Physical properties of cometary dust: Relation of DIDSY data to grain properties

The Dust Impact Detection System (DIDSY) for the Giotto Halley Mission consists of two types of sensors for the detection of cometary dust particles: two impact plasma sensors and five piezo-electric momentum sensors. One sensor of each type is covered by a penetration film. A 1 μm thick aluminum film covers an impact plasma sensor. One momentum sensor is mounted onto the rear shield behind the 1 mm front shield made from aluminum. The parameters measured are the total charge released upon impact and the amplitude of the acoustic signal generated by the impact. Both quantities depend on the mass and speed of the impacting particles. At the impact speed of 68 km/sec the mass of cometary dust particles can be determined in the mass range from 10⁻¹⁷ g to 10⁻³ g. From the difference in the countrates measured by the sensors with and without penetration film the average bulk density of dust particles of masses 10⁻¹⁴ g and 10⁻⁶ g can be determined. With appropriate calibration an accuracy of a factor of 2 for both the mass and density determination can be obtained.     

S3 and S4 absorption cross sections in the range of 340 to 600 nm and evaluation of the S3 abundance in the lower atmosphere of Venus

S₃ absorption cross section equals 6×10⁻¹⁷ cm² at 400 nm, 6 × 10⁻¹⁹ cm² at 500 nm (less by a factor of 4 than that given by Sanko), 4×10⁻²⁰ cm² at 600 nm. That of S₄ equals 1.5 × 10⁻¹⁷ cm² at 450 nm, 8 × 10⁻¹⁷ cm² at 500 nm, and 4.7 × 10⁻¹⁷ cm² at 600 nm. Preliminary evaluation of the S₃ mixing ratio in the lower atmosphere of Venus is (8±3)×10⁻¹¹ at 5 to 25km according to the Venera 14 measurements and several times lower at the locations of the Veneras-11 and -13.     

Cometary particle impact simulation using pulsed lasers

Calibration of the DIDSY experiment momentum sensors for the GIOTTO Mission to Comet Halley requires laboratory simulation of impacts at 68 km s⁻¹ for particle mass values in the range 10⁻⁶ g to 10⁻¹⁰ g. Existing techniques for particle acceleration cannot simultaneously attain these extreme values of velocity and particle mass, making it necessary to adopt some less direct method of impact simulation. This paper considers the application of high power pulsed lasers for laboratory simulation of the momentum impulse produced by a cometary dust particle impact on the GIOTTO spacecraft.     

Experimental drop tube of the metallurgy department of Grenoble

The drop tube which will be available in the “Centre d'Etudes Nucléaires de Grenoble” is described. Its main features are the following: - Dimensions : Drop height : 47.1 m Drop time : 3.1 s Tube inside diameter : 0.2 m - Experimental atmosphere : 1 Ultra-vacuum : 10⁻⁶ to 10⁻⁷ Pa - Residual gravity level : 10⁻⁸ to 10⁻⁹ g according to the vacuum level and drop diameter.

This facility is unique insofar as it enables experiments to be performed under ultra-vacuum conditions which, by delaying the formation of surface oxides, should contribute to improving maximum undercooling values.

The techniques used for obtaining small metallic drops (0.5 to 3 mm) are described. The availability of this instrument for the scientific community is also foreseen by the french sponsoring organizations (CEA, CNES, CNRS) ; some practicle informations will be given to potential experimenters.     

Earth encounter velocities for interplanetary meteoroids

The speed distribution of meteoroids encountering the Earth is shown to be similar for all meteoroid masses in the range 1 g to 10⁻¹² g. The speed distribution of interplanetary meteoroids encountering the Earth has usually been inferred from meteor observations. This paper reviews commonly quoted distributions and introduces more recent estimates. The influence quoted measurement uncertainties have on the distribution of Earth encounter velocities presented by Sekanina and Southworth (1975) and Erickson (1968) is presented. The Divine (1993) model of interplanetary meteoroids fits a set of orbital distributions to a wide range of spacecraft and ground based dust detector observations. By ‘flying’ the Earth through this model the distribution of geocentric encounter velocities has been obtained for typical particle masses, 10⁻⁹ and 10⁻¹² g while those at 10⁻⁴ and 10⁻⁵ g are shown to be in error.     

Fluctuation analyses of the X-ray background

We review recent progress in the use of analyses of fluctuations in the cosmic X-ray background (XRB) to determine the source counts below the detection thresholds. Three flux domains are discussed: the range around 10⁻¹²erg cm⁻² s⁻¹ where the Ginga and Einstein Observatory results remain inconsistent unless the sources at these fluxes (mainly Seyfert galaxies) are highly absorbed at low energies, the 10⁻¹⁴erg cm⁻² s⁻¹ zone where the flattening of the source counts predicted by fluctuation analyses of Einstein Observatory images is now confirmed by Rosat, and the 10⁻¹⁵erg cm⁻² s⁻¹ flux domain, where fluctuation analyses of Rosat images show that source counts remain subeuclidean with very little contribution to the XRB coming from these sources.     

Geminga pulsar observations with gamma-telescope Gamma-1

The Geminga light curve obtained with the “Gamma-1” telescope features two peaks separated by 0.5 ± 0.03 period. The light curve is pronounced for γ-quanta energies higher than 400 MeV. The pulsed flux upper limit (1σ) in the energy interval 50 – 300 MeV is 6·10⁻⁷ cm⁻²sec⁻¹. For energies >300 MeV the pulsed component power law spectrum has an exponent 1.1 _−0.3^+1.1 and an integral flux (1.1±0.3)·10⁻⁶ cm⁻²sec⁻¹.     

Use of combined light flash and plasma measurements to study hypervelocity impact processes

We present new measurements concerning generation of light flash during hypervelocity impacts. We use iron particles (10⁻¹³ to 10⁻¹⁷ kg) with velocities over the range 1 to 42 km/s impacting semi-infinite targets (aluminium and molybdenum). The main results of previous work in the field are found to be reproduced with some slight deviations. For iron projectiles with given mass and velocity the energy of the flash (normalized to mass) is proportional to velocity to the power of 3.5 for aluminium targets and 3.9 for molybdenum targets. The duration of the flash is of order 1 microsecond. Simultaneous measurements of the generation of impact plasma do not change this. The onset of plasma generation of the bulk target material does not affect the total light flash energy. We discuss the duration of the flash compared to a simple calculation of temperature in the target and plasma vs time.     

CMB anisotropy testing from small satellites

We review the advantages and possibilities of small satellites. New results of data reduction of the satellite-borne experiment RELICT-1 are presented. For the inflation spectrum of primordial perturbations we obtained the estimate for quadrupole component 6·10⁻⁶ <ΔT₂/T<3.3·10⁻⁶. The RELICT-2 mission will provide a possibility of measurement of CBR anisotropy down to the level less than ΔT₂/T = 10⁻⁸. We present the results of engineering testing of RELICT-2 measurement system and discuss ways of improving of the radiometers sensitivities.     

Calcium abundance measurements using the Yohkoh BCS

Soft X-ray observations by SMM and other spacecraft have shown that the abundance of certain elements in solar corona varies from flare to flare. In this study, observations made by the Yohkoh Bragg Crystal Spectrometer (BCS) in helium-like Ca XIX have been analysed, and Ca abundance determined for 177 flares observed during the first four years of the mission (1991–1995). The average abundance of Ca relative to H for all flares is A_Ca = (3.64±0.39) × 10⁻⁶. As with an earlier study of SMM data, the abundance is found to be enhanced compared to the photosphere ((2.24±0.10) × 10⁻⁶), and with only minor variation from flare to flare. However, the absolute value and range of values determined by this study is smaller than in the previous study; these differences are discussed.     

Modeling of the very high velocity impact process with respect to in-situ ionization measurements

This overview deals with very high impact velocities, where complete vaporization of an impacting cosmic dust particle is to be expected upon expansion from the high pressure high temperature state behind the stopping shock (v > 15 km/s). The topics discussed are the mechanics and thermodynamics of compression, adiabatic release, equation of state and nonequilibrium states upon expansion. The case of very high particle porosity (ρ 1 g/cm³) and the case of very small dust masses (m < 10⁻¹⁷ g) are discussed from what one presently knows. The possibility of three body collisions in the expanding gas phase is discussed briefly. The effect of oblique impact is discussed with respect to its relevance to the ionization process. The numbers communicated are up to the highest “experimental” impact velocities (80 km/s, Halley mission). As one goes to lower impact velocities (20 < v < 30 km/s) there is still complete vaporization of the dust particle but ionization out of the bulk of the particle becomes low. Other than thermal processes may become important. Ideas are outlined to understand their physical nature.     

Current fluctuations in the Earth''s magnetosphere caused by lower-hybrid-drift turbulence

Nonlinear calculations of the anomalous electrical conductivity in the plasma of the earth's plasma mantle, the tail plasma sheet boundary layer and the ionospheric F-region density-trough are presented provided that lower-hybrid-drift turbulence exists. It is shown that in these regions the stabilization of the wave growth is mainly caused by current relaxation. Further, the fluctuations of the electrical currents are estimated via Ampère's and Ohm's laws. It is found that the lower-hybrid-drift turbulence causes maximum anomalous collision frequencies of the order of 10⁻² Hz in the magnetosphere. The maximum current fluctuations are about 3 10⁻⁹ A/m². The theoretical results are in agreement with ISEE and Prognoz-8 measurements.     

Near infrared observations of comet Halley from Vega 2

Spatial distribution of the continuum radiation in the range of 0.95–1.9 μm presumes total dust production rate of the comet of 10ρ tonne s⁻¹ (ρ is the dust material density) and its angular distribution proportional cos . Observations of the water vapor band at 1.38 μ m reveal strong jets, their time shift from the dust jet measured in situ is consistent with gas velocity of 0.82±0.1 km s⁻¹ and dust velocity of 0.55±0.08 km s⁻¹. The OH vibrational-rotational bands observed are excided directly via photolysis of water vapor. Water vapor production rate deduced from the H₂O band and OH band intensities is 8×10²⁹ s⁻¹. Intensity of the CN(0,0) band result in the CN column density of 9×10¹² cm⁻², i.e. larger by a factor of 3 than given by the violet band.     

Interaction of artificially injected plasma flows with ionospheric plasma

Results of rocket experiments on study of plasma flows (PF) artificially injected by sources separated from vehicles and their effect on medium parameters in ionosphere at altitudes 160:230 km are presented.PF were injected comprising lithium ions with velocities 1,2 x 10⁴ m/sec. and cesium-potassium ions with velocities (1,4–1,5)x10³ m/sec. Mass flow rate in case of lithium PS is 2 mg/sec, and in case of cesium-potassium PS is 0,2 g/sec. During experiments mass-spectrometer measurements of ion medium content in ranges of different ion masses were held, disturbancies of electric fields with frequencies up to 20 kHz and electron flows with energies 0,7keV, 4,6keV and over 40 keV were controlled at distancies from 150m to (500–600)m between plasma source and scientific equipment.     

Interplanetary scintillations of PKS 2314+03 during occultation by comet Halley

Strong interplanetary scintillations (IPS) of the quasar 2314+03 were recorded at 103 MHz at Thaltej-Ahmedabad, India with a transit type correlation interferometer on 18, 19 and 20 December 1985, as the radio source was predicted to be occulted by the ion tail of the comet Halley.

On 18th through 20th very strong scintillations, with periodicities of 1 sec average were observed, their amplitude progressively decreasing as the source approached the tail-end. The rms variations of scintillating flux of the source on 18, 19 & 20 were about 18, 11 & 4.7 Jy, as against 3.3 Jy on control days 17 and 21 December for solar elongation of 87°.

Assuming Gaussian irregularities with weak scattering, the rms density variations, ΔN, of 10, 6, 3 and 1 elec./cm³ on 18 through 21 December, from the comet nucleus towards its tail-end, varied as (ΔN) ∝ r^−3.3 as against (ΔN) ∝ r⁻² in the solar plasma.

Quasi-periodic modulations of the enhanced scintillating flux possibly imply 10⁴ km scale-size ion condensations and mean electron density of 10³ to 10⁴ electrons/cm³ in the Halley's plasma tail.     

Radiative forces and LAGEOS' orbit

The orbit analysis of LAGEOS satellite has resulted into the discovery and/or reassessment of several very small sources of perturbation on satellite orbits. The analysis of orbital arcs of duration ranging from one month to several years has revealed that perturbative effects are present, having unpredicted long-term or secular components down to the 10⁻¹² m/s² acceleration level. It was soon realized that those perturbations have a non-gravitational origin.

In recent years, we have devoted some effort to the physical modelling of radiative perturbations, caused by momentum exchanges with an appropriate radiative field, and have considered their potential role in the analysis of LAGEOS orbit residuals. These perturbations include: (i) direct solar radiation pressure; (ii) radiation pressure from the Earth's emitted/reflected/diffused radiation flux; (iii) the so-called thermal thrust force.

The main results of this work are reviewed, discussing its relationships with models developed by other research groups. In particular, we present a list of the physical processes which still appear to need more detailed and realistic modelling to reach a better understanding of LAGEOS dynamics at the 10⁻¹² m/s² level.     

Interplanetary dust interaction with comets: Production of x-rays

Recent results of theoretical investigations related to generation of high-energy (0.1-1 keV) photons in comets due to production of high-temperature (3×10⁵-10⁷ K) plasma clots from collisions of cometary and interplanetary grains at high relative velocities (70-700 km s⁻¹ at heliocentric distances R=0.01-1 AU) are summarized and main features of the process are marked.     

Hard X-ray focusing optics up to 80 keV for the future missions

X-ray telescopes have been providing high sensitivity X-ray observations in numerous missions. For X-ray telescopes in the future, one of the key technologies is to expand the energy band beyond 10 keV. We designed depth-graded multilayer, so-called supermirrors, for a hard X-ray telescope in the energy band up to 40 keV using lightweight thin-foil optics. They were successfully flown in a balloon flight and obtained a hard X-ray image of Cyg X-1 in the 20–40 keV band. Now supermirrors are promising to realize a hard X-ray telescope. We have estimated the performance of a hard X-ray telescope using a platinum–carbon supermirror for future satellite missions, such as NeXT (Japan) and XEUS (Europe). According to calculations, they will have a significant effective area up to 80 keV, and their effective areas will be more than 280 cm² even at 60 keV. Limiting sensitivity will be down to 1.7 × 10⁻¹³ erg cm⁻² s⁻¹ in the 10–80 keV band at a 100 ks observation. In this paper, we present the results of the balloon experiment with the first supermirror flown and projected effective areas of hard X-ray telescopes and action items for future missions.     

A serendipitous survey for galaxy clusters by the XMM-Newton Survey Science Center

We describe the initial results of a programme to detect and identify extended X-ray sources found serendipitously in XMM-Newton observations. We have analyzed 186 EPIC-PN images at high galactic latitude with a limiting flux of 1 × 10⁻¹⁴ erg cm⁻² s⁻¹ and found 62 cluster candidates. Thanks to the enhanced sensitivity of the XMM-Newton telescopes, the new clusters found in this pilot study are on the average fainter, more compact, and more distant than those found in previous X-ray surveys. At our survey limit the surface density of clusters is about 5 deg⁻². We also present the first results of an optical follow-up programme aiming at the redshift measurement of a large sample of clusters. The results of this pilot study give a first glimpse on the potential of serendipitous cluster science with XMM-Newton based on real data. The largest, yet to be fulfilled promise is the identification of a large number of high-redshift clusters for cosmological studies up to z = 1 or 1.5.