101,080 research outputs found
Energy spectrum, dissipation and spatial structures in reduced Hall magnetohydrodynamic
We analyze the effect of the Hall term in the magnetohydrodynamic turbulence
under a strong externally supported magnetic field, seeing how this changes the
energy cascade, the characteristic scales of the flow and the dynamics of
global magnitudes, with particular interest in the dissipation.
Numerical simulations of freely evolving three-dimensional reduced
magnetohydrodynamics (RHMHD) are performed, for different values of the Hall
parameter (the ratio of the ion skin depth to the macroscopic scale of the
turbulence) controlling the impact of the Hall term. The Hall effect modifies
the transfer of energy across scales, slowing down the transfer of energy from
the large scales up to the Hall scale (ion skin depth) and carrying faster the
energy from the Hall scale to smaller scales. The final outcome is an effective
shift of the dissipation scale to larger scales but also a development of
smaller scales. Current sheets (fundamental structures for energy dissipation)
are affected in two ways by increasing the Hall effect, with a widening but at
the same time generating an internal structure within them. In the case where
the Hall term is sufficiently intense, the current sheet is fully delocalized.
The effect appears to reduce impulsive effects in the flow, making it less
intermittent.Comment: 17 pages, 10 figure
Effects of electron inertia in collisionless magnetic reconnection
We present a study of collisionless magnetic reconnection within the
framework of full two-fluid MHD for a completely ionized hydrogen plasma,
retaining the effects of the Hall current, electron pressure and electron
inertia. We performed 2.5D simulations using a pseudo-spectral code with no
dissipative effects. We check that the ideal invariants of the problem are
conserved down to round-off errors. Our results show that the change in the
topology of the magnetic field lines is exclusively due to the presence of
electron inertia. The computed reconnection rates remain a fair fraction of the
Alfv\'en velocity, which therefore qualifies as fast reconnection
Phase Diagram and Incommensurate Phases in Undoped Manganites
We study the existence of incommensurate phases in the phase diagram of the
two orbital double exchange model coupled with Jahn-Teller phonons and with
superexchange interactions. In agreement with experimental results, we find
that undoped manganites ( being some rare earth element) show
temperature induced commensurate-incommensurate phase transitions. In the
incommensurate phase the magnetic wave vector varies with temperature. The
incommensurate phase arises from the competition between the short range
antiferromagnetic superexchange interaction and the long range ferromagnetic
double exchange interaction
Geometric Langevin equations on submanifolds and applications to the stochastic melt-spinning process of nonwovens and biology
In this article we develop geometric versions of the classical Langevin
equation on regular submanifolds in euclidean space in an easy, natural way and
combine them with a bunch of applications. The equations are formulated as
Stratonovich stochastic differential equations on manifolds. The first version
of the geometric Langevin equation has already been detected before by
Leli\`evre, Rousset and Stoltz with a different derivation. We propose an
additional extension of the models, the geometric Langevin equations with
velocity of constant absolute value. The latters are seemingly new and provide
a galaxy of new, beautiful and powerful mathematical models. Up to the authors
best knowledge there are not many mathematical papers available dealing with
geometric Langevin processes. We connect the first version of the geometric
Langevin equation via proving that its generator coincides with the generalized
Langevin operator proposed by Soloveitchik, Jorgensen and Kolokoltsov. All our
studies are strongly motivated by industrial applications in modeling the fiber
lay-down dynamics in the production process of nonwovens. We light up the
geometry occuring in these models and show up the connection with the spherical
velocity version of the geometric Langevin process. Moreover, as a main point,
we construct new smooth industrial relevant three-dimensional fiber lay-down
models involving the spherical Langevin process. Finally, relations to a class
of self-propelled interacting particle systems with roosting force are
presented and further applications of the geometric Langevin equations are
given
Solution of differential equations by application of transformation groups
Report applies transformation groups to the solution of systems of ordinary differential equations and partial differential equations. Lies theorem finds an integrating factor for appropriate invariance group or groups can be found and can be extended to partial differential equations
Positive current noise cross-correlations in capacitively coupled double quantum dots with ferromagnetic leads
We examine cross-correlations (CCs) in the tunneling currents through two
parallel interacting quantum dots coupled to four independent ferromagnetic
electrodes. We find that when either one of the two circuits is in the parallel
configuration with sufficiently strong polarization strength, a new mechanism
of dynamical spin blockade, i.e., a spin-dependent bunching of tunneling
events, governs transport through the system together with the inter-dot
Coulomb interaction, leading to a sign-reversal of the zero-frequency current
CC in the dynamical channel blockade regime, and to enhancement of positive
current CC in the dynamical channel anti-blockade regimes, in contrast to the
corresponding results for the case of paramagnetic leads.Comment: 9 pages, 3 figure
Inactivation of Mandelate Racemase by 3-Hydroxypyruvate Reveals a Potential Mechanistic Link between Enzyme Superfamilies
Mandelate racemase (MR), a member of the enolase superfamily, catalyzes the Mg2+-dependent interconversion of the enantiomers of mandelate. Several α-keto acids are modest competitive inhibitors of MR [e.g., mesoxalate (Ki = 1.8 ± 0.3 mM) and 3-fluoropyruvate (Ki = 1.3 ± 0.1 mM)], but, surprisingly, 3-hydroxypyruvate (3-HP) is an irreversible, time-dependent inhibitor (kinact/KI = 83 ± 8 M–1 s–1). Protection from inactivation by the competitive inhibitor benzohydroxamate, trypsinolysis and electrospray ionization tandem mass spectrometry analyses, and X-ray crystallographic studies reveal that 3-HP undergoes Schiff-base formation with Lys 166 at the active site, followed by formation of an aldehyde/enol(ate) adduct. Such a reaction is unprecedented in the enolase superfamily and may be a relic of an activity possessed by a promiscuous progenitor enzyme. The ability of MR to form and deprotonate a Schiff-base intermediate furnishes a previously unrecognized mechanistic link to other α/β-barrel enzymes utilizing Schiff-base chemistry and is in accord with the sequence- and structure-based hypothesis that members of the metal-dependent enolase superfamily and the Schiff-base-forming N-acetylneuraminate lyase superfamily and aldolases share a common ancestor
Long-range correlations and trends in Colombian seismic time series
We study long-range correlations and trends in time series extracted from the
data of seismic events occurred from 1973 to 2011 in a rectangular region that
contains mainly all the continental part of Colombia. The long-range
correlations are detected by the calculation of the Hurst exponents for the
time series of interevent intervals, separation distances, depth differences
and magnitude differences. By using a modification of the classical
method that has been developed to detect short-range correlations in time
series, we find the existence of persistence for all the time series considered
except for magnitude differences. We find also, by using the until the
third order, that the studied time series are not influenced by trends.
Additionally, an analysis of the Hurst exponent as a function of the number of
events in the time and the maximum window size is presented.Comment: 21 pages, 6 figures, 2 figures added, types corrected, accepted to be
published in Physica
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