25,494 research outputs found
MHD wave propagation from the sub-photosphere to the corona in an arcade-shaped magnetic field with a null point
The aim of this work is to study the energy transport by means of MHD waves
propagating in quiet Sun magnetic topology from layers below the surface to the
corona. Upward propagating waves find obstacles, such as the equipartition
layer with plasma b=1 and the transition region, and get converted, reflected
and refracted. Understanding the mechanisms by which MHD waves can reach the
corona can give us information about the solar atmosphere and the magnetic
structures. We carry out two-dimensional numerical simulations of wave
propagation in a magnetic field structure that consists of two vertical flux
tubes separated by an arcade shaped magnetic field. This configuration contains
a null point in the corona, that significantly modifies the behaviour of the
waves. We describe in detail the wave propagation through the atmosphere under
different driving conditions. We also present the spatial distribution of the
mean acoustic and magnetic energy fluxes and the spatial distribution of the
dominant frequencies in the whole domain. We conclude that the energy reaches
the corona preferably along vertical magnetic fields, inside the flux tubes,
and it has an acoustic nature. Most of the magnetic energy keeps concentrated
below the transition region due to the refraction of the magnetic waves and the
continuous conversion of acoustic-like waves into fast magnetic waves in the
equipartition layer located in the photosphere. However, part of the magnetic
energy reaches the low corona when propagating in the region where the arcades
are located, but waves are sent back downwards to the lower atmosphere at the
null point surroundings. This phenomenon, together with the reflection and
refraction of waves in the TR and the lower turning point, act as a re-feeding
of the atmosphere. In the frequency distribution, we find that high frequency
waves can reach the corona outside the vertical flux tubes.Comment: 13 pages, 13 figure
Effect of liquid nitrogen pre-treatment on various types of wool waste fibres for biogas production
This study investigated the role of liquid nitrogen (LN2) in increasing microbial accessibility of wool proteins for biogas production. It involves a mechanical size reduction of four different types of raw wool fibres, namely, Blackface, Bluefaced Leicester, Texel and Scotch Mule, in presence of liquid nitrogen, followed by the determination of the methane production potential of the pre-treated wool fibres. The highest methane yield, 157.3 cm3 g−1 VS, was obtained from pre-treated Scotch mule wool fibre culture, and represented more than 80% increase when compared to the yield obtained from its raw equivalent culture. The increase in biogas yield was attributed to the effectiveness of LN2 in enhancing particle size reduction and the consequent increase in wool solubility and bioavailability. Results also showed that LN2 pre-treatment can enhance size reduction but has limited effect on the molecular structure. The study also showed that the biogas potential of waste wool fibres varies with the type and source of wool
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A Real Time Tax Smoothing Based Fiscal Policy Rule
In this paper we consider the real-time implementation of a fiscal policy rule based on tax smoothing (Barro (1979) and Bohn (1998)). We show that the tax smoothing approach, augmented by fiscal habit considerations, provides a surprisingly accurate description of US budget surplus movements. In order to investigate the robustness of the policy implications of the rule, we construct a real-time US fiscal data set, complementing the data documented by Croushore and Stark (2001). For each variable we record the different vintages, reflecting the remeasurements that occur over time. We demonstrate that the easily constructed rule provided a useful benchmark for policy analysis that is robust to real-time remeasurements
Parametric study of the interface behavior between two immiscible liquids flowing through a porous medium
When two immiscible liquids that coexist inside a porous medium are drained
through an opening, a complex flow takes place in which the interface between
the liquids moves, tilts and bends. The interface profiles depend on the
physical properties of the liquids and on the velocity at which they are
extracted. If the drainage flow rate, the liquids volume fraction in the
drainage flow and the physical properties of the liquids are known, the
interface angle in the immediate vicinity of the outlet (theta) can be
determined. In this work, we define four nondimensional parameters that rule
the fluid dynamical problem and, by means of a numerical parametric analysis,
an equation to predict theta is developed. The equation is verified through
several numerical assessments in which the parameters are modified
simultaneously and arbitrarily. In addition, the qualitative influence of each
nondimensional parameter on the interface shape is reported.Comment: 7 pages, 12 figure
Retrieving Infinite Numbers of Patterns in a Spin-Glass Model of Immune Networks
The similarity between neural and immune networks has been known for decades,
but so far we did not understand the mechanism that allows the immune system,
unlike associative neural networks, to recall and execute a large number of
memorized defense strategies {\em in parallel}. The explanation turns out to
lie in the network topology. Neurons interact typically with a large number of
other neurons, whereas interactions among lymphocytes in immune networks are
very specific, and described by graphs with finite connectivity. In this paper
we use replica techniques to solve a statistical mechanical immune network
model with `coordinator branches' (T-cells) and `effector branches' (B-cells),
and show how the finite connectivity enables the system to manage an extensive
number of immune clones simultaneously, even above the percolation threshold.
The system exhibits only weak ergodicity breaking, so that both multiple
antigen defense and homeostasis can be accomplished.Comment: Editor's Choice 201
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