7,708 research outputs found
Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars
Many fully convective stars exhibit a wide variety of surface magnetism,
including starspots and chromospheric activity. The manner by which bundles of
magnetic field traverse portions of the convection zone to emerge at the
stellar surface is not especially well understood. In the Solar context, some
insight into this process has been gleaned by regarding the magnetism as
consisting partly of idealized thin flux tubes (TFT). Here, we present the
results of a large set of TFT simulations in a rotating spherical domain of
convective flows representative of a 0.3 solar-mass, main-sequence star. This
is the first study to investigate how individual flux tubes in such a star
might rise under the combined influence of buoyancy, convection, and
differential rotation. A time-dependent hydrodynamic convective flow field,
taken from separate 3D simulations calculated with the anelastic equations,
impacts the flux tube as it rises. Convective motions modulate the shape of the
initially buoyant flux ring, promoting localized rising loops. Flux tubes in
fully convective stars have a tendency to rise nearly parallel to the rotation
axis. However, the presence of strong differential rotation allows some
initially low latitude flux tubes of moderate strength to develop rising loops
that emerge in the near-equatorial region. Magnetic pumping suppresses the
global rise of the flux tube most efficiently in the deeper interior and at
lower latitudes. The results of these simulations aim to provide a link between
dynamo-generated magnetic fields, fluid motions, and observations of starspots
for fully convective stars.Comment: 20 pages, 15 figures, accepted to Astrophysical Journa
A Theory on the Convective Origins of Active Longitudes on Solar-like Stars
Using a thin flux tube model in a rotating spherical shell of turbulent,
solar-like convective flows, we find that the distribution of emerging flux
tubes in our simulation is inhomogeneous in longitude, with properties similar
to those of active longitudes on the Sun and other solar-like stars. The
large-scale pattern of flux emergence our simulations produce exhibits
preferred longitudinal modes of low order, drift with respect to a fixed
reference system, and alignment across the Equator at low latitudes between 15
degrees. We suggest that these active-longitude-like emergence patterns are the
result of columnar, rotationally aligned giant cells present in our convection
simulation at low latitudes. If giant convecting cells exist in the bulk of the
solar convection zone, this phenomenon, along with differential rotation, could
in part provide an explanation for the behavior of active longitudes.Comment: This paper was accepted to The Astrophysical Journal on May 6, 201
Theoretical limits on magnetic field strengths in low-mass stars
Observations have suggested that some low-mass stars have larger radii than
predicted by 1-D structure models. Some theoretical models have invoked very
strong interior magnetic fields (of order 1 MG or more) as a possible cause of
such large radii. Whether fields of that strength could in principle by
generated by dynamo action in these objects is unclear, and we do not address
the matter directly. Instead, we examine whether such fields could remain in
the interior of a low mass object for a significant time, and whether they
would have any other obvious signatures. First, we estimate timescales for the
loss of strong fields by magnetic buoyancy instabilities. We consider a range
of field strengths and simple morphologies, including both idealized flux tubes
and smooth layers of field. We confirm some of our analytical estimates using
thin flux tube magnetohydrodynamic (MHD) simulations of the rise of buoyant
fields in a fully-convective M-dwarf. Separately, we consider the Ohmic
dissipation of such fields. We find that dissipation provides a complementary
constraint to buoyancy: while small-scale, fibril fields might be regenerated
faster than they rise, the dissipative heating associated with such fields
would in some cases greatly exceed the luminosity of the star. We show how
these constraints combine to yield limits on the internal field strength and
morphology in low-mass stars. In particular, we find that for stars of 0.3
solar masses, no fields in flux tubes stronger than about 800 kG are
simultaneously consistent with both constraints.Comment: 19 pages, 10 figures, accepted to Ap
Comparing Simulations of Rising Flux Tubes Through the Solar Convection Zone with Observations of Solar Active Regions: Constraining the Dynamo Field Strength
We study how active-region-scale flux tubes rise buoyantly from the base of
the convection zone to near the solar surface by embedding a thin flux tube
model in a rotating spherical shell of solar-like turbulent convection. These
toroidal flux tubes that we simulate range in magnetic field strength from 15
kG to 100 kG at initial latitudes of 1 degree to 40 degrees in both
hemispheres. This article expands upon Weber, Fan, and Miesch (Astrophys. J.,
741, 11, 2011) (Article 1) with the inclusion of tubes with magnetic flux of
10^20 Mx and 10^21 Mx, and more simulations of the previously investigated case
of 10^22 Mx, sampling more convective flows than the previous article, greatly
improving statistics. Observed properties of active regions are compared to
properties of the simulated emerging flux tubes, including: the tilt of active
regions in accordance with Joy's Law as in Article 1, and in addition the
scatter of tilt angles about the Joy's Law trend, the most commonly occurring
tilt angle, the rotation rate of the emerging loops with respect to the
surrounding plasma, and the nature of the magnetic field at the flux tube apex.
We discuss how these diagnostic properties constrain the initial field strength
of the active region flux tubes at the bottom of the solar convection zone, and
suggest that flux tubes of initial magnetic field strengths of \geq 40 kG are
good candidates for the progenitors of large (10^21 Mx to 10^22 Mx) solar
active regions, which agrees with the results from Article 1 for flux tubes of
10^22 Mx. With the addition of more magnetic flux values and more simulations,
we find that for all magnetic field strengths, the emerging tubes show a
positive Joy's Law trend, and that this trend does not show a statistically
significant dependence on the magnetic flux.Comment: Accepted to Solar Physics Topical Issue: Solar Dynamics and Magnetism
from the Interior to the Atmospher
Evaluation of an integrated system for classification, assessment and comparison of services for long-term care in Europe: the eDESDE-LTC study
The harmonization of European health systems brings with it a need for tools to allow the standardized collection of information about medical care. A common coding system and standards for the description of services are needed to allow local data to be incorporated into evidence-informed policy, and to permit equity and mobility to be assessed. The aim of this project has been to design such a classification and a related tool for the coding of services for Long Term Care (DESDE-LTC), based on the European Service Mapping Schedule (ESMS). Methods. The development of DESDE-LTC followed an iterative process using nominal groups in 6 European countries. 54 researchers and stakeholders in health and social services contributed to this process. In order to classify services, we use the minimal organization unit or "Basic Stable Input of Care" (BSIC), coded by its principal function or "Main Type of Care" (MTC). The evaluation of the tool included an analysis of feasibility, consistency, ontology, inter-rater reliability, Boolean Factor Analysis, and a preliminary impact analysis (screening, scoping and appraisal). Results: DESDE-LTC includes an alpha-numerical coding system, a glossary and an assessment instrument for mapping and counting LTC. It shows high feasibility, consistency, inter-rater reliability and face, content and construct validity. DESDE-LTC is ontologically consistent. It is regarded by experts as useful and relevant for evidence-informed decision making. Conclusion: DESDE-LTC contributes to establishing a common terminology, taxonomy and coding of LTC services in a European context, and a standard procedure for data collection and international comparison
Giant protists (xenophyophores, Foraminifera) are exceptionally diverse in parts of the abyssal eastern Pacific licensed for polymetallic nodule exploration
Xenophyophores, giant, fragile, agglutinated foraminifera (protists), are major constituents of the abyssal megafauna in the equatorial Pacific Clarion-Clipperton Zone (CCZ), a region where seabed mining of polymetallic nodules may occur in the future. As part of a baseline study of benthic communities we made extensive collections of xenophyophores in two areas (UK-1 and OMS) licensed for exploration by the International Seabed Authority. Based on test morphology, we distinguished 36 morphospecies (34 new to science) among 130 specimens. Twenty of these morphospecies yielded 184 DNA sequences, a 14-fold increase in genetic data for xenophyophores that confirms their high diversity in the eastern CCZ. A further 15 morphospecies (8 new to science) were recognised in samples from two other areas (APEI-6 and Russian exploration license area) within or adjacent to the CCZ. This large number of species confirms that the CCZ is a focal area for xenophyophore diversity. More broadly, it represents an unprecedented increase in the known global diversity of xenophyophores and suggests that many species remain undiscovered in the World's oceans. Xenophyophores are often sessile on nodules in the CCZ, making these delicate organisms particularly vulnerable to mining impacts. They can also play a crucial role in deep-sea ecosystems, providing habitat structures for meiofaunal and macrofaunal organisms and enhancing the organic content of sediments surrounding their tests. The loss of xenophyophores due to seabed mining may therefore have wider implications for the recovery of benthic communities following major human disturbances on the abyssal seafloor
Five new species and two new genera of xenophyophores (Foraminifera: Rhizaria) from part of the abyssal equatorial Pacific licensed for polymetallic nodule exploration
Based on a combination of morphological and molecular data, we describe five new species and two new genera of xenophyophores from the Clarion–Clipperton Zone (abyssal eastern Pacific), an area with commercially valuable seafloor deposits of polymetallic nodules. Bizarria bryiformis gen. et sp. nov. displays unusual features, notably an organic-walled test, largely devoid of agglutinated particles, comprising interconnected branches growing upwards from the nodule substrate; the bases of the branches contain dark masses of waste material (stercomare) and pale strands of cytoplasm (granellare), the whitish, tuft-like extremities contain sediment particles. Tendalia reteformis gen. et sp. nov. forms a delicate network of agglutinated tubes. Shinkaiya contorta sp. nov. is characterized by a contorted, partly reticulated plate-like test while the simpler plate-like test of Galatheammina interstincta sp. nov. combines characters typical of Galatheammina and Psammina. In Semipsammina mattaeformis sp. nov., a thin, delicate test with one or more tubular extensions forms a flat canopy over the mat-like stercomare encrusting the nodule substrate. Tendalia reteformis and S. contorta are free-living; the other species are sessile on nodules. Together, they illustrate the considerable morphological diversity of xenophyophores in a region where they dominate the megafauna, and highlight some major taxonomic challenges posed by these giant monothalamous foraminifera
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