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| Modulated reconnection rate and energy conversion at the magnetopause under steady IMF conditions |
| We use the multi-spacecraft mission Cluster to make observational estimates of the local energy conversion across the dayside high-latitude magnetopause. The energy conversion is estimated during eleven complete magnetopause crossings under steady south-dawnward interplanetary magnetic field (IMF). We describe a new method to determine the reconnection rate from the magnitude of the local energy conversion. The reconnection rate as well as the energy conversion varies during the course of the eleven crossings and is typically much higher for the outbound crossings. This supports the previous interpretation that reconnection is continuous but its rate is modulated. |
| Publication date: 30 Apr 2008 |
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| Cluster observations of an ion-scale current sheet in the magnetotail under the presence of a guide field |
| We report on Cluster observations of a thin current sheet interval under the presence of a strong |BY| during a fast earthward flow interval between 1655 UT and 1703 UT on 17 August 2003. The strong |BY| in the tail could be associated with a strong IMF |BY|, but the large fluctuations in BY, not seen in the IMF, suggest that a varying reconnection rate causes a varying transport of BY-dominated magnetic flux and/or a change in BY due to the Hall-current system. During the encounter of the high-speed flow, an intense current layer was observed around 1655:53 UT with a peak current density of 182 nA/m², the largest current density observed by the Cluster four-spacecraft magnetic field measurement in the magnetotail. The half width of this current layer was estimated to be ~290 km, which was comparable to the ion-inertia length. Its unique signature is that the strong current is mainly field-aligned current flowing close to the center of the plasma sheet. The event was associated with parallel heating of electrons with asymmetries, which suggests that electrons moving along the field lines can contribute to a strong dawn-to-dusk current when the magnetotail current sheet becomes sufficiently thin and active in a strong guide field case. |
| Publication date: 25 Apr 2008 |
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| Study of reconnection-associated multiscale fluctuations with Cluster and Double Star |
| The objective of the paper is to asses the specific spectral scaling properties of magnetic reconnection associated fluctuations/turbulence at the earthward and tailward outflow regions observed simultaneously by the Cluster and Double Star (TC-2) spacecraft on 26 September 2005. Systematic comparisons of spectral characteristics, including variance anisotropy and scale-dependent spectral anisotropy features in wave vector space were possible due to the well-documented reconnection events, occurring between the positions of Cluster (X = - 14-16 Re) and TC-2 (X = - 6.6 Re). Another factor of key importance is that the magnetometers on the spacecraft are similar. The comparisons provide further evidence for asymmetry of physical processes in earthward/tailward reconnection outflow regions. Variance anisotropy and spectral anisotropy angles estimated from the multiscale magnetic fluctuations in the tailward outflow region show features which are characteristic for magnetohydrodynamic cascading turbulence in the presence of a local mean magnetic field. The multiscale magnetic fluctuations in the earthward outflow region are not only exhibiting more power, lack of variance, and scale-dependent anisotropies but also are having larger anisotropy angles. In this region the magnetic field is more dipolar and the main processes driving turbulence are flow breaking/mixing, perhaps combined with turbulence ageing and noncascade-related multiscale energy sources. |
| Publication date: 25 Apr 2008 |
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| Time Variability of the Neutral Iron Lines from the Sgr B2 Region and its Implication of a Past Outburst of Sgr A* |
| We investigate long-term X-ray behaviors from the Sgr B2 complex using archival data of the X-ray satellites Suzaku, XMM-Newton, Chandra and ASCA. The observed region of the Sgr B2 complex includes two prominent spots in the Fe I K-alpha line at 6.40 keV, a giant molecular cloud M0.66-0.02 known as the "Sgr B2 cloud" and an unusual X-ray source G0.570-0.018. Although these 6.40 keV spots have spatial extensions of a few pc scale, the morphology and flux of the 6.40 keV line has been time variable for 10 years, in contrast to the constant flux of the Fe XXV K-alpha line at 6.67 keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M0.66-0.02; the 6.40 keV line flux declined in 2001 and decreased to 60% in the time span 1994-2005. The other spot G0.570-0.018 is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability (~10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A* was X-ray bright in the past 300 year and exhibited a time variability with a period of a few years. |
| Publication date: 15 Apr 2008 |
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| Modeling PSBL high speed ion beams observed by Cluster and Double Star |
| On October 8, 2004, the Cluster and Double Star spacecraft crossed the near-Earth (12-19 RE) magnetotail neutral sheet during the recovery phase of a small, isolated substorm. Although they were separated in distance by ~7 RE and in time by ~30 min, both Cluster and Double Star observed steady, but highly structured Earthward moving >1000 km/s high speed H+ beams in the PSBL. This paper utilizes a global magnetohydrodynamic (MHD) simulation driven by Wind spacecraft solar wind input to model the large-scale structure of the PSBL and large-scale kinetic (LSK) particle tracing calculations to investigate the similarities and differences in the properties of the observed beams. This study finds that the large-scale shape of the PSBL is determined by the MHD configuration. On smaller scales, the LSK calculations, in good qualitative agreement with both Cluster and Double Star observations, demonstrated that the PSBL is highly structured in both time and space, on time intervals of less than 2 min, and spatial distances of the order of 0.2-0.5 RE. This picture of the PSBL is different from the ordered and structured region previously reported in observations. |
| Publication date: 14 Apr 2008 |
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| Comparison of periodic substorms at Jupiter and Earth |
| The Energetic Particles Detector and magnetometer measurements on Galileo showed that the Jovian magnetosphere undergoes reconfiguration processes which are very similar to the characteristics of a terrestrial substorm. At Jupiter the reconfiguration process occurs quasi-periodically with a repetition period of several days. In the terrestrial magnetosphere periodic substorms have been observed during magnetic storms. The comparison of the periodic magnetospheric disturbances at Jupiter and Earth shows that they are similar in dynamic features as well as in spatial distribution but have different energy sources. In the case of Earth, the well-established energy source is the solar wind. In the case of the Jovian magnetosphere, it is believed that internal energy is supplied by the fast planetary rotation and the moon Io which releases ~1000 kg s-1 of plasma into the magnetosphere. It is established that the energy accumulation and subsequent release lead to similar features in the magnetospheres of both planets. The particle data show periodic intensity fluctuations and plasma pressure variations. In addition, recurring signatures of stretching and dipolarization are observed in the magnetic field at the terrestrial and Jovian magnetospheres. Furthermore, the release process is associated with an intensification of auroral emissions. The typical phases for terrestrial substorms like growth, expansion and recovery are also found in the periodic substorms at Jupiter. As a lesson taken from the Jovian magnetosphere it is proposed that under certain conditions periodic magnetospheric substorms at Earth can be driven by mass-loading from the plasmasphere. |
| Publication date: 12 Apr 2008 |
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| Study of waves in the magnetotail region with cluster and DSP |
| The study of the neutral sheet is of fundamental importance in understanding the dynamics of the Earth's magnetosphere. From the earliest observation of the magnetotail, it has been found that the neutral sheet frequently appears to be in motion due to changing solar wind conditions and geomagnetic activity. Multiple crossings of the neutral sheet by spacecraft have been attributed to a flapping motion of the neutral sheet in the north-south direction, a wavy profile either along the magnetotail or the dawn-dusk direction. Cluster observations have revealed that the flapping motions of the Earth's magnetotail are of internal origin and that kink-like waves are emitted from the central part of the tail and propagate toward the tail flanks. This flapping motion is shown here to propagate at an angle of ~45° with xGSM. A possible assumption that the flapping could be created by a wake travelling away from a fast flow in the current sheet is rejected. Other waves in the magnetotail are found in the ULF range. One conjunction event between Cluster and DoubleStar TC1 is presented where all spacecraft show ULF wave activity at a period of approximately 5 min during fast Earthward flow. These waves are shown to be Kelvin-Helmholtz waves on the boundaries of the flow channel. Calculations show that the conversion of flow energy into magnetic energy through the Kelvin-Helmholtz instability can contribute to a significant part of flow breaking between Cluster and DoubleStar TC1. |
| Publication date: 11 Apr 2008 |
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| Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z~2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse |
| Using deep near-infrared spectroscopy, Kriek et al. found that 45% of massive galaxies at z~2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)-assisted Keck/adaptive optics (AO). Considering that their median stellar mass is 1.7 x 1011 MSun, the galaxies are remarkably small, with a median effective radius re = 0.9 kpc. Galaxies of similar mass in the nearby universe have sizes of H5 kpc and average stellar densities that are 2 orders of magnitude lower than the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm previous studies at z>2 that lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most 10% of the population of K-selected quiescent galaxies at z~2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z~2.3, through dry mergers or other processes, consistent with predictions from hierarchical models. |
| Publication date: 10 Apr 2008 |
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| Cluster multispacecraft determination of AKR angular beaming |
| Simultaneous observations of AKR emission using the four-spacecraft Cluster array were used to make the first direct measurements of the angular beaming patterns of individual bursts. By comparing the spacecraft locations and AKR burst locations, the angular beaming pattern was found to be narrowly confined to a plane containing the magnetic field vector at the source and tangent to a circle of constant latitude. Most rays paths are confined within 15° of this tangent plane, consistent with numerical simulations of AKR k-vector orientation at maximum growth rate. The emission is also strongly directed upward in the tangent plane, which we interpret as refraction of the rays as they leave the auroral cavity. The narrow beaming pattern implies that an observer located above the polar cap can detect AKR emission only from a small fraction of the auroral oval at a given location. This has important consequences for interpreting AKR visibility at a given location. It also helps re-interpret previously published Cluster VLBI studies of AKR source locations, which are now seen to be only a subset of all possible source locations. The observations are inconsistent with either filled or hollow cone beaming models. |
| Publication date: 09 Apr 2008 |
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| Science Requirements Document for Cross-Scale |
| The contents of this Science Requirements Document (Sci-RD) are agreed by all the contributors to be the scientific requirements for the Cross-Scale mission against which the Reference Payload has been
designed and the mission profile established.
[This is an abbreviated version of the original document abstract.] |
| Publication date: 07 Apr 2008 |
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| Payload Definition Document for Cross-Scale |
| This Payload Definition Document (PDD) is a compilation of the Cross-Scale strawman payload requirements and of their related reference design. The PDD plays a key role in defining the resources
required by the Cross-Scale instruments and in providing the information necessary to conduct the mission assessment study and the preliminary spacecraft design.
[This is an abbreviated version of the original document abstract.]
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| Publication date: 04 Apr 2008 |
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| Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in omega Centauri |
| The globular cluster omega Centauri is one of the largest and most massive members of the galactic system. However, its classification as a globular cluster has been challenged making it a candidate for being the stripped core of an accreted dwarf galaxy; this together with the fact that it has one of the largest velocity dispersions for star clusters in our galaxy makes it an interesting candidate for harboring an intermediate-mass black hole. We measure the surface brightness profile from integrated light on an HSTACS image of the center, and find a central power-law cusp of logarithmic slope
-0.08.We also analyze Gemini GMOS-IFU kinematic data for a 5"x5" field centered on the nucleus of the cluster, as well as for a field 14" away.We detect a clear rise in the velocity dispersion from 18.6 km s-1 at 14" to 23 km s-1 in the center. A rise in the velocity dispersion could be due to a central black hole, a central concentration of stellar remnants, or a central orbital structure that is radially biased.We discuss each of these possibilities. An isotropic, spherical dynamical model implies a black hole mass of 4.0+0.75-1.0 x 104 MSun, and excludes the no black hole case at greater than 99% significance. We have also run flattened, orbit-based models and find similar results. While our preferred model is the existence of a central black hole, detailed numerical simulations are required to confidently rule out the other possibilities. |
| Publication date: 02 Apr 2008 |
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| PLATO Payload Definition Document |
| The purpose of the Payload Description Document (PDD) is to provide a reference design for the industrial assessment of the PLATO payload. On this basis the PLATO PDD represents a Reference Document to the Statement of Work released for the industrial assessment study of the PLATO Project.
[This is an abbreviated version of the original document abstract.] |
| Publication date: 01 Apr 2008 |
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| XMM-Newton Observations of Broad Absorption Line Quasars with Polar Outflows |
| We have selected a sample of broad absorption line (BAL) quasars which show significant radio variations, indicating the presence of polar BAL outflows. We obtained snapshot XMM observations of four polar BAL QSOs, to check whether strong X-ray absorption, one of the most prominent characteristics of most BAL QSOs, also exists in polar outflows. Two of the sources are detected in the X-ray. Spectral fittings show that they are X-ray normal with no intrinsic X-ray absorption, suggesting the X-ray shielding gas might be absent in polar BAL outflows. Comparing to non-BAL QSOs, one of two X-ray-nondetected sources remains consistent with X-ray normal, while the other one, which is an iron low-ionization BAL (FeLoBAL), shows an X-ray weakness factor of >19, suggesting strong intrinsic X-ray absorption. Alternative explanations for the nondetection of strong X-ray absorption in the two X-ray-detected sources are that (1) the absorption is more complex than a simple neutral absorber, such as partial covering absorption or ionized absorption; (2) there might be significant jet contribution to the detected X-ray emission. Current data are insufficient to test these possibilities, and further observations are required to understand the X-ray nature of polar BAL outflows. |
| Publication date: 01 Apr 2008 |
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