264 research outputs found
Progress in Absorber R&D for Muon Cooling
A stored-muon-beam neutrino factory may require transverse ionization cooling
of the muon beam. We describe recent progress in research and development on
energy absorbers for muon-beam cooling carried out by a collaboration of
university and laboratory groups.Comment: 7 pages, 1 figure, presented at the 3rd International Workshop on
Neutrino Factory Based on Muon Storage Rings (NuFACT'01), May 24-30, 2001,
Tsukuba, Japa
Team performance indicators that predict match outcome and points difference in professional rugby league
Performance indicators allow for the objective quantification of performance, however, limited PI research for professional rugby league exists. Therefore, this paper assessed 24 relative PIs (home value minus away) from all 27 rounds of the 2012, 2013 and 2014 European Super League seasons, collected by Opta, amounting to 567 matches. Backwards logistic (match outcome) and linear (points difference) regression models were used alongside exhaustive Chi-Square Automatic Interaction Detection decision trees to identify performance indicators (PIs) and key performance indicators. Teams had a higher chance of winning and would gain more points when they scored first (OR = 1.6, β = 2.4) and increased completed sets (OR = 1.2, β = 1.2) by one unit. Conversely, teams had a lower chance of winning when they increased scoots (OR = 0.9, β = −0.2). However, some PIs which were thought to be important (as identified by previous literature) were removed from the analysis thus calling into question the appropriateness of stepwise methods. Future research may consider utilising dimension reduction techniques when analysing large data-sets that encompass multiple variables
Alfvénic wave heating of the upper chromosphere in flares
We have developed a numerical model of flare heating due to the dissipation
of Alfv\'enic waves propagating from the corona to the chromosphere. With this
model, we present an investigation of the key parameters of these waves on the
energy transport, heating, and subsequent dynamics. For sufficiently high
frequencies and perpendicular wave numbers, the waves dissipate significantly
in the upper chromosphere, strongly heating it to flare temperatures. This
heating can then drive strong chromospheric evaporation, bringing hot and dense
plasma to the corona. We therefore find three important conclusions: (1)
Alfv\'enic waves, propagating from the corona to the chromosphere, are capable
of heating the upper chromosphere and the corona, (2) the atmospheric response
to heating due to the dissipation of Alfv\'enic waves can be strikingly similar
to heating by an electron beam, and (3) this heating can produce explosive
evaporation.Comment: Accepted to ApJ
A Hydrodynamic Model of Alfvénic Wave Heating in a Coronal Loop and Its Chromospheric Footpoints
Alfv\'enic waves have been proposed as an important energy transport
mechanism in coronal loops, capable of delivering energy to both the corona and
chromosphere and giving rise to many observed features, of flaring and
quiescent regions. In previous work, we established that resistive dissipation
of waves (ambipolar diffusion) can drive strong chromospheric heating and
evaporation, capable of producing flaring signatures. However, that model was
based on a simplified assumption that the waves propagate instantly to the
chromosphere, an assumption which the current work removes. Via a ray tracing
method, we have implemented traveling waves in a field-aligned hydrodynamic
simulation that dissipate locally as they propagate along the field line. We
compare this method to and validate against the magnetohydrodynamics code
Lare3D. We then examine the importance of travel times to the dynamics of the
loop evolution, finding that (1) the ionization level of the plasma plays a
critical role in determining the location and rate at which waves dissipate;
(2) long duration waves effectively bore a hole into the chromosphere, allowing
subsequent waves to penetrate deeper than previously expected, unlike an
electron beam whose energy deposition rises in height as evaporation reduces
the mean-free paths of the electrons; (3) the dissipation of these waves drives
a pressure front that propagates to deeper depths, unlike energy deposition by
an electron beam.Comment: Accepted to Ap
Sunquake generation by coronal magnetic restructuring
Sunquakes are the surface signatures of acoustic waves in the Sun's interior
that are produced by some but not all flares and coronal mass ejections (CMEs).
This paper explores a mechanism for sunquake generation by the magnetic field
changes that occur during flares and CMEs, using MHD simulations with a
semiempirical FAL-C atmosphere to demonstrate the generation of acoustic waves
in the interior in response to changing magnetic tilt in the corona. We find
that Alfven-sound resonance combined with the ponderomotive force produces
acoustic waves in the interior with sufficient energy to match sunquake
observations when the magnetic field angle changes by the order of 10 degrees
in a region where the coronal field strength is a few hundred gauss or more.
The most energetic sunquakes are produced when the coronal field is strong,
while the variation of magnetic field strength with height and the time scale
of the tilt change are of secondary importance.Comment: 6 pages, 3 figures; accepted to Ap
Neural antecedents of self-initiated actions in secondary motor cortex
The neural origins of spontaneous or self-initiated actions are not well understood and their interpretation is controversial. To address these issues, we used a task in which rats decide when to abort waiting for a delayed tone. We recorded neurons in the secondary motor cortex (M2) and interpreted our findings in light of an integration-to-bound decision model. A first population of M2 neurons ramped to a constant threshold at rates proportional to waiting time, strongly resembling integrator output. A second population, which we propose provide input to the integrator, fired in sequences and showed trial-to-trial rate fluctuations correlated with waiting times. An integration model fit to these data also quantitatively predicted the observed inter-neuronal correlations. Together, these results reinforce the generality of the integration-to-bound model of decision-making. These models identify the initial intention to act as the moment of threshold crossing while explaining how antecedent subthreshold neural activity can influence an action without implying a decision.info:eu-repo/semantics/publishedVersio
75th Anniversary of ‘Existence of Electromagnetic-Hydrodynamic Waves’
We have recently passed the 75th anniversary of one of the most important
results in solar and space physics: Hannes Alfv\'en's discovery of Alfv\'en
waves and the Alfv\'en speed. To celebrate the anniversary, this article
recounts some major episodes in the history of MHD waves. Following an
initially cool reception, Alfv\'en's ideas were propelled into the spotlight by
Fermi's work on cosmic rays, the new mystery of coronal heating and, as
scientific perception of interplanetary space shifted dramatically and the
space race started, detection of Alfv\'en waves in the solar wind. From then
on, interest in MHD waves boomed, laying the foundations for modern remote
observations of MHD waves in the Sun, coronal seismology and some of today's
leading theories of coronal heating and solar wind acceleration. In 1970,
Alfv\'en received the Nobel Prize for his work in MHD, including these
discoveries. The article concludes with some reflection about what the history
implies about the way we do science, especially the advantages and pitfalls of
idealised mathematical models.Comment: 10 pages, accepted by Solar Physic
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