2,588 research outputs found
Impact of observational uncertainties on universal scaling of MHD turbulence
Scaling exponents are the central quantitative prediction of theories of
turbulence and in-situ satellite observations of the high Reynolds number solar
wind flow have provided an extensive testbed of these. We propose a general,
instrument independent method to estimate the uncertainty of velocity field
fluctuations. We obtain the systematic shift that this uncertainty introduces
into the observed spectral exponent. This shift is essential for the correct
interpretation of observed scaling exponents. It is sufficient to explain the
contradiction between spectral features of the Elsasser fields observed in the
solar wind with both theoretical models and numerical simulations of
Magnetohydrodynamic turbulence
A Kinetic Alfven wave cascade subject to collisionless damping cannot reach electron scales in the solar wind at 1 AU
(Abridged) Turbulence in the solar wind is believed to generate an energy
cascade that is supported primarily by Alfv\'en waves or Alfv\'enic
fluctuations at MHD scales and by kinetic Alfv\'en waves (KAWs) at kinetic
scales . Linear Landau damping of KAWs increases with
increasing wavenumber and at some point the damping becomes so strong that the
energy cascade is completely dissipated. A model of the energy cascade process
that includes the effects of linear collisionless damping of KAWs and the
associated compounding of this damping throughout the cascade process is used
to determine the wavenumber where the energy cascade terminates. It is found
that this wavenumber occurs approximately when ,
where and are, respectively, the real frequency and
damping rate of KAWs and the ratio is evaluated in the limit as
the propagation angle approaches 90 degrees relative to the direction of the
mean magnetic field.Comment: Submitted to Ap
A Universal Law for Solar-Wind Turbulence at Electron Scales
The interplanetary magnetic fluctuation spectrum obeys a Kolmogorovian power
law at scales above the proton inertial length and gyroradius which is well
regarded as an inertial range. Below these scales a power law index around
is often measured and associated to nonlinear dispersive processes.
Recent observations reveal a third region at scales below the electron inertial
length. This region is characterized by a steeper spectrum that some refer to
it as the dissipation range. We investigate this range of scales in the
electron magnetohydrodynamic approximation and derive an exact and universal
law for a third-order structure function. This law can predict a magnetic
fluctuation spectrum with an index of which is in agreement with the
observed spectrum at the smallest scales. We conclude on the possible existence
of a third turbulence regime in the solar wind instead of a dissipation range
as recently postulated.Comment: 11 pages, will appear in Astrophys.
Paradoxes in Fair Computer-Aided Decision Making
Computer-aided decision making--where a human decision-maker is aided by a
computational classifier in making a decision--is becoming increasingly
prevalent. For instance, judges in at least nine states make use of algorithmic
tools meant to determine "recidivism risk scores" for criminal defendants in
sentencing, parole, or bail decisions. A subject of much recent debate is
whether such algorithmic tools are "fair" in the sense that they do not
discriminate against certain groups (e.g., races) of people.
Our main result shows that for "non-trivial" computer-aided decision making,
either the classifier must be discriminatory, or a rational decision-maker
using the output of the classifier is forced to be discriminatory. We further
provide a complete characterization of situations where fair computer-aided
decision making is possible
Liver transplantation: an unfinished product.
Liver transplantation has become an extraordinarily valuable and useful operation, but one that is not perfect and that has not been exploited to anything like its full potential. Better immunosuppression may become available soon as exemplified by developments with the Japanese drug, FK506. Improved preservation with the UW solution is already here. With these advantages, liver transplantation is certain to become far more widely used than at any time in the past. Examples were cited of innovative approaches using liver transplantation for the treatment of hepatic malignancies
Amelioration of normothermic canine liver ischemia with prostacyclin.
A model of hepatic ischemia was developed in dogs using a pump-driven splanchnic-to-jugular vein bypass during crossclamping of the portal triad. An LD50 was established with three hours of ischemia. PGI2 given for one hour before the ischemic insult ameliorated the ischemic injury and increased survival
Strong magnetohydrodynamic turbulence with cross helicity
Magnetohydrodynamics (MHD) provides the simplest description of magnetic
plasma turbulence in a variety of astrophysical and laboratory systems. MHD
turbulence with nonzero cross helicity is often called imbalanced, as it
implies that the energies of Alfv\'en fluctuations propagating parallel and
anti-parallel the background field are not equal. Recent analytical and
numerical studies have revealed that at every scale, MHD turbulence consists of
regions of positive and negative cross helicity, indicating that such
turbulence is inherently locally imbalanced. In this paper, results from high
resolution numerical simulations of steady-state incompressible MHD turbulence,
with and without cross helicity are presented. It is argued that the inertial
range scaling of the energy spectra (E^+ and E^-) of fluctuations moving in
opposite directions is independent of the amount of cross-helicity. When cross
helicity is nonzero, E^+ and E^- maintain the same scaling, but have differing
amplitudes depending on the amount of cross-helicity.Comment: To appear in Physics of Plasma
A new Fermi smearing approach for scattering of multi-GeV electrons by nuclei
The cross section for electron scattering by nuclei at high momentum
transfers is calculated within the Fermi smearing approximation (FSA), where
binding effects on the struck nucleon are introduced via the relativistic
Hartree approximation (RHA). The model naturally preserves current
conservation, since the response tensor for an off-shell nucleon conserves the
same form that for a free one but with an effective mass. Different
parameterizations for the inelastic nucleon structure function, are analyzed.
The smearing at the Fermi surface is introduced through a momentum distribution
obtained from a perturbative nuclear matter calculation. Recent CEBAF data on
inclusive scattering of 4.05 GeV electrons on Fe are well reproduced for
all measured geometries for the first time, as is evident from the comparison
with previous calculations.Comment: 8 pages in Revtex4 style, 6 eps figures, to appear in Physical Review
Nanotribological characterization of industrial Polytetrafluorethylene-based coatings by atomic force microscopy
We present the result of a systematic study of the tribological properties of
industrial Polytetrafluorethylene (PTFE)-based coatings carried out with an
atomic force microscope. A new characterization protocol allowed the reliable
and quantitative assessment of the friction coefficient and adhesion forces at
the sub-micrometer scale even for highly corrugated industrial samples. We have
studied and compared PTFE coatings charged with different additives in dry and
humid environment. The influence of additives and humidity on the friction
coefficient and on adhesion forces has been investigated using standard silicon
nitride tips as sliders in the low-load regime.Comment: to be published in Thin Solid Films 18 pages, 2 tables, 3 figure
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