6,104 research outputs found
On the mean field dynamo with Hall effect
We study in the present paper how Hall effect modifies the quenching process
of the electromotive force (e.m.f.) in Mean Field Dynamo (MFD) theories. We
write down the evolution equations for the e.m.f. and for the large and small
scale magnetic helicity, treat Hall effect as a perturbation and integrate the
resulting equations assuming boundary conditions such that the total
divergencies vanish. For force-free large scale magnetic fields, Hall effect
acts by coupling the small scale velocity and magnetic fields. For the range of
parameters considered, the overall effect is a stronger quenching of the e.m.f.
than in standard MHD and a damping of the inverse cascade of magnetic helicity.
In astrophysical environments characterized by the parameters considered here,
Hall effect would produce an earlier quenching of the e.m.f. and consequently a
weaker large scale magnetic field.Comment: 8 pages, 4 figures. Accepted by A&
GRBs from unstable Poynting dominated outflows
Poynting flux driven outflows from magnetized rotators are a plausible
explanation for gamma-ray burst engines. We suggest a new possibility for how
such outflows might transfer energy into radiating particles. We argue that the
Poynting flux drives non-linearly unstable large amplitude electromagnetic
waves (LAEMW) which ``break'' at radii cm where the MHD
approximation becomes inapplicable. In the ``foaming'' (relativisticly
reconnecting) regions formed during the wave breaks the random electric fields
stochastically accelerate particles to ultrarelativistic energies which then
radiate in turbulent electromagnetic fields. The typical energy of the emitted
photons is a fraction of the fundamental Compton energy with plus additional boosting due to the bulk motion
of the medium. The emission properties are similar to synchrotron radiation,
with a typical cooling time sec. During the wave break, the
plasma is also bulk accelerated in the outward radial direction and at larger
radii can produce afterglows due to the interactions with external medium. The
near equipartition fields required by afterglow models maybe due to magnetic
field regeneration in the outflowing plasma (similarly to the field generation
by LAEMW of laser-plasma interactions) and mixing with the upstream plasma.Comment: 15 pages, 1 figur
Public Art Discourse: A Case Study of Gateshead, England
This paper discusses how the purposes of public art are understood in official discourses. Discourses legitimise desired courses of action, such as whether public art is commissioned compared to other spending priorities, and the processes that surround its use as a type of intervention. The multiple meanings attached to this art genre constitute it as an argumentation field with alternative and possibly conflicting objectives. This makes it particularly interesting to approach public art sociologically as a constructed practice. The focus of the study is a local authority with an international reputation for public art: Gateshead in England, home to Antony Gormley’s Angel of the North. The study sourced a range of municipal documents and undertook an analysis informed by a grounded theory approach to identify important themes and connections between them. Four coherent discourses are revealed, not easily discernible from the often fragmented references to public art across various schemes, projects and strategies described in the documents. These were ‘venue’, ‘inclusion’, ‘quality of life’, and ‘civic pride’. The paper shows how these discourses relate to wider sociological and policy concerns, especially regarding municipal improvement
Three Dimensional Evolution of a Relativistic Current Sheet : Triggering of Magnetic Reconnection by the Guide Field
The linear and non-linear evolution of a relativistic current sheet of pair
() plasmas is investigated by three-dimensional particle-in-cell
simulations. In a Harris configuration, it is obtained that the magnetic energy
is fast dissipated by the relativistic drift kink instability (RDKI). However,
when a current-aligned magnetic field (the so-called "guide field") is
introduced, the RDKI is stabilized by the magnetic tension force and it
separates into two obliquely-propagating modes, which we call the relativistic
drift-kink-tearing instability (RDKTI). These two waves deform the current
sheet so that they trigger relativistic magnetic reconnection at a crossover
thinning point. Since relativistic reconnection produces a lot of non-thermal
particles, the guide field is of critical importance to study the energetics of
a relativistic current sheet.Comment: 12 pages, 4 figures; fixed typos and added a footnote [24
Quantification of Mandatory Sustainment Requirements
To emphasize the importance of sustainment, the DoD Joint Requirements Oversight Council addressed sustained Materiel readiness and established a mandatory Key Performance Parameter (KPP) for Materiel Availability; it also established supporting Key System Attributes (KSAs) for Materiel Reliability and Ownership Cost (Chairman of the Joint Chiefs of Staff Manual (CJCSM) 3170.01C, 2007). Current guidance requires two numbers: a threshold value and an objective value (Chairman of the Joint Chiefs of Staff Manual (CJCSM) 3170.01C, 2007). No distinction is made between the approaches in establishing these values for major system acquisitions, versus smaller, modification-focused efforts for existing systems. The Joint Staff proposed guidance to assist in determining these values for major acquisition programs, but the guidance has yet to be tested on modification contracts. To assess its applicability, we performed a case study of a recent acquisition program under consideration by Air Mobility Command. We sought to apply the principles put forth in this draft guide prepared by the Office of the Secretary of Defense in Collaboration with the Joint Staff. This research seeks to assist the combat developer and program manager to develop an objective, standard, repeatable method for quantifying the mandatory Materiel Availability KPP and the associated Materiel Reliability KSA values established by the Joint Requirements Oversight Council
Fast Shocks From Magnetic Reconnection Outflows
Magnetic reconnection is commonly perceived to drive flow and particle
acceleration in flares of solar, stellar, and astrophysical disk coronae but
the relative roles of different acceleration mecha- nisms in a given
reconnection environment are not well understood. We show via direct numerical
simulations that reconnection outflows produce weak fast shocks, when
conditions for fast recon- nection are met and the outflows encounter an
obstacle. The associated compression ratios lead to a Fermi acceleration
particle spectrum that is significantly steeper than the strong fast shocks
commonly studied, but consistent with the demands of solar flares. While this
is not the only acceleration mechanism operating in a reconnection environment,
it is plausibly a ubiquitous one
Reconnecting Magnetic Flux Tubes as a Source of In Situ Acceleration in Extragalactic Radio Sources
Many extended extragalactic radio sources require a local {\it in situ\/}
acceleration mechanism for electrons, in part because the synchrotron lifetimes
are shorter than the bulk travel time across the emitting regions. If the
magnetic field in these sources is localized in flux tubes, reconnection may
occur between regions of plasma \be (ratio of particle to magnetic pressure)
, even though averaged over the plasma volume may be \gsim 1.
Reconnection in low regions is most favorable to acceleration from
reconnection shocks. The reconnection X-point regions may provide the injection
electrons for their subsequent non-thermal shock acceleration to distributions
reasonably consistent with observed spectra. Flux tube reconnection might
therefore be able to provide acceleration required by large scale
jets and lobes.Comment: 14 pages, plain TeX, accepted to Ap.J.Let
Growth mechanism of planar or nanorod structured tungsten oxide thin films deposited via aerosol assisted chemical vapour deposition (AACVD)
Aerosol assisted chemical vapour deposition (AACVD) is used to deposit tungsten oxide thin films from tungsten hexacarbonyl (W(CO)6) at 339 to 358 °C on quartz substrate. The morphologies of as-deposited thin films, which are comprised of two phases (W25O73 and W17O47), vary from planar to nanorod (NR) structures as the distance from the inlet towards the outlet of the reactor is traversed. This is related to variation of the actual temperature on the substrate surface (ΔT = 19 °C), which result in a change in growth mode due to competition between growth rate (perpendicular to substrate) and nucleation rate (parallel to substrate). When the ratio of perpendicular growth rate to growth rate contributed by nucleation is higher than 7.1, the as-deposited tungsten oxide thin film forms as NR. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Thermodynamic Entropy And The Accessible States of Some Simple Systems
Comparison of the thermodynamic entropy with Boltzmann's principle shows that
under conditions of constant volume the total number of arrangements in simple
thermodynamic systems with temperature-independent heat capacities is TC/k. A
physical interpretation of this function is given for three such systems; an
ideal monatomic gas, an ideal gas of diatomic molecules with rotational motion,
and a solid in the Dulong-Petit limit of high temperature. T1/2 emerges as a
natural measure of the number of accessible states for a single particle in one
dimension. Extension to N particles in three dimensions leads to TC/k as the
total number of possible arrangements or microstates. The different microstates
of the system are thus shown a posteriori to be equally probable, with
probability T-C/k, which implies that for the purposes of counting states the
particles of the gas are distinguishable. The most probable energy state of the
system is determined by the degeneracy of the microstates.Comment: 9 pages, 1 figur
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