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| CoRoT Measures Solar-Like Oscillations and Granulation in Stars Hotter Than the Sun |
| Oscillations of the Sun have been used to understand its interior structure. The extension of similar studies to more distant stars has raised many difficulties despite the strong efforts of the international community over the past decades. The CoRoT (Convection Rotation and Planetary Transits) satellite, launched in December 2006, has now measured oscillations and the stellar granulation signature in three main sequence stars that are noticeably hotter than the sun. The oscillation amplitudes are about 1.5 times as large as those in the Sun; the stellar granulation is up to three times as high. The stellar amplitudes are about 25% below the theoretic values, providing a measurement of the nonadiabaticity of the process ruling the oscillations in the outer layers of the stars. |
| Publication date: 24 Oct 2008 |
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| Studying Resolved Stellar Populations with the James Webb Space Telescope |
| The James Webb Space Telescope (JWST) will be an exciting, highly capable tool, able
to make important contributions to studies of stellar populations in nearby galaxies,
including our own. JWST observations will contribute to: (1) the study of the star
formation histories of nearby galaxies, exploiting the large lever arm provided by visibleinfrared
colors; (2) the derivation of the properties of stellar populations from the study of
the bright red stellar component out to the Virgo cluster and beyond; and (3) the
derivation of the white dwarf cooling sequence age of globular clusters in the Milky Way
from the study of deep visible red color magnitude diagrams. |
| Publication date: 22 Oct 2008 |
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| Transients in oxygen outflow above the polar cap as observed by the Cluster spacecraft |
| Oxygen ion outflow associated with the cusp and cleft give rise to persistent oxygen ion beams which can be observed over the polar cap. For high altitude spacecraft such as Cluster these beams are often observed for several hours on each occasion. This allows for a study of typical temporal structures on the time scale of minutes. We have used 3 years of data from spring, January to May of years 2001 to 2003, for a study of the oxygen number flux variation in the polar cap ion outflow. The source of these oxygen ion beams is the cusp and cleft, and variations in ionospheric upflow on time scales of around 8 min have been reported from ground based studies using incoherent scatter radar. Such upflows typically do not reach escape velocity, and further energization above the ionosphere is required for outflow to occur. Our study shows that a typical time scale between sudden number flux enhancements observed by Cluster in a geocentric distance range of 5 RE to 12 RE is 5 to 10 min. A superposed epoch study does not reveal any significant convection velocity or temperature changes around the flux enhancement events. Sudden temperature enhancements occur with a typical time interval of about 4 min, A superposed epoch study does not reveal any number flux enhancements associated with the temperature enhancements. The clear modulation of the high altitude number flux in a manner which resembles the modulation of the ionospheric upflow indicates that this is the main limiting factor determining the total outflow. The process behind transient upflow events in the ionosphere is therefore important for the total ionospheric outflow. Subsequent heating above the ionosphere appears to be common enough in the cusp/cleft region that it does not significantly modulate the oxygen ion number flux. |
| Publication date: 21 Oct 2008 |
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| Resonant Cyclotron Scattering in Magnetars' Emission |
| We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of 10 magnetars, including canonical and transient anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs). In this scenario, nonthermal magnetar spectra in the soft X-rays (i.e., below 10 keV) result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while using the same number of free parameters as in the commonly used empirical blackbody plus power-law model. However, while the RCS model can alone reproduce the soft X-ray spectra of AXPs, the much harder spectra of SGRs below 10 keV require the addition of a power-law component (the latter being the same component responsible for their hard X-ray emission). Although this model in its present form does not explain the hard X-ray emission (i.e., above 20 keV) of a few of these sources, we took this further component into account in our modeling not to overlook its contribution in the 4-10 keV band. We find that the entire class of sources is characterized by magnetospheric plasma with a density which, at resonant radius, is about 3 orders of magnitude higher than the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities are also in better agreement with more recent estimates. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars shows that the RCS model is capable of catching the main features of the spectra observed below 10 keV. |
| Publication date: 20 Oct 2008 |
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| Iterative inversion of global magnetospheric information from energy neutral atom (ENA) images recorded by the TC-2/NUADU instrument |
| A method is presented for retrieving the magnetospheric ion distribution from Energetic Neutral Atom (ENA) measurements made by the NUADU instrument on the TC-2 spacecraft. Based on the already well-established method of constrained linear inversion, an iterance technique suitable for the low count ENA measurements has been developed which is tolerant of the noise background. By the iterance technique, it is possible for the first time to simultaneously retrieve the magnetospheric ion distribution and the exospheric neutral density, and further to recover global ENA emissions in three dimensions. The technique is applied to a representative ENA image recorded in energy channel 2 (protons: 50-81 keV) of the NUADU instrument during a major geomagnetic storm and it is, thereby, shown that the retrieval method developed provides a useful tool for extracting ion distribution information from ENA data. |
| Publication date: 15 Oct 2008 |
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| Solar wind transport into magnetosphere caused by magnetic reconnection at high latitude magnetopause during northward IMF: Cluster-DSP conjunction observations |
| An event of Cluster-Double Star conjunction observations of magnetic reconnection at high latitude magnetopause nightside of both cusps and solar wind transport into magnetosphere caused by such reconnection process has been investigated. During northward IMF, Cluster/SC1 observed accelerated flows and ion heating associated with magnetic reconnection at high latitude magnetopause nightside of southern cusp. And Double Star observed cold dense solar wind plasma transported into dayside magnetosphere. The analysis on such conjunction observations shows that: (1) during northward IMF, magnetic reconnection occurs at high latitude nightside of southern cusp, accompanied by accelerated flows that are observed by Cluster/SC1; (2) the direction of the accelerated flows, with its sunward component Vx, dawnward component Vy, northward component Vz, is quite consistent with the theoretical anticipation under the condition of northward IMF with dawnward component By; (3) reconnection can heat plasma more in parallel direction than in perpendicular direction, to a level of about 4 keV; (4) with reconnection taking place at high latitude magnetopause nightside of the southern cusp, TC-1 observed cold and dense plasma transported into magnetosphere; (5) by reconnection at high latitude magnetopause nightside of both cusps, solar wind flux tube can be captured by magnetosphere and pulled into dayside magnetosphere. This event presents further observational evidence for magnetic reconnection at high latitude magnetopause nightside of both cusps as an important mechanism of solar wind transport into magnetosphere. |
| Publication date: 15 Oct 2008 |
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| Surveys on magnetospheric plasmas based by the Double Star Project (DSP) exploration |
| The equatorial and polar satellites of the Double Star Project (DSP) were launched successfully on December 29, 2003 and July 25, 2004, respectively, and both of them are operating smoothly. The DSP provides a good opportunity for investigating the structure of the magnetosphere. Based on the DSP data collected during 2004, we have surveyed the distribution of the magnetic fields and plasmas in the magnetosphere. It is found that: (1) Near the Earth's equatorial plane within geocentric distances of less than 7 RE, the Earth's magnetic field is dipolar. In the vicinity of the magnetopause, the magnetic field is enhanced by a factor of about 1.5, and on the nightside, the magnetic field can vary significantly from the Earth's dipole field, likely caused by the presence of the near-Earth tail current sheet. (2) In the day-side magnetosheath, the electron and ion densities are usually both in the range of 10-30 cm-3; the ion and electron temperatures are usually about 200 and 50 eV, respectively. The flow pattern is usually smooth, with a low velocity in the subsolar region and with significantly higher velocities in the dawn and dusk flanks. (3) In the region between the magnetopause and plasmasphere the density is low, approximately 0.5-5 cm-3, and the temperature is high, about 1-10 keV for ions and 0.1-5 keV for electrons. The ion temperature has an apparent anisotropy, with the ratio of the perpendicular and parallel temperatures being about 1.0-1.3 for the night-and dusk-side magnetosphere and about 1.3-2.0 for the day-and dawn-side magnetosphere. -- Remainder of abstract is truncated -- |
| Publication date: 15 Oct 2008 |
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| Ultra low frequency waves observed by Double Star TC-1 in the plasmasphere boundary layer |
| The characteristic and properties of ULF waves in the plasmasphere boundary layer during two very quiet periods are present. The ULF waves were detected by Double Star TC-1 when the spacecraft passed through the plasmasphere in an outbound and inbound trajectories, respectively. A clear association between the ULF waves and periodic variations of energetic ions fluxes was observed. The observations showed that the wave frequency was higher inside the plasmasphere than outside. The mechanism generating these ULF waves and possible diagnosing of the "classical plasmapause" location with the ULF wave were discussed. |
| Publication date: 15 Oct 2008 |
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| Recent in-situ observations of magnetic reconnection in near-Earth space |
| The paper presents a brief review of recent in-situ observations of reconnection in space, with emphasis on results pertaining to the question of anti-parallel versus component reconnection, the implied spatial and temporal scales, the location of the reconnection sites, particle acceleration, reconnection rates, the dependence on plasma beta, and the properties of the diffusion region. |
| Publication date: 11 Oct 2008 |
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| Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs |
Context INTEGRAL has two sensitive gamma-ray instruments that have detected and localised 47 gamma-ray bursts (GRBs) from its launch in October 2002 up to July 2007.
Aims. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, and spectrometer, SPI.
Methods. Spectral properties of the GRBs are determined using power-law and, where appropriate, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy gamma-rays with respect to high-energy gamma-rays, are measured for 31 of the GRBs.
Results. The photon index distribution of power-law fits to the prompt emission spectra is presented and is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ~0.15 ph/cm²/s is ~ 1400 per year in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags <0.75s (between 25-50 keV and 50-300 keV) and one with long lags >0.75s . Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies, and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q=-0.225 +/- 0.090 and hence reflect the local large-scale structure of the Universe. |
| Publication date: 10 Oct 2008 |
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