11,710 research outputs found
On the turbulent energy cascade in anisotropic magnetohydrodynamic turbulence
The problem of the occurrence of an energy cascade for Alfv\'enic turbulence
in solar wind plasmas was hystorically addressed by using phenomenological
arguments based to the weakness of nonlinear interactions and the anisotropy of
the cascade in wave vectors space. Here, this paradox is reviewed through the
formal derivation of a Yaglom relation from anisotropic Magnetohydrodynamic
equation. The Yaglom relation involves a third-order moment calculated from
velocity and magnetic fields and involving both Els\"asser vector fields, and
is particularly useful to be used as far as spacecraft observations of
turbulence are concerned
Measuring the LISA test mass magnetic proprieties with a torsion pendulum
Achieving the low frequency LISA sensitivity requires that the test masses
acting as the interferometer end mirrors are free-falling with an unprecedented
small degree of deviation. Magnetic disturbances, originating in the
interaction of the test mass with the environmental magnetic field, can
significantly deteriorate the LISA performance and can be parameterized through
the test mass remnant dipole moment and the magnetic susceptibility
. While the LISA test flight precursor LTP will investigate these effects
during the preliminary phases of the mission, the very stringent requirements
on the test mass magnetic cleanliness make ground-based characterization of its
magnetic proprieties paramount. We propose a torsion pendulum technique to
accurately measure on ground the magnetic proprieties of the LISA/LTP test
masses.Comment: 6 pages, 3 figure
Integral group actions on symmetric spaces and discrete duality symmetries of supergravity theories
For a split, simply connected, semisimple Lie group of rank
and the maximal compact subgroup of , we give a method for computing
Iwasawa coordinates of using the Chevalley generators and the Steinberg
presentation. When is a scalar coset for a supergravity theory in
dimensions , we determine the action of the integral form
on . We give explicit results for the action of the
discrete --duality groups and on the
scalar cosets and
for type IIB supergravity
in ten dimensions and 11--dimensional supergravity in dimensions,
respectively. For the former, we use this to determine the discrete U--duality
transformations on the scalar sector in the Borel gauge and we describe the
discrete symmetries of the dyonic charge lattice. We determine the
spectrum--generating symmetry group for fundamental BPS solitons of type IIB
supergravity in dimensions at the classical level and we propose an
analog of this symmetry at the quantum level. We indicate how our methods can
be used to study the orbits of discrete U--duality groups in general
Mixture or mosaic? Genetic patterns in UK grey squirrels support a human-mediated ‘long-jump’ invasion mechanism
Aim Clarifying whether multiple introductions of a species remain relatively isolated or merge and interbreed is essential for understanding the dynamics of invasion processes. Multiple introductions from different sources can result in a mixture of genetically distinct populations, increasing the total genetic diversity. This mixing can resolve the ‘genetic paradox’, whereby in spite of the relatively small numbers of introduced individuals, the augmented diversity due to this mixing increases adaptability and the ability of the species to spread in new environments. Here, we aim to assess whether the expansion of a successful invader, the Eastern grey squirrel, was partly driven by the merger of multiple introductions and the effects of such a merger on diversity. Location UK, Ireland. Methods We analysed the genetic variation at 12 microsatellite loci of 381 individuals sampled from one historical and 14 modern populations of grey squirrels. Results Our data revealed that current UK population structure resembles a mosaic, with minimal interpopulation mixing and each element reflecting the genetic make-up of historic introductions. The genetic diversity of each examined population was lower than a US population or a historical UK population. Numbers of releases in a county did not correlate with county-level genetic diversity. Inbreeding coefficients remain high, and effective population sizes remain small. Main conclusions Our results support the conclusion that rapid and large-scale expansion in this species in the UK was not driven by a genetic mixing of multiple introduced populations with a single expansion front, but was promoted by repeated translocations of small propagules. Our results have implications for the management of grey squirrels and other invasive species and also demonstrate how invaders can overcome the genetic paradox, if spread is facilitated by human-mediated dispersal
Current voltage characteristics and excess noise at the trap filling transition in polyacenes
Experiments in organic semiconductors (polyacenes) evidence a strong super
quadratic increase of the current-voltage (I-V) characteristic at voltages in
the transition region between linear (Ohmic) and quadratic (trap free
space-charge-limited-current) behaviours. Similarly, excess noise measurements
at a given frequency and increasing voltages evidence a sharp peak of the
relative spectral density of the current noise in concomitance with the strong
super-quadratic I-V characteristics. Here we discuss the physical
interpretation of these experiments in terms of an essential contribution from
field assisted trapping-detrapping processes of injected carriers. To this
purpose, the fraction of filled traps determined by the I-V characteristics is
used to evaluate the excess noise in the trap filled transition (TFT) regime.
We have found an excellent agreement between the predictions of our model and
existing experimental results in tetracene and pentacene thin films of
different length in the range .Comment: 20 pg, 13 figures, in pres
Characterization of disturbance sources for LISA: torsion pendulum results
A torsion pendulum allows ground-based investigation of the purity of
free-fall for the LISA test masses inside their capacitive position sensor.
This paper presents recent improvements in our torsion pendulum facility that
have both increased the pendulum sensitivity and allowed detailed
characterization of several important sources of acceleration noise for the
LISA test masses. We discuss here an improved upper limit on random force noise
originating in the sensor. Additionally, we present new measurement techniques
and preliminary results for characterizing the forces caused by the sensor's
residual electrostatic fields, dielectric losses, residual spring-like
coupling, and temperature gradients.Comment: 11 pages, 8 figures, accepted for publication Classical and Quantum
Gravit
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