Magnetic fields and the structure of the solar corona

During the eclipse of 12 November 1966, the solar corona was photographed at an effective wavelength of 6500 Å with an f/16, 11.1 cm aperture camera. Reduction of the observations yields coronal radiances and polarizations from 1.4 to 3.5 solar radii. Standard techniques are used for the separation of F and K-coronas, determination of coronal electron densities and temperatures, and estimation of the orientation of the major streamers in space.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Confidence bounds on risk assessments for underground nuclear waste repositories

The estimation of risk confidence bounds is an important element of a comprehensive probabilistic risk assessment for a radioactive waste repository. Normal distribution bounds may be used in the asymptotic limit of a very large number of Monte Carlo simulations, but sharp skewness of the risk distribution may severely retard the convergence process. The Tchebycheff bounds are parameter-free and may be applied regardless of distribution, save for the finiteness of variance. These bounds may be generally applicable, but they are invariably very broad. Better parameter-free bounds for mean risk are presented here, based on an inequality originally derived by Guttman.     

Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En₇₆Fs₂₁ Wo₃～En₅₈Fs₃₀Wo₁₂) than those in lithology A of EETA79001 (En₇₃Fs₂₂Wo₅～En₄₅Fs₄₃Wo₁₂), rather similar to pyroxenes in lherzolitic martian meteorites (En₇₆Fs₂₁ Wo₃～En₆₃Fs₂₂Wo₁₅). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo_76‐58) than EETA79001 olivine (Fo_81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo_74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).     

THE CORREO AND SUWANEE SPRING METEORITES: TWO NEW ORDINARY CHONDRITE FINDS

Two new ordinary chondrites were found about 40 km west of Albuquerque, New Mexico. Correo is an H4 chondrite with distinct chondrules and major olivine (Fo_81.4), orthopyroxene (En_82.3) and plagioclase (An₁₂). Suwanee Spring is an L5 chondrite with few distinct chondrules and a highly recrystallized matrix. Major minerals are olivine (Fo_75.4), orthopyroxene (En_77.7) and plagioclase (An₉). The metallic Ni-Fe phases of both meteorites are typical of slowly-cooled ordinary chondrites.     

A comprehensive model current system for high-latitude magnetic activity - I. The steady state system

Summary. Magnetic variations at high latitudes represent the combination of magnetic perturbations from several different current systems. The relative effects of each of the contributory current systems vary from event to event, resulting in changes in magnetic perturbation patterns in a given local time sector which are a function of several variables, e.g. level of magnetospheric activity, spectrum of precipitating particles responsible for ionospheric conductivity, configuration of interplanetary magnetic field, etc. In this paper we extend the work of Hughes & Rostoker in which the steady state perturbation pattern in each local time sector was established from ground-based meridian line magnetometer data. We shall define the various current systems which contribute to the high-latitude magnetic perturbation pattern and we shall construct the model composite current system which best explains the high-latitude observations in the winter months. We shall discuss changes in this pattern as a function of season and as a function of the configuration of the interplanetary magnetic field. Finally, we shall relate the geometrical configuration of the ionosphere—magnetosphere current flow to the convection of plasma in the magnetosphere.     

Can the Einstein-de Sitter model adequately describe the Universe at any epoch?

The characteristics of Friedmann model universes containing decoupled matter and radiation are investigated to establish the range in redshift over which the frequently assumed Einstein-de Sitter approximation is valid. This approximation is found to have decidedly doubtful value for small, yet entirely plausible, values of the material density parameter.     

Collisional heating by nonthermal electrons in a tapered magnetic loop

We examine the behavior of non-thermal electrons injected into a tapered magnetic loop, under the action of both Coulomb collisional and magnetic field gradient forces. An approximate analytic formula for the heating rate as a function of distance along the loop is developed, and found to be in good agreement with exact numerical solutions of the relevant equations. Such a formula is useful as a source term in many situations, such as hydrodynamic simulations of atmospheric response to flare energy input.Presidential Young Investigator.