1,257 research outputs found
Magnetic helicity in magnetohydrodynamic turbulence with a mean magnetic field
A computational investigation of magnetic helicity of the fluctuatingmagnetic fieldHm in ideal and freely decaying threeâdimensional (3âD) magnetohydrodynamics (MHD) in the presence of a uniform mean magnetic field is performed. It is shown that for ideal 3âD MHDHm, which is a rugged invariant in the absence of a mean magnetic field [Frisch et al., J. Fluid Mech. 77, 796 (1975)], decays from its initial value and proceeds to oscillate about zero. The decay of Hm is shown to result from the presence of a new ââgeneralizedââ helicity invariant, which includes contributions from the uniform magnetic field. The loss of invariance of Hm will diminish the effects of inverse transfer of Hm on freely decaying turbulence. This is demonstrated in a discussion of the selective decay relaxation process
Measurement of the electric fluctuation spectrum of magnetohydrodynamic turbulence
Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show
the spectral behavior of classical Kolmogorov fluid turbulence over an inertial
subrange and departures from this at short wavelengths, where energy should be
dissipated. Here we present the first measurements of the electric field
fluctuation spectrum over the inertial and dissipative wavenumber ranges in a
plasma. The inertial subrange is observed and
agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed
in this regime is shown to be consistent with the Alfv\'en speed. At smaller
wavelengths the electric spectrum is softer and is consistent
with the expected dispersion relation of short-wavelength kinetic Alfv\'en
waves. Kinetic Alfv\'en waves damp on the solar wind ions and electrons and may
act to isotropize them. This effect may explain the fluid-like nature of the
solar wind.Comment: submitted; 4 pages + 3 figure
Spin-polarization-induced structural selectivity in Pd and Pt () compounds
Spin-polarization is known to lead to important {\it magnetic} and {\it
optical} effects in open-shell atoms and elemental solids, but has rarely been
implicated in controlling {\it structural} selectivity in compounds and alloys.
Here we show that spin-polarized electronic structure calculations are crucial
for predicting the correct crystal structures for Pd and Pt
compounds. Spin-polarization leads to (i) stabilization of the structure
over the structure in PtCr, PdCr, and PdMn, (ii) to the
stabilization of the structure over the structure in PdCo
and to (iii) ordering (rather than phase-separation) in PtCo and PdCr.
The results are analyzed in terms of first-principles local spin density
calculations.Comment: 4 pages, REVTEX, 3 eps figures, to appear in PR
Dynamo generated field emergence through recurrent plasmoid ejections
Magnetic buoyancy is believed to drive the transport of magnetic flux tubes
from the convection zone to the surface of the Sun. The magnetic fields form
twisted loop-like structures in the solar atmosphere. In this paper we use
helical forcing to produce a large-scale dynamo-generated magnetic field, which
rises even without magnetic buoyancy. A two layer system is used as
computational domain where the upper part represents the solar atmosphere.
Here, the evolution of the magnetic field is solved with the stress--and--relax
method. Below this region a magnetic field is produced by a helical forcing
function in the momentum equation, which leads to dynamo action. We find
twisted magnetic fields emerging frequently to the outer layer, forming
arch-like structures. In addition, recurrent plasmoid ejections can be found by
looking at space--time diagrams of the magnetic field. Recent simulations in
spherical coordinates show similar results.Comment: 4 pages, 8 figures, To appear in the proceedings of the IAU273
"Physics of Sun and Star Spots
Relativistic Proton Production During the 14 July 2000 Solar Event: The Case for Multiple Source Mechanisms
Protons accelerated to relativistic energies by transient solar and
interplanetary phenomena caused a ground-level cosmic ray enhancement on 14
July 2000, Bastille Day. Near-Earth spacecraft measured the proton flux
directly and ground-based observatories measured the secondary responses to
higher energy protons. We have modelled the arrival of these relativistic
protons at Earth using a technique which deduces the spectrum, arrival
direction and anisotropy of the high-energy protons that produce increased
responses in neutron monitors. To investigate the acceleration processes
involved we have employed theoretical shock and stochastic acceleration
spectral forms in our fits to spacecraft and neutron monitor data. During the
rising phase of the event (10:45 UT and 10:50 UT) we find that the spectrum
between 140 MeV and 4 GeV is best fitted by a shock acceleration spectrum. In
contrast, the spectrum at the peak (10:55 UT and 11:00 UT) and in the declining
phase (11:40 UT) is best fitted with a stochastic acceleration spectrum. We
propose that at least two acceleration processes were responsible for the
production of relativistic protons during the Bastille Day solar event: (1)
protons were accelerated to relativistic energies by a shock, presumably a
coronal mass ejection (CME). (2) protons were also accelerated to relativistic
energies by stochastic processes initiated by magnetohydrodynamic (MHD)
turbulence.Comment: 38 pages, 9 figures, accepted for publication in the Astrophysical
Journal, January, 200
Single-Molecule Dynamics of the Calcium-Dependent Activation of Plasma-Membrane Ca2+-ATPase by Calmodulin
AbstractThe plasma membrane calcium-ATPase (PMCA) helps to control cytosolic calcium levels by pumping out excess Ca2+. PMCA is regulated by the Ca2+ signaling protein calmodulin (CaM), which stimulates PMCA activity by binding to an autoinhibitory domain of PMCA. We used single-molecule polarization methods to investigate the mechanism of regulation of the PMCA by CaM fluorescently labeled with tetramethylrhodamine. The orientational mobility of PMCA-CaM complexes was determined from the extent of modulation of single-molecule fluorescence upon excitation with a rotating polarization. At a high Ca2+ concentration, the distribution of modulation depths reveals that CaM bound to PMCA is orientationally mobile, as expected for a dissociated autoinhibitory domain of PMCA. In contrast, at a reduced Ca2+ concentration a population of PMCA-CaM complexes appears with significantly reduced orientational mobility. This population can be attributed to PMCA-CaM complexes in which the autoinhibitory domain is not dissociated, and thus the PMCA is inactive. The presence of these complexes demonstrates the inadequacy of a two-state model of Ca2+ pump activation and suggests a regulatory role for the low-mobility state of the complex. When ATP is present, only the high-mobility state is detected, revealing an altered interaction between the autoinhibitory and nucleotide-binding domains
Integration of formal fault analysis in ASSERT: Case studies and lessons learnt
International audienceThe ASSERT European Integrated Project (Automated proof-based System and Software Engineering for Real-Time systems; EC FP6, IST-004033) has investigated, elaborated and experimented advanced methods based on the AltaRica language and support tool OCAS for architecture and fault approach propagation description analysis, and integrated in the complete ASSERT process. The paper describes lessons learnt from three case studies: safety critical spacecraft, autonomous deep exploration spacecraft, and civil aircraft
On the Estimation of Solar Energetic Particle Injection Timing from Onset Times near Earth
We examine the accuracy of a common technique for estimating the start time
of solar energetic particle injection based on a linear fit to the observed
onset time versus 1/(particle velocity). This is based on a concept that the
first arriving particles move directly along the magnetic field with no
scattering. We check this by performing numerical simulations of the transport
of solar protons between 2 and 2000 MeV from the Sun to the Earth, for several
assumptions regarding interplanetary scattering and the duration of particle
injection, and analyzing the results using the inverse velocity fit. We find
that in most cases, the onset times align close to a straight line as a
function of inverse velocity. Despite this, the estimated injection time can be
in error by several minutes. Also, the estimated path length can deviate
greatly from the actual path length along the interplanetary magnetic field.
The major difference between the estimated and actual path lengths implies that
the first arriving particles cannot be viewed as moving directly along the
interplanetary magnetic field.Comment: 19 pages, 3 Postscript figures. Astrophys. J., in pres
On contribution of three-body forces to interaction at intermediate energies
Available data on large-angle nucleon-deuteron elastic scattering
below the pion threshold give a signal for three-body forces. There is a
problem of separation of possible subtle aspects of these forces from off-shell
effects in two-nucleon potentials.
By considering the main mechanisms of the process, we show qualitatively that
in the quasi-binary reaction with the final spin singlet
NN-pair in the S-state the relative contribution of the 3N forces differs
substantially from the elastic channel.
It gives a new testing ground for the problem in question.Comment: 9 pages, Latex, 3 Postscript figure
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