24,609 research outputs found
Directly comparing coronal and solar wind elemental fractionation
As the solar wind propagates through the heliosphere, dynamical processes
irreversibly erase the signatures of the near-Sun heating and acceleration
processes. The elemental fractionation of the solar wind should not change
during transit however, making it an ideal tracer of these processes. We aimed
to verify directly if the solar wind elemental fractionation is reflective of
the coronal source region fractionation, both within and across different solar
wind source regions. A backmapping scheme was used to predict where solar wind
measured by the Advanced Composition Explorer (ACE) originated in the corona.
The coronal composition measured by the Hinode Extreme ultraviolet Imaging
Spectrometer (EIS) at the source regions was then compared with the in-situ
solar wind composition. On hourly timescales there was no apparent correlation
between coronal and solar wind composition. In contrast, the distribution of
fractionation values within individual source regions was similar in both the
corona and solar wind, but distributions between different sources have
significant overlap. The matching distributions directly verifies that
elemental composition is conserved as the plasma travels from the corona to the
solar wind, further validating it as a tracer of heating and acceleration
processes. The overlap of fractionation values between sources means it is not
possible to identify solar wind source regions solely by comparing solar wind
and coronal composition measurements, but a comparison can be used to verify
consistency with predicted spacecraft-corona connections.Comment: Accepted version; 8 pages, 7 figure
Reentrance of disorder in the anisotropic shuriken Ising model
For a material to order upon cooling is common sense. What is more seldom is
for disorder to reappear at lower temperature, which is known as reentrant
behavior. Such resurgence of disorder has been observed in a variety of
systems, ranging from Rochelle salts to nematic phases in liquid crystals.
Frustration is often a key ingredient for reentrance mechanisms. Here we shall
study a frustrated model, namely the anisotropic shuriken lattice, which offers
a natural setting to explore an extension of the notion of reentrance between
magnetic disordered phases. By tuning the anisotropy of the lattice, we open a
window in the phase diagram where magnetic disorder prevails down to zero
temperature. In this region, the competition between multiple disordered ground
states gives rise to a double crossover where both the low- and
high-temperature regimes are less correlated than the intervening classical
spin liquid. This reentrance of disorder is characterized by an entropy
plateau, a multi-step Curie law crossover and a rather complex diffuse
scattering in the static structure factor. Those results are confirmed by
complementary numerical and analytical methods: Monte Carlo simulations,
Husimi-tree calculations and an exact decoration-iteration transformation.Comment: 16 pages, 13 figure
Living on the edge : ground-state selection in quantum spin-ice pyrochlores
The search for new quantum phases, especially in frustrated magnets, is
central to modern condensed matter physics. One of the most promising places to
look is in rare-earth pyrochlore magnets with highly-anisotropic exchange
interactions, materials closely related to the spin ices Ho2Ti2O7 and Dy2Ti2O7.
Here we establish a general theory of magnetic order in these materials. We
find that many of their most interesting properties can be traced back to the
accidental degeneracies where phases with different symmetry meet. These
include the ordered ground state selection by fluctuations in Er2Ti2O7, the
dimensional-reduction observed in Yb2Ti2O7, and the absence of magnetic order
in Er2Sn2O7.Comment: A long-paper version of this preprint, "Living on the Edge", appears
as arXiv:1603.09466 [accepted for publication in Physical Review B]. The text
of v2 is otherwise unchanged from v1 (Submitted on 14 Nov 2013
Experimental studies of hypersonic boundary-layer transition and effects of wind tunnel disturbances
Boundary layer transition data on cones and free stream disturbance levels were measured in a hypersonic wind tunnel and Mach 8 variable density hypersonic tunnel. Transition data were obtained by using different conical models and techniques for detecting the location of transition. The disturbance levels were measured by using hot wire anemometry and pressure transducers. The transition Reynolds numbers obtained from the tests correlated well when the fluctuating pressures measured at the surface of conical models were used as a correlating parameter
Evaluation of flow quality in two large NASA wind tunnels at transonic speeds
Wind tunnel testing of low drag airfoils and basic transition studies at transonic speeds are designed to provide high quality aerodynamic data at high Reynolds numbers. This requires that the flow quality in facilities used for such research be excellent. To obtain a better understanding of the characteristics of facility disturbances and identification of their sources for possible facility modification, detailed flow quality measurements were made in two prospective NASA wind tunnels. Experimental results are presented of an extensive and systematic flow quality study of the settling chamber, test section, and diffuser in the Langley 8 foot transonic pressure tunnel and the Ames 12 foot pressure wind tunnel. Results indicate that the free stream velocity and pressure fluctuation levels in both facilities are low at subsonic speeds and are so high as to make it difficult to conduct meaningful boundary layer control and transition studies at transonic speeds
Would Two Dimensions be World Enough for Spacetime?
We consider various curious features of general relativity, and relativistic
field theory, in two spacetime dimensions. In particular, we discuss: the
vanishing of the Einstein tensor; the failure of an initial-value formulation
for vacuum spacetimes; the status of singularity theorems; the non-existence of
a Newtonian limit; the status of the cosmological constant; and the character
of matter fields, including perfect fluids and electromagnetic fields. We
conclude with a discussion of what constrains our understanding of physics in
different dimensions.Comment: 31 pages, 1 figur
Direct Dark Matter Detection with Velocity Distribution in the Eddington approach
Exotic dark matter together with the vacuum energy (associated with the
cosmological constant) seem to dominate the Universe. Thus its direct detection
is central to particle physics and cosmology. Supersymmetry provides a natural
dark matter candidate, the lightest supersymmetric particle (LSP). One
essential ingredient in obtaining the direct detection rates is the density and
the velocity distribution of the LSP in our vicinity. In the present paper we
study simultaneously density profiles and velocity distributions in the context
of the Eddington approach. In such an approach, unlike the commonly assumed
Maxwell-Boltzmann (M-B) distribution, the upper bound of the velocity arises
naturally from the potential.Comment: 21 LaTex pages, 27 figure
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