Neutral points on the boundary of the closed magnetosphere

Theoretical studies of a field-free plasma incident upon a magnetic dipole lead to a closed magnetosphere with two neutral points in the noon magnetic meridian, at a latitude of ± 70°–75° and a geocentric distance of approximately 10 R_E. The position of the neutral points with respect to the dipole axis is not greatly affected by the angle of incidence of the solar wind. Although the field magnitude near the neutral points is only a fraction of the dipole field, the direction is seen to reverse on opposite sides of the neutral point. Near the boundary the field direction is parallel to the boundary and tends to point towards the neutral point in the Northern hemisphere.     

“We can lick gravity,but…”: What trajectory for space in Australia?

In 2008 the Australian Senate Economics Committee held an inquiry into the state of Australia's space science and industry sector. One of the most important problems identified by the committee was that Australia lacked both a space agency and a national space policy in order to formulate a whole-of-government approach to space issues. This article briefly discusses Australia's historical political lacuna in relation to space before considering how space issues fit within broader contemporary policy frameworks at both the domestic and international levels. Using some of the findings from the inquiry, the author then addresses some core issues that the Australian government should consider when constructing a national space policy.     

Mercury’s Weather-Beaten Surface: Understanding Mercury in the Context of Lunar and Asteroidal Space Weathering Studies

Mercury’s regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury’s exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury’s regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury’s regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury’s regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury’s dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury’s relatively featureless visible–near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury’s surface composition.     

Digital system design in the presence of single event upsets

The authors consider the effects of single event upsets (SEUs) on digital systems, and show techniques for designing reliable systems with current levels of SEU protection. Three main systems are discussed: main memory, logic, and cache memory. A design for the main and cache memory subsystems that are SEU protected is also described. With SEU defined in bit days p, and using single error correction, it is shown that for all subsystems considered, an effective upset rate which is proportional to the product of p² and the time between corrections, or scrub time, can be obtained. Data for memory chip size and performance derived from the gallium-arsenide (GaAs) pilot lines funded by the Defense Advanced Research Projects Agency (DARPA) throughout the 1980s are used     

International participation and the space station : A contractor's perspective

This article presents the views of the Boeing company on the benefits and disadvantages of international participation in the space station. The author outlines the company's goals within the project, and makes some recommendations on how to reduce the concerns which arise from international cooperation, in order to maximize the benefits which could be achieved.     

Aerosols and clouds in the upper troposphere–lower stratosphere region detected by GOMOS and ACE: Intercomparison and analysis of the years 2004 and 2005

Satellite-based limb occultation measurements are well suited for the detection and mapping of polar stratospheric clouds (PSCs) and cirrus clouds. PSCs are of fundamental importance for the formation of the Antarctic ozone hole that occurs every year since the early 1980s in Southern Hemisphere spring. Despite progress in the observation, modeling and understanding of PSCs in recent years, there are still important questions which remain to be resolved, e.g. PSC microphysics, composition, formation mechanisms and long-term changes in occurrence. In addition, it has recently become clear that cirrus clouds significantly affect the global energy balance and climate, due to their influence on atmospheric thermal structure.     

Gram-Schmidt回归及在刀具磨损预报中的应用

多元线性回归是一种应用广泛的统计分析方法.在实际应用中,当自变量集合存在严重多重相关性时,普通最小二乘方法就会失效.为解决这一问题,利用Gram-Schmidt 正交变换,提出一种新的多元线性回归建模方法——Gram-Schmidt回归.该方法可实现多元线性回归中的变量筛选,同时也解决了自变量多重相关条件下的有效建模问题.将该方法应用于机械加工过程中刀具磨损的预报分析,有效地进行了变量筛选,并得到了解释性强同时拟合优度也很高的模型结果.      

The Need for a Wholistic Design Approach when using Gallium Arsenide Technology

The emerging Gallium Arsenide digital integrated circuit technology is rapidly becoming well enough established that designers of signal and data processors are planning its incorporation into advanced computational engines of all types. An examination of the characteristics of present and future generation GaAs integrated circuits indicates that they will emphasize moderate on-chip gate counts and high gate speeds. Complete advantage can be taken of GaAs digital technology only if traditional signal processor architectures are completely recast at the memory layout, logic design, arithmetic implementation, and system architecture levels, and if these issues are considered in combination with system, logic board, and chip layout and packaging constraints in a single integrated approach.     

Electrical Charging of Volcanic Plumes

Many explosive terrestrial volcanic eruptions are accompanied by lightning and other atmospheric electrical phenomena. The plumes produced generate large perturbations in the surface atmospheric electric potential gradient and high charge densities have been measured on falling volcanic ash particles. The complex nature of volcanic plumes (which contain gases, solid particles, and liquid drops) provides several possible charging mechanisms. For plumes rich in solid silicate particles, fractoemission (the ejection of ions and atomic particles during fracture events) is probably the dominant source of charge generation. In other plumes, such as those created when lava enters the sea, different mechanisms, such as boiling, may be important. Further charging mechanisms may also subsequently operate, downwind of the vent. Other solar system bodies also show evidence for volcanism, with activity ongoing on Io. Consequently, volcanic electrification under different planetary scenarios (on Venus, Mars, Io, Moon, Enceladus, Tethys, Dione and Triton) is also discussed.     

Partially Ionized Plasmas in Astrophysics

Partially ionized plasmas are found across the Universe in many different astrophysical environments. They constitute an essential ingredient of the solar atmosphere, molecular clouds, planetary ionospheres and protoplanetary disks, among other environments, and display a richness of physical effects which are not present in fully ionized plasmas. This review provides an overview of the physics of partially ionized plasmas, including recent advances in different astrophysical areas in which partial ionization plays a fundamental role. We outline outstanding observational and theoretical questions and discuss possible directions for future progress.