17,178 research outputs found
(k,q)-Compressed Sensing for dMRI with Joint Spatial-Angular Sparsity Prior
Advanced diffusion magnetic resonance imaging (dMRI) techniques, like
diffusion spectrum imaging (DSI) and high angular resolution diffusion imaging
(HARDI), remain underutilized compared to diffusion tensor imaging because the
scan times needed to produce accurate estimations of fiber orientation are
significantly longer. To accelerate DSI and HARDI, recent methods from
compressed sensing (CS) exploit a sparse underlying representation of the data
in the spatial and angular domains to undersample in the respective k- and
q-spaces. State-of-the-art frameworks, however, impose sparsity in the spatial
and angular domains separately and involve the sum of the corresponding sparse
regularizers. In contrast, we propose a unified (k,q)-CS formulation which
imposes sparsity jointly in the spatial-angular domain to further increase
sparsity of dMRI signals and reduce the required subsampling rate. To
efficiently solve this large-scale global reconstruction problem, we introduce
a novel adaptation of the FISTA algorithm that exploits dictionary
separability. We show on phantom and real HARDI data that our approach achieves
significantly more accurate signal reconstructions than the state of the art
while sampling only 2-4% of the (k,q)-space, allowing for the potential of new
levels of dMRI acceleration.Comment: To be published in the 2017 Computational Diffusion MRI Workshop of
MICCA
Wind-driven Accretion in Protoplanetary Disks. I: Suppression of the Magnetorotational Instability and Launching of the Magnetocentrifugal Wind
We perform local, vertically stratified shearing-box MHD simulations of
protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account
the effects of both Ohmic resistivity and ambipolar diffusion (AD). The
magnetic diffusion coefficients are evaluated self-consistently from a look-up
table based on equilibrium chemistry. We first show that the inclusion of AD
dramatically changes the conventional picture of layered accretion. Without net
vertical magnetic field, the system evolves into a toroidal field dominated
configuration with extremely weak turbulence in the far-UV ionization layer
that is far too inefficient to drive rapid accretion. In the presence of a weak
net vertical field (plasma beta~10^5 at midplane), we find that the MRI is
completely suppressed, resulting in a fully laminar flow throughout the
vertical extent of the disk. A strong magnetocentrifugal wind is launched that
efficiently carries away disk angular momentum and easily accounts for the
observed accretion rate in PPDs. Moreover, under a physical disk wind geometry,
all the accretion flow proceeds through a strong current layer with thickness
of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4
times the sound speed. Both Ohmic resistivity and AD are essential for the
suppression of the MRI and wind launching. The efficiency of wind transport
increases with increasing net vertical magnetic flux and the penetration depth
of the FUV ionization. Our laminar wind solution has important implications on
planet formation and global evolution of PPDs.Comment: 23 pages, 13 figures, accepted to Ap
Dynamics of Solids in the Midplane of Protoplanetary Disks: Implications for Planetesimal Formation
(Abridged) We present local 2D and 3D hybrid numerical simulations of
particles and gas in the midplane of protoplanetary disks (PPDs) using the
Athena code. The particles are coupled to gas aerodynamically, with
particle-to-gas feedback included. Magnetorotational turbulence is ignored as
an approximation for the dead zone of PPDs, and we ignore particle self-gravity
to study the precursor of planetesimal formation. Our simulations include a
wide size distribution of particles, ranging from strongly coupled particles
with dimensionless stopping time tau_s=Omega t_stop=1e-4 to marginally coupled
ones with tau_s=1 (where Omega is the orbital frequency, t_stop is the particle
friction time), and a wide range of solid abundances. Our main results are: 1.
Particles with tau_s>=0.01 actively participate in the streaming instability,
generate turbulence and maintain the height of the particle layer before
Kelvin-Helmholtz instability is triggered. 2. Strong particle clumping as a
consequence of the streaming instability occurs when a substantial fraction of
the solids are large (tau_s>=0.01) and when height-integrated solid to gas mass
ratio Z is super-solar. 3. The radial drift velocity is reduced relative to the
conventional Nakagawa-Sekiya-Hayashi (NSH) model, especially at high Z. We
derive a generalized NSH equilibrium solution for multiple particle species
which fits our results very well. 4. Collision velocity between particles with
tau_s>=0.01 is dominated by differential radial drift, and is strongly reduced
at larger Z. 5. There exist two positive feedback loops with respect to the
enrichment of local disk solid abundance and grain growth. All these effects
promote planetesimal formation.Comment: 25 pages (emulate apj), accepted to Ap
Dynamics of entanglement in the transverse Ising model
We study the evolution of nearest-neighbor entanglement in the one
dimensional Ising model with an external transverse field. The system is
initialized as the so called "thermal ground state" of the pure Ising model. We
analyze properties of generation of entanglement for different regions of
external transverse fields. We find that the derivation of the time at which
the entanglement reaches its first maximum with respect to the reciprocal
transverse field has a minimum at the critical point. This is a new indicator
of quantum phase transition.Comment: To be published in PR
The Hilbert Space of Quantum Gravity Is Locally Finite-Dimensional
We argue in a model-independent way that the Hilbert space of quantum gravity
is locally finite-dimensional. In other words, the density operator describing
the state corresponding to a small region of space, when such a notion makes
sense, is defined on a finite-dimensional factor of a larger Hilbert space.
Because quantum gravity potentially describes superpo- sitions of different
geometries, it is crucial that we associate Hilbert-space factors with spatial
regions only on individual decohered branches of the universal wave function.
We discuss some implications of this claim, including the fact that quantum
field theory cannot be a fundamental description of Nature.Comment: Essay written for the Gravity Research Foundation 2017 Awards for
Essays on Gravitation. 6 page
Millet agriculture dispersed from Northeast China to the Russian Far East: integrating archaeology, genetics, and linguistics
Broomcorn and foxtail millets were being cultivated in the West Liao River basin in Northeast China by at least the sixth millennium BCE. However, when and how millet agriculture spread from there to the north and east remains poorly understood. Here, we trace the dispersal of millet agriculture from Northeast China to the Russian Far East and weigh demic against cultural diffusion as mechanisms for that dispersal. We compare two routes for the spread of millet into the Russian Far East discussed in previous research—an inland route across Manchuria, and a coastal/inland route initially following the Liaodong Peninsula and Yalu River—using an archaeological dataset including millet remains, pottery, stone tools, spindle whorls, jade and figurines. We then integrate the archaeological evidence with linguistic and genetic findings in an approach we term ‘triangulation’. We conclude that an expansion of agricultural societies in Northeast China during the Middle to Late Hongshan (4000–3000 BCE) coincided with the arrival of millet cultivation in eastern Heilongjiang and the Primorye province of the Russian Far East. Our findings support the inland, Manchuria route for the dispersal of millet to the Primorye and suggest that, as well as long-distance cultural exchange, demic diffusion was also involved. Our results are broadly compatible with the farming/language dispersal hypothesis and consistent with a link between the spread of millet farming and proto-Tungusic, the language ancestral to the contemporary Tungusic languages, in late Neolithic Northeast Asia. © 2020 The Author
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Ecological thresholds and large carnivores conservation: Implications for the Amur tiger and leopard in China
The ecological threshold concept describes how changes in one or more factors at thresholds can result in a large shift in the state of an ecosystem. This concept focuses attention on limiting factors that affect the tolerance of systems or organisms and changes in them. Accumulating empirical evidence for the existence of ecological thresholds has created favorable conditions for practical application to wildlife conservation. Applying the concept has the potential to enhance conservation of two large carnivores, Amur tiger and leopard, and the knowledge gained could guide the construction of a proposed national park. In this review, ecological thresholds that result from considering a paradigm of bottom-up control were evaluated for their potential to contribute to the conservation of Amur tiger and leopard. Our review highlights that large carnivores, as top predators, are potentially affected by ecological thresholds arising from changes in climate (or weather), habitat, vegetation, prey, competitors, and anthropogenic disturbances. What's more, interactions between factors and context dependence need to be considered in threshold research and conservation practice, because they may amplify the response of ecosystems or organisms to changes in specific drivers. Application of the threshold concept leads to a more thorough evaluation of conservation needs, and could be used to guide future Amur tiger and leopard research and conservation in China. Such application may inform the conservation of other large carnivores worldwide
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