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| Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster |
| The immediate effect of the rotation of the interplanetary magnetic field (IMF) from southward to northward on cusp precipitation has been rarely observed by a polar orbiting satellite in the past. The four Cluster spacecraft observed such an event on 23 September 2004 as they were crossing the polar cusp within 2-16 min from each other. Between the first three and the last spacecraft crossing the cusp, the IMF rotated from southward to northward with a dominant By (GSM) component. For the first time we can examine the changes in the particle precipitation immediately after such IMF change. The first two spacecraft observed typical IMF-southward ion dispersion, while the last one observed both an IMF-southward-like dispersion in the boundary layer and an IMF-northward dispersion in the cusp. After the IMF turning, the cusp is shown to have grown in size in both the poleward and equatorward directions. A three-dimensional magnetohydrodynamic simulation is used to determine the locations of the sources of the ions and the topology of the magnetic field during the event. |
| Publication date: 02 May 2008 |
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| Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions |
| Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma. |
| Publication date: 02 May 2008 |
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| Accurate Mars Express orbits to improve the determination of the mass and ephemeris of the Martian moons |
| The determination of the ephemeris of the Martian moons has benefited from observations of their plane-of-sky positions derived from images taken by cameras onboard spacecraft orbiting Mars. Images obtained by the Super Resolution Camera (SRC) onboard Mars Express (MEX) have been used to derive moon positions relative to Mars on the basis of a fit of a complete dynamical model of their motion around Mars. Since, these positions are computed from the relative position of the spacecraft when the images are taken, those positions need to be known as accurately as possible. An accurate MEX orbit is obtained by fitting two years of tracking data of the Mars Express Radio Science (MaRS) experiment onboard MEX. The average accuracy of the orbits has been estimated to be around 20-25 m. From these orbits, we have re-derived the positions of Phobos and Deimos at the epoch of the SRC observations and compared them with the positions derived by using the MEX orbits provided by the ESOC navigation team. After fit of the orbital model of Phobos and Deimos, the gain in precision in the Phobos position is roughly 30 m, corresponding to the estimated gain of accuracy of the MEX orbits. A new solution of the GM of the Martian moons has also been obtained from the accurate MEX orbits, which is consistent with previous solutions and, for Phobos, is more precise than the solution from the Mars Global Surveyor (MGS) and Mars Odyssey (ODY) tracking data. It will be further improved with data from MEX-Phobos closer encounters (at a distance less than 300 km). This study also demonstrates the advantage of combining observations of the moon positions from a spacecraft and from the Earth to assess the real accuracy of the spacecraft orbit. In turn, the natural satellite ephemerides can be improved and participate to a better knowledge of the origin and evolution of the Martian moons. |
| Publication date: 01 May 2008 |
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| ESA's report to the 37th COSPAR meeting - Cross-Scale section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the Cosmic Vision candidate, Cross-Scale. |
| Publication date: 01 May 2008 |
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| ESA's report to the 37th COSPAR meeting - Laplace section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the Cosmic Vision candidate, Laplace. |
| Publication date: 01 May 2008 |
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| ESA's report to the 37th COSPAR meeting - Marco Polo section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the Cosmic Vision candidate, Marco-Polo. |
| Publication date: 01 May 2008 |
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| ESA's report to the 37th COSPAR meeting - PLATO section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the Cosmic Vision candidate, PLATO. |
| Publication date: 01 May 2008 |
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| ESA's report to the 37th COSPAR meeting - SPICA section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the Cosmic Vision candidate, SPICA. |
| Publication date: 01 May 2008 |
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| ESA's report to the 37th COSPAR meeting - TandEM section |
| ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the Cosmic Vision candidate, TandEM.
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| Publication date: 01 May 2008 |
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| Evidence That Solar Flares Drive Global Oscillations in the Sun |
| Solar flares are large explosions on the Sun's surface caused by a sudden release of magnetic energy. They are known to cause local short-lived oscillations traveling away from the explosion like water rings. Here we show that the energy in the solar acoustic spectrum is correlated with flares. This means that the flares drive global oscillations in the Sun in the same way that the entire Earth is set ringing for several weeks after a major earthquake such as the 2004 December Sumatra-Andaman one. The correlation between flares and energy in the acoustic spectrum of disk-integrated sunlight is stronger for high-frequency waves than for ordinary p-modes which are excited by the turbulence in the near-surface convection zone immediately beneath the photosphere. |
| Publication date: 01 May 2008 |
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