The inherent displacive flexibility of the hexacelsian tetrahedral framework and its relationship to polymorphism in Ba-hexacelsian

 The results of a rigid unit mode (RUM) analysis of the inherent displacive structural flexibility of the tetrahedral framework of the ideal hexacelsian structure type are presented. One of the three types of RUM found to exist is characterized by modulation wave vectors perpendicular to <110> and atomic displacement patterns involving tetrahedral rotation around the parent c axis while a second type of RUM is found to be soft at any modulation wave vector and to involve tetrahedral rotation about in-plane rotation axes. It is shown how a combination of these two types of RUM motion associated with the same q=1/2<1ˉ101>^* modulation wave vector enables the outstanding crystal chemical problems and apparently mutually contradictory results as regards polymorphism in Ba-hexacelsian to be resolved. Received: 30 December 1999 / Accepted: 16 May 2000     

Wonders of Nature in each Quarter of the World: a nineteenth century geological game

Wonders of Nature in each Quarter of the World was an early nineteenth century educational board game designed to teach children about some of the natural wonders of the world, such as volcanoes. The game was produced at a time of advances in geological thinking and geographical expeditions and this study places such changes and events within the context of the pastime, and presents an interesting window on the way geology was perceived almost two centuries ago.     

The compressibility and high pressure structure of diopside from first principles simulation

The structure of diopside (CaMgSi₂O₆) has been calculated at pressures between 0 and 25 GPa using the planewaves and pseudopotentials approach to density functional theory. After applying a pressure correction of 4.66 GPa to allow for the under-binding usually associated with the generalized gradient approximation, cell parameters are in good agreement with experiment. Fitting to the third-order Birch–Murnaghan equation of state yields values of 122 GPa and 4.7 for the bulk modulus and its pressure derivative. In addition to cell parameters, our calculations provide all atomic positional parameters to pressures considerably beyond those currently available from experiment. We have analyzed these data in terms of polyhedral rigidity and regularity and find that the most compressible Ca polyhedron becomes markedly less anisotropic above 10 GPa. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Retreat dynamics of the eastern sector of the British–Irish Ice Sheet during the last glaciation

The findings of BRITICE-CHRONO Transect 2 through the North Sea Basin and eastern England are reported. We define ice-sheet marginal oscillation between ~31 and 16 ka, with seven distinctive former ice-sheet limits (L1–7) constrained by Bayesian statistical analysis. The southernmost limit of the North Sea Lobe is recorded by the Bolders Bank Formation (L1; 25.8–24.6 ka). L2 represents ice-sheet oscillation and early retreat to the northern edge of the Dogger Bank (23.5–22.2 ka), with the Garret Hill Moraine in north Norfolk recording a significant regional readvance to L3 at 21.5–20.8 ka. Ice-marginal oscillations at ~26–21 ka resulted in L1, L2 and L3 being partially to totally overprinted. Ice-dammed lakes related to L1–3, including Lake Humber, are dated at 24.1–22.3 ka. Ice-sheet oscillation and retreat from L4 to L5 occurred between 19.7 and 17.3 ka, with grounding zone wedges marking an important transition from terrestrial to marine tidewater conditions, triggered by the opening of the Dogger Lake spillway between 19.9 and 17.5 ka. L6 relates to ice retreat under glacimarine conditions and final ice retreat into the Firth of Forth by 15.8 ka. L7 (~15 ka) represents an ice retreat from Bosies Bank into the Moray Firth.     

Coronal Cavity Survey: Morphological Clues to Eruptive Magnetic Topologies

We present a survey on coronal prominence cavities conducted using 19 months of data from the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO) satellite. Coronal cavities are elliptical regions of rarefied density lying above and around prominences. They can be long-lived (weeks to months) but are often observed to eventually erupt as part of a coronal mass ejection (CME). We determine morphological properties of the cavities both by qualitatively assessing their shape, and quantitatively fitting them with ellipses. We demonstrate consistency between these two approaches, and find that fitted ellipses are taller than they are wide for almost all cavities studied, in agreement with an earlier analysis of white-light cavities. We examine correlations between cavity shape, aspect ratio, and propensity for eruption. We find that cavities with a teardrop-shaped morphology are more likely to erupt, and we discuss the implications of this morphology for magnetic topologies associated with CME models. We provide the full details of the survey for broad scientific use as supplemental material.     

Relativistic electrons in solar particle events

We present here the results of the first systematic study of electrons of energies greater than 10 MeV associated with solar flares. We have made direct measurements of the frequency with which these particles are found in solar flare related events, the spectra of the particles, and the evolution of the spectra during these events. In addition the nature of the propagation of these electrons is studied and the degree of anisotropy in their diffusion is measured for the first time.The observations were made aboard the spacecraft OGO-5 from 1968, March through 1969, August. It is found that electrons in the energy range 12–45 MeV are normally present in major solar particle events. The time-intensity profiles of the fluxes indicate diffusive propagation; and the time-to-maximum intensity is found to vary with solar longitude in a way which can only be the result of anisotropic propagation with the perpendicular diffusion coefficient comparable in magnitude to, but smaller than, the parallel diffusion coefficient. Spectra during the observed events fit the power law AE ^– with 2.5 3.8. The time evolution of the spectrum throughout the course of the 4 most intense events shows that the spectrum steepens rapidly during the initial phase but retains a constant slope through the decay phase.Events which behave in a manner which is not described by the normal diffusion picture are also discussed. These examples show phenomena other than direct flare injection followed by anisotropic diffusion.This work was supported in part by the National Aeronautics and Space Administration under Contract NAS 5-9096 and Grant NGL 14-001-005.Submitted to the Department of Physics, University of Chicago, Chicago, Illinois in partial fulfillment of the requirements for the Ph. D. degree.NASA Trainee 1967–1969.     

X-Ray bursts from solar flares behind the limb

From the UCSD OSO-7 X-ray experiment data, we have identified 54 X-ray bursts with 5.1–6.6 keV flux greater than 10³ photon cm^?2 keV^?1 which were not accompanied by visible Hα flare on the solar disk. By studying OSO-5 X-ray spectroheliograms, Hα activity at the limb and the emergence and disappearance of sunspot groups at the limb, we found 17 active centers as likely seats of the X-ray bursts beyond the limb. We present the analysis of 37 X-ray bursts and their physical parameters. We compare our results with those published by Datlowe et al. (1974a, b) for disk events. The distributions of maximum temperature, maximum emission measure, and characteristic cooling time of the over-the-limb events do not significantly differ from those of disk events. We show that of conduction and radiation, the former is the dominant cooling mechanism for the hot flare plasma. Since the disk and over-the-limb bursts are similar, we conclude that the scale height for X-ray emission in the 5–10 keV range is large and is consistent with that of Catalano and Van Allen (1973), 11000 km, for primarily 1–3 keV emission. Twenty-five or about 2/3 of the over-the-limb events had a non-thermal component. The distribution of peak 20 keV flux is not significantly different from that of disk events. However, the spectral index at the time of maximum flux is significantly different for events over the limb and for events near the center of the disk; the spectral index for over-the-limb events is larger by about δγ = 3/4. If hard X-ray emission came only from localized sources low in the chromosphere we would expect that hard X-ray emission, would be occulted over the limb; on the contrary, the observation show that the fraction of soft X-ray bursts which have a nonthermal component is the same on and off of the disk. Thus hard X-ray emission over extended regions is indicated.     

Heating and cooling of the thermal X-ray plasma in solar flares

Characteristic times for heating and cooling of the thermal X-ray plasma in solar flares are estimated from the time profile of the thermal X-ray burst and from the temperature, emission measure and over-all length scale of the flare-heated plasma at thermal X-ray maximum. The heating is assumed to be due to magnetic field reconnection, and the cooling is assumed to be due to heat conduction and radiation. Temperatures and emission measures derived from UCSD OSO-7 X-ray flare observations are used, and length scales are obtained from Big Bear large-scale Hα filtergrams for 17 small (subflare to Class 1) flares. The empirical values obtained for the characteristic times imply (1) that flares are produced by magnetic field reconnection, (2) that conduction cooling of the thermal X-ray plasma dominates radiative cooling and (3) that reconnection heating and conduction cooling of the thermal X-ray plasma are approximately in balance at thermal X-ray maximum. This model in combination with the data gives estimates for the electron number density (10¹⁰–10¹¹ cm^?3) and the magnetic field strength (10–100 G) in the thermal X-ray plasma and for the total thermal energy generated in a subflare (≈ 10³⁰ erg for an Hα area ≈ 1 square degree) which agree with previous observational and theoretical estimates obtained by others.