereas in the flux rope, EY are much smaller. Waves around lower hybrid frequency do not show a clear correlation with energetic electrons. We interpret the field-aligned electrons at the separatrices as directly accelerated by the reconnection electric field in the diffusion region, whereas we interpret the perpendicular electrons as trapped within the flux rope and accelerated by a combination of betatron acceleration with nonadiabatic pitch-angle scattering. Our observations indicate that thin current sheets during dynamic reconnection are important for in situ production of energetic electrons and that simultaneous measurements of electrons and electromagnetic fields within thin sheets are crucial to understand the acceleration mechanisms. |
| Publication date: 27 Dec 2008 |
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| AKARI/IRC Deep Survey in the North Ecliptic Pole Region |
| We have made a deep and wide imaging survey with all nine AKARI/IRC bands from 2 to 24 micron within a half degree of the North Ecliptic Pole. The survey covered acircular area of 0.38deg2 centered at RA = 17h56m, Dec = 66°37' where a deep optical multi-band survey has been conducted by Subaru/Suprime-Cam. The 5-sigma sensitivity of the survey for point sources is 11.0, 48, 117, and 275 microJansky at wavelengths 3, 7, 15, and 24 micron, respectively. The survey is limited by sky noise at wavelengths from 7 to 24 micron, and limited by source confusion from 2 to 4 micron. We have also made a point source catalog at each band, consisting of more than 5000 sources with a 50% completeness limit of 93 microJansky at 15 micron. Our results are consistent with the pilot survey at faint fluxes and has better statistical significance at the bright fluxes. This is the first near- and mid-infrared sub-milliJansky extragalactic survey with contiguous wavelength coverage from 2 to 24 micron and a large band-merged catalogue. It will provide powerful and unique diagnostics of obscured star-formation and AGN activities in the galaxies up to z = 2. |
| Publication date: 25 Dec 2008 |
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| Introduction to the special section on Venus Express: Results of the Nominal Mission |
More than 25 spacecraft from the United States and the Soviet Union visited Venus in the 20th century, but in spite of the many successful measurements they made, a great number of fundamental problems in the physics of the planet remained unsolved [Taylor, 2006; Titov et al., 2006]. In particular, a systematic and long-term survey of the atmosphere was missing, and most aspects of atmospheric behavior remained puzzling. After the Magellan radar mapping mission ended in 1994, there followed a hiatus of more than a decade in Venus research, until the European Space Agency took up the challenge and sent its own spacecraft to our planetary neighbor. The goal of this mission, Venus Express, is to carry out a global, long-term remote and in situ investigation of the atmosphere, the plasma environment, and some aspects of the surface of Venus from orbit [Titov et al., 2001; Svedhem et al., 2007].
Venus Express continues and extends the investigations of earlier missions by providing detailed monitoring of processes and phenomena in the atmosphere and near-space environment of Venus. Radio, solar, and stellar occultation, together with thermal emission spectroscopy, sound the atmospheric structure in the altitude range from 150 to 40 km with vertical resolution of few hundred meters, revealing strong temperature variations driven by radiation and dynamical processes.
- The remainder of the abstract is truncated -
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| Publication date: 17 Dec 2008 |
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| IXO Mission Concept |
Presentation of the IXO mission concept at the IXO Coordination Group meeting on 20 November 2008.
Contents:
- Introduction
- IXO mission requirements
- IXO mission analysis
- IXO configuration
- IXO instrument module
- IXO service module
- IXO mirror assembly
- Options
- Conclusion
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| Publication date: 16 Dec 2008 |
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| ISOC Newsletter #21 |
Contents:-
Foreword
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AO-6 and AO-7
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The 7th INTEGRAL Workshop
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Science Operations
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Contacting ISOC
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| Publication date: 15 Dec 2008 |
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| ST-ECF Newsletter 45 |
Contents:- Hubble Status
- The Hubble Cache
- Footprint Finder
- HLA ACS Grism Data
- Scisoft 7.2
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| Publication date: 15 Dec 2008 |
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| The science of space weather |
| The basic physics underpinning space weather is reviewed, beginning with a brief overview of the main causes of variability in the near-Earth space environment. Although many plasma phenomena contribute to space weather, one of the most important is magnetic reconnection, and recent cutting edge research in this field is reviewed. We then place this research in context by discussing a number of specific types of space weather in more detail. As society inexorably increases its dependence on space, the necessity of predicting and mitigating space weather will become ever more acute. This requires a deep understanding of the complexities inherent in the plasmas that fill space and has prompted the development of a new generation of scientific space missions at the international level. |
| Publication date: 13 Dec 2008 |
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| Aurora (Robotic Exploration) Programme Update |
Presentation at Royal Astronomical Society, London.
Contents:
- Current ExoMars objectives
- Recap of Enhanced ExoMars Baseline
- Present transfer timelines to avoid dust storm landing
- Present and future TLC capability at Mars
- Financial Situation
- Descoping strategy
- Next Steps
- MREP(MarsRobotic Exploration Preparation
- Conclusions
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| Publication date: 12 Dec 2008 |
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| Mineralogical composition, structure, morphology, and geological history of Aram Chaos crater fill on Mars derived from OMEGA Mars Express data |
| Aram Chaos is a crater 280 km in diameter centered at 2.5°N, 338.5°E. It is filled by chaotic terrains overlain by a dome-shaped, layered 900 m thick formation displaying
spectral signatures of ferric oxides on Thermal Emission Spectrometer (TES) and Observatoire pour la Mineralogie, L'Eau, les Glaces et L'Activite (OMEGA) medium spatial resolution data. We describe in detail the mineralogical composition, structure, and morphology of this crater fill using high-resolution data (OMEGA, Mars Orbiter Laser
Altimeter, Mars Orbiter Camera, TES, Thermal Emission Imaging System, and High-Resolution Imaging Science Experiment). We infer the following formation scenario: the crater was first filled by a geological formation, the composition of which remains unclear because it is covered by dust. Widespread fracturing of this formation led to the development of chaotic terrains. Later, a second layered formation, presently dome
shaped, was emplaced unconformably on the chaotic terrains. This younger unit is composed of a bright, poorly consolidated material that contains both monohydrated sulfates and ferric oxides according to OMEGA data. The surface of this formation is partially covered by dust and displays landforms indicating that the bright material has
been mobilized by wind during or after its deposition. After its emplacement, this formation has been grooved down to various depths by large eolian erosion corridors. In these corridors, eolian removal of the bright material with a sulfate-rich matrix has left debris fans, sand sheets, and dunes, which display some of the strongest spectral
signatures of ferric oxides on Mars. Similar residual deposits enriched in ferric oxides, overlying a layered formation containing both ferric oxides and sulfates, have been observed by the Opportunity rover in Meridiani Planum, suggesting a common formation process. |
| Publication date: 09 Dec 2008 |
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| Tailward propagation of Pi2 waves in the Earth's magnetotail lobe |
| Pi2 waves are an intergral part of the substorm process and have been observed on the ground and in space. Using the special ability of Cluster to determine the propagation direction of signals measured in the magnetometer data, it is found that in the lobes of the Earth's magnetotail, for the cases in this study, the Pi2 waves are travelling tailward. The polarization of the waves in the lobes corresponds well with the polarization observed in the highest latitude ground station. The propagation velocity of the Pi2 waves in the lobes is basically Alfvénic. |
| Publication date: 09 Dec 2008 |
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| Complexities of a 3-D plasmoid flux rope as shown by an MHD simulation |
| The results of a global magnetohydrodynamic (MHD) simulation of a pair of magnetospheric substorms on 11 August 2002 are presented. Comparisons of data with simulation results reveal a good agreement regarding the sequence of events during substorm development. We give particular emphasis to results in the simulation of a flux rope formed during the second substorm. Unlike standard 2-D depictions of reconnection and plasmoid release during the substorm sequence, the simulation shows a highly complex structure that has considerable winding of both closed and open field lines. Additionally, the simulated flux rope does not move tailward uniformly, but rather it has asymmetric motion in which the dawn flank portion moves tailward prior to the dusk portion of the flux rope. This results in a skewed flux rope structure that runs almost parallel to the tail axis instead of perpendicular to it. The simulation compares well with both prior flux rope simulations as well as satellite observations of flux ropes. We use the global simulation to map flux tube properties to the ionosphere, which allows the complexity of the mapping of the magnetic field structure from the tail to the ionosphere to be seen in a novel manner. |
| Publication date: 05 Dec 2008 |
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| Atmospheric structure and dynamics as the cause of ultraviolet markings in the clouds of Venus |
| When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds and seem to trace dynamical activity in the middle atmosphere. It has long been unclear whether the global pattern arises from differences in cloud top altitude (which was earlier estimated to be 66-72 km), compositional variations or temperature contrasts. Here we report multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing which brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the 'cold collar', an annulus of cold air characterized by 30 K lower temperatures with a positive lapse rate, which suppresses vertical mixing and cuts off the supply of ultraviolet absorbers from below. In low and middle latitudes, the visible cloud top is located at a remarkably constant altitude of 72±1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment rather than by elevation changes. The cloud top descends to 64 km in the eye of the hemispheric vortex, which appears as a depression in the upper cloud deck. The ultraviolet dark circular streaks enclose the vortex eye and are dynamically connected to it. |
| Publication date: 04 Dec 2008 |
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| Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data |
The Hipparcos satellite, developed and launched by the European Space Agency (ESA) in 1989, was the first space mission dedicated to astrometry - the accurate measurement of positions, distances, and proper motions of stars. Amongst the key achievements of its measurements are refining the cosmic distance scale, characterising the large-scale kinematic motions in the Solar neighbourhood, providing precise luminosities for stellar modelling, and confirming Einstein's prediction of the effect of gravity on starlight. This authoritative account of the Hipparcos contributions over the last decade is an outstanding reference for astronomers, astrophysicists and cosmologists. It reviews the applications of the data in different areas, describing the subject and the state-of-the-art before Hipparcos, and summarising all major contributions to the topic made by Hipparcos. It contains a detailed overview of the Hipparcos and Tycho Catalogues, their annexes and their updates. Each chapter ends with comprehensive references to relevant literature.
Table of Contents
1. The Hipparcos and Tycho catalogues;
2. Derived catalogues and applications;
3. Double and multiple stars;
4. Photometry and variability;
5. Luminosity calibration and distance scale;
6. Open clusters, groups and associations;
7. Stellar structure and evolution;
8. Specific stellar types and the ISM;
9. Structure of the Galaxy;
10. Solar System and exo-planets;
Index.
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| Publication date: 01 Dec 2008 |
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| Optical Images of an Exosolar Planet 25 Light-Years from Earth |
| Fomalhaut, a bright star 7.7 parsecs (25 light-years) from Earth, harbors a belt of cold dust with a structure consistent with gravitational sculpting by an orbiting planet. Here, we present optical observations of an exoplanet candidate, Fomalhaut b. Fomalhaut b lies about 119 astronomical units (AU) from the star and 18 AU of the dust belt, matching predictions of its location. Hubble Space Telescope observations separated by 1.73 years reveal counterclockwise orbital motion. Dynamical models of the interaction between the planet and the belt indicate that the planet's mass is at most three times that of Jupiter; a higher mass would lead to gravitational disruption of the belt, matching predictions of its location. The flux detected at 0.8 µm is also consistent with that of a planet with mass no greater than a few times that of Jupiter. The brightness at 0.6 µm and the lack of detection at longer wavelengths suggest that the detected flux may include starlight reflected off a circumplanetary disk, with dimension comparable to the orbits of the Galilean satellites. We also observe variability of unknown origin at 0.6 µm. |
| Publication date: 28 Nov 2008 |
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| Water vapour jets inside the plume of gas leaving Enceladus |
| A plume of water vapour escapes from fissures crossing the south polar region of the Saturnian moon Enceladus. Tidal deformation of a thin surface crust above an internal ocean could result in tensile and compressive stresses that would affect the width of the fissures; therefore, the quantity of water vapour released at different locations in Enceladus' eccentric orbit is a crucial measurement of tidal control of venting. Here we report observations of an occultation of a star by the plume on 24 October 2007 that revealed four high-density gas jets superimposed on the background plume. The gas jet positions coincide with those of dust jets reported elsewhere inside the plume. The maximum water column density in the plume is about twice the density reported earlier. The density ratio does not agree with predictions - we should have seen less water than was observed in 2005. The ratio of the jets' bulk vertical velocities to their thermal velocities is 1.5 0.2, which supports the hypothesis that the source of the plume is liquid water, with gas accelerated to supersonic velocity in nozzle-like channels. |
| Publication date: 27 Nov 2008 |
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| Integral Field Spectroscopy of (U)LIRGs. From VLT to JWST |
| In the book "Science with the VLT in the ELT Era", Moorwood, Alan F.M. (Ed.), book series "Astrophysics and Space Science Proceedings", ISSN:1570-6591, doi:10.1007/978-1-4020-9190-2, part V, pp. 301-305
Some first results from the Integral Field Spectroscopy Survey of (U)LIRGs using VLT instruments VIMOS and SINFONI are presented. Detailed studies of the two-dimensional ionization structure and kinematics of the stars and different gas phases are within reach with IFS techniques on 8 m class telescopes. The perspectives of extending these studies to high-z galaxies with future IFS instruments (NIRSpec and MIRI) onboard JWST are considered. |
| Publication date: 21 Nov 2008 |
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| The statistics of foreshock cavities: results of a Cluster survey |
| We use Cluster magnetic field, thermal ion, and energetic particle observations upstream of the Earth's bow shock to investigate the occurrence patterns of foreshock cavities. Such cavities are thought to form when bundles of magnetic field connect to the quasi-parallel bow shock. Shock-processed suprathermal ions can then stream along the field, back against the flow of the solar wind. These suprathermals enhance the pressure on shock-connected field lines causing them to expand into the surrounding ambient solar wind plasma. Foreshock cavities exhibit depressions in magnetic field magnitude and thermal ion density, associated with enhanced fluxes of energetic ions. We find typical cavity duration to be few minutes with interior densities and magnetic field magnitudes dropping to ~60% of those in the surrounding solar wind. Cavities are found to occur preferentially in fast, moderate magnetic field strength solar wind streams. Cavities are observed in all parts of the Cluster orbit upstream of the bow shock. When localised in a coordinate system organised by the underlying physical processes in the foreshock, there is a systematic change in foreshock cavity location with IMF cone angle. At low (high) cone angles foreshock cavities are observed outside (inside) the expected upstream boundary of the intermediate ion foreshock. |
| Publication date: 20 Nov 2008 |
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| Complex structure within Saturn's infrared aurora |
| The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission and ionize the hydrogen, leading to H3+ infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown. |
| Publication date: 13 Nov 2008 |
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| GALS - Gradient Analysis by Least Squares |
| We present a method, GALS (Gradient Analysis by Least Squares) for estimating the gradient of a physical field from multi-spacecraft observations. To obtain the best possible spatial resolution, the gradient is estimated in the frame of reference where structures in the field are essentially locally stationary. The estimates are refined iteratively by a least squares method.
We show that GALS is not very sensitive to the spacecraft configuration and resolves structures much smaller than the characteristic size of the spacecraft distribution. Furthermore, GALS requires little user input.
GALS has been tested on synthetic magnetic field data and data from the Cluster FGM instrument. GALS will also be useful for other types of data. The results indicate that GALS is robust and superior to the curlometer method for estimating the current from magnetic field measurements. |
| Publication date: 10 Nov 2008 |
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| Evolution of Kelvin-Helmholtz activity on the dusk flank magnetopause |
| Our purpose is to characterize the evolution of the magnetopause Kelvin-Helmholtz (KH) wave activity with changes in thickness of the adjacent boundary layer, geomagnetic latitude and interplanetary magnetic field (IMF) orientation. As the IMF turns northward, wave activity may be generated at the dayside before propagating down the tail, where the boundary layer is expected to support longer wavelengths. We use two-point observations on the dusk magnetopause at low latitudes, from Geotail on the dayside and Cluster tailward of the dusk terminator. We quantify the wavelength, power, wavefront steepness and propagation direction at Cluster. An estimate of the thickness of the low-latitude boundary layer (LLBL) is obtained by correlating normal distances to the magnetopause, derived from two empirical solar-wind-driven models, with a systematic relationship (the "transition parameter") found between the electron number density and temperature; the correlation factor is used to infer the temporal evolution of the thickness of the locally sampled layer. We find that wavelengths are controlled by the IMF clock angle, as expected when generated by the KH mechanism at the dayside, although amplitudes, wavefront steepness and propagation directions are more closely correlated with the layer thickness. A survey of parameter space provides evidence of the contribution of the KH mechanism to the widening of the electron LLBL. |
| Publication date: 05 Nov 2008 |
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