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| Radar observations and the shape of near-Earth asteroid 2008 EV5 |
| We observed the near-Earth ASTEROID 2008 EV5 with the Arecibo and Goldstone planetary radars and the Very Long Baseline Array during December 2008. EV5 rotates retrograde and its overall shape is a 400 ± 50 m oblate spheroid. The most prominent surface feature is a ridge parallel to the asteroid's equator that is broken by a concavity about 150 m in diameter. Otherwise the asteroid's surface is notably smooth on decameter scales. EV5's radar and optical albedos are consistent with either rocky or stony-iron composition. The equatorial ridge is similar to structure seen on the rubble-pile near-Earth asteroid (66391) 1999 KW4 and is consistent with YORP spin-up reconfiguring the asteroid in the past. We interpret the concavity as an impact crater. Shaking during the impact and later regolith redistribution may have erased smaller features, explaining the general lack of decameter-scale surface structure. |
| Publication date: 30 Apr 2011 |
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| Grad-Shafranov Reconstruction of Magnetic Flux Ropes in the Near-Earth Space |
| Electric currents permeate space plasmas and often have a significant component along the magnetic field to form magnetic flux ropes. A larger spatial perspective of these structures than from the direct observation along the satellite path is crucial in visualizing their role in plasma dynamics. For magnetic flux ropes that are approximately two-dimensional equilibrium structures on a certain plane, Grad-Shafranov reconstruction technique, developed by Bengt Sonnerup and his colleagues (see Sonnerup et al. in J. Geophys. Res. 111:A09204, 2006), can be used to reveal two-dimensional maps of associated plasma and field parameters. This review gives a brief account of the technique and its application to magnetic flux ropes near the Earth's magnetopause, in the solar wind, and in the magnetotail. From this brief survey, the ranges of the total field-aligned current and the total magnetic flux content for these magnetic flux ropes are assessed. The total field-aligned current is found to range from ~0.14 to ~9.7×104 MA, a range of nearly six orders of magnitude. The total magnetic flux content is found to range from ~0.25 to ~2.3×106 MWb, a range of nearly seven orders of magnitude. To the best of our knowledge, this review reports the largest range of both the total field-aligned current and the total magnetic flux content for magnetic flux ropes in space plasmas. |
| Publication date: 22 Apr 2011 |
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| The auroral footprint of Enceladus on Saturn |
| Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh, about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters - and as such is probably indicative of variable plume activity. |
| Publication date: 21 Apr 2011 |
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| The auroral footprint of Enceladus on Saturn |
Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh, about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters' and as such is probably indicative of variable plume activity. |
| Publication date: 21 Apr 2011 |
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| Plasma jet braking: Energy dissipation and nonadiabatic electrons |
| We report in situ observations by the Cluster spacecraft of wave-particle interactions in a magnetic flux pileup region created by a magnetic reconnection outflow jet in Earth's magnetotail. Two distinct regions of wave activity are identified: lower-hybrid drift waves at the front edge and whistler-mode waves inside the pileup region. The whistler-mode waves are locally generated by the electron temperature anisotropy, and provide evidence for ongoing betatron energization caused by magnetic flux pileup. The whistler-mode waves cause fast pitch-angle scattering of electrons and isotropization of the electron distribution, thus making the flow braking process nonadiabatic. The waves strongly affect the electron dynamics and thus play an important role in the energy conversion chain during plasma jet braking. |
| Publication date: 18 Apr 2011 |
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| Characterizing interstellar filaments with Herschel in IC 5146 |
| We provide a first look at the results of the Herschel Gould Belt survey toward the IC 5146 molecular cloud and present a preliminary analysis of the filamentary structure in this region. The column density map, derived from our 70-500 micron Herschel data, reveals a complex network of filaments and confirms that these filaments are the main birth sites of prestellar cores. We analyze the column density profiles of 27 filaments and show that the underlying radial density profiles fall off as r-1.5 to r-2.5 at large radii. Our main result is that the filaments seem to be characterized by a narrow distribution of widths with a median value of 0.10 ± 0.03 pc, which is in stark contrast to a much broader distribution of central Jeans lengths. This characteristic width of ~0.1 pc corresponds to within a factor of ~2 to the sonic scale below which interstellar turbulence becomes subsonic in diffuse gas, which supports the argument that the filaments may form as a result of the dissipation of large-scale turbulence. |
| Publication date: 13 Apr 2011 |
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| Cluster observations of multiple dipolarization fronts |
| We present Cluster observations of a series of dipolarization fronts (DF 1 to 6) at the central current sheet in Earth's magnetotail. The velocities of fast earthward flow following behind each DF 1-3 are comparable to the Alfvén velocity, indicating that the flow bursts might have been generated by bursty reconnection that occurred tailward of the spacecraft. Based on multispacecraft timing analysis, DF normals are found to propagate mainly earthward at 160-335 km/s with a thickness of 900-1500 km, which corresponds to the ion inertial length or gyroradius scale. Each DF is followed by significant fluctuations in the x and y components of the magnetic field whose peaks are found 1-2 min after the DF passage. These (Bx, By) fluctuations propagate dawnward (mainly) and earthward. Strongly enhanced field-aligned beams are observed coincidently with (Bx, By) fluctuations, while an enhancement of cross-tail currents is associated with the DFs. From the observed pressure imbalance and flux tube entropy changes between the two regions separated by the DF, we speculate that interchange instability destabilizes the DFs and causes the deformation of the midtail magnetic topology. This process generates significant field-aligned currents and might power the auroral brightening in the ionosphere. However, this event is associated with neither the main substorm auroral breakup nor the poleward expansion, which might indicate that the observed multiple DFs have been dissipated before they reach the inner plasma sheet boundary. |
| Publication date: 13 Apr 2011 |
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| Cassini SAR, radiometry, scatterometry and altimetry observations of Titan's dune fields |
| Large expanses of linear dunes cover Titan's equatorial regions. As the Cassini mission continues, more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation (SAR, radiometry, scatterometry, altimetry) and with an increasing variety of observational geometries. In this paper, we report on Cassini's radar instrument observations of the dune fields mapped through May 2009 and present our key findings in terms of Titan's geology and climate. We estimate that dune fields cover ~12.5% of Titan's surface, which corresponds to an area of ~10 million km2, roughly the area of the United States. If dune sand-sized particles are mainly composed of solid organics as suggested by VIMS observations (Cassini Visual and Infrared Mapping Spectrometer) and atmospheric modeling and supported by radiometry data, dune fields are the largest known organic reservoir on Titan. Dune regions are, with the exception of the polar lakes and seas, the least reflective and most emissive features on this moon. Interestingly, we also find a latitudinal dependence in the dune field microwave properties: up to a latitude of ~11°, dune fields tend to become less emissive and brighter as one moves northward. Above ~11° this trend is reversed. The microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion (relative to that of the dune), roughness and degree of sand cover. In agreement with radiometry and scatterometry observations, SAR images suggest that the fraction of interdunes increases northward up to a latitude of ~14°. |
| Publication date: 07 Apr 2011 |
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| Venus's southern polar vortex reveals precessing circulation |
Published online in Science Express, 7 April 2011. Initial images of Venus's South Pole by the Venus Express mission showed the presence of a bright, highly variable vortex, similar to that at the planet's North Pole. Using high-resolution infrared measurements of polar winds from the Venus Express's Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, we show the vortex to have a constantly varying internal structure, with a centre of rotation displaced from the geographic South Pole by ~3 degrees of latitude, and which drifts around the pole with a period of 5 to 10 Earth days. This is indicative of a nonsymmetric and varying precession of the polar atmospheric circulation with respect to the planetary axis. |
| Publication date: 07 Apr 2011 |
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| Swift follow-up of unidentified X-ray sources in the XMM–Newton Slew Survey |
We present deep Swift follow-up observations of a sample of 94 unidentified X-ray sources from the XMM-Newton Slew Survey. The X-ray Telescope (XRT) on-board Swift detected 29 per cent of the sample sources; the flux limits for undetected sources suggest the bulk of the Slew Survey sources are drawn from one or more transient populations. We report revised X-ray positions for the XRT-detected sources, with typical uncertainties of 2.9 arcsec, reducing the number of catalogued optical matches to just a single source in most cases. We characterize the sources detected by Swift through their X-ray spectra and variability and via Ultraviolet-Optical Telescope photometry and using catalogued near-infrared, optical and radio observations of potential counterparts. Six sources can be associated with known objects and eight sources may be associated with unidentified ROSAT sources within the 3 sigma error radii of our revised X-ray positions. We find 10 of the 30 XRT- and/or Burst Alert Telescope (BAT)-detected sources are clearly stellar in nature, including one periodic variable star and two high proper motion stars. For 11 sources we propose an active galactic nucleus (AGN) classification, among which four are detected in hard X-rays and three have redshifts spanning z= 0.2-0.9 obtained from the literature or from optical spectroscopy presented here. A further three sources are suspected AGN and one is a candidate Galactic hard X-ray flash, while five sources remain unclassified. The 67 Slew Survey sources we do not detect with Swift XRT or BAT are studied via their characteristics in the Slew Survey observations and by comparison with the XRT- and BAT-detected population. We suggest that these are mostly if not all extragalactic, though unlikely to be highly absorbed sources in the X-rays such as Compton thick AGN.
- The remainder of the abstract is truncated - |
| Publication date: 01 Apr 2011 |
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| Cluster observations of a cusp diamagnetic cavity: Structure, size, and dynamics |
| We have analyzed Cluster magnetic field and plasma data during high-altitude cusp crossing and compared them with high-resolution MHD simulations. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as the IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side of the cusp because the reconnection site had changed location. We found evidence of magnetic reconnection both during northward and southward IMF conditions. The Cluster separation was ~5000 km, enabling for the first time measurements both inside the DMC and surrounding boundaries that allowed us to construct the structure of the DMC and put the observations of ion pitch angle distributions in context of local reconnection topology and gradients of the boundaries. The cavity is characterized by strong magnetic field fluctuations and high-energy particles. At the magnetosheath boundary the high-energy particle fluxes reduced by several orders of magnitude. Throughout the magnetosheath, the high-energy proton fluxes remained low except during brief intervals when sc4 and sc1 dropped back into the cavity due to changes in solar wind dynamic pressure. However, the high-energy O+ fluxes did not drop as much in the magnetosheath and were mostly at 60°-120° pitch angles, indicative of a trapped population in the DMC which is observed in the magnetosheath due to a large gyroradius. Significant fluxes of protons and ionized oxygen were also observed escaping from the diamagnetic cavity antiparallel to the magnetic field in a time scale more consistent with the local DMC source than with a reflected bow shock source. |
| Publication date: 31 Mar 2011 |
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| The evolution of Titan's detached haze layer near equinox in 2009 |
Saturn's moon Titan has a massive atmosphere laden with layers of photochemical haze. We report a recent dramatic change in the vertical structure of this haze, with a persistent 'detached' layer dropping in altitude from over 500 km to only 380 km between 2007 and 2010. The detached haze layer appears to be a well-defined tracer for Titan's meridional stratospheric circulation, models of which suggest that a pole-to-pole meridional cell weakens during equinox as solar heating becomes more symmetric. These measurements connect the Cassini observations with those made by Voyager almost one seasonal cycle earlier. They place detailed constraints on the seasonal circulation, on the sources of photochemical aerosols, on the microphysical processes and on the complex interplay of these components. |
| Publication date: 31 Mar 2011 |
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| Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1 |
Published online in Science Express, 24 March 2011.
Because of their inherently high flux allowing the detection of clear signals, black hole x-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the INTEGRAL/IBIS telescope. Spectral modeling of the data reveals two emission mechanisms: The 250-400 keV data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400keV-2MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band. |
| Publication date: 24 Mar 2011 |
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| Workshop on Landing Sites for Exploration Missions - Summary Outcome and Recommendations |
The first European workshop on Landing Sites for Exploration Missions took place at the Lorentz Centre, in Leiden (NL) on 17-21 January 2011. The workshop was organised in the framework of a Europlanet JRA1 (Support to Future Missions) grant. It gathered more than sixty participants from the space science and engineering communities in Europe, the United States, and Asia.
The workshop's programme combined a series of lectures presenting different perspectives on landing sites with hands-on sessions involving small, interdisciplinary teams focussing on specific mission scenarios.
The goal of the workshop was to bring together the international landing site community to start preparing for landing site selection and characterisation activities serving missions having a European component. The week was concluded with a conference at the European Space Research and Technology Centre (ESTEC), focussing on the international programmatic context of robotic exploration missions.
This report presents the workshop's outcome, recommendations, and detailed results of the various mission scenarios that were studied: three Mars mission scenarios (2016 Entry, Descent and Landing
Demonstrator Module, 2018 Dual Rover Mission, and Mars Sample Return); a mission to a small solar system body, such as Phobos or an asteroid; and a Google Lunar X-prize mission to consider landing site selection approaches for commercial missions). |
| Publication date: 20 Mar 2011 |
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| Rapid and Extensive Surface Changes Near Titan's Equator: Evidence of April Showers |
| Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation. |
| Publication date: 18 Mar 2011 |
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| Rapid and Extensive Surface Changes Near Titan's Equator: Evidence of April Showers |
| Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation. |
| Publication date: 18 Mar 2011 |
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| Monitoring the solar UV irradiance spectrum from the observation of a few passbands |
| Context. The solar irradiance in the UV is a key ingredient in space weather applications; however, because of the lack of continuous and long-term observations, various indices are still used today as surrogates for the solar spectral irradiance.
Aims. As an alternative to current spectrometers we use a few radiometers with properly chosen passbands and reconstruct the solar spectral irradiance from their outputs. The feasibility of such a reconstruction is justified by the high redundancy in the spectral variability.
Methods. Using a multivariate statistical approach, we first compared six years of daily-averaged UV spectra and a selection of passbands (from existing radiometers) and solar indices. This leads to a strategy for defining those passbands that are most appropriate for reconstructing the spectrum.
Results. With four passdbands chosen from already existing instruments, we reconstruct the UV spectrum with a relative error of about 20%. Better performance is achieved with a combination of passbands than with a combination of indices. |
| Publication date: 03 Mar 2011 |
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| Halo Models of Large Scale Structure |
| We review the formalism and applications of the halo-based description of non-linear gravitational clustering. In this approach, all mass is associated with virialized dark matter halos; models of the number and spatial distribution of the halos, and the distribution of dark matter within each halo, are used to provide estimates of how the statistical properties of large scale density and velocity fields evolve as a result of non-linear gravitational clustering. We first describe the model, and demonstrate its accuracy by comparing its predictions with exact results from numerical simulations of non-linear gravitational clustering. We then present several astrophysical applications of the halo model: these include models of the spatial distribution of galaxies, the non-linear velocity, momentum and pressure fields, descriptions of weak gravitational lensing, and estimates of secondary contributions to temperature fluctuations in the cosmic microwave background. |
| Publication date: 16 Feb 2011 |
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| Submillimetre galaxies reside in dark matter haloes with masses greater than 3×10^11 solar masses |
| The extragalactic background light at far-infrared wavelengths comes from optically faint, dusty, star-forming galaxies in the Universe with star formation rates of a few hundred solar masses per year. These faint, submillimetre galaxies are challenging to study individually because of the relatively poor spatial resolution of far-infrared telescopes. Instead, their average properties can be studied using statistics such as the angular power spectrum of the background intensity variations. A previous attempt at measuring this power spectrum resulted in the suggestion that the clustering amplitude is below the level computed with a simple ansatz based on a halo model. Here we report excess clustering over the linear prediction at arcminute angular scales in the power spectrum of brightness fluctuations at 250, 350 and 500 ¼m. From this excess, we find that submillimetre galaxies are located in dark matter haloes with a minimum mass, Mmin, such that log10[Mmin/Msolar] = 11.5 (+0.7-0.2) at 350 µm, where Msolar is the solar mass. This minimum dark matter halo mass corresponds to the most efficient mass scale for star formation in the Universe, and is lower than that predicted by semi-analytical models for galaxy formation. |
| Publication date: 16 Feb 2011 |
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| On the origin of the energetic ion events measured upstream of the Earth's bow shock by STEREO, Cluster, and Geotail |
| In 2007 during the declining phase of the solar cycle the energetic upstream ion events occurred mainly after a corotating interaction region passed the Earth's magnetosphere. We study the relation between these upstream events observed from about 70 to 1750 RE away from the Earth and observations in the vicinity of the terrestrial bow shock (up to 30 RE). For this purpose, simultaneous measurements of energetic ions from STEREO A and STEREO B (far upstream region) and from Cluster and Geotail (near the bow shock) are used. In all cases the energetic ions far upstream are associated with the upstream ion events near the bow shock. The upstream events are observed simultaneously mainly when the magnetic field is pointing along the line joining those satellites in the far upstream region with those near the terrestrial bow shock. The upstream events near the bow shock often coincide with sunward directed electron bursts, increased AE index (>200 nT), nonexponential proton spectra, and most important the presence of O+ ions, all of which imply at least partly a magnetospheric origin. In ~57% of cases the upstream ion events near the bow shock are associated with electron bursts and/or with the presence of O+, and ~40% of the latter events are associated with electron bursts at STEREO A. Although we present strong evidence that the events are partially of magnetospheric origin, we do not exclude the presence of the ions accelerated at the bow shock. |
| Publication date: 11 Feb 2011 |
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