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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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