Interplanetary shock waves: Ulysses observations in and out of the ecliptic plane

Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forwardreverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction.     

Dynamic imaging and RCS measurements of aircraft

Results on radar cross section (RCS) measurements and inverse synthetic aperture radar images of a Mooney 231 aircraft using a ground-to-air measurement system (GTAMS) and a KC-135 airplane using an airborne radar are presented. The Mooney 231 flew in a controlled path in both clockwise and counterclockwise orbits, and successively with the gear down, flaps in the take-off position and with the speed brakes up. The data indicates that RCS pattern measurements from both ground-based and airborne radar of flying aircraft are useful and that the inverse synthetic aperture radar (ISAR) images obtained are valuable for signature diagnostics     

Dynamic dissipative compensators for multibody flexible spacestructures

The stability characteristics of dynamic dissipative compensators are investigated for multibody flexible space structures having nonlinear dynamics. The problem addressed is that of proving asymptotic stability of dynamic dissipative compensators. The stability proof uses the Lyapunov approach and exploits the inherent passivity of such systems. For such systems these compensators are shown to be robust to parametric uncertainties and unmodeled dynamics. The results are applicable to a large class of structures such as satellites with multiple payloads and space-based manipulators     

The Solar Chemical Composition

We present our current knowledge of the solar chemical composition based on the recent significant downward revision of the solar photospheric abundances of the most abundant metals. These new solar abundances result from the use of a 3D hydrodynamic model of the solar atmosphere instead of the classical 1D hydrostatic models, accounting for departures from LTE, and improved atomic and molecular data. With these abundances, the new solar metallicity, Z, decreases to Z=0.012, almost a factor of two lower than earlier widely used values. We compare our values with data from other sources and analyse a number of impacts of these new photospheric abundances. While resolving a number of longstanding problems, the new 3D-based solar photospheric composition also poses serious challenges for the standard solar model as judged by helioseismology.     

Elemental Abundances of the Bulk Solar Wind: Analyses from Genesis and ACE

Analysis of the Genesis samples is underway. Preliminary elemental abundances based on Genesis sample analyses are in good agreement with in situ-measured elemental abundances made by ACE/SWICS during the Genesis collection period. Comparison of these abundances with those of earlier solar cycles indicates that the solar wind composition is relatively stable between cycles for a given type of flow. ACE/SWICS measurements for the Genesis collection period also show a continuum in compositional variation as a function of velocity for the quasi-stationary flow that defies the simple binning of samples into their sources of coronal hole (CH) and interstream (IS).     

A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

The Grain Impact Analyser and Dust Accumulator (GIADA) Experiment for the Rosetta Mission: Design, Performances and First Results

The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10⁻¹⁰ ÷ 4.0 × 10⁻⁴ kg m s⁻¹ for velocities up to about 300 m s⁻¹. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10⁻¹⁰ g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts demonstrate that GIADA is behaving as expected according to the design specifications. The International GIADA Consortium (I, E, UK, F, D, USA).     

Problems of application of face gasodynamic seals in aircraft engines

Problems arising in introduction of gasodynamic seals in aircraft engines are considered. The operation of a face gasodynamic seal as part of a natural gas pump is analyzed and its efficiency in the presence of oil is shown.     

Optimal friction control on a circular cylinder taking into account the reverse effect of the boundary layer

We analyze the effect of injection both of uniformly distributed over the entire cylinder surface and of the optimal one on the velocity distribution at the outer border of the boundary layer and, as a result, on friction.     

The storm-time ring current

In this paper I am reviewing recent advances and open disputes in the study of the terrestrial ring current, with emphasis on its storm-time dynamics. The ring current is carried by energetic charged particles flowing toroidally around the Earth, and creating a ring of westward electric current, centered at the equatorial plane and extending from geocentric distances of about 2 R _E to roughly 9 R _E. This current has a permanent existence due to the natural properties of charged particles in the geospace environment, yet its intensity is variable. It becomes more intense during global electromagnetic disturbances in the near-Earth space, which are known as space (or magnetic or geomagnetic) storms. Changes in this current are responsible for global decreases in the Earth's surface magnetic field, which is the defining feature of geomagnetic storms. The ring current is a critical element in understanding the onset and development of space weather disturbances in geospace. Ring current physics has long been driven by several paradigms, similarly to other disciplines of space physics: the solar origin paradigm, the substorm-driver paradigm, the large-scale symmetry paradigm, the charge-exchange decay paradigm. The paper addresses these paradigms through older and recent important investigations.