23 research outputs found
Radial evolution of the wave-vector anisotropy of solar wind turbulence between 0.3 and 1 AU
We present observations of the power spectral anisotropy in wave-vector space
of solar wind turbulence, and study how it evolves in interplanetary space with
increasing heliocentric distance. For this purpose we use magnetic field
measurements made by the Helios-2 spacecraft at three positions between 0.29
and 0.9 AU. To derive the power spectral density (PSD) in -space based on single-satellite measurements is a challenging task not
yet accomplished previously. Here we derive the spectrum
(, ) from the spatial
correlation function by a
transformation according to the projection-slice theorem. We find the so
constructed PSDs to be distributed in k-space mainly along a ridge that is more
inclined toward the than axis, a new result
which probably indicates preferential cascading of turbulent energy along the
direction. Furthermore, this ridge of the distribution is found
to gradually get closer to the axis, as the outer scale length of
the turbulence becomes larger while the solar wind flows further away from the
Sun. In the vicinity of the axis, there appears a minor
spectral component that probably corresponds to quasi-parallel Alfv\'enic
fluctuations. Their relative contribution to the total spectral density tends
to decrease with radial distance. These findings suggest that solar wind
turbulence undergoes an anisotropic cascade transporting most of its magnetic
energy towards larger , and that the anisotropy in the inertial
range is radially developing further at scales that are relatively far from the
ever increasing outer scale
Cactus Representations in Polylogarithmic Max-flow via Maximal Isolating Mincuts
A cactus representation of a graph, introduced by Dinitz et al. in 1976, is
an edge sparsifier of size that exactly captures all global minimum cuts
of the graph. It is a central combinatorial object that has been a key
ingredient in almost all algorithms for the connectivity augmentation problems
and for maintaining minimum cuts under edge insertions (e.g. [NGM97], [CKL+22],
[Hen97]). This sparsifier was generalized to Steiner cactus for a vertex set
, which can be seen as a vertex sparsifier of size that captures
all partitions of corresponding to a -Steiner minimum cut, and also
hypercactus, an analogous concept in hypergraphs. These generalizations further
extend the applications of cactus to the Steiner and hypergraph settings.
In a long line of work on fast constructions of cactus and its
generalizations, a near-linear time construction of cactus was shown by [Karger
and Panigrahi 2009]. Unfortunately, their technique based on tree packing
inherently does not generalize. The state-of-the-art algorithms for Steiner
cactus and hypercactus are still slower than linear time by a factor of
[DV94] and [CX17], respectively.
We show how to construct both Steiner cactus and hypercactus using
polylogarithmic calls to max flow, which gives the first almost-linear time
algorithms of both problems. The constructions immediately imply
almost-linear-time connectivity augmentation algorithms in the Steiner and
hypergraph settings, as well as speed up the incremental algorithm for
maintaining minimum cuts in hypergraphs by a factor of .
The key technique behind our result is a novel variant of the influential
isolating mincut technique [LP20, AKL+21] which we called maximal isolating
mincuts. This technique makes the isolating mincuts to be "more balanced"
which, we believe, will likely be useful in future applications.Comment: To appear in SODA 202
Cactus Representation of Minimum Cuts: Derandomize and Speed up
Given an undirected weighted graph with vertices and edges, we give
the first deterministic -time algorithm for constructing the cactus
representation of \emph{all} global minimum cuts. This improves the current
-time state-of-the-art deterministic algorithm, which can be
obtained by combining ideas implicitly from three papers [Karger JACM'2000, Li
STOC'2021, and Gabow TALG'2016] The known explicitly stated deterministic
algorithm has a runtime of [Fleischer 1999, Nagamochi and Nakao
2000]. Using our technique, we can even speed up the fastest randomized
algorithm of [Karger and Panigrahi, SODA'2009] whose running time is at least
to .Comment: SODA 202
Proton Heating in Solar Wind Compressible Turbulence with Collisions between Counter-propagating Waves
Magnetohydronamic turbulence is believed to play a crucial role in heating
the laboratorial, space, and astrophysical plasmas. However, the precise
connection between the turbulent fluctuations and the particle kinetics has not
yet been established. Here we present clear evidence of plasma turbulence
heating based on diagnosed wave features and proton velocity distributions from
solar wind measurements by the Wind spacecraft. For the first time, we can
report the simultaneous observation of counter-propagating magnetohydrodynamic
waves in the solar wind turbulence. Different from the traditional paradigm
with counter-propagating Alfv\'en waves, anti-sunward Alfv\'en waves (AWs) are
encountered by sunward slow magnetosonic waves (SMWs) in this new type of solar
wind compressible turbulence. The counter-propagating AWs and SWs correspond
respectively to the dominant and sub-dominant populations of the imbalanced
Els\"asser variables. Nonlinear interactions between the AWs and SMWs are
inferred from the non-orthogonality between the possible oscillation direction
of one wave and the possible propagation direction of the other. The associated
protons are revealed to exhibit bi-directional asymmetric beams in their
velocity distributions: sunward beams appearing in short and narrow patterns
and anti-sunward broad extended tails. It is suggested that multiple types of
wave-particle interactions, i.e., cyclotron and Landau resonances with AWs and
SMWs at kinetic scales, are taking place to jointly heat the protons
perpendicularly and parallel
Combining Multiple Query Interface Matchers Using Dempster-Shafer Theory of Evidence
Matching query interfaces is a crucial step in data integration across multiple Web databases. The problem is closely related to schema matching that typically exploits different features of schemas. Relying on a particular feature of schemas is not sufficient. We propose an evidential approach to combining multiple matchers using Dempster-Shafer theory of evidence. First, our approach views the match results of an individual matcher as a source of evidence that provides a level of confidence on the validity of each candidate attribute correspondence. Second, it combines multiple sources of evidence to get a combined mass function that represents the overall level of confidence, taking into account the match results of different matchers. Our combination mechanism does not require use of weighing parameters, hence no setting and tuning of them is needed. Third, it selects the top k attribute correspondences of each source attribute from the target schema based on the combined mass function. Our experimental results show that our approach is highly accurate and effective.
Adaptive Finite-Time Command Filtered Fault-Tolerant Control for Uncertain Spacecraft with Prescribed Performance
In this paper, an adaptive finite-time fault-tolerant control scheme is proposed for the attitude stabilization of rigid spacecrafts. A first-order command filter is presented at the second step of the backstepping design to approximate the derivative of the virtual control, such that the singularity problem caused by the differentiation of the virtual control is avoided. Then, an adaptive fuzzy finite-time backstepping controller is developed to achieve the finite-time attitude stabilization subject to inertia uncertainty, external disturbance, actuator saturation, and faults. Through using an error transformation, the prescribed performance boundary is incorporated into the controller design to guarantee the prescribed performance of the system output. Numerical simulations demonstrate the effectiveness of the proposed scheme
Influence of extra La and annealing temperature on microstructure and magnetocaloric properties of La-Fe-Co-Si alloys
The microstructure, phase constitution and magnetocaloric properties of non-stoichiometric La1.7Fe11Co0.8Si1.2 alloys were investigated. Compared with stoichiometric La(Fe,Co,Si)(13) alloys, a complex phase, including La(Fe,Co,Si)(13) phase, La5Si3 phase, La(Fe,Co)Si phase, alpha-Fe phase and La oxide, can rapidly form upon annealing at 1323 K for 24 h in the present alloy. The resulting alloy exhibits a large magnetic entropy change of 7.5 J kg(-1) K-1 at 271 K in a magnetic field change of 2 T. In addition, the annealed La1.7Fe11Co0.8Si1.2 alloy shows a reversible magnetic phase transition with zero hysteresis. These characteristics indicate that the present La-rich La-Fe-Co-Si compounds have potential for the applications as ambient magnetic refrigeration materials. (C) 2015 Elsevier B.V. All rights reserved