R. Grard

In situ measurements of the physical characteristics of Titan's environment

On the basis of previous ground-based and fly-by information, we knew that Titans 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.

The dust distribution within the inner coma of comet P/Halley 1982i - Encounter by Giotto's impact detectors

Analysis of the data from Giottos Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus. In order to improve the data coverage (and especially for the smallest grains, to approximately 10(-19) kg particle mass), data from the PIA instrument has been combined with DIDSY data. Flux profiles are presented for the various mass channels showing, to a first approximation, a 1/R2 flux dependence, where R is the distance of the detection point from the cometary nucleus, although significant differences are noted. Deviations from this dependence are observed, particularly close to the nucleus. From the flux profiles, mass and geometrical area distributions for the dust grains are derived for the trajectory through the coma. Groundbased CCD imaging of the dust continuum in the inner coma at the time of encounter is also used to derive the area of grains intercepted by Giotto. The results are consistent with the area functions derived by Giotto data and the low albedo of the grains deduced from infrared emission. For the close encounter period (-5 min to +5 min), the cumulative mass distribution function has been investigated, initially in 20 second periods; there is strong evidence from the data for a steepening of the index of the mass distribution for masses greater than 10(-13) kg during passage through dust jets which is not within the error limits of statistical uncertainty. The fluences for dust grains along the entire trajectory is calculated; it is found that extrapolation of the spectrum determined at intermediate masses (cumulative mass index alpha = 0.85) is not able to account for the spacecraft deceleration as observed by the Giotto Radio Science Experiment and by ESOC tracking operations. Data at large masses (>10(-8) kg) recently analysed from the DIDSY data set show clear evidence of a decrease in the mass distribution index at these masses within the coma, and it is shown that such a value of the mass index can provide sufficient mass for consistency with the observed deceleration. The total particulate mass output from the nucleus of comet P/Halley at the time of encounter would be dependent on the maximum mass emitted if this change in slope observed in the coma were also applicable to the emission from the nucleus; this matter is discussed in the text. The flux time profiles have been converted through a simple approach to modeling of the particle trajectories to yield an indication of nucleus surface activity. There is indication of an enhancement in flux at t approximately -29 s corresponding to crossing of the dawn terminator, but the flux detected prior to crossing of the dawn terminator is shown to be higher than predicted by simple modelling. Further enhancements corresponding to jet activity are detected around +190 s and +270 s.

VLF Electromagnetic Waves Observed Onboard GEOS-1

This paper is concerned mainly with the information which can be extracted from frequency-time spectra in the VLF range. The instrument used is the correlator which has a good frequency resolution (50 Hz) and time resolution (30 ms) in one magnetic and one electric component simultaneously. By suitable computer analysis, it is possible for instance to distinguish between the two dominant electromagnetic emissions, hiss and chorus, as well as to display the complete spectra. This treatment is applied to the Survey periods, which are a fixed sequence of modes, repeated every hour on the hour in order to have reference data from GEOS analogous to many ground-based observatories. One result of this treatment obtained already is that hiss and chorus normally appear together, although one or the other may be dominating in intensity. The occurrence rate of these emissions in local time is also given.For continuous surveillance the filterbank data are used. There are 16 frequency filters supplying magnetic and electric amplitude at few different frequencies. Using these data, a storm sudden commencement can be followed with good time resolution (1 s), and an interesting correlation has been found in a few cases between the VLF signal amplitude and the cold plasma density (as measured by the active part of the S-300 experiment).

First measurements of plasma waves near Mars

HERE we report preliminary results from electic field measurements in the environment of Mars using the plasma-wave system on board the Soviet spacecraft Phobos 2. It also includes a Lang-muir probe which measured plasma densities. Electron-plasma oscillations observed upstream of the bow shock correspond to a solar-wind density of 2 cm-3. The shock-foot boundary was crossed up to three times on each orbit. The shock ramp was detected at altitudes between 0.45 and 0.75 Mars radii (RM) above the planetary surface. The density increased by about a factor of two at the ramp. The shock position, although variable, seemed to be consistent with previous measurements. The downstream mag-netosheath contained broadband electric-field noise below the plasma frequency. The boundary of the obstacle, or planetopause, was crossed at altitudes of the order of 0.28 RM; the cold plasma density was highly variable within the planetopause and reached the unexpected value of 700 cm-3 on the third orbit, at 0.25 Rm altitude. Bursts of waves with frequencies below the electron cyclotron frequency, possibly in the whistler mode, occur within the planetopause.

THE CHARACTERISATION OF TITAN'S ATMOSPHERIC PHYSICAL PROPERTIES BY THE HUYGENS ATMOSPHERIC STRUCTURE INSTRUMENT (HASI)

The Huygens Atmospheric Structure Instrument (HASI) is a multi-sensor package which has been designed to measure the physical quantities characterising the atmosphere of Titan during the Huygens probe descent on Titan and at the surface. HASI sensors are devoted to the study of Titans atmospheric structure and electric properties, and to provide information on its surface, whether solid or liquid.

Martian planetopause as seen by the plasma wave system onboard Phobos 2

The existence of a plasma boundary between the bow shock and the atmosphere of Mars has been confirmed by the Phobos 2 observations. This boundary is called planetopause, magnetopause, ion-composition boundary or protonopause, depending upon the authors. A careful examination of plasma wave system (PWS) data has revealed that planetopause signatures are generally clear and not questionable, enabling the calculation of the mean planetopause position up to 16 Martian radii in the distant tail. The planetopause model derived from PWS measurements is compared with models calculated from data sets obtained with other instruments on Phobos 2. With the exception of the transition plasma layer, called mass-loading boundary by the Automatic Space Plasma Experiment with a rotating analyzer (ASPERA) investigators, the published planetopause, magnetopause/areomagnetopause and ion-composition boundary locations are similar to the planetopause locations deduced from PWS data. This would suggest that, despite their different names, these boundaries correspond to the same plasma transition region. The PWS observations show that the planetopause position varies in the same way as does the bow shock position and is not significantly affected by the solar wind ram pressure. The absence of correlation between the planetopause position and solar wind ram pressure raises serious questions about the role that possible intrinsic magnetic field may play at the Martian planetopause. Finally, the planetopause is believed to be an ion-composition discontinuity rather than the “obstacle” predicted by gasdynamic models.

Active Spacecraft Potential Control

Charging of the outer surface or of the entire structure of a spacecraft in orbit can have a severe impact on the scientific output of the instruments. Typical floating potentials for magnetospheric satellites (from +1 to several tens of volts in sunlight) make it practically impossible to measure the cold (several eV) component of the ambient plasma. Effects of spacecraft charging are reduced by an entirely conductive surface of the spacecraft and by active charge neutralisation, which in the case of Cluster only deals with a positive potential. The Cluster spacecraft are instrumented with ion emitters of the liquid-metal ion-source type, which will produce indium ions at 5 to 8 keV energy. The operating principle is field evaporation of indium in the apex field of a needle. The advantages are low power consumption, compactness and high mass efficiency. The ion current will be adjusted in a feedback loop with instruments measuring the spacecraft potential (EFW and PEACE). A stand-alone mode is also foreseen as a back-up. The design and principles of the operation of the active spacecraft potential control instrument (ASPOC) are presented in detail. Flight experience with a similar instrument on the Geotail spacecraft is outlined.

Plasma and waves around Mars

Abstract The Plasma Wave System (PWS) on board the Soviet Spacecraft Phobos 2 has performed electron density and, for the first time, electric field measurements in the environment of the planet Mars. Electron plasma oscillations are observed upstream of the bow shock and yield a solar wind density of the order of 2 cm−3; the shock foot and/or ion foreshock are detected on each orbit. The altitude of the bow shock in the noon sector fluctuates between 0.45 and 0.75 Mars radii (Rms) above the planetary surface. The downstream solar wind, in the planetosheath, is characterized by increased plasma density and broadband electrostatic noise. The planetopause is crossed at altitudes of the order of 0.28 Rms. Electromagnetic waves with frequencies below the local gyrofrequency, propagating in the whistler mode, are recorded within the planetosphere, where the electron plasma density reaches unexpectedly large values, of up to 700 cm−3 at an altitude of 0.25 Rms. Intense electrostatic emissions generated by heavy planetary ions are observed upstream of the shock ; these waves are linked with the erosion process of the Martian atmosphere by the solar wind.

Returns to Mercury: science and mission objectives

Abstract As the inner end-member of the planetary system, Mercury plays an important role in constraining and testing dynamical and compositional theories of planetary formation. With its companions Venus, Earth and Mars, it forms the family of terrestrial planets, a category of celestial objects in which each member holds information essential for retracing the history of the whole group. For example, knowledge about the origin and evolution of these planets is one of the keys to understanding how conditions to support life have been met in the Solar System and, possibly, elsewhere. This quest is all the more important as terrestrial-like objects orbiting other stars are not yet accessible; our own solar system remains the only laboratory where we can test models that are also applicable to other planetary systems. The exploration of Mercury is therefore of fundamental importance for answering questions of astrophysical and philosophical significance, such as: ‘Are terrestrial bodies a common feature of most planetary systems in the Galaxy?’

Measurements of quasi-static electric fields between 3 and 7 Earth radii on GEOS-1

AbstractQuasi-static electric fields have been measured with two spherical probes supported by cable booms providing a baseline of 42 m for the measurement. The performance of the experiment is outlined to demonstrate that electric fields can be measured with accuracies of ±0.7 mV m-1 and ±1.0 mV m-1 in the dawn-dusk and satellite-sun directions respectively. These uncertainties can be considerably reduced under favourable plasma conditions. Examples of typical observations are described.(a)The average electric field is always characterized by an irregular structure with time scales 0.5–5 min and with amplitudes of a few mV m-1.(b)During substorms dawn-dusk electric fields up to 20–30 mV m-1 have been observed over intervals of 30–60 s.(c)Oscillating electric fields with peak-to-peak amplitudes up to 10 mV m-1 and periods of 3–10 min have been observed following magnetospheric disturbances. The observations are discussed in terms of plasma motions and possible spatial scale sizes of the phenomena, standing magnetospheric wave modes and electrostatic potentials.