7,844 research outputs found
Delayed feedback as a means of control of noise-induced motion
Time--delayed feedback is exploited for controlling noise--induced motion in
coherence resonance oscillators. Namely, under the proper choice of time delay,
one can either increase or decrease the regularity of motion. It is shown that
in an excitable system, delayed feedback can stabilize the frequency of
oscillations against variation of noise strength. Also, for fixed noise
intensity, the phenomenon of entrainment of the basic oscillation period by the
delayed feedback occurs. This allows one to steer the timescales of
noise-induced motion by changing the time delay.Comment: 4 pages, 4 figures. In the replacement file Fig. 2 and Fig. 4(b),(d)
were amended. The reason is numerical error found, that affected the
quantitative estimates of correlation time, but did not affect the main
messag
Magnetic Helicity Conservation and Astrophysical Dynamos
We construct a magnetic helicity conserving dynamo theory which incorporates
a calculated magnetic helicity current. In this model the fluid helicity plays
a small role in large scale magnetic field generation. Instead, the dynamo
process is dominated by a new quantity, derived from asymmetries in the second
derivative of the velocity correlation function, closely related to the `twist
and fold' dynamo model. The turbulent damping term is, as expected, almost
unchanged. Numerical simulations with a spatially constant fluid helicity and
vanishing resistivity are not expected to generate large scale fields in
equipartition with the turbulent energy density. The prospects for driving a
fast dynamo under these circumstances are uncertain, but if it is possible,
then the field must be largely force-free. On the other hand, there is an
efficient analog to the dynamo. Systems whose turbulence is
driven by some anisotropic local instability in a shearing flow, like real
stars and accretion disks, and some computer simulations, may successfully
drive the generation of strong large scale magnetic fields, provided that
. We show that this
criterion is usually satisfied. Such dynamos will include a persistent,
spatially coherent vertical magnetic helicity current with the same sign as
, that is, positive for an accretion disk and negative for
the Sun. We comment on the role of random magnetic helicity currents in storing
turbulent energy in a disordered magnetic field, which will generate an
equipartition, disordered field in a turbulent medium, and also a declining
long wavelength tail to the power spectrum. As a result, calculations of the
galactic `seed' field are largely irrelevant.Comment: 28 pages, accepted by The Astrophysical Journa
An Alternative Parameterization of R-matrix Theory
An alternative parameterization of R-matrix theory is presented which is
mathematically equivalent to the standard approach, but possesses features
which simplify the fitting of experimental data. In particular there are no
level shifts and no boundary-condition constants which allows the positions and
partial widths of an arbitrary number levels to be easily fixed in an analysis.
These alternative parameters can be converted to standard R-matrix parameters
by a straightforward matrix diagonalization procedure. In addition it is
possible to express the collision matrix directly in terms of the alternative
parameters.Comment: 8 pages; accepted for publication in Phys. Rev. C; expanded Sec. IV,
added Sec. VI, added Appendix, corrected typo
Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1
We show that dramatic spectral evolution associated with dips occurring near
phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times
heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying
faint component which is not attenuated by the variable column and whose flux
is ~10% of that of the unabsorbed bright component. A prominent Fe emission
line at ~6.5 keV is evident during the dips. The absolute line flux outside the
dips is similar to that during the dips, indicating that the line is associated
with the faint component. These results are consistent with a model in which
the bright component is radiation received directly from a compact source while
the faint component may be attributed to scattered radiation. Our results are
also generally consistent with those of Brandt et al., who found that a
partial- covering model could explain ASCA spectra of a low-to-high transition
in Cir X-1. The relative brightness of the two components in our model requires
a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson
scattering in material that mostly surrounds the source. We find that
illumination of such a scattering cloud by the observed direct component would
produce an Fe K-alpha fluorescence flux that is in rough agreement with the
flux of the observed emission line. We also conclude that if the scattering
medium is not highly ionized, our line of sight to the compact source does not
pass through it. Finally, we discuss simple pictures of the absorbers
responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages,
including 11 figures
Evolution of helicity in NOAA 10923 over three consecutive solar rotations
We have studied the evolution of magnetic helicity and chirality in an active
region over three consecutive solar rotations. The region when it first
appeared was named NOAA10923 and in subsequent rotations it was numbered NOAA
10930, 10935 and 10941. We compare the chirality of these regions at
photospheric, chromospheric and coronal heights. The observations used for
photospheric and chromospheric heights are taken from Solar Vector Magnetograph
(SVM) and H_alpha imaging telescope of Udaipur Solar Observatory (USO),
respectively. We discuss the chirality of the sunspots and associated H_alpha
filaments in these regions. We find that the twistedness of superpenumbral
filaments is maintained in the photospheric transverse field vectors also. We
also compare the chirality at photospheric and chromospheric heights with the
chirality of the associated coronal loops, as observed from the HINODE X-Ray
Telescope.Comment: 8 pages, 4 figure
Stuttering Min oscillations within E. coli bacteria: A stochastic polymerization model
We have developed a 3D off-lattice stochastic polymerization model to study
subcellular oscillation of Min proteins in the bacteria Escherichia coli, and
used it to investigate the experimental phenomenon of Min oscillation
stuttering. Stuttering was affected by the rate of immediate rebinding of MinE
released from depolymerizing filament tips (processivity), protection of
depolymerizing filament tips from MinD binding, and fragmentation of MinD
filaments due to MinE. Each of processivity, protection, and fragmentation
reduces stuttering, speeds oscillations, and reduces MinD filament lengths.
Neither processivity or tip-protection were, on their own, sufficient to
produce fast stutter-free oscillations. While filament fragmentation could, on
its own, lead to fast oscillations with infrequent stuttering; high levels of
fragmentation degraded oscillations. The infrequent stuttering observed in
standard Min oscillations are consistent with short filaments of MinD, while we
expect that mutants that exhibit higher stuttering frequencies will exhibit
longer MinD filaments. Increased stuttering rate may be a useful diagnostic to
find observable MinD polymerization in experimental conditions.Comment: 21 pages, 7 figures, missing unit for k_f inserte
Generalized Hermite-Gauss decomposition of the two-photon state produced by spontaneous parametric down-conversion
We provide a general decomposition of the two-photon state produced by
spontaneous parametric down-conversion in Hermite-Gaussian modes, in the case
that the pump beam is described by a Hermite-Gaussian beam of any order. We
show that the spatial correlations depend explicitly on the order of the pump
beam, as well as other experimental parameters. We use the decomposition to
demonstrate a few interesting cases. Our results are applicable to the
engineering of two-photon spatial entanglement, in particular for non-Gaussian
states.Comment: 14 page draft, 5 figure
Influence of shock wave propagation on dielectric barrier discharge plasma actuator performance
Interest in plasma actuators as active flow control devices is growing rapidly due to their lack of mechanical parts, light weight and high response frequency. Although the flow induced by these actuators has received much attention, the effect that the external flow has on the performance of the actuator itself must also be considered, especially the influence of unsteady high-speed flows which are fast becoming a norm in the operating flight envelopes. The primary objective of this study is to examine the characteristics of a dielectric barrier discharge (DBD) plasma actuator when exposed to an unsteady flow generated by a shock tube. This type of flow, which is often used in different studies, contains a range of flow regimes from sudden pressure and density changes to relatively uniform high-speed flow regions. A small circular shock tube is employed along with the schlieren photography technique to visualize the flow. The voltage and current traces of the plasma actuator are monitored throughout, and using the well-established shock tube theory the change in the actuator characteristics are related to the physical processes which occur inside the shock tube. The results show that not only is the shear layer outside of the shock tube affected by the plasma but the passage of the shock front and high-speed flow behind it also greatly influences the properties of the plasma
Elastic p-3He and n-3H scattering with two- and three-body forces
We report on a microscopic calculation of n-3H and p-3He scattering employing
the Argonne v_{18} and v_8' nucleon-nucleon potentials with and without
additional three-nucleon force. An R-matrix analysis of the p-3He and n-3H
scattering data is presented. Comparisons are made for the phase shifts and a
selection of measurements in both scattering systems. Differences between our
calculation and the R-matrix results or the experimental data can be attributed
to only two partial waves (3P0 and 3P2). We find the effect of the Urbana IX
and the Texas-Los Alamos three-nucleon forces on the phase shifts to be
negligible.Comment: submitted to Phys. Rev.
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