Calculation of secondary-electron escape currents from inclined-spacecraft surfaces in a magnetic field

In low Earth orbit, the geomagnetic field B(vector) is strong enough that secondary electrons emitted from spacecraft surfaces have an average gyroradius much smaller than typical dimensions of large spacecraft. This implies that escape of secondaries will be strongly inhibited on surfaces which are nearly parallel to B(vector), even if a repelling electric field exists outside them. This effect is likely to make an important contribution to the current balance and hence the equilibrium potential of such surfaces, making high voltage charging of them more likely. Numerically calculated escaping secondary electron fluxes are presented for these conditions. For use in numerical spacecraft charging simulations, an analytic curve fit to these results is given which is accurate to within 3% of the emitted current

Numerical simulation of spacecraft charging phenomena

A numerical simulation program is being constructed having the following features: (1) infinite circular cylindrical geometry with angle-dependence, (2) inclusion of incident particles, photoelectrons, secondary electrons, backscattered electrons, any gun emissions, and any internal current pathways including surface conductive layers, (3) quasistatic time-dependent iteration, in which sheath potential changes during particle transit times are ignored, (4) use of approximate, locally-dependent space charge density expressions in solving Poisson's equation for sheath potentials, with use of numerical orbit-following to determine surface currents, and (5) incident particle velocity distributions isotropic or beam-like, or some superposition of these. Rationales for each of these features are discussed

Current collection in a magnetoplasma

The authors present a survey of a very incomplete subject, current collection in a magnetoplasma. The best-developed and simplest theories for current collection are steady-state collisionless theories, and these must be understood before departures from them can be analyzed usefully. Thus, the authors begin with a review of them. The authors include some recent numerical results which indicate that steady-state collisionless Laplace-limit currents remain substantially below the Parker-Murphy (1967) canonical upper bound out to very large electrode potentials, and approach it as a limit only very slowly if at all. Attempts to correct this theory for space-charge effects lead to potential disturbances which extend to infinite distance along the electrode's magnetic shadow, unless collisional effects are also taken into account. However, even a small amount of relative plasma drift motion, such as that involved in a typical rocket experiment, can change this conclusion fundamentally. It is widely believed that time-averaged current collection may be increased by effects of plasma turbulence, and the authors review the available evidence for and against this contention. Steady-state collisionless particle dynamics predicts the existence of a toroidal region of trapped orbits which surrounds the electrode. Light emissions from this region have been photographed, indicating that collisional ionization may also occur there, and this, and/or scattering by collisions or possibly turbulent fluctuations in this region, may also increase current collection by the electrode. The authors also discuss effects on particle motions near the electrode, associated with breakdown of magnetic insulation in the region of large electric fields near it

Prediction of large negative shaded-side spacecraft potentials

A calculation by Knott, for the floating potential of a spherically symmetric synchronous-altitude satellite in eclipse, was adapted to provide simple calculations of upper bounds on negative potentials which may be achieved by electrically isolated shaded surfaces on spacecraft in sunlight. Large (approximately 60 percent) increases in predicted negative shaded-side potentials are obtained. To investigate effective potential barrier or angular momentum selection effects due to the presence of less negative sunlit-side or adjacent surface potentials, these expressions were replaced by the ion random current, which is a lower bound for convex surfaces when such effects become very severe. Further large increases in predicted negative potentials were obtained, amounting to a doubling in some cases

Images d’un meurtre : La Genèse des artistes

Jacopo della Quercia, Lorenzo Ghiberti, le Titien et combien de leurs prédécesseurs aussi bien que de leurs contemporains ont-ils représenté la scène où Caïn tue son frère Abel. Que pouvait signifier, à la Renaissance, la représentation de cet épisode de la Genèse ? Quelles étaient les moyens et les limites de la représentation figurative lorsqu’elle se donnait pour objectif la relation à un récit ? Cet article veut suggérer l’ampleur des réseaux de signification à prendre en compte quand vient le temps d’interpréter les images dites narratives : les usages de la tradition figurative, les nouvelles conceptions de l’art qui émergent et les fonctions qu’on attribue aux oeuvres, la richesse de la pensée humaniste, les attentes des théologiens aussi bien que parfois des aspects politiques.Jacopo della Quercia, Lorenzo Ghiberti, Titian and many more before them have depicted Cain slaying Abel. What could be the meaning of this depiction of the Genesis narrative at Renaissance times ? What were the means and limits of pictorial representation when it had the reference to a narrative as its very goal ? This paper points at the multiple meanings that have to be taken into account when it is time to interpret the so-called narrative images —the ways of figurative tradition, the new emerging concepts in art, the function attributed to works of art, the depth of human imagination, the theologians’ expectations, as well as (sometimes) political considerations

“Comme des bons pères de famille:” Masculinity, Patriarchy, and the Transnational Violences of Slavery in French Colonial North America

French colonial Louisiana has long captured the imaginations of academic and amateur historians alike. However, the histories of French Louisiana and New Orleans have often been analyzed within the canon of American history, overlooking its transnational and transcontinental connections to New France and the French Empire. This paper mobilizes 18th Century court documents from French colonial New Orleans that detail an assault perpetrated by a plantation overseer against enslaved workers. Jacques Charpentier dit le Roy migrated to Louisiana from what is now known as Canada, and was employed as an overseer by plantation owner and Superior Council member Amyault d’Auseville. Charpentier’s violent conduct led to the death of an enslaved man by the name of Brunet, and perpetrated multiple physical and sexual assaults against enslaved women, including Brunet’s wife, Bizao. The d’Auseville vs. Charpentier case not only illustrates the violence of slavery within the French Empire, but also the ways in which class differences were mobilized to entrench racial hierarchies. Above all, the case shows that the institution of slavery was sustained by migrations within the French Empire in North America, and provides concrete evidence of the transnational and transcontinental nature of slaveholding. This paper problematizes historiographical arguments that slavery was ‘less brutal’ in the French Empire by bringing the d’Auseville vs. Charpentier case in conversation with the Codes Noirs and its patriarchal foundations

Arbitrating the Great Writ: Resolving Federal Habeas Corpus Disputes Through Arbitration

Published in cooperation with the American Bar Association Section of Dispute Resolutio

Developmental Spelling in Fourth Grade: An Analysis of What Poor Readers Do

Since Carol Chomsky (1971a, 1971b) and Charles Read (1971) published their pioneer reports on the development of writing behaviors in young children, examinations of the developmental or invented spellings of emergent writers have contributed to changes in emphases in early literacy instruction. Before that time educators seldom advocated writing experiences for children before they learned to read (Adams, 1990). During the past twenty-five years, there have been careful descriptions and analyses of the developmental stages and strategies of young children who experiment with and work through patterns of spelling while discovering written language. As a result of this body of work, more teachers have learned to decipher and assess the development of spellings of preschoolers and primary grade students. The increased ability to understand beginning attempts with print of the youngest writers has no doubt contributed to the encouragement of story writing, journals, and other writing activities from the earliest school years. Fortunately, the increase in opportunities to write also enhances the development of phonemic awareness and word recognition, both of which are predictors of future reading success (Gill, 1992; Juel, Griffith, and Gough, 1986; Perfetti, 1985; Tunmer and Nesdale, 1985)

Oral Participation in Shared Reading and Writing By Limited English Proficient Students in a Multiethnic Class Setting

Meeting the educational needs of students with limited English proficiency is a challenge that is changing and will continue to change the direction of educational programs. There has been a huge influx of limited English proficient (LEP) students across all geographic regions of the United States (United States Department of Education, 1992). Of the 25 largest school districts in the country, 23 have a majority of minority students (Multicultural Education Review Task Force, 1991). The impact of this increase in LEP students has resulted in individual states and school districts examining their resources, priorities, and curricula to meet their needs