Publication archive

Publication archive

The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission and ionize the hydrogen, leading to H3+ infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.
Published: 13 November 2008
We present a method, GALS (Gradient Analysis by Least Squares) for estimating the gradient of a physical field from multi-spacecraft observations. To obtain the best possible spatial resolution, the gradient is estimated in the frame of reference where structures in the field are essentially locally stationary. The estimates are refined iteratively by a least squares method. We show that GALS is not very sensitive to the spacecraft configuration and resolves structures much smaller than the characteristic size of the spacecraft distribution. Furthermore, GALS requires little user input. GALS has been tested on synthetic magnetic field data and data from the Cluster FGM instrument. GALS will also be useful for other types of data. The results indicate that GALS is robust and superior to the curlometer method for estimating the current from magnetic field measurements.
Published: 10 November 2008
Our purpose is to characterize the evolution of the magnetopause Kelvin-Helmholtz (KH) wave activity with changes in thickness of the adjacent boundary layer, geomagnetic latitude and interplanetary magnetic field (IMF) orientation. As the IMF turns northward, wave activity may be generated at the dayside before propagating down the tail, where the boundary layer is expected to support longer wavelengths. We use two-point observations on the dusk magnetopause at low latitudes, from Geotail on the dayside and Cluster tailward of the dusk terminator. We quantify the wavelength, power, wavefront steepness and propagation direction at Cluster. An estimate of the thickness of the low-latitude boundary layer (LLBL) is obtained by correlating normal distances to the magnetopause, derived from two empirical solar-wind-driven models, with a systematic relationship (the "transition parameter") found between the electron number density and temperature; the correlation factor is used to infer the temporal evolution of the thickness of the locally sampled layer. We find that wavelengths are controlled by the IMF clock angle, as expected when generated by the KH mechanism at the dayside, although amplitudes, wavefront steepness and propagation directions are more closely correlated with the layer thickness. A survey of parameter space provides evidence of the contribution of the KH mechanism to the widening of the electron LLBL.
Published: 05 November 2008

Our understanding of the Universe has come under increased scrutiny over the last 25 years. New instruments have opened fascinating perspectives for testing General Relativity, alternative theories of gravitation, as well as studying quantum mechanics and exploring the boundaries of quantum gravity. Violations of the principle laws of the currently underlying theories can give clues to aid the unification of the four physical forces, or lead the way for the discovery of new interactions and particles.

The aim of the Fundamental Physics Explorer (FPE) is to provide the means to test the the foundations of modern physics in a cost effective and efficient manner. The FPE programme could consist of up to three spacecrafts, each re-using a small platform, accessing space to take advantage of an almost constantly unperturbed environment, thus improving the precision of current measurements. More specifically, the FPE Technology Reference Study (TRS) aims to identify the key technologies required and the technical challenges associated with fundamental physics missions.

Published: 12 December 2007
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.
Published: 25 October 2008
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.
Published: 22 October 2008
The Science Payload and Advanced Concepts Office (SCI-A) of the ESA Science Directorate conducts a number of Technology Reference Studies (TRS) on hypothetical scientific missions that are not part of the approved Science programme. Such TRS activities allow identifying, at an early stage, technology development needs as well as exploring future mission scenarios. As part of this effort, the Gamma Ray Lens (GRL) mission, a future generation gamma-ray observatory, has been the subject of a preliminary internal investigation. The present paper provides an overview of the science goals assumed for this study, the selection of the reference mission profile, together with a preliminary description of the spacecraft design. The reference payload is also described, as well as the list of technology development activities derived from the study.
Published: 15 December 2005
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 --
Published: 16 October 2008
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.
Published: 16 October 2008
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.
Published: 16 October 2008
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.
Published: 16 October 2008
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.
Published: 12 October 2008

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.

Published: 11 October 2008
A new method for remote sensing of the quasiperpendicular part of the bow shock surface is presented. The method is based on analysis of high frequency electric field fluctuations corresponding to Langmuir, upshifted, and downshifted oscillations in the electron foreshock. Langmuir waves usually have maximum intensity at the upstream boundary of this region. All these waves are generated by energetic electrons accelerated by quasiperpendicular zone of the shock front. Nonstationary behavior of the shock, in particular due to rippling, should result in modulation of energetic electron fluxes, thereby giving rise to variations of Langmuir waves intensity. For upshifted and downshifted oscillations, the variations of both intensity and central frequency can be observed. For the present study, WHISPER measurements of electric field spectra obtained aboard Cluster spacecraft are used to choose 48 crossings of the electron foreshock boundary with dominating Langmuir waves and to perform for the first time a statistical analysis of nonstationary behavior of quasiperpendicular zone of the Earth's bow shock. Analysis of hidden periodicities in plasma wave energy reveals shock front nonstationarity in the frequency range 0.33 fBi < f < fBi, where fBi is the proton gyrofrequency upstream of the shock, and shows that the probability to observe such a nonstationarity increases with Mach number. The profiles observed aboard different spacecraft and the dominating frequencies of the periodicities are usually different. Hence nonstationarity and/or rippling seem to be rather irregular both in space and time rather than resembling a quasiregular wave propagating on the shock surface.
Published: 24 September 2008
Observations of solar wind from both large polar coronal holes (PCHs) during Ulysses' third orbit showed that the fast solar wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum (first) orbit. In addition, while much more variable, measurements in the slower, in-ecliptic wind match quantitatively with Ulysses and show essentially identical trends. Thus, these combined observations indicate significant, long-term variations in solar wind output from the entire Sun. The significant, long-term trend to lower dynamic pressures means that the heliosphere has been shrinking and the heliopause must be moving inward toward the Voyager spacecraft. In addition, our observations suggest a significant and global reduction in the mass and energy fed in below the sonic point in the corona. The lower supply of mass and energy may result naturally from a reduction of open magnetic flux during this period.
Published: 19 September 2008
The main task of this Working Group has been to review the state-of-the art knowledge of the Milky Way galaxy, to identify the future challenges, and to propose which tools (in terms of facilities, infrastructures, instruments, science policies) would be needed to successfully tackle and solve the remaining open questions. Considering the leadership position that Europe has reached in the field of Galactic astronomy (thanks to the Hipparcos mission and the Very Large Telescope) and looking at the (near-)future major initiatives it has undertaken (VISTA and VST survey telescopes, Gaia mission), this work clearly has been very timely. It is of uttermost importance for European astronomy to keep and further consolidate its leading position. This Working Group has made recommendations that would allow dissecting our backyard laboratory, the Galaxy, even further. ESO survey telescopes about to become operational and the upcoming ESA Gaia mission are a guarantee for opening new horizons and making new discoveries. We, the astronomers, with the support of our funding agencies, are ready to fully commit to the best exploitation of the treasure that is ahead of us. The main recommendations this Working Group has made to ESA and ESO are to guarantee the expected tremendous capabilities of these new facilities, to vigourously organise their synergies and to jointly give ways to European astronomers to be leaders in the exploitation of their output data.
Published: 02 May 2008
The magnetic field variations are analyzed in the range of time periods from 4 s to 240 s in the magnetosheath observed by the Double Star TC-1 and Cluster in 2004. The characteristics of the magnetic field fluctuations are strongly controlled by the angle between the upstream interplanetary magnetic field (IMF) and the normal of the bow shock. Generally speaking, the magnetic field fluctuations in the quasi-parallel magnetosheath are more intense than those in the quasi-perpendicular ones. Almost purely compressional waves are found in the quasi-perpendicular magnetosheath. With the increase of the local plasma beta, both the magnitude and direction of the magnetic field fluctuate more intensely. There exists an inverse correlation between the local temperature anisotropy Tperp./Tparallel and the plasma beta.
Published: 15 September 2008
In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes >80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00-15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ~-10° to -15° clock angle for the North (South) Hemisphere. - Remainder of abstract truncated -
Published: 13 September 2008

Aims. We present the first albedo determination of 2867 Steins, the asteroid target of the Rosetta space mission together with 21 Lutetia.

Methods.The data were obtained in polarimetric mode at the ESO-VLT telescope with the FORS1 instrument in the V and R filters. Observations were carried out from June to August 2005 covering the phase angle range from 10.3 degrees to 28.3 degrees, allowing the determination of the asteroid albedo by the well known experimental relationship between the albedo and the slope of the polarimetric curve at the inversion angle.

Results. The measured polarization values of Steins are small, confirming an E-type classification for this asteroid, as already suggested from its spectral properties. The inversion angle of the polarization curve in the V and R filters is respectively of 17.3±1.5 degrees and 18.4±1.0 degrees, and the corresponding slope parameter is of 0.037±0.003%/deg and 0.032±0.003%/deg. On the basis of its polarimetric slope value, we have derived an albedo of 0.45±0.1, that gives an estimated diameter of 4.6 km, assuming an absolute V magnitude of 13.18 mag.

Published: 02 March 2006
24-Jun-2024 03:40 UT

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