Publication archive

Publication archive

Magnetic reconnection has a crucial role in a variety of plasma environments in providing a mechanism for the fast release of stored magnetic energy. During reconnection the plasma forms a 'magnetic nozzle', like the nozzle of a hose, and the rate is controlled by how fast plasma can flow out of the nozzle. But the traditional picture of reconnection has been unable to explain satisfactorily the short timescales associated with the energy release, because the flow is mediated by heavy ions with a slow resultant velocity. Recent theoretical work has suggested that the energy release is instead mediated by electrons in waves called 'whistlers', which move much faster for a given perturbation of the magnetic field because of their smaller mass. Moreover, the whistler velocity and associated plasma velocity both increase as the 'nozzle' becomes narrower. A narrower nozzle therefore no longer reduces the total plasma flow-the outflow is independent of the size of the nozzle. Here we report observations demonstrating that reconnection in the magnetosphere is driven by whistlers, in good agreement with the theoretical predictions
Published: 30 March 2001
We discuss the detector requirements for future X-ray astrophysics missions and present preliminary results from our compound semiconductor program designed to produce X-ray detectors with high spatial and spectral resolution across the energy range 1 keV to 200 keV. Several prototype detectors have been fabricated from monocrystalline TlBr and tested at hard X-ray wavelengths in our laboratories and at the ESRF synchrotron research facility. Energy resolutions of 1.6 keV (fwhm) at 5.9 keV and 2.6 keV (fwhm) at 26 keV have been achieved, although we find that performance is highly variable due to polarisation effects. The resolution function is dominated by high leakage current at all energies. From pulse height measurements of Am241 as a function of detector bias, we derive the electron mobility-lifetime product at -2 °C to be (2.9±0.2) x 10-4 cm² V-1. This is about an order of magnitude higher than previously reported values.
Published: 01 February 2001
The XMM-OM instrument extends the spectral coverage of the XMM-Newton observatory into the ultraviolet and optical range. It provides imaging and time-resolved data on targets simultaneously with observations in the EPIC and RGS. It also has the ability to track stars in its field of view, thus providing an improved post-facto aspect solution for the spacecraft. An overview of the XMM-OM and its operation is given, together with current information on the performance of the instrument.
Published: 05 January 2001
The EPIC focal plane imaging spectrometers on XMM-Newton use CCDs to record the images and spectra of celestial X-ray sources focused by the three X-ray mirrors. There is one camera at the focus of each mirror; two of the cameras contain seven MOS CCDs, while the third uses twelve PN CCDs, defining a circular field of view of 30' diameter in each case. The CCDs were specially developed for EPIC, and combine high quality imaging with spectral resolution close to the Fano limit. A filter wheel carrying three kinds of X-ray transparent light blocking filter, a fully closed, and a fully open position, is fitted to each EPIC instrument. The CCDs are cooled passively and are under full closed loop thermal control. A radio-active source is fitted for internal calibration. Data are processed on-board to save telemetry by removing cosmic ray tracks, and generating X-ray event files; a variety of different instrument modes are available to increase the dynamic range of the instrument and to enable fast timing. The instruments were calibrated using laboratory X-ray beams, and synchrotron generated monochromatic X-ray beams before launch; in-orbit calibration makes use of a variety of celestial X-ray targets. The current calibration is better than 10% over the entire energy range of 0.2 to 10 keV. All three instruments survived launch and are performing nominally in orbit. In particular full field-of-view coverage is available, all electronic modes work, and the energy resolution is close to pre-launch values. Radiation damage is well within pre-launch predictions and does not yet impact on the energy resolution. The scientific results from EPIC amply fulfil pre-launch expectations.
Published: 05 January 2001
he European Photon Imaging Camera (EPIC) consortium has provided the focal plane instruments for the three X-ray mirror systems on XMM-Newton. Two cameras with a reflecting grating spectrometer in the optical path are equipped with MOS type CCDs as focal plane detectors (Turner 2001), the telescope with the full photon flux operates the novel pn-CCD as an imaging X-ray spectrometer. The pn-CCD camera system was developed under the leadership of the Max-Planck-Institut für extraterrestrische Physik (MPE), Garching. The concept of the pn-CCD is described as well as the different operational modes of the camera system. The electrical, mechanical and thermal design of the focal plane and camera is briefly treated. The in-orbit performance is described in terms of energy resolution, quantum efficiency, time resolution, long term stability and charged particle background. Special emphasis is given to the radiation hardening of the devices and the measured and expected degradation due to radiation damage of ionizing particles in the first 9 months of in orbit operation.
Published: 05 January 2001
The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Grating Spectrometers (RGS) behind two of its three nested sets of Wolter I type mirrors. The instrument allows high-resolution (E/DE 100 to 500) measurements in the soft X-ray range (6 to 38 Å or 2.1 to 0.3 keV) with a maximum effective area of about 140 cm² at 15 Å. Its design is optimized for the detection of the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, and silicon, as well as the L shell transitions of iron. The present paper gives a full description of the design of the RGS and its operational modes. We also review details of the calibrations and in-orbit performance including the line spread function, the wavelength calibration, the effective area, and the instrumental background.
Published: 05 January 2001
This Special Letters Issue of Astronomy & Astrophysics is dedicated to the first results from XMM-Newton. One year after the launch of the X-Ray space observatory, this series of 56 publications describes the spacecraft and the instruments onboard, and provides a first look at their exciting scientific capabilities.
Published: 05 January 2001
XMM is the largest scientific observatory developed by ESA and dedicated to exploring the Universe in the soft-X-ray portion of the electromagnetic spectrum. This article presents XMM as a complete system, composed by the space segment (the satellite and its launch vehicle) and the ground segment (all of ground-based infrastructure needed to control the satellite and gather the scientific data). The definition of the XMM orbit, which is also described, is a key element in meeting the primary mission objectives and satisfying the project programmatic requirements and constraints.
Published: 01 December 1999
The Rosetta spacecraft will fly-by a few asteroids during its course to the final cometary target. The candidate asteroids presently are 3840 Ministrobel (S-type), 2703 Siwa and 140 (C-type).With the limited data presently available on these bodies we calculated some approximate quantities which may be useful to select the fly-by trajectories of the ROSETTA probe. In particular we derived the zones in which particles could stably orbit by analyzing Hills problem of three hierarchical masses--the sun, the asteroid and the orbiting particle. Then, following the approach of Hamilton and Burns, the effects of solar radiation pressure and of the ellipticity of the orbits were also taken into account. In this way for each asteroid we could calculate not only a classical quantity like the radius of the Hill sphere, but also the critical starting orbital distance (as a function of orbital inclination) within which most orbits remain bound to the asteroid, and outside which most escape as a consequence of perturbations. Moreover we determined the orbital stability zone, defined as the union of all the numerically integrated orbits showing long-term stability, for each of the target asteroids. The particular shape of these zones would suggest to have the spacecrafts close approach out of the orbital plane of the asteroids.
Published: 02 May 1999
Amorphous silicate dust grains have been produced in the laboratory by means of laser ablation of solid targets in different ambient atmospheres. In this work we show that, if the condensation occurs in the presence of hydrogen, the spectra of silicate grains, together with the characteristic 10 and 20 µm features, exhibit an absorption band around 4.6 µm. Such features, absent in the spectra of the same silicate grains produced in an oxygen atmosphere, may be attributed to a fundamental stretching vibration of -SiH functional groups bound into the grains or on their surface.
Published: 02 May 1999
The Sun from Space is a comprehensive account of solar astrophysics and how our perception and knowledge of this star have gradually evolved as mankind has elucidated ever more of its mysteries. The emphasis is on the last decade, which has seen three successful solar spacecraft missions: SOHO, Ulysses and Yohkoh. Together these have confirmed many aspects of the SUN and its output, and provided new clues to the numerous open questions that remain. The author, a leading researcher in the field, writes in a clear and concise style. Known also for his famous books "Astrophysical Formulae", "Sun, Earth and Sky", and the prize-winning "Wanderers in Space", he has succeeded once again in addressing a complex scientific topic in a very approachable way. Hence, this generously illustrated book, whilst primarily addressing students, will also be of interest to a broader readership covering all levels from the amateur to the expert.
Published: 02 May 2000
The photochemistry of hydrocarbons in Titan's atmosphere is modeled by a comprehensive kinetic scheme, containing 732 elementary reactions and 147 species up to C60. Four groups of the hydrocarbons are considered : Polyacetylenes (PA), Polyvinyles (PV), Vinylacetylenes (VA) and Allenes (Polyenes).
Published: 02 April 1997
We develop a semiempirical grey radiative model to quantify Titan's surface temperature as a function of pressure and composition of a nitrogen-methane-hydrogen atmosphere, solar flux and atmospheric haze. We then use this model, together with non-ideal gas-liquid equilibrium theory to investigate the behavior of the coupled surface-atmosphere system on Titan.
Published: 15 December 1999
Several instruments have been described at the annual meetings of the EGS (European Geophysica Society) and the DPS (Division for Planetary Sciences) The numerous abstracts which were published can be found in:

Bulletin of the American Astronomical Society, Vol 31, No. 4 (DPS)

Published: 02 September 1999
The aim of this work is to deduce physical characteristics of the aerosols that compose the thin detached haze layer of Titan, for which the composition and origin are currently unknown. We have used four images at three different phase angles and three different wavelengths where the detached haze clearly appears. The low optical path in the layer permits assumptions that greatly simplify the treatment and allow us to consider the scattered intensity as the product of the scattering cross section, the phase function at the considered phase angle, and the number of aerosols. Physical arguments are given to support fractal aerosols in the detached haze, and we investigate a wide range of monomer radii, monomer numbers, and imaginary refractive indexes. This study yields information about the spectral variation of the refractive index for the detached haze tholins that are clearly different from those for the main haze. On the other hand, no firm values are given for the monomer radius and number, but these two parameters are strongly linked through a simple relation
Published: 02 August 2000
We estimate the wind speeds in Titan's thermosphere by considering the various terms of the wind equation, without actually solving it, with a view to anticipating what might be observed by the Cassini spacecraft in 2004. The winds, which are driven by horizontal pressure gradients produced by solar heating, are controlled in the Earth's thermosphere by ion-drag and coriolis force, but in Titan's thermosphere they are mainly controlled by the nonlinear advection and curvature forces.
Published: 01 January 2000
Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we present a detailed energy budget analysis for the surface of a model comet in the orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effective energy input. This yields lower gas fluxes from the surface. For an 100% active, non-dust covered surface we obtain a water gas flux on the order of about 1.5×1028 molecules s-1 at perihelion, assuming a radius of 600 m. The calculated gas flux of water is within the order of measured values for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon—not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the radius of comet 46P/Wirtanen may be much larger than the accepted value of 600 m. A radius in the order of 2 km seems more likely to explain the measurements. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effective surface area of sublimation.
Published: 22 November 1998
23-Nov-2020 16:47 UT

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