2,331 research outputs found
Mathematic & mathematics education: searching for common ground, edited by M. Fried and T. Dreyfus, New York, Springer, 2014, 402 pp., £90, ISBN 978-94-007-7472-8
This is an Accepted Manuscript of an article published by Taylor & Francis in Research in Mathematics Education on 22nd Aug 2014, available online: http://dx.doi.org/10.1080/14794802.2014.93735
Covariant Symmetry Classifications for Observables of Cosmological Birefringence
Polarizations of electromagnetic waves from distant galaxies are known to be
correlated with the source orientations. These quantities have been used to
search for signals of cosmological birefringence. We review and classify
transformation properties of the polarization and source orientation
observables. The classifications give a firm foundation to certain practices
which have sprung up informally in the literature. Transformations under parity
play a central role, showing that parity violation in emission or in the
subsequent propagation is an observable phenomenon. We also discuss statistical
measures, correlations and distributions which transform properly and which can
be used for systematic data analysis.Comment: 8 pages, revtex, 1 postscript figur
The Greisen Equation Explained and Improved
Analytic description of the evolution of cosmic ray showers is dominated by
the Greisen equation nearly five decades old. We present an alternative
approach with several advantages. Among the new features are a prediction of
the differential distribution, replacing Greisen's form which fails to be
positive definite. Explicit comparison with Monte Carlo simulations shows
excellent agreement after a few radiation lengths of development. We find a
clear connection between Monte Carlo adjustment of Greisen's form and
underlying physics, and present a concise derivation with all steps explicit.
We also reconstruct the steps needed to reproduce Greisen's approximate
formula, which appears not to have been published previously.Comment: 8 pages, 7 figures, revised version, accepted for publication in
Phys. Rev.
The Virgo Alignment Puzzle in Propagation of Radiation on Cosmological Scales
We reconsider analysis of data on the cosmic microwave background on the
largest angular scales. Temperature multipoles of any order factor naturally
into a direct product of axial quantities and cosets. Striking coincidences
exist among the axes associated with the dipole, quadrupole, and octupole CMB
moments. These axes also coincide well with two other axes independently
determined from polarizations at radio and optical frequencies propagating on
cosmological scales. The five coincident axes indicate physical correlation and
anisotropic properties of the cosmic medium not predicted by the conventional
Big Bang scenario. We consider various mechanisms, including foreground
corrections, as candidates for the observed correlations. We also consider
whether the propagation anomalies may be a signal of ``dark energy'' in the
form of a condensed background field. Perhaps {\it light propagation} will
prove to be an effective way to look for the effects of {\it dark energy}.Comment: 24 pages, 4 figures, minor changes, no change in result or
conclusions. to appear in IJMP
Knudsen gas provides nanobubble stability
We provide a model for the remarkable stability of surface nanobubbles to
bulk dissolution. The key to the solution is that the gas in a nanobubble is of
Knudsen type. This leads to the generation of a bulk liquid flow which
effectively forces the diffusive gas to remain local. Our model predicts the
presence of a vertical water jet immediately above a nanobubble, with an
estimated speed of , in good agreement with our
experimental atomic force microscopy measurement of . In
addition, our model also predicts an upper bound for the size of nanobubbles,
which is consistent with the available experimental data
Evidence for Adsorption of Chlorine Species on Iron(III) (hydr)oxides in the Sheepbed Mudstone, Gale Crater, Mars
Chlorine is a widespread element on Mars present in dust, soils and rocks, including the Sheepbed mudstone at Yellowknife Bay, Gale crater. Combined elemental and volatile analyses of two drilled samples, Cumberland and John Klein, indicated that chloride (Cl-) and perchlorate (ClO4 -) are likely present in the mudstone. The nature of chlorine species in Sheepbed mudstone is still not well constrained. It has been proposed that both are present as amorphous or crystalline salts physically mixed with mudstone minerals. We alternatively hypothesize that adsorbed perchlorate and chloride exist in the mudstone and adsorption could occur, in particular, on Fe(III) (hydr)oxide phases as supported by laboratory observations on terrestrial materials. Mineralogical and compositional analyses of the drilled Cumberland mudstone sample revealed the presence of ~30 wt% of a Fe-rich X-ray amorphous phase. Ferrihydrite has been proposed as a component of the Fe-rich X-ray amorphous material. The objectives of this work were to determine adsorption of perchlorate and chloride on ferrihydrite and to enable data comparison by characterizing adsorbed chloride and perchlorate with thermal and evolved gas analysis run under operating conditions similar to the SAM instrument onboard the Curiosity rover
Shear and turbulence production across subtidal channels
Author Posting. © Sears Foundation for Marine Research, 2006. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 64 (2006): 147-171, doi:10.1357/002224006776412359.In intertidal regions with subtidal channels, effects of bathymetry on overlying flow vary greatly with tidal stage. Around low water when mudflats and marsh are exposed, flow is constrained to channels, but when water depths are greater, tidal forcing may not necessarily be aligned with meandering channel axes. Flow across the channel can generate strong shear and turbulence at the elevation of the channel banks and can significantly increase turbulent energy in the middle of the water column. Field observations in a mudflat channel of San Francisco Bay indicate that cross-channel shear regularly occurs there early in ebb tides. With increased freshwater flow, baroclinic forcing can enhance shear by decoupling flow between dense water flooding in the channel and fresher water ebbing above the channel banks. A water column numerical model with κ-ε turbulence closure is modified to represent the cross-channel shear production. Numerical results with uniform density indicate that turbulence production increases with the angle between the barotropic tidal forcing and the channel axis. When a longitudinal salinity gradient is imposed, cross-channel shear production contributes to breakdown of periodic stratification. Turbulence produced at the channel banks locally exceeds dissipation, and the excess energy is either lost to buoyancy or diffuses vertically to lower energy regions near the surface and near the bed. The balance among shear production, buoyancy production, and diffusion of turbulence depends on the flow angle and the strength of the longitudinal salinity gradient.This research was funded by National
Institutes of Health grant no. P42ES0475 from the National Institute of Environmental Health
Sciences
Tidal and meteorological forcing of sediment transport in tributary mudflat channels
Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 27 (2007): 1510-1527, doi:10.1016/j.csr.2007.01.010.Field observations of flow and sediment transport in a tributary channel through intertidal
mudflats indicate that suspended sediment was closely linked to advection and dispersion
of a tidal salinity front. During calm weather when tidal forcing was dominant, high
concentrations of suspended sediment advected up the mudflat channel in the narrow
region between salty water from San Francisco Bay and much fresher runoff from the
small local watershed. Salinity and suspended sediment dispersed at similar rates
through each tidal inundation, such that during receding ebbs the sediment pulse had
spread spatially and maximum concentrations had decreased. Net sediment transport was
moderately onshore during the calm weather, as asymmetries in stratification due to tidal
straining of the salinity front enhanced deposition, particularly during weaker neap tidal
forcing. Sediment transport by tidal forcing was periodically altered by winter storms.
During storms, strong winds from the south generated wind waves and temporarily
increased suspended sediment concentrations. Increased discharge down the tributary
channels due to precipitation had more lasting impact on sediment transport, supplying
both buoyancy and fine sediment to the system. Net sediment transport depended on the
balance between calm weather tidal forcing and perturbations by episodic storms. Net
transport in the tributary channel was generally off-shore during storms and during calm
weather spring tides, and on-shore during calm weather neap tides.The research was funded by
National Institutes of Health grant P42ES0475 from the National Institute of Environmental Health
Sciences.
Reference
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