ESA Science & Technology - Publication Archive
In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices--such as water (the dominant constituent), CO2, CO- and of dust grains embedded in the ice matrix. By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed.
Published: 02 May 1999
An improved unidimensional model of the heat transport and gas diffusion within a porous cometary nucleus is presented, in which the time-dependent gas diffusion equation is coupled with the heat diffusion equation to describe the energy transport due to sublimation and recondensation of volatiles, but is solved independently using a different discrete time step. Also, the erosion of interfaces within the nucleus, due to the sublimation of ices and the removal of dust, is now treated by means of a continuous adaptation of the discrete grid to the interfaces positions, removing numerical stability problems associated with the variation of structure and composition of the discrete layers. The results of this model are then compared with those of another unidimensional model which does not make use of the above-mentioned numerical methods, both computed for the same set of physical parameters describing comet P/Wirtanen, and the effects of the different modelling assumptions on the results are discussed. A new bidimensional model of the heat transport within a porous comet nucleus is presented, and its results are compared with those obtained from the above-mentioned unidimensional model (modified to include the same physics of the bidimensional model). The ability of bidimensional models to better describe the effects of variations in the local physical conditions on the comet activity is then discussed.
Published: 02 May 1999
Proceedings of the 3rd Integral Workshop, Taormina 1998
Published: 02 May 1999
Published: 02 May 1999
Saturn's large moon Titan is unique among planetary satellites in that it possesses a thick atmosphere and a haze layer that is opaque to visible light. This haze is believed to be composed of organic compounds produced by the photolysis of methane. It has been suggested that the photochemical products of methane photolysis, primarily ethane, would "rain out" over time and may produce reservoirs of liquid hydrocarbons on Titan's surface. Such material would appear very dark, with an albedo =<0.02 (Khare et al. 1990, Bull. Am. Astron. Soc.22, 1033). Such low-albedo regions have not been previously detected on Titan's surface. Here we report observations of Titan at a resolution of 0.04 arcsec (0.02 arcsec/pixel) using the technique of speckle imaging from the 10-m Keck I Telescope. By observing Titan at specific infrared wavelengths which are windows through its atmosphere, we have made both an albedo map of Titan's surface at 1.6 and 2.1 µm and an estimate of Titan's haze optical depth at these wavelengths. We clearly distinguish low-albedo features (reflectance <0.05) on Titan's surface.
Published: 02 May 1999
The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of the electron mass. The similarities of the gradients as well as the behavior of the electron flow velocity can be derived from the electron trapping scale and the electron mass independence of the reconnection electric field. A further model which includes a significant guide magnetic field exhibits almost identical behavior. The explanation of this result lies in a Hall-type electric field which locally eliminates the magnetizing effect on the electrons of the guide magnetic field. The resulting electron dynamics is nearly identical to the one found in the model without guide magnetic field. --- Remainder of abstract truncated ---
Published: 16 May 1999
This article is based on a talk about the history of our universe given by Professor Gustav Andreas Tammann to the Pro ISSI.
Published: 02 April 1999
Observations of outflow velocities in coronal holes (regions of open coronal magnetic field) have recently been obtained with the Solar and Heliospheric Observatory (SOHO) spacecraft. Velocity maps of Ne7+ from its bright resonance line at 770 angstroms, formed at the base of the corona, show a relationship between outflow velocity and chromospheric magnetic network structure, suggesting that the solar wind is rooted at its base to this structure, emanating from localized regions along boundaries and boundary intersections of magnetic network cells. This apparent relation to the chromospheric magnetic network and the relatively large outflow velocity signatures will improve understanding of the complex structure and dynamics at the base of the corona and the source region of the solar wind.
Published: 05 February 1999
A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2-10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.
Published: 15 January 1999
Published: 01 November 1998
Presented at International Conference On Environmental Systems, July 1998, Danvers, MA, USA, Session: Satellite, Payload, Instrument and Launch Vehicle Thermal Control. Abstract: A study of the first in-orbit temperatures of Huygens shows that the probe will very likely survive thermally all vacuum cruise and coast phases. Calculated heat fluxes, mass flows of Titan's atmosphere into and out of the probe and temperatures give confidence also for the mission phase proper in 2004, i.e., the 2.5 h descent into Titan's -200
Basotect foam bags insulate the probe from this atmosphere. These bags and their fixation had to be drastically modified between Titan test on STPM (May 95) and on FM (June 96). The mission phases, thermal requirements, thermal design, tests with the probe, special tests for the foam bag development and their modification are presented.
Published: 17 July 1998
The spectral response of a 100 x 100 micron² tantalum based superconducting tunnel junction to 5.9 keV x-ray photons from a 55Fe source has been studied. In full illumination the energy resolution for the Mn Kalpha line complex is 56 eV, dominated by spatial nonuniformity in the response of the detector. When illuminating selectively a 5-10 micron diam spot in the center of the detector, the energy resolution improves to 22 eV, corresponding to 15.7 eV for the individual Mn Kalpha 1 and Mn Kalpha 2 lines. This exceeds the predicted theoretical energy resolution of 7.3 eV for this type of device by only a factor of ~ 2.
Published: 23 June 1998
Photon counting experiments at wavelengths ranging from near infrared to x-ray with niobium based superconducting tunnel junctions with aluminium trapping layers are presented. Single photons can be detected up to a wavelength of 1 micron. The response in the ultraviolet to near-infrared region is characterized by a good energy linearity (< 2.5%), a capability to handle event rates up to ~ 3 kHz, and moderate energy resolving power (E/DeltaE = ~7 for E = 4 eV). The x-ray response at 6 keV is characterized by anomalously high signals compared to the low energy response, a severe energy nonlinearity and a relatively poor energy resolution of ~ 140 eV, full width at half maximum.
Published: 02 May 1998
We report Hubble Space Telescope NICMOS observations of the central region of NGC 5128 at 2.2 mm and in Paa. The continuum images show extended emission typical of an elliptical galaxy and a strong unresolved central source we identify as the nucleus of the galaxy. Its position is consistent with ground-based IR and radio data, and with the peak of reddening found with the first wide field planetary camera. In Paa , we detect a prominent elongated structure, centered on the nucleus, extended by ~2" at a position angle of ~33°, and with a major to minor axis ratio of ~2. We interpret this as an inclined, ~40 pc diameter, thin nuclear disk of ionized gas rather than a jet-gas cloud interaction. We do see several weaker Paa features, some of which may be circumnuclear gas clouds shocked by the X-ray/radio jet. The disk is one of the smallest ever observed at the nucleus of an active galactic nucleus (AGN). It is not perpendicular to the jet but is consistent with being oriented along the major axis of the bulge. If it represents the warped outer portion of an accretion disk around a black hole, we conclude that even on the scale of a few parsecs, the disk is dominated by the galaxy gravitational potential and is not directly related to the symmetry axis of the AGN.
Published: 02 May 1998
Solar flares involve a release of the Suns magnetic energy as X-radiation, particle beams and high-speed plasma flows. But we have discovered, using data from the Solar and Heliospheric Observatory (SOHO), that these flares also affect the Suns interior, generating seismic waves similar to earthquakes. For example, a three-kilometre-high seismic wave was caused by a moderate X-ray flare that occurred on 9 July 1996 and propagated at about 50 kilometres per second to a distance 120 000 kilometres from the flare site.
Published: 29 May 1998
Asymmetric NbN-Nb-Al-AlOx-Al-Nb superconducting tunnel junctions have been investigated as photon counting detectors at x-ray and ultraviolet (UV)-visible wavelengths. The inclusion of a thin NbN passivation layer on the top electrode of the devices in place of the natural niobium oxides has reduced the quasiparticle loss rates, thereby enhancing the probability of multiple tunnel processes. As a consequence, the detector responsivity has increased from 900e-/eV, up to values in excess of 2000e-/eV in the temperature range 0.30-0.8 K. Such a responsivity level has allowed single photon counting performance at wavelengths as long as 700 nm and at operating temperatures as high as 830 mK. The devices show a linear response in the UV-visible range, while at 6 keV the expected nonlinearities in the energy response and moderate energy resolution similar to that found in Nb-Al junctions are observed.
Published: 16 May 1998
It is now over 40 years since the gaseous disc of our galaxy was discovered to be warped from radio observations of neutral hydrogen . Subsequently the warp has been detected in the distribution of galactic dust , molecular clouds , and luminous stars [4,5]. Roughly half of all spiral galaxies have similarly warped discs, which suggests that warps are common and long-lived phenomenon. However, there is still no consensus as to what induces galactic discs to become warped: intergalactic winds, tidal interactions with satellites, magnetic pressure and massive dark halos have all been proposed as causative agents. Here we use data from the Hipparcos satllite  to probe the Milky Way's warp.
Published: 03 April 1998
Published: 02 March 1998
We use absolute trigonometric parallaxes from the Hipparcos Catalogue to determine individual distances to members of the Hyades cluster, from which the 3-dimensional structure of the cluster can be derived. Inertially-referenced proper motions are used to rediscuss distance determinations based on convergent-point analyses. A combination of parallaxes and proper motions from Hipparcos, and radial velocities from ground-based observations, are used to determine the position and velocity components of candidate members with respect to the cluster centre, providing new information on cluster membership: 13 new candidate members within 20 pc of the cluster centre have been identified. Farther from the cluster centre there is a gradual merging between certain cluster members and field stars, both spatially and kinematically.
Published: 01 March 1998
We present the COS B/EGRET 1997 ephemeris for the rotation of the Geminga pulsar. This ephemeris is derived from high-energy gamma -ray observations that span 24 yr. The recently obtained accurate position and proper motion are assumed. A cubic ephemeris predicts the rotational phase of Geminga with errors smaller than 50 milliperiods for all existing high-energy gamma -ray observations that span a 24.2 yr timing baseline. The braking index obtained is 17 +/- 1. Further observation is required to ascertain whether this high value truly reflects the rotational energy loss mechanism, or whether it is a manifestation of timing noise. The ephemeris parameters are sufficiently constrained so that timing noise will be the limitation on forward extrapolation. If Geminga continues to rotate without a glitch, as it has for at least 23 yr, we expect this ephemeris to continue to describe the phase, with an error of less than 100 milliperiods, until 2008. Statistically significant timing residuals are detected in the EGRET data that depart from the cubic ephemeris at a level of 30 milliperiods. Although this could simply be an additional manifestation of timing noise, the EGRET timing residuals appear to have a sinusoidal modulation that is consistent with a planet of mass 1.7/sin i MSun orbiting Geminga at a radius of 3.3 AU.
Published: 01 February 1998
— 20 Items per Page