1,201 research outputs found
On the two-dimensional state in driven magnetohydrodynamic turbulence
The dynamics of the two-dimensional (2D) state in driven tridimensional (3D)
incompressible magnetohydrodynamic turbulence is investigated through
high-resolution direct numerical simulations and in the presence of an external
magnetic field at various intensities. For such a flow the 2D state (or slow
mode) and the 3D modes correspond respectively to spectral fluctuations in the
plan and in the area . It is shown that if
initially the 2D state is set to zero it becomes non negligible in few turnover
times particularly when the external magnetic field is strong. The maintenance
of a large scale driving leads to a break for the energy spectra of 3D modes;
when the driving is stopped the previous break is removed and a decay phase
emerges with alfv\'enic fluctuations. For a strong external magnetic field the
energy at large perpendicular scales lies mainly in the 2D state and in all
situations a pinning effect is observed at small scales.Comment: 11 pages, 11 figure
Comment on "Correlated electron-nuclear dynamics: Exact factorization of the molecular wavefunction" [J. Chem. Phys. 137, 22A530 (2012)]
In spite of the relevance of the proposal introduced in the recent work A.
Abedi, N. T. Maitra and E. K. U. Gross, J. Chem. Phys. 137, 22A530, 2012, there
is an important ingredient which is missing. Namely, the proof that the norms
of the electronic and nuclear wavefunctions which are the solutions to the
nonlinear equations of motion are preserved by the evolution. To prove the
conservation of these norms is precisely the objective of this Comment.Comment: 2 pages, published versio
On spectral scaling laws for incompressible anisotropic MHD turbulence
A heuristic model is given for anisotropic magnetohydrodynamics (MHD)
turbulence in the presence of a uniform external magnetic field B_0 {\bf {\hat
e}_{\pa}}. The model is valid for both moderate and strong and is able
to describe both the strong and weak wave turbulence regimes as well as the
transition between them. The main ingredient of the model is the assumption of
constant ratio at all scales between \add{the} linear wave period and \add{the}
nonlinear turnover timescale. Contrary to the model of critical balance
introduced by Goldreich and Sridhar [P. Goldreich and S. Sridhar, ApJ {\bf
438}, 763 (1995)], it is not assumed in addition that this ratio be equal to
unity at all scales which allows us to use the Iroshnikov-Kraichnan
phenomenology. It is then possible to recover the widely observed anisotropic
scaling law \kpa \propto \kpe^{2/3} between parallel and perpendicular
wavenumbers (with reference to B_0 {\bf {\hat e}_{\pa}}) and to obtain the
universal prediction, , for the total energy spectrum
E(\kpe,\kpa) \sim \kpe^{-\alpha} \kpa^{-\beta}. In particular, with such a
prediction the weak Alfv\'en wave turbulence constant-flux solution is
recovered and, for the first time, a possible explanation to its precursor
found numerically by Galtier et al [S. Galtier et al., J. Plasma Phys. {\bf
63}, 447 (2000)] is given
Spectral features of solar wind turbulent plasma
Spectral properties of a fully compressible solar wind Hall
Magnetohydrodynamic plasma are investigated by means of time dependent three
dimensional Hall MHD simulations. Our simulations, in agreement with spacecraft
data, identify a spectral break in turbulence spectra at characteristic
length-scales associated with electromagnetic fluctuations that are smaller
than the ion gyroradius. In this regime, our 3D simulations show that turbulent
spectral cascades in the presence of a mean magnetic field follow an
omnidirectional anisotropic inertial range spectrum close to . The
onset of the spectral break in our simulations can be ascribed to the presence
of nonlinear Hall interactions that modify the spectral cascades. Our
simulations further show that the underlying charachteristic turbulent
fluctuations are spectrally anisotropic, the extent of which depends critically
on the local wavenumber. The fluctuations associated with length scales smaller
than the ion gyroradius are highly compressible and tend to exhibit a near
equipartition in the velocity and magnetic fields. Finally, we find that the
orientation of velocity and magnetic field fluctuations critically determine
the character of nonlinear interactions that predominantly govern a Hall MHD
plasma, like the solar wind.Comment: This paper is accepted for publication in Monthly Notices of the
Royal Astronomical Society Main Journa
Diversity of large woody lignophytes preceding the extinction of Archaeopteris: new data from the middle Tournaisian of Thuringia (Germany)
International audienceAnatomically preserved axes representing three lignophyte species occur in the middle Tournaisian deposit of Kahlleite in Thuringia. One is characterized by a small oval eustele, short uniseriate rays, and alternate distichous phyllotaxy. It is assigned to the progymnosperm genus Protopitys. The two others share a eustelic primary vascular system comprising a parenchymatous pith and numerous xylem strands in a peripheral position. The secondary xylem comprises rays that are mostly uniseriate and rarely exceed 20 cells in height. One is referred to as Eristophyton sp.; the second, characterized by ray cells showing a wide range of sizes and shapes is assigned to Aporoxylon primigenium. These records extend the stratigraphical range of Protopitys and Eristophyton down to the middle Tournaisian and confirm their great longevity through most of the Mississippian. They suggest that the diversity of putative arborescent lignophytes co-occurring with Archaeopteris around the D/C boundary but surviving successfully above this limit has been underestimated
Parallel electric field generation by Alfven wave turbulence
{This work aims to investigate the spectral structure of the parallel
electric field generated by strong anisotropic and balanced Alfvenic turbulence
in relation with the problem of electron acceleration from the thermal
population in solar flare plasma conditions.} {We consider anisotropic Alfvenic
fluctuations in the presence of a strong background magnetic field. Exploiting
this anisotropy, a set of reduced equations governing non-linear, two-fluid
plasma dynamics is derived. The low- limit of this model is used to
follow the turbulent cascade of the energy resulting from the non-linear
interaction between kinetic Alfven waves, from the large magnetohydrodynamics
(MHD) scales with down to the small "kinetic" scales
with , being the ion sound gyroradius.}
{Scaling relations are obtained for the magnitude of the turbulent
electromagnetic fluctuations, as a function of and ,
showing that the electric field develops a component parallel to the magnetic
field at large MHD scales.} {The spectrum we derive for the parallel electric
field fluctuations can be effectively used to model stochastic resonant
acceleration and heating of electrons by Alfven waves in solar flare plasma
conditions
Spin-orbit torque from the introduction of Cu interlayers in Pt/Cu/Co/Pt nanolayered structures for spintronic devices
Spin currents can modify the magnetic state of ferromagnetic ultrathin films through spin-orbit torque. They may be generated by means of spin-orbit interactions by either bulk or interfacial phenomena. Electrical transport measurements reveal a 6-fold increase of the spin-orbit torque accompanied by a drastic reduction of the spin Hall magnetoresistance upon the introduction of an ultrathin Cu interlayer in a Pt/Cu/Co/Pt structure with perpendicular magnetic anisotropy. We analyze the dependence of the spin Hall magnetoresistance with the thickness of the interlayer, ranging from 0.5 to 15 nm, in the frame of a drift diffusion model that provides information on the expected spin currents and spin accumulations in the system. The results demonstrate that the major responsibility of both effects is spin memory loss at the interface. The enhancement of the spin-orbit torque when introducing an interlayer opens the possibility to design more efficient spintronic devices based on materials that are cheap and abundant such as copper. More specifically, spin-orbit torque magnetic random access memories and spin logic devices could benefit from the spin-orbit torque enhancement and cheaper material usage presented in this study
Anharmonicity And The Fano Effect In The To () Phonon Mode Of Gallium Phosphide
The Raman line shape of GaP has been measured under hydrostatic pressure up to 106 kbar at room temperature. The results are interpreted with a model involving a "Fano-type" resonance between the single TO () phonon mode and several double-phonon densities of states. This model allow us to determine for the first time the sign of the anharmonic interaction. In contradiction to previous interpretations, no anomalous behavior of the parameters used in this model is found as a function of the pressure. © 1983 The American Physical Society.28127334733
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