20,226 research outputs found
The Relation between Solar Eruption Topologies and Observed Flare Features I: Flare Ribbons
In this paper we present a topological magnetic field investigation of seven
two-ribbon flares in sigmoidal active regions observed with Hinode, STEREO, and
SDO. We first derive the 3D coronal magnetic field structure of all regions
using marginally unstable 3D coronal magnetic field models created with the
flux rope insertion method. The unstable models have been shown to be a good
model of the flaring magnetic field configurations. Regions are selected based
on their pre-flare configurations along with the appearance and observational
coverage of flare ribbons, and the model is constrained using pre-flare
features observed in extreme ultraviolet and X-ray passbands. We perform a
topology analysis of the models by computing the squashing factor, Q, in order
to determine the locations of prominent quasi-separatrix layers (QSLs). QSLs
from these maps are compared to flare ribbons at their full extents. We show
that in all cases the straight segments of the two J-shaped ribbons are matched
very well by the flux-rope-related QSLs, and the matches to the hooked segments
are less consistent but still good for most cases. In addition, we show that
these QSLs overlay ridges in the electric current density maps. This study is
the largest sample of regions with QSLs derived from 3D coronal magnetic field
models, and it shows that the magnetofrictional modeling technique that we
employ gives a very good representation of flaring regions, with the power to
predict flare ribbon locations in the event of a flare following the time of
the model
Evaluation of a wind-tunnel gust response technique including correlations with analytical and flight test results
A wind tunnel technique for obtaining gust frequency response functions for use in predicting the response of flexible aircraft to atmospheric turbulence is evaluated. The tunnel test results for a dynamically scaled cable supported aeroelastic model are compared with analytical and flight data. The wind tunnel technique, which employs oscillating vanes in the tunnel throat section to generate a sinusoidally varying flow field around the model, was evaluated by use of a 1/30 scale model of the B-52E airplane. Correlation between the wind tunnel results, flight test results, and analytical predictions for response in the short period and wing first elastic modes of motion are presented
Derivation of reduced two-dimensional fluid models via Dirac's theory of constrained Hamiltonian systems
We present a Hamiltonian derivation of a class of reduced plasma
two-dimensional fluid models, an example being the Charney-Hasegawa-Mima
equation. These models are obtained from the same parent Hamiltonian model,
which consists of the ion momentum equation coupled to the continuity equation,
by imposing dynamical constraints. It is shown that the Poisson bracket
associated with these reduced models is the Dirac bracket obtained from the
Poisson bracket of the parent model
Lie algebraic noncommuting structures from reparametrisation symmetry
We extend our earlier work of revealing both space-space and space-time
noncommuting structures in various models in particle mechanics exhibiting
reparametrisation symmetry. We show explicitly (in contrast to the earlier
results in our paper \cite{sg}) that for some special choices of the
reparametrisation parameter , one can obtain space-space noncommuting
structures which are Lie-algebraic in form even in the case of the relativistic
free particle. The connection of these structures with the existing models in
the literature is also briefly discussed. Further, there exists some values of
for which the noncommutativity in the space-space sector can be made
to vanish. As a matter of internal consistency of our approach, we also study
the angular momentum algebra in details.Comment: 9 pages Latex, some references adde
Hyperfine-mediated gate-driven electron spin resonance
An all-electrical spin resonance effect in a GaAs few-electron double quantum
dot is investigated experimentally and theoretically. The magnetic field
dependence and absence of associated Rabi oscillations are consistent with a
novel hyperfine mechanism. The resonant frequency is sensitive to the
instantaneous hyperfine effective field, and the effect can be used to detect
and create sizable nuclear polarizations. A device incorporating a micromagnet
exhibits a magnetic field difference between dots, allowing electrons in either
dot to be addressed selectively.Comment: related papers available at http://marcuslab.harvard.ed
Coherent spin manipulation in an exchange-only qubit
Initialization, manipulation, and measurement of a three-spin qubit are
demonstrated using a few-electron triple quantum dot, where all operations can
be driven by tuning the nearest-neighbor exchange interaction. Multiplexed
reflectometry, applied to two nearby charge sensors, allows for qubit readout.
Decoherence is found to be consistent with predictions based on gate voltage
noise with a uniform power spectrum. The theory of the exchange-only qubit is
developed and it is shown that initialization of only two spins suffices for
operation. Requirements for full multi-qubit control using only exchange and
electrostatic interactions are outlined.Comment: related work at http://marcuslab.harvard.ed
On the use of projectors for Hamiltonian systems and their relationship with Dirac brackets
The role of projectors associated with Poisson brackets of constrained
Hamiltonian systems is analyzed. Projectors act in two instances in a bracket:
in the explicit dependence on the variables and in the computation of the
functional derivatives. The role of these projectors is investigated by using
Dirac's theory of constrained Hamiltonian systems. Results are illustrated by
three examples taken from plasma physics: magnetohydrodynamics, the
Vlasov-Maxwell system, and the linear two-species Vlasov system with
quasineutrality
Minimizing Strong Telluric Absorption in Near Infra-red Stellar Spectra
We have obtained high resolution spectra (R = 25000) of an A star over
varying airmass to determine the effectiveness of telluric removal in the limit
of high signal to noise. The near infra-red line HeI at 2.058 microns, which is
a sensitive indicator of physical conditions in massive stars, supergiants, HII
regions and YSOs, resides among pressure broadened telluric absorption from
carbon dioxide and water vapor that varies both in time and with observed
airmass.
Our study shows that in the limit of bright stars at high resolution,
accuracies of 5% are typical for high airmass observations (greater than 1.9),
improving to a photon-limited accuracy of 2% at smaller airmasses (less than
1.15). We find that by using the continuum between telluric absorption lines of
a ro-vibrational fan a photon-limited 1% accuracy is achievable.Comment: 14 pages, 7 figures. Accepted for publication in PAS
Painting the ideal home: using art to express visions of technologically supported independent living for older people in North East England
This paper describes the investigation of the development of future technological products to support older people in everyday living through the agency of a community art group. Recent research has identified a number of challenges facing designers seeking to use traditional participatory design approaches to gather technology requirements data from older people. Here, a project is described that sought to get a group of older people to think creatively about their needs and desires for technological support through the medium of paint. The artistic expression technique described in this article allowed the identification of issues that had also been found by previous research that used a range of different techniques. This indicates that the approach shows promise, as it allows information to be gathered in an environment that is comfortable and familiar using methods already known by the participants and which they find enjoyable. It provides a complement (or possible alternative) to standard protocols and has the potential benefit of extracting even richer information as the primary task for participants is enjoyable in its own right and is not associated with an interrogative process. Furthermore, it is argued that some of the key risks of traditional approaches are lessened or removed by the naturalistic setting of this approach
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