Publication archive

Publication archive

After another year of extraordinary success the Director of Science, David Southwood, presents a report on the activities throughout the whole Science Programme.
Published: 20 December 2005
Nineteen electron diffusion regions at magnetic field reconnection sites have been found in one hour of Cluster satellite data near the subsolar magnetopause. Investigations on previously unachieved spatial and temporal scales show the following for the first time: direct conversion of magnetic energy to electron energy (The resulting accelerated electrons, their field-aligned currents, and their post-acceleration fate were measured); the spatial dimensions of the large electric fields in the electron diffusion regions have an average thickness of 0.3 times the electron skin depth, in agreement with theory; and these electron diffusion regions were topological boundaries between open and closed magnetic field geometries that contained different plasma and magnetic field line flows. The electron diffusion regions were located at the magnetospheric separatrix where the magnetic field was large and the electron beta was less than one.
Published: 17 December 2005
We report on a puzzling event occurred during a long BeppoSAX observation of the slow-rotating binary pulsar GX1+4 . During this event, lasting about 1 day, the source X-ray flux was over a factor 10 lower than normal. The low-energy pulsations disappeared while at higher energies they were shifted in phase by 0.25 . The con- tinuum spectrum taken outside this low-intensity event was well fitted by an absorbed cut-o power law, and exhibited a broad iron line at 6.5 keV probably due to the blending of the neutral (6.4 keV) and ionised (6.7 keV) K iron lines. The spectrum during the event was Compton reflection dominated and it showed two narrow iron lines at 6.4 keV and 7.0 keV, the latter never revealed before in this source. We also present a possible model for this event in which a variation of the accretion rate thickens a torus-like accretion disc which hides for a while the direct neutron star emission from our line of sight. In this scenario the Compton reflected emission observed during the event is well explained in terms of emission reflected by the side of the torus facing our line of sight.
Published: 15 December 2005
Markarian (Mkn) 297 is a complex system comprised of two interacting galaxies that has been modelled with a variety of scenarios. Observations of this system were made with the Infrared Space Observatory (ISO) using the ISOCAM, ISOPHOT and LWS instruments.
Published: 15 December 2005
We present an interval of extremely long-lasting narrow-band Pc5 pulsations during the recovery phase of a large geomagnetic storm. These pulsations occurred continuously for many hours and were observed throughout the magnetosphere and in the dusk-sector ionosphere. The subject of this paper is the favorable radial alignment of the Cluster, Polar, and geosynchronous satellites in the dusk sector during a 3-hour subset of this interval that allows extensive analysis of the global nature of the pulsations and the tracing of their energy transfer from the solar wind to the ground. Virtually monochromatic large-amplitude pulsations were observed by the CANOPUS magnetometer chain at dusk for several hours, during which the Cluster spacecraft constellation traversed the dusk magnetopause. The solar wind conditions were very steady, the solar wind speed was fast, and time series analysis of the solar wind dynamic pressure shows no significant power concentrated in the Pc5 band. The pulsations are observed in both geosynchronous electron and magnetic field data over a wide range of local times while Cluster is in the vicinity of the magnetopause providing clear evidence of boundary oscillations with the same periodicity as the ground and geosynchronous pulsations. Furthermore, the Polar spacecraft crossed the equatorial dusk magnetosphere outside of geosynchronous orbit (L ~ 6-9) and observed significant electric and magnetic perturbations around the same quasi-stable central frequency (1.4-1.6 mHz). The Poynting vector observed by the Polar spacecraft associated with these pulsations has strong field-aligned oscillations, as expected for standing Alfvén waves, as well as a nonzero azimuthal component, indicating a downtail component to the energy propagation. - Remainder of abstract truncated -
Published: 15 December 2005
SGR 1806-20 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Götz et al. (2004a). Our sample includes a very bright outburst that occurred on October 5 2004, during which over one hundred bursts were emitted in 10 minutes, involving an energy release of 3x1042 erg. We present a detailed analysis of it and discuss our results in the framework of the magnetar model.
Published: 15 December 2005
Contents: Ageing of Spectral Lamps in Space; Extracting Spectra with Optimal Weights in aXe1.5; ESA-ESO Topical Science Working Groups; NEON Observing Schools
Published: 15 December 2005
Sirius B is the nearest and brightest of all white dwarfs, but it is very difficult to observe at visible wavelengths due to the overwhelming scattered light contribution from Sirius A. However, from space we can take advantage of the superb spatial resolution of the Hubble Space Telescope (HST) to resolve the A and B components. Since the closest approach in 1993, the separation between the two stars has become increasingly favourable and we have recently been able to obtain a spectrum of the complete Balmer line series for Sirius B using the HST Space Telescope Imaging Spectrograph (STIS). The quality of the STIS spectra greatly exceeds that of previous ground-based spectra, and can be used to provide an important determination of the stellar temperature (Teff= 25193 K) and gravity (log g= 8.556). In addition, we have obtained a new, more accurate, gravitational redshift of 80.42 +/- 4.83 km s-1 for Sirius B. Combining these results with the photometric data and the Hipparcos parallax, we obtain new determinations of the stellar mass for comparison with the theoretical mass-radius relation. However, there are some disparities between the results obtained independently from log g and the gravitational redshift which may arise from flux losses in the narrow 50 × 0.2 arcsec² slit. Combining our measurements of Teff and log g with the Wood evolutionary mass-radius relation, we obtain a best estimate for the white dwarf mass of 0.978 Msolar. Within the overall uncertainties, this is in agreement with a mass of 1.02 Msolar obtained by matching our new gravitational redshift to the theoretical mass-radius relation.
Published: 16 October 2005
We present results of the analysis of 1.275 MeV gamma-ray line global distribution derived from the all-sky data accumulated by COMPTEL on board the Compton Gamma Ray Observatory (CGRO) from 1991 to 1997. Previously the 1.275 MeV gamma-ray line was believed to be largely produced in the decay of radioactive isotope 22Na that is synthesised in the classical nova (CN) thermonuclear runaway (TNR). Another way to produce the 1.275 MeV line emission is via the excitation of 22Ne nuclei, e.g. through the low-energy cosmic ray interactions with the nuclei of the interstellar matter that lead to the production of 22Ne*, or of 22Na. This scenario, as we now believe, can be dominant in contributing to the total 1.275 MeV gamma-ray line emission from the galactic bulge.

Unfortunately, systematic uncertainties in the analysis of the COMPTEL data hinder a clear distinction between two alternative scenarios.

Published: 15 November 2005
Observations of four WR galaxies (NGC 5430, NGC 6764, Mrk 309 and VII Zw 19) using the Infrared Space Observatory are presented here. ISOCAM maps of NGC 5430, Mrk 309 and NGC 6764 revealed the location of star formation regions in each of these galaxies. ISOPHOT spectral observations from 4 to 12 microns detected the ubiquitous PAH bands in the nuclei of the targets and several of the disk star forming regions, while LWS spectroscopy detected [OI] and [CII] emission lines from two galaxies, NGC 5430 and NGC 6764.

Using a combination of ISO and IRAS flux densities, a dust model based on the sum of modified blackbody components was successfully fitted to the available data. These models were then used to calculate new values for the total IR luminosities for each galaxy, the size of the various dust populations, and the global SFR.

The derived flux ratios, the SFRs, the high L(PAH)/L(40-120 microns) and F(PAH 7.7 microns)/F(7.7 microns continuum) values suggest that most of these galaxies are home to only a compact burst of star formation. The exception is NGC 6764, whose F(PAH 7.7 microns)/F(7.7 microns continuum) value of 1.22 is consistent with the presence of an AGN, yet the L(PAH)/L(40-120 microns) is more in line with a starburst, a finding in line with a compact low-luminosity AGN dominated by the starburst.

Published: 16 August 2005
Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (420-520 nm) probes one scale height higher than the long-wavelength one (520-620 nm), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 ms-1 at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds.
Published: 15 December 2005
The European Space Agency together with industrial partners has studied a concept for low-cost in-situ exploration of the atmosphere of Venus: the Venus Entry Probe. The Venus Entry Probe is one of ESA's Technology Reference Studies (TRS). TRSs are model science-driven missions that are, although not part of the ESA science programme, able to provide focus to future technology requirements. This is accomplished through the study of several technologically demanding and scientifically meaningful mission concepts, which are strategically chosen to address diverse technological issues. The TRSs complement ESA's current mission specific development programme and allow the ESA Science Directorate to strategically plan the development of technologies that will enable potential future scientific missions. Venus has been targeted because in-situ exploration of the atmosphere of Venus is both scientifically interesting and technologically challenging.
Published: 09 December 2005
On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 ± 0.25 K, and the pressure was 1,467 ± 1 hPa.
Published: 08 December 2005
Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity-probably due to methane-and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details.
Published: 08 December 2005
Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity – probably due to methane – and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details.
Published: 08 December 2005
Aerosols in Titan's atmosphere play an important role in determining its thermal structure. They also serve as sinks for organic vapours and can act as condensation nuclei for the formation of clouds, where the condensation efficiency will depend on the chemical composition of the aerosols. So far, however, no direct information has been available on the chemical composition of these particles. Here we report an in situ chemical analysis of Titan's aerosols by pyrolysis at 600 °C. Ammonia (NH3) and hydrogen cyanide (HCN) have been identified as the main pyrolysis products. This clearly shows that the aerosol particles include a solid organic refractory core. NH3 and HCN are gaseous chemical fingerprints of the complex organics that constitute this core, and their presence demonstrates that carbon and nitrogen are in the aerosols.
Published: 08 December 2005
The surface of Saturn's largest satellite – Titan – is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.
Published: 08 December 2005
Saturn's largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellite's surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.
Published: 08 December 2005
One of Titan's most intriguing attributes is its copious but featureless atmosphere. The Voyager 1 fly-by and occultation in 1980 provided the first radial survey of Titan's atmospheric pressure and temperature and evidence for the presence of strong zonal winds. It was realized that the motion of an atmospheric probe could be used to study the winds, which led to the inclusion of the Doppler Wind Experiment on the Huygens probe. Here we report a high resolution vertical profile of Titan's winds, with an estimated accuracy of better than 1 m s-1. The zonal winds were prograde during most of the atmospheric descent, providing in situ confirmation of superrotation on Titan. A layer with surprisingly slow wind, where the velocity decreased to near zero, was detected at altitudes between 60 and 100 km. Generally weak winds (~1 m s-1) were seen in the lowest 5 km of descent.
Published: 08 December 2005
The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.
Published: 08 December 2005
23-Sep-2020 06:30 UT

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