Publication archive

Publication archive

This article is based on a talk on the subject of planet formation given by Professor Willy Benz of the Physikalisches Institut of the University of Bern to Pro ISSI.
Published: 02 September 2000
On March 24, 1995, the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the low-latitude boundary layer about 15 RE tailward of the dusk meridian. Although the magnetospheric and magnetosheath magnetic fields were strongly northward, the Bz component showed strong short-duration fluctuations in which Bz could even reach negative values. We have used two-dimensional magnetohydrodynamic simulations with magnetospheric and magnetosheath input parameters specifically chosen for this Geotail event to identify the processes which cause the observed boundary properties. It is shown that these fluctuations can be explained by the Kelvin-Helmholtz instability if the k vector of the instability has a component along the magnetic field direction. The simulation results show many of the characteristic properties of the Geotail observations. In particular, the quasi-periodic strong fluctuations are well explained by satellite crossings through the Kelvin-Helmholtz vortices. It is illustrated how the interior structure of the Kelvin-Helmholtz vortices leads to the rapid fluctuations in the Geotail observations. Our results suggest an average Kelvin-Helmholtz wavelength of about 5 RE, with a vortex size of close to 2 RE for an average repetition time of 2.5 min. The growth time for these waves implies a source region of about 10-16 RE upstream from the location of the Geotail spacecraft (i.e., near the dusk meridian). The results also indicate a considerable mass transport of magnetosheath material into the magnetosphere by magnetic reconnection in the Kelvin-Helmholtz vortices.
Published: 16 September 2000
For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15RE behind the Earth while the solar wind remained very quiet (V=330 km s-1, n=14-21 cm-3) with a very steady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n=19 cm-3), cool (Tp=40 eV), rapidly flowing (V=310 km s-1) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km s-1), lower but substantial densities (n=3 cm-3) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3-min periodicity, and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmholtz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.
Published: 16 September 2000
S-Cam is a cryogenic camera for ground based astronomy designed around a 6x6 array of Ta-Al Superconducting Tunnel Junctions (STJs). The camera has been conceived as a technology demonstrator, aiming to prove the potential of this new generation of single photon counting detectors at a ground-based telescope as a possible precursor to space based applications. Following a first test campaign at the William Herschel Telescope in La Palma (Canary Islands, Spain), an improved version of the camera (S-Cam 2) has been developed and tested. In this paper we provide an overview of the latest camera performance, a description of the up-dated S-Cam 2 system and a summary of the main test results. An example of the novel astronomical data obtained during the second test campaign conducted in December 1999 are also shortly described.
Published: 16 August 2000
We report on the design and testing of a new readout scheme for Superconducting Tunnel Junction (STJ) arrays. By grouping the electrodes in rows and columns, this method drastically reduces the number of connections and electronic circuits reruired for reading out a large format array of pixelated detectors. It is a generic scheme in that it could be applied to different kinds of detector arrays. Using charge sensitive amplifiers with junction field-effect transistors (JFETs) we verify that the energy resolution degrades primarily due to capacitance increase at the amplifier's input node. However, since each detector is read-out by two independent circuits, these two outputs can be combined to increase the signal-tonoise level. The measurements reported here were carried out on an array of 6x6 junctions. All junctions were biased but only 2 rows and 2 columns read-out. We compare the results to measurements carried out on a similar 6x6 array fabricated from the same trilayer but with individual pixel read-out. The measurements show that stable biasing of STJs is possible with the new configuration and that the measured optical spectral line resolutions are consistent with our theoretical predictions.
Published: 16 August 2000
Superconducting tunnel junctions (STJs) have been demonstrated as photon counting detectors in the UV-NIR wavelength range (100-1000 nm). They combine a modest wavelength resolving power with fast response and high detection efficiency over a broad wavelength band. This makes this type of detector an interesting alternative to the present generation of detectors used in UV/optical astronomy, such as CCDs and micro-channel plates. Practical applications require imaging detectors with large sensitive area and good spatial resolution. While the feasibillity of small arrays of closely packed STJs which are individually biased and read-out has already been demonstrated, the development of large format arrays is limited by the large number of electronics chains and wire connections to the cold detector which would be required. An alternative approach is to use a large area absorber combined with a few STJs at the edges or corners. A photon's energy as well as its absorption position in the absorber can be derived from the signal amplitudes measured in the STJs. In this paper the performance in terms of wavelength resolving power and position resolution of four different linear geometries of Ta absorbers, read out with Ta-Al STJs, is investigated and compared with single STJs. The UV and optical spectra obtained with the absorbers show resolving powers within a factor of two of the theoretical limit. In particular, a measured resolving power at lambda=300 nm of ~16 with a position resolution of ~9 micron is achieved with a 100x50 micron² absorber in between two 50x50 micron² STJs.
Published: 16 August 2000
We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), 1- and 2- dimensional photon-counting imaging spectrometers, based on Ta/Al-based STJs placed on a Ta absorber. Results obtained with highly collimated illumination with 10 keV X-ray photons clearly demonstrate the imaging capabilities of 2-dimensional DROIDs. The derived spatial FWHM resolution is 7 micron for a 200 x 200 micron² absorber. With a 1-D DROID we have measured an intrinsic energy resolution of 15 eV FWHM for 6 keV photons. At high energies (E > 6 keV) the resolution is limited by spatial fluctuations in the qp recombination rate.
Published: 16 July 2000
We present an application of the generalised proximity effect theory. The theory has been used to determine the energy gap (Delta g) in proximised transition metal - aluminium bilayer structures such as Nb/Al, Ta/Al, V/Al and Mo/Al. These bilayers have different film thicknesses ranging from 5 to 260 nm. For the cases of Nb/Al, Ta/Al and V/Al bilayers, the interface parameters Gamma and GammaBN (here we define Gamma as the ratio of the products of normal state resistivity and coherence length in each film of the bilayer while GammaBN is the ratio of the boundary resistance between film 1 and 2 to the product of the resistivity and coherence length in the second film), which were used as input parameters to the model, were inferred experimentally from an existing bilayer of each kind and then suitably modified for different film thicknesses. This experimental assessment is therefore based on a comparison of measurements of the critical temperature and the energy gap at 300 mK with the predictions from the model for various values of Gamma, GammaBN. The energy gap of the bilayer was experimentally determined by using symmetrical Superconducting Tunnel Junctions (STJs) of the form S-Al-AlOx-Al-S, where each electrode corresponds to a proximised bilayer. However for the case of Mo/Al bilayers the interface parameters were determined theoretically since currently no STJ data for this configuration are available. The results for the Nb/Al, Ta/Al and V/Al bilayers have also then been compared to experimentally determined energy gaps found for a series of STJs with different film thicknesses. The correspondence between experiment and theory is very good.
Published: 16 July 2000
GAIA is proposed for ESA's fifth cornerstone mission which has a prospective launch date of 2009. The objectives of GAIA are many-fold, but the core objective is the discovery of the origin and formation of the Galaxy. To do this GAIA will combine information from astrometry, photometry, and radial velocity instruments using the proven principles of the Hipparcos mission.
Published: 19 June 2000
This article is based on a talk given by Professor Johannes Geiss to the Pro ISSI.
Published: 02 May 2000
We discuss the observational requirements for future X-ray planetary and astrophysics missions and present preliminary laboratory results from our compound semiconductor program. The detectors used in the tests were simple monolithic devices, which are used in conjunction with a detailed material science and technology development program intended to produce near Fano limited, pixilated hard X-ray detectors. In practical terms, this means producing active arrays, comprised of over 10³ pixels each being of order 100 microns in size, with spectral resolving powers, E/DeltaE > 20 at 10 keV and high quantum efficiencies over the energy range 1 to 200 keV. Four materials are currently under study - GaAs, HgI2, TlBr and CdZnTe. In the cases of GaAs and CdZnTe, the detector energy resolution functions are approaching the Fano limit.
Published: 16 July 2000
Thermal Emission Spectrometer (TES) observations of the recession phase of Mars' south polar cap are used to quantitatively map this recession in both thermal and visual appearance. Geographically uniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior. For most of the cap, while temperatures remain near the CO2 frost point, albedos slowly increase with the seasonal rise of the Sun, then drop rapidly as frost patches disappear over a period of ~20 days. A "Cryptic" region remains dark and mottled throughout its cold period. TES observations are compared with the first-order theoretical spectra of solid CO2 frost with mixtures of dust and H2O. The TES spectra indicate that the Cryptic region has much larger grained sold CO2 than the rest of the cap and that the solid CO2 here may be in the form of a slab. The Mountains of Mitchel remain cold and bright well after other areas at comparable latitude, apparently as a result of unusually small size of the CO2 frost grains; we found little evidence for a significant presence of H2O. Although CO2 grain size may be the major difference between these regions, incorporated dust is also required to match the observations; a self-cleaning process carries away the smaller dust grains. Comparisons with Viking observations indicate little difference in the seasonal cycle 12 Martian years later. The observed radiation balance indicates CO2 sublimation budgets of up to 1250 kg m-3. Regional atmospheric dust is common: localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm.
Published: 26 April 2000
S-Cam is a cryogenic camera for ground based astronomy based on a 6x6 array of Superconducting Tunnel Junctions (STJs). The camera has been designed as a technology demonstrator, aiming to prove the potential of this new generation of single photon counting detectors at a ground-based telescope. In this article we provide an overview of the detector performance, a description of the S-Cam system and a summary of the test results. The first astronomical data obtained at the William Herschel Telescope (WHT) in La Palma (Canary Islands, Spain) are also described.
Published: 08 April 2000

Remote sensing observations and the direct sampling of material from a few comets have established the characteristic composition of cometary gas. This gas is ionized by solar ultraviolet radiation and the solar wind to form 'pick-up' ions, ions in a low ionization state that retain the same compositional signatures as the original gas.

The pick-up ions are carried outward by the solar wind, and they could in principle be detected far from the coma. (Sampling of pick-up ions has also been used to study interplanetary dust, Venus' tail and the interstellar medium.)

Here we report the serendipitous detection of cometary pick-up ions, most probably associated with the tail of comet Hyakutake, at a distance of 3.4 au from the nucleus. Previous observations have provided a wealth of physical and chemical information about a small sample of comets, but this detection suggests that remote sampling of comet compositions, and the discovery of otherwise invisible comets, may be possible.

Published: 07 April 2000
Observations of the varying orientations of comet tails led to the suggestion of the existence of the solar wind - a continuous outflow of ionized material from the Sun. It is now well established that gas from comets is ionized by several processes and joins the solar wind, forming an ion (plasma) tail that points away from the Sun. The plasma environments of three comets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous crossing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 au determined for the Great March Comet of 1843 (C/1843 D1). Our measurements reveal that, at this distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter.
Published: 07 April 2000
We report on hard X-ray measurements with two epitaxial GaAs detectors of active areas 2.22 mm² and thicknesses 40 and 400 microns at the ESRF and HASYLAB synchrotron research facilities. The detectors were fabricated using high purity material and in spite of an order of magnitude difference in depletion depths, they were found to have comparable performances with energy resolutions at -45 °C of ~1 keV fwhm at 7 keV rising to ~2 keV fwhm at 200 keV and noise floors in the range 1-1.5 keV. At energies < 30 keV, the energy resolution was dominated by leakage current and electromagnetic pick-up, while at the highest energies measured, the resolutions approach the expected Fano limit (e.g., ~1 keV near 200 keV). Both detectors are remarkably linear, with average rms non-linearities of 0.2% over the energy range 10-60 keV, which, taken in conjunction with Monte-Carlo results indicate that charge collection efficiencies must be in excess of 98%. This is consistent with material science metrology which show that the material used to produce them is of extremely high purity with impurity concentrations < 1013 cm-3.
Published: 11 March 2000
Images of an active region on the far side of the Sun were derived by applying seismic holography to recent helioseismic observations from space. Active regions are the centers of energetic phenomena such as solar flares and coronal mass ejections whose resulting electromagnetic and particle radiation interfere with telecommunications and power transmissions on Earth and can pose significant hazards to astronauts and spacecraft. Synoptic seismic imaging of far-side solar activity will now allow us to anticipate the appearance of large active regions more than a week ahead of their arrival on the east solar limb.
Published: 10 March 2000
Proceedings of 1999 NASA/JPL International Conference on FUNDAMENTAL PHYSICS IN SPACE, April 29,30 and May 1, 1999, Washington DC, NASA Document D-18925. "GALILEO GALILEI" (GG) is a proposal for a small, low orbit satellite devoted to testing the Equivalence Principle (EP) of Galileo, Newton and Einstein. The GG Report on Phase A Study recently carried out with funding from ASI (Agenzia Spaziale Italiana) concludes that GG can test the Equivalence Principle to 1 part in 1017 at room temperature. The main novelty is to modulate the expected differential signal of an EP violation at the spin rate of the spacecraft (2 Hz). As compared to other experiments, the modulation frequency is increased by more than a factor 104, thus reducing 1/f (low frequency) electronic and mechanical noise. The challenge in this field is to fly an experiment able to improve by many orders of magnitude the current best sensitivity (of about 1 part 1012). This requires spurious relative motions of the test bodies to be greatly reduced, leaving them essentially motionless. Doing that with more than one pair of bodies appears to be an unnecessary complication. This is why GG is now proposed with a single pair of test masses. -- Remainder of abstract truncated --
Published: 15 January 2000
We present the first results obtained with a 2-dimensional X-ray imaging spectrometer consisting of a 200x200 micron² Ta absorber and read out by four Ta/Al superconducting tunnel junctions (STJs). A preliminary image reconstruction algorithm allows the visualisation of the diffraction pattern from a 5 micron pinhole illuminated with 10 keV X-rays. The image suggests a spatial resolution better than 10 micron. The algorithm does not take into account quasi-particle losses in the absorber. Hence, the pulse height reconstruction is not optimal and the energy resolution varies significantly across the absorber. The best energy resolution is obtained for a 20x20 micron² area in the centre of the absorber, and amounts to ~77 eV at a photon energy of 5895 eV, with a 70 eV electronic noise contribution.
Published: 15 January 2000
We present two independent experiments, each of which suggests that the local energy gap in Ta (and Nb) has a lateral spatial variation on a scale of several micron. The first experiment is a series of current-voltage characterizations of Nb/Al/AlOx and Ta/Al/AlOx Josephson junctions, which reveals a dependence of the measured energy gap on the size of the junction. This implies a geometrical dependence of the energy gap. An extended version of the current theory of the proximity effect could explain this phenomenon when a lateral coherence length is introduced, which is of the order of the bulk coherence length of the materials. The second experiment is a series of coincidence measurements of photon absorption events in a Ta absorber between two Ta/Al junctions. There is a clear distinction in the pulse-height characteristics between events detected in the absorber and the junctions. Interestingly, there are also events indicating the presence of a transition region between the absorber and the junction. Event statistics imply that this region has a size of ~6 micron, independent of photon energy, which is quite a bit larger than even the bulk coherence length in Ta. It is argued that an additional effect due to 'smearing' by the relaxed quasi-particle cloud must also be present. These effects are interesting and intriguing; not only from a theoretical viewpoint, but also for energy-gap engineering of superconducting materials for practical applications, e.g. in a variety of photon and particle detectors.
Published: 15 January 2000
28-Sep-2020 19:00 UT

ShortUrl Portlet

Shortcut URL

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