K. Schwingenschuh

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 Cluster Magnetic Field Investigation

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.

Ions of planetary origin in the Martian magnetosphere (Phobos 2/TAUS experiment)

Abstract The measurements onboard the Phobos 2 Martian orbiter revealed one more physical process of Martian neutral atmosphere dissipation—outflow of heavy ions of planetary origin through the magnetic tail of Mars. The distribution of heavy ions through the cross-section of the Martian magnetotail is studied based on TAUS spectrometer data. Average loss rate of heavy ions through the plasmasheet (separating magnetotail lobes) is evaluated as ∼ 5 × 10 24 s −1 . The revealed process of Martian atmosphere dissipation is important for cosmological time and constitutes ∼ 10% of non-thermal oxygen dissipation due to dissociative recombination of molecular oxygen ions near exobase.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

A comparison of induced magnetotails of planetary bodies: Venus, Mars, and Titan

The Pioneer Venus orbiter (PVO), PHOBOS 2, and Voyager 1 spacecraft have together provided observations of three planetary bodies with induced magnetotails: Venus, Mars, and Titan. During the extended mission of PVO, the tail of Venus was probed at an altitude of ∼1.3 planetary radii, which provided a more appropriate basis for comparison with the Mars data (at ∼2.7 planetary radii), and Titan data (∼2.5 planetary radii downstream), than the previously analyzed Venus tail data obtained near PVO apoapsis (∼12 planetary radii). A parallel examination of the magnetic properties of these tails at downstream distances within 3 planetary radii reveals the following similarities and differences. In the cases of Venus and Mars, which are always embedded in the supermagnetosonic solar wind flow, the tail lobe fields are smoothly joined to the draped magnetosheath fields at their outer boundaries, but separated in the center by a distinct, and sometimes narrow, current sheet. The tail of Mars has a cross section that is wider, when scaled by the planet radius, than that at Venus (as found by earlier MARS spacecraft experiments), a lobe field strength that is about the same as that at Venus in spite of the factor of ∼3 smaller interplanetary field at Mars, and a cross-tail field strength that exceeds that at Venus by ∼1.5 times. The tail of Titan appears similar to the others except that there is no bow shock and little or no draped magnetosheath field signature since the surrounding magnetospheric plasma flow is submagnetosonic (although super-Alfvenic). The lobe field strengths are about half those at Venus and Mars, while the cross-tail field is almost negligible. The near-Titan tail diameter is close to the body diameter. In place of the smooth transition to a draped magnetosheath field at the tail boundaries, as seen at Venus and Mars, the Titan observations show current sheets where the field rotates to its external orientation. It is shown that the Titan wake magnetic signature can be simulated with a model field composed of a cylindrical boundary containing antiparallel axial “tail lobe” fields, surrounded by a field described by streamlines of incompressible flow around a cylinder. Simulation of the magnetic fields observed at Mars and Venus, on the other hand, requires a draped magnetosheath field model with an appropriately oriented comet-tail-like model in its interior.

The solar wind interaction with Mars: Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape

Observations taken by Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 are used to model the shape, position, and variability of the Martian bow shock for the purpose of better understanding the interaction of this planet with the solar wind. Emphasis is placed upon comparisons with the results of similar analyses at Venus, the only planet known to have no significant intrinsic magnetic field. Excellent agreement is found between Mars bow shock models derived from the earlier Mariner-Mars data set (24 crossings in 1964–1974) and the far more extensive observations recently returned by Phobos 2 (94 crossings in 1989). The best fit model to the aggregate data set locates the subsolar bow shock at a planetocentric distance of 1.56±0.04 RM. Mapped into the terminator plane, the average distance to the Martian bow shock is 2.66±0.05 RM. Compared with Venus, the bow wave at Mars is significantly more distant in the terminator plane, 2.7 RM versus 2.4 RV, and over twice as variable in location with a standard deviation of 0.49 RM versus 0.21 RV at Venus. The Mars 2, 3, and 5 and Phobos 2 data also contain a small number of very distant dayside shock crossings with inferred subsolar obstacle radii derived from gasdynamic modeling of 2000 to 4000 km. Such distant bow shock occurrences do not appear to take place at Venus and may be associated with the expansion of a small Martian magnetosphere under the influence of unusually low solar wind pressure. Finally, the altitude of the Venus bow shock has a strong solar cycle dependence believed to be due to the effect of solar EUV on the neutral atmosphere and mass loading. Comparison of the Phobos 2 shock observations near solar maximum (Rz = 141) with the Mariner-Mars measurements taken much farther from solar maximum (Rz = 59) indicates that the Martian bow shock location is independent of solar cycle phase and, hence, solar EUV flux. These results are interpreted in terms of a hybrid solar wind interaction model in which this planet possesses a weak, but significant, intrinsic magnetic field.

Ionospheric layer induced by meteoric ionization in Titan's atmosphere

Abstract The ablation of meteoroids and the ionization of metallic ions in the atmosphere of Titan has been investigated. The ionization rates of the most abundant metals in cometary meteoroids, Si + ,Mg + and Fe + , are calculated from the meteoroid mass loss rate and the ionization probability of each metal. We have modeled the ion-neutral chemistry of metallic ions and calculated the concentration of the most abundant metal ions and electrons. We found that long-lived metallic ions considerably change the predictions of the electron density by the models which only consider solar radiation and electrons trapped in the magnetosphere of Saturn. The inclusion of metallic ions in the upper ionospheric models leads to an increase in the electron concentration below 800 km. We conclude that an ionospheric layer should be present at around 700 km with an electron density peak similar in magnitude to the one produced by solar radiation at 1000 km or by cosmic rays at 90 km.

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.

The dependence of the Martian magnetopause and bow shock on solar wind ram pressure according to Phobos 2 TAUS ion spectrometer measurements

The location of the Martian magnetopause and that of the bow shock are studied on the basis of three-dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thickness D on the solar wind ram pressure: D∼(ϱυ²)−1/k, the power index turned out to be k∼5.9±0.5. The close coincidence of this index with k = 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar magnetopause.

Comparison of observed plasma and magnetic field structures in the wakes of Mars and Venus

Plasma and magnetic field observations from the Phobos 2 spacecraft at Mars and the Pioneer Venus orbiter (PVO) at Venus show that there are some notable similarities in the structure of the low-altitude magnetotails at both of these weakly magnetized planets. In particular, it is found that when conditions in the interplanetary medium are steady and the orbit sampling geometry is appropriate, two magnetic tail lobes, with an intervening “plasma sheet” or “central tail ray” in the approximate location of the dividing current sheet, are present. This behavior is seen in both the Phobos 2 ASPERA plasma analyzer data and in the PVO Langmuir probe data. In the Phobos 2 data, the tail ray is found to be composed primarily of antisunward streaming oxygen (O+) plasma which has a bulk velocity consistent with an energy close to that of the upstream solar wind plasma. The PVO Langmuir probe experiment also detected two (or more) additional cold plasma structures flanking the central feature; Phobos 2 data, on the other hand, show a proton plasma “boundary layer” flanking the central (mostly O+) tail ray or plasma sheet, with sporadic fluxes or rays of O+ ions. If the latter considered is to be the magnetosheath (solar wind plasma) at the tail boundary, it is mainly the common central tail O+ features that suggest that there are common planetary ion acceleration and magnetotail formation processes at work in the low-altitude wakes of Mars and Venus. On the other hand, an important contribution from picked-up exospheric hydrogen in the wake at Mars cannot be ruled out.