Energetic particle influence on the Earth's atmosphere

This manuscript gives an up-to-date and comprehensive overview of the effects of energetic particle precipitation (EPP) onto the whole atmosphere, from the lower thermosphere/mesosphere through the stratosphere and troposphere, to the surface. The paper summarizes the different sources and energies of particles, principally galactic cosmic rays (GCRs), solar energetic particles (SEPs) and energetic electron precipitation (EEP). All the proposed mechanisms by which EPP can affect the atmosphere are discussed, including chemical changes in the upper atmosphere and lower thermosphere, chemistry-dynamics feedbacks, the global electric circuit and cloud formation. The role of energetic particles in Earth’s atmosphere is a multi-disciplinary problem that requires expertise from a range of scientific backgrounds. To assist with this synergy, summary tables are provided, which are intended to evaluate the level of current knowledge of the effects of energetic particles on processes in the entire atmosphere

Magnetic anisotropy in the U/sub x/Th/sub 1-x/Zn/sub 8. 5/ system

We have investigated the source of the anisotropic susceptibility of UZn/sub 8.5/ by preparing single crystals of U/sub x/Th/sub 1-x/Zn/sub 8.5/ for small x. Preliminary indications are that anisotropic exchange, rather than crystal field effects, are responsible for the anisotropy, but possible impurity contributions to the (anisotropic) susceptibility of the ThZn/sub 8.5/ do not permit a stronger statement. Further measurements for small x show that both the electronic specific heat and the susceptibility at low temperature are strongly enhanced, and that Kondo-like resistivity minima are observed. The enhancements are somewhat reduced in the concentrated system UZn/sub 8.5/

The mechanical response of a Uranium-Niobium alloy: A comparison of cast versus wrought processing

A rigorous experimentation and validation program is being undertaken to develop “process aware” constitutive models that elucidate the fundamental mechanisms controlling plasticity in uranium-6 wt.% niobium alloys (U-6Nb). The first alloy is a “wrought” material produced, by processing a cast ingot via forging and rolling into plate. The second material investigated is a direct cast U-6Nb alloy. The purpose of the investigation is to determine the principal differences, or more importantly, similarities, between the two materials due to processing. It is well known that parameters like grain size, impurity size and chemistry affect the deformation and failure characteristics of materials. Metallography conducted on these materials revealed that the microstructures are quite different. Characterization techniques including tension, compression, and shear testing were performed to quantify the principal differences between the materials as a function of stress state. Dynamic characterization using a split Hopkinson pressure bar in conjunction with Taylor impact testing was conducted to derive and thereafter validate constitutive material models. The primary differences between the materials will be described and predictions about material behavior will be made

Kinetic equations for concurrent size and shape coarsening by the ledge mechanism

The kinetic equations describing concurrent size and shape coarsening of plate-and rod-shaped particles having shapes that deviate from equilibrium are presented. In the derivations, the assumption is made that some of the interfaces are fully or partially coherent and migrate by the ledge mechanism. Three different interfacial character combinations are considered. The analysis also assumes a small and constant volume fraction of particles so that the average matrix composition can be estimated from knowledge of the particle size distribution, the surface area available for atomic attachment/detachment, and the diffusion distance. The resultant flux equations are then used in a computer model to predict the coarsening behavior of an ensemble of nonequilibrium-shaped particles. Comparison of these results with those obtained from the traditional coarsening theory of Lifshitz and Slyosov1 and Wagner2 (LSW) show significant discrepancies. These differences are attributed to the invalidity of many assumptions made in the LSW theory when applied to solid:solid coarsening systems. © 1991 The Minerals, Metals and Materials Society, and ASM International