1,950 research outputs found
On a limitation of Zeeman polarimetry and imperfect instrumentation in representing solar magnetic fields with weaker polarization signal
Full disk vector magnetic fields are used widely for developing better
understanding of large-scale structure, morphology, and patterns of the solar
magnetic field. The data are also important for modeling various solar
phenomena. However, observations of vector magnetic fields have one important
limitation that may affect the determination of the true magnetic field
orientation. This limitation stems from our ability to interpret the differing
character of the Zeeman polarization signals which arise from the photospheric
line-of-sight vs. the transverse components of the solar vector magnetic field,
and is likely exacerbated by unresolved structure (non-unity fill fraction) as
well as the disambiguation of the 180 degeneracy in the
transverse-field azimuth. Here we provide a description of this phenomenon, and
discuss issues, which require additional investigation.Comment: 13 pages, 5 figure, Journal of Space Weather and Space Climate,
accepted, 202
Global variation of the dust-to-gas ratio in evolving protoplanetary discs
Recent theories suggest planetesimal formation via streaming and/or
gravitational instabilities may be triggered by localized enhancements in the
dust-to-gas ratio, and one hypothesis is that sufficient enhancements may be
produced in the pile-up of small solid particles inspiralling under aerodynamic
drag from the large mass reservoir in the outer disc. Studies of particle
pile-up in static gas discs have provided partial support for this hypothesis.
Here, we study the radial and temporal evolution of the dust-to-gas ratio in
turbulent discs, that evolve under the action of viscosity and
photoevaporation. We find that particle pile-ups do not generically occur
within evolving discs, particularly if the introduction of large grains is
restricted to the inner, dense regions of a disc. Instead, radial drift results
in depletion of solids from the outer disc, while the inner disc maintains a
dust-to-gas ratio that is within a factor of ~2 of the initial value. We
attribute this result to the short time-scales for turbulent diffusion and
radial advection (with the mean gas flow) in the inner disc. We show that the
qualitative evolution of the dust-to-gas ratio depends only weakly upon the
parameters of the disc model (the disc mass, size, viscosity, and value of the
Schmidt number), and discuss the implications for planetesimal formation via
collective instabilities. Our results suggest that in discs where there is a
significant level of midplane turbulence and accretion, planetesimal formation
would need to be possible in the absence of large-scale enhancements. Instead,
trapping and concentration of particles within local turbulent structures may
be required as a first stage of planetesimal formation.Comment: Accepted by Monthly Notices of the Royal Astronomical Society on the
8th of March 2012. 20 pages. 16 figure
Particle transport in evolving protoplanetary disks: Implications for results from Stardust
Samples returned from comet 81P/Wild 2 by Stardust confirm that substantial
quantities of crystalline silicates were incorporated into the comet at
formation. We investigate the constraints that this observation places upon
protoplanetary disk physics, assuming that outward transport of particles
processed at high temperatures occurs via advection and turbulent diffusion in
an evolving disk. We also look for constraints on particle formation locations.
Our results are based upon 1D disk models that evolve with time under the
action of viscosity and photoevaporation, and track solid transport using an
ensemble of individual particle trajectories. We find that two classes of disk
model are consistent with the Stardust findings. One class features a high
particle diffusivity (a Schmidt number Sc < 1), which suffices to diffuse
particles up to 20 microns in size outward against the mean gas flow. For Sc >
1, such models are unlikely to be viable, and significant outward transport
requires that the particles of interest settle into a midplane layer that
experiences an outward gas flow. In either class of models, the mass of inner
disk material that reaches the outer disk is a strong function of the disk's
initial compactness. Hence, models of grain transport within steady-state disks
underestimate the efficiency of outward transport. Neither model results in
sustained outward transport of very large particles exceeding a mm in size. We
show that the transport efficiency generally falls off rapidly with time.
Hence, high-temperature material must be rapidly incorporated into icy bodies
to avoid fallback, and significant radial transport may only occur during the
initial phase of rapid disk evolution. It may also vary substantially between
disks depending upon their initial mass distributions. We discuss implications
for Spitzer observations of crystalline silicates in T Tauri disks.Comment: ApJ, in pres
Microstructural enrichment functions based on stochastic Wang tilings
This paper presents an approach to constructing microstructural enrichment
functions to local fields in non-periodic heterogeneous materials with
applications in Partition of Unity and Hybrid Finite Element schemes. It is
based on a concept of aperiodic tilings by the Wang tiles, designed to produce
microstructures morphologically similar to original media and enrichment
functions that satisfy the underlying governing equations. An appealing feature
of this approach is that the enrichment functions are defined only on a small
set of square tiles and extended to larger domains by an inexpensive stochastic
tiling algorithm in a non-periodic manner. Feasibility of the proposed
methodology is demonstrated on constructions of stress enrichment functions for
two-dimensional mono-disperse particulate media.Comment: 27 pages, 12 figures; v2: completely re-written after the first
revie
Full genome sequence and sfRNA interferon antagonist activity of Zika virus from Recife, Brazil
Background:
The outbreak of Zika virus (ZIKV) in the Americas has transformed a previously obscure mosquito-transmitted arbovirus of the Flaviviridae family into a major public health concern. Little is currently known about the evolution and biology of ZIKV and the factors that contribute to the associated pathogenesis. Determining genomic sequences of clinical viral isolates and characterization of elements within these are an important prerequisite to advance our understanding of viral replicative processes and virus-host interactions.
Methodology/Principal findings:
We obtained a ZIKV isolate from a patient who presented with classical ZIKV-associated symptoms, and used high throughput sequencing and other molecular biology approaches to determine its full genome sequence, including non-coding regions. Genome regions were characterized and compared to the sequences of other isolates where available. Furthermore, we identified a subgenomic flavivirus RNA (sfRNA) in ZIKV-infected cells that has antagonist activity against RIG-I induced type I interferon induction, with a lesser effect on MDA-5 mediated action.
Conclusions/Significance:
The full-length genome sequence including non-coding regions of a South American ZIKV isolate from a patient with classical symptoms will support efforts to develop genetic tools for this virus. Detection of sfRNA that counteracts interferon responses is likely to be important for further understanding of pathogenesis and virus-host interactions
Rapidly Changing Range Limits in a Warming World: Critical Data Limitations and Knowledge Gaps for Advancing Understanding of Mangrove Range Dynamics in the Southeastern USA
Climate change is altering species’ range limits and transforming ecosystems. For example, warming temperatures are leading to the range expansion of tropical, cold-sensitive species at the expense of their cold-tolerant counterparts. In some temperate and subtropical coastal wetlands, warming winters are enabling mangrove forest encroachment into salt marsh, which is a major regime shift that has significant ecological and societal ramifications. Here, we synthesized existing data and expert knowledge to assess the distribution of mangroves near rapidly changing range limits in the southeastern USA. We used expert elicitation to identify data limitations and highlight knowledge gaps for advancing understanding of past, current, and future range dynamics. Mangroves near poleward range limits are often shorter, wider, and more shrublike compared to their tropical counterparts that grow as tall forests in freeze-free, resource-rich environments. The northern range limits of mangroves in the southeastern USA are particularly dynamic and climate sensitive due to abundance of suitable coastal wetland habitat and the exposure of mangroves to winter temperature extremes that are much colder than comparable range limits on other continents. Thus, there is need for methodological refinements and improved spatiotemporal data regarding changes in mangrove structure and abundance near northern range limits in the southeastern USA. Advancing understanding of rapidly changing range limits is critical for foundation plant species such as mangroves, as it provides a basis for anticipating and preparing for the cascading effects of climate-induced species redistribution on ecosystems and the human communities that depend on their ecosystem services
Search for CP Violation in the Decay Z -> b (b bar) g
About three million hadronic decays of the Z collected by ALEPH in the years
1991-1994 are used to search for anomalous CP violation beyond the Standard
Model in the decay Z -> b \bar{b} g. The study is performed by analyzing
angular correlations between the two quarks and the gluon in three-jet events
and by measuring the differential two-jet rate. No signal of CP violation is
found. For the combinations of anomalous CP violating couplings, and , limits of \hat{h}_b < 0.59h^{\ast}_{b} < 3.02$ are given at 95\% CL.Comment: 8 pages, 1 postscript figure, uses here.sty, epsfig.st
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