7,689 research outputs found
A necessary extension of the surface flux transport model
Customary two-dimensional flux transport models for the evolution of the
magnetic field at the solar surface do not account for the radial structure and
the volume diffusion of the magnetic field. When considering the long-term
evolution of magnetic flux, this omission can lead to an unrealistic long-term
memory of the system and to the suppression of polar field reversals. In order
to avoid such effects, we propose an extension of the flux transport model by a
linear decay term derived consistently on the basis of the eigenmodes of the
diffusion operator in a spherical shell. A decay rate for each eigenmode of the
system is determined and applied to the corresponding surface part of the mode
evolved in the flux transport model. The value of the volume diffusivity
associated with this decay term can be estimated to be in the range 50--100
km^2/s by considering the reversals of the polar fields in comparison of flux
transport simulations with observations. We show that the decay term prohibits
a secular drift of the polar field in the case of cycles of varying strength,
like those exhibited by the historical sunspot record.Comment: for further information visit: http://solweb.oma.be/users/baumann
A note on dark energy induced by D-brane motion
In this note we study the possibility of obtaining dark energy solution in a
D-brane scenario in a warped background that includes brane-position dependent
corrections for the non-perturbative superpotential. The volume modulus is
stabilized at instantaneous minima of the potential. Though the model can
account for the existence of dark energy within present observational bound -
fine-tuning of the model parameters becomes unavoidable. Moreover, the model
does not posses a tracker solution.Comment: 5 pages, 4 figure
Decreased well-being after job loss : testing omitted causes
Job loss is widely known to lead to a substantial decrease in workers' subjective well-being. Functionalist theories explain this fact by arguing that the fundamental needs that work fulfills are absent during unemployment. Recent evidence from longitudinal studies however contradicts this approach, showing that workers who find a new job do not fully regain their former level of well-being upon reemployment. Therefore other mechanisms must be at work. We suggest that changes in social or economic domains of workers' lives - triggered by job displacement - lead to the observed changes in well-being. Drawing on a unique data set from a survey of workers displaced by plant closure in Switzerland after the financial crisis of 2008, our analysis confirms the previous result that finding a job after displacement does not completely restore workers' pre-displacement level of well-being. The factors that best explain this outcome are changes in social domains, notably changes in workers' job - related social status and their relationships to friends. This result provides valuable insights about the long lasting scars job displacement leaves on workers' lives
A possible signature of terrestrial planet formation in the chemical composition of solar analogs
Recent studies have shown that the elemental abundances in the Sun are
anomalous when compared to most (about 85%) nearby solar twin stars. Compared
to its twins, the Sun exhibits a deficiency of refractory elements (those with
condensation temperatures Tc>900K) relative to volatiles (Tc<900K). This
finding is speculated to be a signature of the planet formation that occurred
more efficiently around the Sun compared with the majority of solar twins.
Furthermore, within this scenario, it seems more likely that the abundance
patterns found are specifically related to the formation of terrestrial
planets. In this work we analyze abundance results from six large independent
stellar abundance surveys to determine whether they confirm or reject this
observational finding. We show that the elemental abundances derived for solar
analogs in these six studies are consistent with the Tc trend suggested as a
planet formation signature. The same conclusion is reached when those results
are averaged heterogeneously. We also investigate the dependency of the
abundances with first ionization potential (FIP), which correlates well with
Tc. A trend with FIP would suggest a different origin for the abundance
patterns found, but we show that the correlation with Tc is statistically more
significant. We encourage similar investigations of metal-rich solar analogs
and late F-type dwarf stars, for which the hypothesis of a planet formation
signature in the elemental abundances makes very specific predictions. Finally,
we examine a recent paper that claims that the abundance patterns of two stars
hosting super-Earth like planets contradict the planet formation signature
hypothesis. Instead, we find that the chemical compositions of these two stars
are fully compatible with our hypothesis.Comment: To appear in Astronomy and Astrophysic
A Field Range Bound for General Single-Field Inflation
We explore the consequences of a detection of primordial tensor fluctuations
for general single-field models of inflation. Using the effective theory of
inflation, we propose a generalization of the Lyth bound. Our bound applies to
all single-field models with two-derivative kinetic terms for the scalar
fluctuations and is always stronger than the corresponding bound for slow-roll
models. This shows that non-trivial dynamics can't evade the Lyth bound. We
also present a weaker, but completely universal bound that holds whenever the
Null Energy Condition (NEC) is satisfied at horizon crossing.Comment: 16 page
Electrical characterization of a Mapham inverter using pulse testing techniques
Electric power requirements for aerospace missions have reached megawatt power levels. Within the next few decades, it is anticipated that a manned lunar base, interplanetary travel, and surface exploration of the Martian surface will become reality. Several research and development projects aimed at demonstrating megawatt power level converters for space applications are currently underway at the NASA Lewis Research Center. Innovative testing techniques will be required to evaluate the components and converters, when developed, at their rated power in the absence of costly power sources, loads, and cooling systems. Facilities capable of testing these components and systems at full power are available, but their use may be cost prohibitive. The use of a multiple pulse testing technique is proposed to determine the electrical characteristics of large megawatt level power systems. Characterization of a Mapham inverter is made using the proposed technique and conclusions are drawn concerning its suitability as an experimental tool to evaluate megawatt level power systems
On the predictive power of Local Scale Invariance
Local Scale Invariance (LSI) is a theory for anisotropic critical phenomena
designed in the spirit of conformal invariance. For a given representation of
its generators it makes non-trivial predictions about the form of universal
scaling functions. In the past decade several representations have been
identified and the corresponding predictions were confirmed for various
anisotropic critical systems. Such tests are usually based on a comparison of
two-point quantities such as autocorrelation and response functions. The
present work highlights a potential problem of the theory in the sense that it
may predict any type of two-point function. More specifically, it is argued
that for a given two-point correlator it is possible to construct a
representation of the generators which exactly reproduces this particular
correlator. This observation calls for a critical examination of the predictive
content of the theory.Comment: 17 pages, 2 eps figure
Electrical properties of teflon and ceramic capacitors at high temperatures
Space power systems and components are often required to operate efficiently and reliably in harsh environments where stresses, such as high temperature, are encountered. These systems must, therefore, withstand exposure to high temperature while still providing good electrical and other functional properties. Experiments were carried out to evaluate Teflon and ceramic capacitors for potential use in high temperature applications. The capacitors were characterized in terms of their capacitance and dielectric loss as a function of temperature, up to 200 C. At a given temperature, these properties were obtained in a frequency range of 50 Hz to 100 kHz. DC leakage current measurements were also performed in a temperature range from 25 to 200 C. The results obtained are discussed and conclusions are made concerning the suitability of the capacitors studied for high temperature applications
- …