1,139 research outputs found
Helical coronal ejections and their role in the solar cycle
The standard theory of the solar cycle in terms of an alpha-Omega dynamo
hinges on a proper understanding of the nonlinear alpha effect. Boundary
conditions play a surprisingly important role in determining the magnitude of
alpha. For closed boundaries, the total magnetic helicity is conserved, and
since the alpha effect produces magnetic helicity of one sign in the large
scale field, it must simultaneously produce magnetic helicity of the opposite
sign. It is this secondary magnetic helicity that suppresses the dynamo in a
potentially catastrophic fashion. Open boundaries allow magnetic helicity to be
lost. Simulations are presented that allow an estimate of alpha in the presence
of open or closed boundaries, either with or without solar-like differential
rotation. In all cases the sign of the magnetic helicity agrees with that
observed at the solar surface (negative in the north, positive in the south),
where significant amounts of magnetic helicity can be ejected via coronal mass
ejections. It is shown that open boundaries tend to alleviate catastrophic
alpha quenching. The importance of looking at current helicity instead of
magnetic helicity is emphasized and the conceptual advantages are discussed.Comment: 8 pages, 7 figs, IAU Symp. 223, In: Multi-Wavelength Investigations
of Solar Activity. Eds: A.V. Stepanov, E.E. Benevolenskaya & A.G. Kosoviche
The initial singularity of ultrastiff perfect fluid spacetimes without symmetries
We consider the Einstein equations coupled to an ultrastiff perfect fluid and
prove the existence of a family of solutions with an initial singularity whose
structure is that of explicit isotropic models. This family of solutions is
`generic' in the sense that it depends on as many free functions as a general
solution, i.e., without imposing any symmetry assumptions, of the
Einstein-Euler equations. The method we use is a that of a Fuchsian reduction.Comment: 16 pages, journal versio
Perfect fluids and generic spacelike singularities
We present the conformally 1+3 Hubble-normalized field equations together
with the general total source equations, and then specialize to a source that
consists of perfect fluids with general barotropic equations of state.
Motivating, formulating, and assuming certain conjectures, we derive results
about how the properties of fluids (equations of state, momenta, angular
momenta) and generic spacelike singularities affect each other.Comment: Considerable changes have been made in presentation and arguments,
resulting in sharper conclusion
p3d: a general data-reduction tool for fiber-fed integral-field spectrographs
The reduction of integral-field spectrograph (IFS) data is demanding work.
Many repetitive operations are required in order to convert raw data into,
typically a large number of, spectra. This effort can be markedly simplified
through the use of a tool or pipeline, which is designed to complete many of
the repetitive operations without human interaction. Here we present our
semi-automatic data-reduction tool p3d that is designed to be used with
fiber-fed IFSs. Important components of p3d include a novel algorithm for
automatic finding and tracing of spectra on the detector, and two methods of
optimal spectrum extraction in addition to standard aperture extraction. p3d
also provides tools to combine several images, perform wavelength calibration
and flat field data. p3d is at the moment configured for four IFSs. In order to
evaluate its performance we have tested the different components of the tool.
For these tests we used both simulated and observational data. We demonstrate
that for three of the IFSs a correction for so-called cross-talk due to
overlapping spectra on the detector is required. Without such a correction
spectra will be inaccurate, in particular if there is a significant intensity
gradient across the object. Our tests showed that p3d is able to produce
accurate results. p3d is a highly general and freely available tool. It is
easily extended to include improved algorithms, new visualization tools and
support for additional instruments. The program code can be downloaded from the
p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&
1-2-3-flavor color superconductivity in compact stars
We suggest a scenario where the three light quark flavors are sequentially
deconfined under increasing pressure in cold asymmetric nuclear matter, e.g.,
as in neutron stars. The basis for our analysis is a chiral quark matter model
of Nambu--Jona-Lasinio (NJL) type with diquark pairing in the spin-1 single
flavor (CSL) and spin-0 two/three flavor (2SC/CFL) channels, and a
Dirac-Brueckner Hartree-Fock (DBHF) approach in the nuclear matter sector. We
find that nucleon dissociation sets in at about the saturation density, n_0,
when the down-quark Fermi sea is populated (d-quark dripline) due to the flavor
asymmetry imposed by beta-equilibrium and charge neutrality. At about 3n_0
u-quarks appear forming a two-flavor color superconducting (2SC) phase, while
the s-quark Fermi sea is populated only at still higher baryon density. The
hybrid star sequence has a maximum mass of 2.1 M_sun. Two- and three-flavor
quark matter phases are found only in gravitationally unstable hybrid star
solutions.Comment: 4 pages, 2 figures, to appear in the proceedings of Quark Matter
2008: 20th International Conference on Ultra-Relativistic Nucleus Nucleus
Collisions (QM 2008), Jaipur, India, 4-10 Feb 200
Tilted two-fluid Bianchi type I models
In this paper we investigate expanding Bianchi type I models with two tilted
fluids with the same linear equation of state, characterized by the equation of
state parameter w. Individually the fluids have non-zero energy fluxes w.r.t.
the symmetry surfaces, but these cancel each other because of the Codazzi
constraint. We prove that when w=0 the model isotropizes to the future. Using
numerical simulations and a linear analysis we also find the asymptotic states
of models with w>0. We find that future isotropization occurs if and only if . The results are compared to similar models investigated previously
where the two fluids have different equation of state parameters.Comment: 14 pages, 3 figure
Unexpected Structures for Intercalation of Sodium in Epitaxial Graphene-SiC Interfaces
We show using scanning tunneling microscopy, spectroscopy, and ab initio
calculations that several intercalation structures exist for Na in epitaxial
graphene on SiC(0001). Intercalation takes place at room temperature and Na
electron-dopes the graphene. It intercalates in-between single-layer graphene
and the carbon-rich interfacial layer. It also penetrates beneath the
interfacial layer and decouples it to form a second graphene layer. This
decoupling is accelerated by annealing and is verified by direct Na deposition
onto the interface layer. Our observations show that intercalation in graphene
is fundamentally different than in graphite and is a versatile means of
electronic control.Comment: 10 pages text, 2 pages, references, and 4 figure page
On the Epistemology of the Precautionary Principle: Reply to Steglich-Petersen
In a recent paper in this journal, we proposed two novel puzzles associated with the precautionary principle. Both are puzzles that materialise, we argue, once we investigate the principle through an epistemological lens, and each constitutes a philosophical hurdle for any proponent of a plausible version of the precautionary principle. Steglich-Petersen claims, also in this journal, that he has resolved our puzzles. In this short note, we explain why we remain skeptica
Ecophysiology of coral reef primary producers across an upwelling gradient in the tropical central Pacific
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Johnson, M. D., Fox, M. D., Kelly, E. L. A., Zgliczynski, B. J., Sandin, S. A., & Smith, J. E. Ecophysiology of coral reef primary producers across an upwelling gradient in the tropical central Pacific. Plos One, 15(2), (2020): e0228448, doi:10.1371/journal.pone.0228448.Upwelling is an important source of inorganic nutrients in marine systems, yet little is known about how gradients in upwelling affect primary producers on coral reefs. The Southern Line Islands span a natural gradient of inorganic nutrient concentrations across the equatorial upwelling region in the central Pacific. We used this gradient to test the hypothesis that benthic autotroph ecophysiology is enhanced on nutrient-enriched reefs. We measured metabolism and photophysiology of common benthic taxa, including the algae Porolithon, Avrainvillea, and Halimeda, and the corals Pocillopora and Montipora. We found that temperature (27.2–28.7°C) was inversely related to dissolved inorganic nitrogen (0.46–4.63 μM) and surface chlorophyll a concentrations (0.108–0.147 mg m-3), which increased near the equator. Contrary to our prediction, ecophysiology did not consistently track these patterns in all taxa. Though metabolic rates were generally variable, Porolithon and Avrainvillea photosynthesis was highest at the most productive and equatorial island (northernmost). Porolithon photosynthetic rates also generally increased with proximity to the equator. Photophysiology (maximum quantum yield) increased near the equator and was highest at northern islands in all taxa. Photosynthetic pigments also were variable, but chlorophyll a and carotenoids in Avrainvillea and Montipora were highest at the northern islands. Phycobilin pigments of Porolithon responded most consistently across the upwelling gradient, with higher phycoerythrin concentrations closer to the equator. Our findings demonstrate that the effects of in situ nutrient enrichment on benthic autotrophs may be more complex than laboratory experiments indicate. While upwelling is an important feature in some reef ecosystems, ancillary factors may regulate the associated consequences of nutrient enrichment on benthic reef organisms.This work was supported by funding from the Moore Family Foundation, the Gordon and Betty Moore Foundation, the Scripps family, and anonymous donors. The funders had no role in study design, data collection and analysis, or preparation of the manuscript
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