1,062 research outputs found
White paper: A plan for cooperation between NASA and DARPA to establish a center for advanced architectures
Large, complex computer systems require many years of development. It is recognized that large scale systems are unlikely to be delivered in useful condition unless users are intimately involved throughout the design process. A mechanism is described that will involve users in the design of advanced computing systems and will accelerate the insertion of new systems into scientific research. This mechanism is embodied in a facility called the Center for Advanced Architectures (CAA). CAA would be a division of RIACS (Research Institute for Advanced Computer Science) and would receive its technical direction from a Scientific Advisory Board established by RIACS. The CAA described here is a possible implementation of a center envisaged in a proposed cooperation between NASA and DARPA
The Whole Heliosphere Interval in the Context of a Long and Structured Solar Minimum: An Overview from Sun to Earth
Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low as, or lower than, observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI 1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minimum (e.g., WHI 2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June – July 2009. At this point (e.g., WHI 3: CR 2085) the Sun–Earth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008 – 2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun’s interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth’s space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere
Magnetocaloric properties of (RE)GaO (RE=Tb,Gd,Nd,Dy)
We report the characteristic magnetic properties of several members of the
rare earth garnet family, GdGaO (GGG), DyGaO
(DGG), TbGaO (TGG), and NdGaO (NGG), and compare
their relative potential utility for magnetocaloric cooling, including their
minimal adiabatic demagnetisation refrigeration (ADR) temperatures and relative
cooling parameters. A main objective of this work was to find potential
improvements over the magnetocaloric properties of GGG for use in low
temperature ADR cryostats. Using Tb and Dy in the RE-site offers
in principle higher saturation magnetisation and Nd gives a lower de
Gennes factor and therefore potentially low transition temperature. Our results
show that DyGaO yields an optimal relative cooling parameter
() at low applied fields and a low transition temperature, which would
allow for the design of more efficient ADR cryostats.Comment: 10 pages, 10 figures, submitted to Physical Review Applie
Magnetic excitations of the Cu quantum spin chain in SrCuPtO
We report the magnetic excitation spectrum as measured by inelastic neutron
scattering for a polycrystalline sample of SrCuPtO. Modeling the data
by the 2+4 spinon contributions to the dynamical susceptibility within the
chains, and with interchain coupling treated in the random phase approximation,
accounts for the major features of the powder-averaged structure factor. The
magnetic excitations broaden considerably as temperature is raised, persisting
up to above 100 K and displaying a broad transition as previously seen in the
susceptibility data. No spin gap is observed in the dispersive spin excitations
at low momentum transfer, which is consistent with the gapless spinon continuum
expected from the coordinate Bethe ansatz. However, the temperature dependence
of the excitation spectrum gives evidence of some very weak interchain
coupling.Comment: 9 pages, 5 figure
Optimized signal deduction procedure for the MIEZE neutron spectroscopy technique
We report a method to determine the phase and amplitude of sinusoidally
modulated event rates, binned into 4 bins per oscillation. The presented
algorithm relies on a reconstruction of the unknown parameters. It omits a
calculation intensive fitting procedure and avoids contrast reduction due to
averaging effects. It allows the current data acquisition bottleneck to be
relaxed by a factor of 4. Here, we explain the approach in detail and compare
it to the established fitting procedures of time series having 4 and 16 time
bins per oscillation. In addition we present the empirical estimates of the
errors of the three methods and compare them to each other. We show that the
reconstruction is unbiased, asymptotic, and efficient for estimating the phase.
Reconstructing the contrast, which corresponds to the amplitude of the
modulation, is roughly 10% less efficient than fitting 16 time binned
oscillations. Finally, we give analytical equations to estimate the error for
phase and contrast as a function of their initial values and counting
statistics.Comment: 14 pages, 5 figures, submitted to IOP Measurement Science and
Technolog
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