334,767 research outputs found
Investigation of intermittency in magnetohydrodynamics and solar wind turbulence: scale-dependent kurtosis
The behavior of scale-dependent (or filtered) kurtosis is studied in the solar wind using magnetic field measurements from the ACE and Cluster spacecraft at 1 AU. It is also analyzed numerically with high-resolution magnetohydrodynamic spectral simulations. In each case the filtered kurtosis increases with wavenumber, implying the presence of coherent structures at the smallest scales. This phase coupling is related to intermittency in solar wind turbulence and the emergence of non-Gaussian statistics. However, it is inhibited by the presence of upstream waves and other phase-randomizing structures, which act to reduce the growth of kurtosis
Photon Statistics of Filtered Resonance Fluorescence
Spectral filtering of resonance fluorescence is widely employed to improve
single photon purity and indistinguishability by removing unwanted backgrounds.
For filter bandwidths approaching the emitter linewidth, complex behaviour is
predicted due to preferential transmission of components with differing photon
statistics. We probe this regime using a Purcell-enhanced quantum dot in both
weak and strong excitation limits, finding excellent agreement with an extended
sensor theory model. By changing only the filter width, the photon statistics
can be transformed between antibunched, bunched, or Poissonian. Our results
verify that strong antibunching and a sub-natural linewidth cannot
simultaneously be observed, providing new insight into the nature of coherent
scattering.Comment: Main manuscript 7 pages with 4 figures, supplementary material of 4
page
Reproducibility of a Titanium Plasma Vacuum Spark Discharge
The results of an extensive operation of a Vacuum Spark plasma using Titanium
electrodes in a 120 ns 150 kA discharge are presented. The hot spots are found
to form with a regular spacing in a zippering Z-pinch plasma, which forms close
to the cathode and extends to approximately two thirds of the anode separation
over a period of a few ns. The axis of the discharge is well defined by an
initial plasma from a Nd:YAG laser focussed onto the cathode electrode surface.
The statistics of the formation of the hot spots are given for the life of one
anode electrode. Between one and three hotspots form and the favored positions
are at 1.5 and 3.0 mm from the cathode and the strongest emission, as observed
in a filtered X-ray pinhole camera, comes from the hot spot closest to the
cathode. The emission spectra resolved between 50 and 350 \AA shows a wide
range of Ti ionization which allows the temperatures of the anode blow off
plasma, the Z-pinch and the hot spot plasma to be distinguished. These results
are compared with filtered PIN diode signals and filtered pinhole images.Comment: 5 pages, 4 figure
Connecting blazars with ultra high energy cosmic rays and astrophysical neutrinos
We present a strong hint of a connection between high energy -ray
emitting blazars, very high energy neutrinos, and ultra high energy cosmic
rays. We first identify potential hadronic sources by filtering -ray
emitters %from existing catalogs that are in spatial coincidence with the high
energy neutrinos detected by IceCube. The neutrino filtered -ray
emitters are then correlated with the ultra high energy cosmic rays from the
Pierre Auger Observatory and the Telescope Array by scanning in -ray
flux () and angular separation () between sources and
cosmic rays. A maximal excess of 80 cosmic rays (42.5 expected) is found at
from the neutrino filtered -ray emitters
selected from the second hard {\it Fermi}-LAT catalogue (2FHL) and for
.
The probability for this to happen is , which translates to
after compensation for all the considered trials. No
excess of cosmic rays is instead observed for the complement sample of
-ray emitters (i.e. not in spatial connection with IceCube neutrinos).
A likelihood ratio test comparing the connection between the neutrino filtered
and the complement source samples with the cosmic rays favours a connection
between neutrino filtered emitters and cosmic rays with a probability of
( after compensation for all the considered
trials. The neutrino filtered -ray sources that make up the cosmic rays
excess are blazars of the high synchrotron peak type. More statistics is needed
to further investigate these sources as candidate cosmic ray and neutrino
emitters.Comment: Accepted for publication in MNRAS, added one figure (redshift
distribution), new IceCube data, and penalty factor for subsets within single
catalogue
Statistical properties of an ideal subgrid-scale correction for Lagrangian particle tracking in turbulent channel flow
One issue associated with the use of Large-Eddy Simulation (LES) to
investigate the dispersion of small inertial particles in turbulent flows is
the accuracy with which particle statistics and concentration can be
reproduced. The motion of particles in LES fields may differ significantly from
that observed in experiments or direct numerical simulation (DNS) because the
force acting on the particles is not accurately estimated, due to the
availability of the only filtered fluid velocity, and because errors accumulate
in time leading to a progressive divergence of the trajectories. This may lead
to different degrees of inaccuracy in the prediction of statistics and
concentration. We identify herein an ideal subgrid correction of the a-priori
LES fluid velocity seen by the particles in turbulent channel flow. This
correction is computed by imposing that the trajectories of individual
particles moving in filtered DNS fields exactly coincide with the particle
trajectories in a DNS. In this way the errors introduced by filtering into the
particle motion equations can be singled out and analyzed separately from those
due to the progressive divergence of the trajectories. The subgrid correction
term, and therefore the filtering error, is characterized in the present paper
in terms of statistical moments. The effects of the particle inertia and of the
filter type and width on the properties of the correction term are
investigated.Comment: 15 pages,24 figures. Submitted to Journal of Physics: Conference
Serie
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