Publication archive

Publication archive

In light of Huygens measurements, we present our improved model of thermal and photochemical evolution of Titan's atmosphere. Atreya et. al (1978) demonstrated that photolysis of ammonia on primordial Titan is capable of producing a nitrogen atmosphere substantially thicker than that measured by Voyager. E. Wilson (2001) carried this calculation one step further by including methane and water vapor explicitly in the ammonia photochemistry model, and arrived at a preliminary estimate of time required to accumulate different amounts of nitrogen. However, both models assumed an isothermal atmosphere. Since chemistry leading up to nitrogen occurs in the stratosphere, both the thermal structure and saturation effects are important for determining the time constants and amounts of nitrogen production. In this presentation, we discuss preliminary results of a radiative equilibrium model for the primordial middle and lower atmosphere of Titan. It includes CH4, NH3 and H2O in solar proportions for its initial composition, and CH4-CH4 pressure induced absorption, which presently controls the thermal structure in the troposphere. The temperature in the stratosphere is controlled by the haze, and we explore the effects of a haze layer at various altitudes for accelerating conversion of ammonia to nitrogen. Furthermore, we include the effects of enhanced solar flux during the T-Tauri phase, which could speed up both the loss of nitrogen and conversion of ammonia to nitrogen. We are in the process of coupling the radiative transfer model to a comprehensive photochemical model (Wilson and Atreya, 2004) to access the roles of trace species other than those included in this calculation.
Published: 28 May 2005
Titan's nitrogen-rich atmosphere is directly bombarded by energetic ions, due to its lack of a significant intrinsic magnetic field. Singly-charged energetic ions from Saturn's magnetosphere undergo charge exchange collisions with neutral atoms in Titan's exosphere, being transformed into energetic neutral atoms (ENAs). The Ion and Neutral Camera (INCA), one of the three sensors that comprise the Magnetosphere Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions from various ion/gas interaction regions in the Saturnian magnetosphere. During Cassini's second orbit around Saturn the spacecraft performed the Ta Titan flyby (October 26, 2004), at an altitude of only 1174 km. INCA data acquired during this targeted close flyby confirm model predictions of dominant finite ion gyroradii effects, but also reveal a much more complex interaction: maximum ENA emissions are originating at higher altitudes than predicted by a simple Chamberlain-type model of the Titan exosphere. These observations will be analyzed and a simulation will be presented of some of the exospheric features they reveal.
Published: 28 May 2005
During the Huygens probe mission at Titan on 14th January 2005, the Huygens Atmospheric Structure Instrument (HASI) obtained measurements of atmospheric properties from up above 1400 km down to the ground, thus inferring the atmospheric structure. The atmospheric profile along the Huygens probe trajectory during entry phase have been retrieved from the accelerometers data, while below 160 km direct pressure and temperature measurements have been performed. The vertical temperature profile retrieved from HASI data is in very good agreement with the model derived from Voyager's observations, confirms the evidence for a stratopause and the inversion layers in the upper atmosphere as observed during stellar occultations and yielded new details on atmospheric structure.
Published: 28 May 2005
1. The very low abundances of Ar, Kr and Xe in Titan's atmosphere can be easily explained by our experimental findings. These gases are trapped in the aerosols, which are formed by UV photolysis of acetylene in their presence. When the aerosols fall down to the surface, they clean the atmosphere of these gases. A continuous supply of the radiogenic produced 40Ar from the interior can explain its small abundance in the atmosphere. 2. The originally soft and sticky photochemical aerosols, as found by us experimentally, were calculated to harden by spontaneous and radiation induces chemical cross-linking. Indeed the camera and other detectors were not covered by sticky aerosols and the intake ports were not clogged. 3. As we predicted, no lightning discharges were detected in the quiescent Titan atmosphere. Therefore, Titan's atmospheric chemistry is driven mainly by solar UV irradiation and not by electrical discharges. 4. The mixing ratios of the major gas phase species produced by UV photolysis of acetylene, as found experimentally: methylacetylene ; diacetylene ; divinyl ; and benzene were observed by the Cassini spacecraft in Titan's upper atmosphere, with an agreement within better than an order of magnitude. 5. The N:C ratio in Titan's aerosols was measured by the Huygens probe, but no results were published yet. UV photolysis of gas mixtures containing C2H2:HCN=10 yield aerosols with a ratio N:C=0.007 up to 0.01. Electrical discharges through a N2:CH4~10 gas mixtures yield a much higher N:C ratio. 6. We anticipate mountains not higher than 1900 m on Titan's surface.
Published: 28 May 2005
The magnetospheres of Earth and Saturn have similarities in terms of the highest energy radiation belt components from Cosmic Ray Albedo Neutron Decay (CRAND) but have otherwise been expected to differ on the role of charged particle convection driven by solar wind interactions with these magnetospheres. Saturn's inner and middle magnetosphere has been assumed to be dominated by corotation with little direct penetration by solar wind and magnetotail plasma. Since Saturn's planetary magnetic field characterized by the Z3 model is axisymmetric, although slightly offset northward from the ring plane, it has been difficult to understand previous Pioneer and Voyager measurements of local time asymmetry in energetic particle populations, including just outside the main rings as found by Pioneer 11. Small scale features (microsignatures) of charged particle absorption by Saturn moons and possible 'ghost' clouds of co-orbiting debris show no consistent patterns in the context of symmetric models for longitudinal drift shells. Since the 100-MeV CRAND proton drift shells are highly symmetric, it is apparent that lower energy electrons and ions showing substantial local time asymmetry are influenced by forces other than simple corotation. Cassini Huygens neutral atom observations show clear evidence of substorm injections reaching into the middle magnetosphere of Saturn preferentially on the nightside.
Published: 28 May 2005
We present observations of four flares that occurred during coordinated observations between the Coronal Diagnostic Spectrometer (CDS) on board SOHO and the Domeless Solar Telescope (DST) at Hida Observatory. We studied the evolution of relative Doppler velocities in the flare kernels by using He I (3.5x104 K), O V (2.2x105 K), and Mg IX (1.0x106 K) spectra obtained with high time cadence (42 s) SOHO CDS observations and the Halpha monochromatic images obtained with the DST. We found that the transition region plasma of O V showed strong downward velocities up to 87 km s-1 simultaneously with the downflows in the lower temperature chromospheric emissions in He I and Halpha during the impulsive phase of all four flares. From these results we suggest that the downflows in the transition region and the chromosphere are a common feature in the impulsive phase of flares. For the Mg IX line we did not detect any significant change in velocity, which suggests that the 106 K plasma was close to the intermediate temperature between the upflowing plasma (107 K) and the downflowing plasma (104-105 K). These are important for understanding the dynamics of the solar atmosphere in response to the sudden energy deposition of a flare.
Published: 02 May 2005
There is a controversy about how features protruding laterally from filaments, called barbs, are magnetically structured. On 2004 August 3, we observed a filament that had well-developed barbs. The observations were performed using the 10 inch refractor of the Big Bear Solar Observatory. A fast camera was employed to capture images at five different wavelengths of the Halpha line and successively record them on the basis of frame selection. The terminating points of the barbs were clearly discernable in the Halpha images without any ambiguity. The comparison of the Halpha images with the magnetograms taken by SOHO MDI revealed that the termination occurred above the minor polarity inversion line dividing the magnetic elements of the major polarity and those of the minor polarity. There is also evidence that the flux cancellation proceeded on the polarity inversion line. Our results together with similar other recent observations support the idea that filament barbs are cool matter suspended in local dips of magnetic field lines, formed by magnetic reconnection in the chromosphere.
Published: 11 June 2005
We present the results of a long (~93 ks) XMM-Newton observation of the bright BL-Lac object PKS 0548-322 (z= 0.069). Our Reflection Grating Spectrometer (RGS) spectrum shows a single absorption feature at an observed wavelength lambda= 23.33 +/- 0.01 Å, which we interpret as OVI Kalpha absorption at z= 0.058, i.e. ~3000 km s-1 from the background object. The observed equivalent width of the absorption line, ~30 mÅ, coupled with the lack of the corresponding absorption edge in the EPIC pn data, implies a column density of NOVI ~ 2 x 1016 cm-2 and turbulence with a Doppler velocity parameter b > 100 km s-1. Within the limitations of our RGS spectrum, no OVII or OV Kalpha absorption are detected. Under the assumption of ionization equilibrium by both collisions and the extragalactic background, this is only marginally consistent if the gas temperature is ~2.5 x 105 K, with significantly lower or higher values being excluded by our limits on OV or OVII. If confirmed, this would be the first X-ray detection of a large amount of intervening warm absorbing gas through OVI absorption. The existence of such a high column density absorber, much stronger than any previously detected one in OVI, would place stringent constraints on the large-scale distribution of baryonic gas in the Universe.
Published: 16 June 2005
We present lightcurves obtained in X-ray by the XMM-Newton EPIC cameras and simultaneous radio lightcurves obtained with the VLA for five active M-type flare stars. A number of flare events were observed, and by comparing radio with X-ray data, we consider various possible flare mechanisms. In cases where there seems to be a clear correlation between radio and X-ray activity, we use an energy budget argument to show that the heating which leads to the X-ray emission could be due to the same particles emitting in the radio. In cases where there is radio activity without corresponding X-ray activity, we argue that the radio emission is likely to arise from coherent processes involving comparatively few particles. In one case, we are able to show from polarization of the radio emission that this is almost certainly the case. Cases for which X-ray activity is seen without corresponding radio activity are more difficult to explain. We suggest that the heating particles may be accelerated to very high energy, and the resulting synchrotron radio emission may be beamed in directions other than the line of sight.
Published: 16 June 2005
The transient X-ray accreting millisecond pulsar XTE J1807-294 was observed during its February/March 2003 outburst by INTEGRAL, partly simultaneously with the XMM-Newton and RXTE satellites. We present here the first study of the 0.5-200 keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature ~40 keV, considerably higher than the value (~10 keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200 keV with a luminosity in the energy band (0.1-200) keV of 1.3 x 1037 erg s-1 (assuming a distance of 8 kpc). 22 days later the luminosity dropped to 3.6 x 1036 erg s-1. A re-analysis of XMM-Newton data yields the orbital Doppler variations of the pulse period and refines the previous ephemeris. For this source, with shortest orbital period of any known binary radio or X-ray millisecond pulsar, we constrain the companion mass MC < 0.022~MSun, assuming minimum mass transfer driven by gravitational radiation. Only evolved dwarfs with a C/O composition are consistent with the Roche lobe and gravitational radiation constraints, while He dwarfs require an unlikely low inclination.
Published: 16 June 2005
Data from the COS-B mission and H I and CO surveys of the entire galactic plane are used to explore the correlation between diffuse galactic gamma-rays and gas tracers. An average ratio of H2 column density to integrated CO temperature of about 2.3 + or - 0.3 x 1020 molecules cm-2 (K km/s)-1 is obtained. It is suggested that the previously noted soft gamma-ray spectrum toward the inner Galaxy may be due to a soft gamma-ray spectrum of the emission which is distributed like molecular gas.
Published: 15 December 1988
Energetic (exceeding 35 MeV) gamma-rays have been observed from the direction of Cygnus X-3 with the SAS-2 gamma-ray telescope. The statistical significance of the excess above the galactic and diffuse radiation is approximately 4.5 sigma. In addition, the gamma-ray flux is modulated at the 4.8-hr period observed in the X-ray and infrared regions, and within the statistical error is in phase with this emission. The flux above 100 MeV has an average value of about 4.4 x 10-6 photons cm-2 s-1. If the distance to Cygnus X-3 is 10 kpc, this flux implies a luminosity of more than 1037 erg/s if the radiation is isotropic and about 1036 erg/s if the radiation is restricted to a cone of 1 steradian, as it might be in a pulsar. Upper limits are presented for the gamma-ray flux from other known or suspected periodic X-ray sources.
Published: 01 March 1977
A list of 25 high-energy (greater than 100 MeV) gamma-ray sources detected by COS B is presented. Only four sources are identified with well-known objects. Of the remaining sources, 20 are at low galactic latitude, and they may represent a new galactic population. Their luminosity is estimated to be in the range of (0.4-5) x 1036 erg s-1. It is seen that several hundred such sources may exist in the galaxy. Their nature is not understood.
Published: 15 January 1981
AIAA-2004-3437, presented at the 40th AIAA Joint-Propulsion-Conference

SMART-1, launched in fall 2003, is Europe's first moon satellite. It shall demonstrate Solar-Electric Propulsion using a PPS-1350 hall thruster. One of the main mission investigations is the characterization of the thruster's charge-exchange ion environment. Two instruments support this analysis: EPDP, consisting of a Langmuir probe, RPA analyser and a solar cell sample, and SPEDE, consisting of two current collection spheres supported by two short booms. ARC Seibersdorf research developed a Particle-In-Cell plasma simulation to support and predict the thruster's induced plasma environment around SMART-1. This paper will give an overview of the modeling approach and a comparison of the model will test results gained during the STENTOR ground test campaign using a similar thruster. We will also report a first interpretation of the measurements from EPDP and SPEDE on SMART-1 and will compare them with the actual model predictions. This analysis shall be used to actually validate the simulation tool to reliably predict charge exchange plasma environments on future missions using electric propulsion.

Published: 12 July 2004
In the high-latitude regions of Earth, aurorae are the often spectacular visual manifestation of the interaction between electrically charged particles (electrons, protons or ions) with the neutral upper atmosphere, as they precipitate along magnetic field lines. More generally, auroral emissions in planetary atmospheres "are those that result from the impact of particles other than photoelectrons". Auroral activity has been found on all four giant planets possessing a magnetic field as well as on Venus, which has no magnetic field. On the nightside of Venus, atomic O emissions at 130.4nm and 135.6nm appear in bright patches of varying sizes and intensities, which are believed to be produced by electrons with energy < 300 eV. Here we report the discovery of an aurora in the martian atmosphere, using the ultraviolet spectrometer SPICAM on board Mars Express. It corresponds to a distinct type of aurora not seen before in the Solar System: it is unlike aurorae at Earth and the giant planets, which lie at the foot of the intrinsic magnetic field lines near the magnetic poles, and unlike venusian auroras, which are diffuse, sometimes spreading over the entire disk. Instead, the martian aurora is a highly concentrated and localized emission controlled by magnetic field anomalies in the martian crust.
Published: 10 June 2005
Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar and, similar to the Earth, N2 is the main component of the atmosphere. Methane is the second most important component, but it is photodissociated on a timescale of 107 years. This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure, 30 km in diameter that does not resemble any features seen on other icy satellites.We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere.
Published: 10 June 2005
Presentation at the 5th SPINE meeting, held 17 September 2003 at ESTEC in Noordwijk, the Netherlands.
Published: 18 September 2003
In the high-latitude regions of Earth, aurorae are the often-spectacular visual manifestation of the interaction between electrically charged particles (electrons, protons or ions) with the neutral upper atmosphere, as they precipitate along magnetic field lines. More generally, auroral emissions in planetary atmospheres "are those that result from the impact of particles other than photoelectrons". Auroral activity has been found on all four giant planets possessing a magnetic field (Jupiter, Saturn, Uranus and Neptune), as well as on Venus, which has no magnetic field. On the nightside of Venus, atomic O emissions at 130.4nm and 135.6nm appear in bright patches of varying sizes and intensities, which are believed to be produced by electrons with energy <300eV. Here we report the discovery of an aurora in the martian atmosphere, using the ultraviolet spectrometer SPICAM on board Mars Express. It corresponds to a distinct type of aurora not seen before in the Solar System: it is unlike aurorae at Earth and the giant planets, which lie at the foot of the intrinsic magnetic field lines near the magnetic poles, and unlike venusian auroras, which are diffuse, sometimes spreading over the entire disk. Instead, the martian aurora is a highly concentrated and localized emission controlled by magnetic field anomalies in the martian crust.
Published: 10 June 2005
The European Space Agency has a strong track record and plenty of ambition to propel it into its next 30 years, says Giovanni Bignami. But key decisions must be made in the context of a new Europe.
Published: 03 June 2005
The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a simulation of the growth of dark matter structure using 2160³ particles, following them from redshift z = 127 to the present in a cube-shaped region 2230 billion lightyears on a side. In postprocessing, we also follow the formation and evolution of the galaxies and quasars. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies.
Published: 03 June 2005
20-Jan-2021 04:46 UT

ShortUrl Portlet

Shortcut URL

https://sci.esa.int/p/dAGeRrW