425 research outputs found
An exploration of the effectiveness of artificial mini-magnetospheres as a potential solar storm shelter for long term human space missions
If mankind is to explore the solar system beyond the confines of our Earth and Moon the problem of radiation protection must be addressed. Galactic cosmic rays and highly variable energetic solar particles are an ever-present hazard in interplanetary space.
Electric and/or magnetic fields have been suggested as deflection shields in the past, but these treated space as an empty vacuum. In fact it is not empty. Space contains a plasma known as the solar wind; a constant flow of protons and electrons coming from the Sun.
In this paper we explore the effectiveness of a âmini-magnetosphereâ acting as a radiation protection shield. We explicitly include the plasma physics necessary to account for the solar wind and its induced effects. We show that, by capturing/containing this plasma, we enhance the effectiveness of the shield. Further evidence to support our conclusions can be obtained from studying naturally occurring âmini-magnetospheresâ on the Moon. These magnetic anomalies (related to âlunar swirlsâ) exhibit many of the effects seen in laboratory experiments and computer simulations. If shown to be feasible, this technology could become the gateway to manned exploration of interplanetary space
Translational groups as generators of gauge transformations
We examine the gauge generating nature of the translational subgroup of
Wigner's little group for the case of massless tensor gauge theories and show
that the gauge transformations generated by the translational group is only a
subset of the complete set of gauge transformations. We also show that, just
like the case of topologically massive gauge theories, translational groups act
as generators of gauge transformations in gauge theories obtained by extending
massive gauge noninvariant theories by a Stuckelberg mechanism. The
representations of the translational groups that generate gauge transformations
in such Stuckelberg extended theories can be obtained by the method of
dimensional descent. We illustrate these with the examples of Stuckelberg
extended first class versions of Proca, Einstein-Pauli-Fierz and massive
Kalb-Ramond theories in 3+1 dimensions. A detailed analysis of the partial
gauge generation in massive and massless 2nd rank symmetric gauge theories is
provided. The gauge transformations generated by translational group in 2-form
gauge theories are shown to explicitly manifest the reducibility of gauge
transformations in these theories.Comment: Latex, 20 pages, no figures, Version to appear in Physical Review
Compaction of Hyaloclastite from the Active Geothermal System at Krafla Volcano, Iceland
Hyaloclastites commonly form high-quality reservoir rocks in volcanic geothermal provinces. Here, we investigated the effects of confinement due to burial following prolonged accumulation of eruptive products on the physical and mechanical evolution of surficial and subsurface (depths of 70âm, 556âm, and 732âm) hyaloclastites from Krafla volcano, Iceland. Upon loading in a hydrostatic cell, the porosity and permeability of the surficial hyaloclastite decreased linearly with mean effective stress, as pores and cracks closed due to elastic (recoverable) compaction up to 22-24âMPa (equivalent to ~1.3âkm depth in the reservoir). Beyond this mean effective stress, denoted as Pâ, we observed accelerated porosity and permeability reduction with increasing confinement, as the rock underwent permanent inelastic compaction. In comparison, the porosity and permeability of the subsurface core samples were less sensitive to mean effective stress, decreasing linearly with increasing confinement as the samples compacted elastically within the conditions tested (to 40âMPa). Although the surficial material underwent permanent, destructive compaction, it maintained higher porosity and permeability than the subsurface hyaloclastites throughout the experiments. We constrained the evolution of yield curves of the hyaloclastites, subjected to different effective mean stresses in a triaxial press. Surficial hyaloclastites underwent a brittle-ductile transition at an effective mean stress of ~10.5âMPa, and peak strength (differential stress) reached 13âMPa. When loaded to effective mean stresses of 33 and 40âMPa, the rocks compacted, producing new yield curves with a brittle-ductile transition at ~12.5 and ~19âMPa, respectively, but showed limited strength increase. In comparison, the subsurface samples were found to be much stronger, displaying higher strengths and brittle-ductile transitions at higher effective mean stresses (i.e., 37.5âMPa for 70âm sample, >75âMPa for 556âm, and 68.5âMPa for 732âm) that correspond to their lower porosities and permeabilities. Thus, we conclude that compaction upon burial alone is insufficient to explain the physical and mechanical properties of the subsurface hyaloclastites present in the reservoir at Krafla volcano. Mineralogical alteration, quantified using SEM-EDS, is invoked to explain the further reduction of porosity and increase in strength of the hyaloclastite in the active geothermal system at Krafla
Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study
A recent experiment by Shimizu et al. has provided evidence of a
superconducting phase in hcp Fe under pressure. To study the
pressure-dependence of this superconducting phase we have calculated the phonon
frequencies and the electron-phonon coupling in hcp Fe as a function of the
lattice parameter, using the linear response (LR) scheme and the full potential
linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the
Eliashberg functions indicate that conventional s-wave
electron-phonon coupling can definitely account for the appearance of the
superconducting phase in hcp Fe. However, the observed change in the transition
temperature with increasing pressure is far too rapid compared with the
calculated results. For comparison with the linear response results, we have
computed the electron-phonon coupling also by using the rigid muffin-tin (RMT)
approximation. From both the LR and the RMT results it appears that
electron-phonon interaction alone cannot explain the small range of volume over
which superconductivity is observed. It is shown that
ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from
magnetic impurities (spin-ordered clusters) can account for the observed values
of the transition temperatures but cannot substantially improve the agreeemnt
between the calculated and observed presure/volume range of the superconducting
phase. A simplified treatment of p-wave pairing leads to extremely small ( K) transition temperatures. Thus our calculations seem to rule out
both - and - wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR
Angular momenta creation in relativistic electron-positron plasma
Creation of angular momentum in a relativistic electron-positron plasma is
explored. It is shown that a chain of angular momentum carrying vortices is a
robust asymptotic state sustained by the generalized nonlinear Schrodinger
equation characteristic to the system. The results may suggest a possible
electromagnetic origin of angular momenta when it is applied to the MeV epoch
of the early Universe.Comment: 20 pages, 6 figure
Topological Defects, Orientational Order, and Depinning of the Electron Solid in a Random Potential
We report on the results of molecular dynamics simulation (MD) studies of the
classical two-dimensional electron crystal in the presence disorder. Our study
is motivated by recent experiments on this system in modulation doped
semiconductor systems in very strong magnetic fields, where the magnetic length
is much smaller than the average interelectron spacing , as well as by
recent studies of electrons on the surface of helium. We investigate the low
temperature state of this system using a simulated annealing method. We find
that the low temperature state of the system always has isolated dislocations,
even at the weakest disorder levels investigated. We also find evidence for a
transition from a hexatic glass to an isotropic glass as the disorder is
increased. The former is characterized by quasi-long range orientational order,
and the absence of disclination defects in the low temperature state, and the
latter by short range orientational order and the presence of these defects.
The threshold electric field is also studied as a function of the disorder
strength, and is shown to have a characteristic signature of the transition.
Finally, the qualitative behavior of the electron flow in the depinned state is
shown to change continuously from an elastic flow to a channel-like, plastic
flow as the disorder strength is increased.Comment: 31 pages, RevTex 3.0, 15 figures upon request, accepted for
publication in Phys. Rev. B., HAF94MD
Regulation and Function of the Interleukin 13 Receptor α 2 During a T Helper Cell Type 2âdominant Immune Response
Highly polarized type 2 cytokine responses can be harmful and even lethal to the host if they are too vigorous or persist too long. Therefore, it is important to elucidate the mechanisms that down-regulate these reactions. Interleukin (IL)-13 has emerged as a central mediator of T helper cell (Th)2-dominant immune responses, exhibiting a diverse array of functional activities including regulation of airway hyperreactivity, resistance to nematode parasites, and tissue remodeling and fibrosis. Here, we show that IL-13 receptor (R)α2 is a critical down-regulatory factor of IL-13âmediated tissue fibrosis induced by the parasitic helminth Schistosoma mansoni. IL-13Rα2 expression was induced after the onset of the fibrotic response, IL-10, IL-13, and Stat6 dependent, and inhibited by the Th1-inducing adjuvant IL-12. Strikingly, schistosome-infected C57BL/6 and BALB/c IL-13Rα2âdeficient mice showed a marked exacerbation in hepatic fibrosis, despite displaying no change in granuloma size, tissue eosinophilia, or mastocytosis. Fibrosis increased despite the fact that IL-13 levels decreased significantly in the liver and serum. Importantly, pathology was prevented when IL-13Rα2âdeficient mice were treated with a soluble IL-13Rα2-Fc construct, formally demonstrating that their exacerbated fibrotic response was due to heightened IL-13 activity. Together, these studies illustrate the central role played by the IL-13Rα2 in the down-regulation of a chronic and pathogenic Th2-mediated immune response
Volume element structure and roton-maxon-phonon excitations in superfluid helium beyond the Gross-Pitaevskii approximation
We propose a theory which deals with the structure and interactions of volume
elements in liquid helium II. The approach consists of two nested models linked
via parametric space. The short-wavelength part describes the interior
structure of the fluid element using a non-perturbative approach based on the
logarithmic wave equation; it suggests the Gaussian-like behaviour of the
element's interior density and interparticle interaction potential. The
long-wavelength part is the quantum many-body theory of such elements which
deals with their dynamics and interactions. Our approach leads to a unified
description of the phonon, maxon and roton excitations, and has noteworthy
agreement with experiment: with one essential parameter to fit we reproduce at
high accuracy not only the roton minimum but also the neighboring local maximum
as well as the sound velocity and structure factor.Comment: 9 pages, 6 figure
An international review of laser Doppler vibrometry:Making light work of vibration measurement
© 2016 In 1964, just a few years after the invention of the laser, a fluid velocity measurement based on the frequency shift of scattered light was made and the laser Doppler technique was born. This comprehensive review paper charts advances in the development and applications of laser Doppler vibrometry (LDV) since those first pioneering experiments. Consideration is first given to the challenges that continue to be posed by laser speckle. Scanning LDV is introduced and its significant influence in the field of experimental modal analysis described. Applications in structural health monitoring and MEMS serve to demonstrate LDV's applicability on structures of all sizes. Rotor vibrations and hearing are explored as examples of the classic applications. Applications in acoustics recognise the versatility of LDV as demonstrated by visualisation of sound fields. The paper concludes with thoughts on future developments, using examples of new multi-component and multi-channel instruments
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues â a realisation of the Tycho-Gaia Astrometric Solution (TGAS) â and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of âŒ3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yrâ1 for the proper motions. A systematic component of âŒ0.3 mas should be added to the parallax uncertainties. For the subset of âŒ94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yrâ1. For the secondary astrometric data set, the typical uncertainty of the positions is âŒ10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to âŒ0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
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