4,568 research outputs found
Is the growth of the Scottish economy the 'first priority' for public spending in Scotland
The Scottish Executive, notably the First Minister, has frequently stated that the growth and development of the Scottish Economy is the first priority of government in Scotland
A Ulysses Detection of Secondary Helium Neutrals
The Interstellar Boundary EXplorer (IBEX) mission has recently studied the
flow of interstellar neutral He atoms through the solar system, and discovered
the existence of a secondary He flow likely originating in the outer
heliosheath. We find evidence for this secondary component in Ulysses data. By
coadding hundreds of Ulysses He beam maps together to maximize signal-to-noise,
we identify a weak signal that is credibly associated with the secondary
component. Assuming a laminar flow from infinity, we infer the following He
flow parameters: V=12.8+/-1.9 km/s, lambda=74.4+/-1.8 deg, beta=-10.5+/-4.1
deg, and T=3000+/-1100 K; where lambda and beta are the ecliptic longitude and
latitude direction in J2000 coordinates. The secondary component has a density
that is 4.9+/-0.9% that of the primary component. These measurements are
reasonably consistent with measurements from IBEX, with the exception of
temperature, where our temperature is much lower than IBEX's T=9500 K. Even the
higher IBEX temperature is suspiciously low compared to expectactions for the
outer heliosheath source region. The implausibly low temperatures are due to
the incorrect assumption of a laminar flow instead of a diverging one, given
that the flow in the outer heliosheath source region will be deflecting around
the heliopause. As for why the IBEX and Ulysses T values are different,
difficulties with background subtraction in the Ulysses data are a potential
source of concern, but the discrepancy may also be another effect of the
improper laminar flow assumption, which could affect the IBEX and Ulysses
analyses differently.Comment: 9 pages, 4 figures, to appear in The Astrophysical Journa
A hierarchy of models for simulating experimental results from a 3D heterogeneous porous medium
In this work we examine the dispersion of conservative tracers (bromide and
fluorescein) in an experimentally-constructed three-dimensional dual-porosity
porous medium. The medium is highly heterogeneous (), and
consists of spherical, low-hydraulic-conductivity inclusions embedded in a
high-hydraulic-conductivity matrix. The bi-modal medium was saturated with
tracers, and then flushed with tracer-free fluid while the effluent
breakthrough curves were measured. The focus for this work is to examine a
hierarchy of four models (in the absence of adjustable parameters) with
decreasing complexity to assess their ability to accurately represent the
measured breakthrough curves. The most information-rich model was (1) a direct
numerical simulation of the system in which the geometry, boundary and initial
conditions, and medium properties were fully independently characterized
experimentally with high fidelity. The reduced models included; (2) a
simplified numerical model identical to the fully-resolved direct numerical
simulation (DNS) model, but using a domain that was one-tenth the size; (3) an
upscaled mobile-immobile model that allowed for a time-dependent mass-transfer
coefficient; and, (4) an upscaled mobile-immobile model that assumed a
space-time constant mass-transfer coefficient. The results illustrated that all
four models provided accurate representations of the experimental breakthrough
curves as measured by global RMS error. The primary component of error induced
in the upscaled models appeared to arise from the neglect of convection within
the inclusions. Interestingly, these results suggested that the conventional
convection-dispersion equation, when applied in a way that resolves the
heterogeneities, yields models with high fidelity without requiring the
imposition of a more complex non-Fickian model.Comment: 27 pages, 9 Figure
Revisiting Ulysses Observations of Interstellar Helium
We report the results of a comprehensive reanalysis of Ulysses observations
of interstellar He atoms flowing through the solar system, the goal being to
reassess the interstellar He flow vector and to search for evidence of
variability in this vector. We find no evidence that the He beam seen by
Ulysses changes at all from 1994-2007. The direction of flow changes by no more
than ~0.3 deg and the speed by no more than ~0.3 km/s. A global fit to all
acceptable He beam maps from 1994-2007 yields the following He flow parameters:
V_ISM=26.08+/-0.21 km/s, lambda=75.54+/-0.19 deg, beta=-5.44+/-0.24 deg, and
T=7260+/-270 K; where lambda and beta are the ecliptic longitude and latitude
direction in J2000 coordinates. The flow vector is consistent with the original
analysis of the Ulysses team, but our temperature is significantly higher. The
higher temperature somewhat mitigates a discrepancy that exists in the He flow
parameters measured by Ulysses and the Interstellar Boundary Explorer, but does
not resolve it entirely. Using a novel technique to infer photoionization loss
rates directly from Ulysses data, we estimate a density of n_He=0.0196+/-0.0033
cm^-3 in the interstellar medium.Comment: to appear in The Astrophysical Journa
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