14,142 research outputs found
Phase perturbation measurements through a heated ionosphere
High frequency radiowaves incident on an overdense (i.e., HF-frequency penetration frequency) ionosphere produce electron density irregularities. The effect of such ionospheric irregularities on the phase of UHF-radiowaves was determined. For that purpose the phase of radiowaves originating from celestial radio sources was observed with two antennas. The radiosources were chosen such that the line of sight to at least one of the antennas (usually both) passed through the modified volume of the ionosphere. Observations at 430 MHz and at 2380 MHz indicate that natural irregularities have a much stronger effect on the UHF phase fluctuations than the HF-induced irregularities for presently achieved HF-power densities of 20-80 uW/sq m. It is not clear whether some of the effects observed are the result of HF-modification of the ionosphere. Upper limits on the phase perturbations produced by HF-modification are 10 deg at 2380 MHz and 80 deg at 430 MHz
Stiff Polymers, Foams and Fiber Networks
We study the elasticity of fibrous materials composed of generalized stiff
polymers. It is shown that in contrast to cellular foam-like structures affine
strain fields are generically unstable. Instead, a subtle interplay between the
architecture of the network and the elastic properties of its building blocks
leads to intriguing mechanical properties with intermediate asymptotic scaling
regimes. We present exhaustive numerical studies based on a finite element
method complemented by scaling arguments.Comment: 4 pages, 5 figure
Bolometric and UV Light Curves of Core-Collapse Supernovae
The Swift UV-Optical Telescope (UVOT) has been observing Core-Collapse
Supernovae (CCSNe) of all subtypes in the UV and optical since 2005. We present
here 50 CCSNe observed with the Swift UVOT, analyzing their UV properties and
behavior. Where we have multiple UV detections in all three UV filters (\lambda
c = 1928 - 2600 \AA), we generate early time bolometric light curves, analyze
the properties of these light curves, the UV contribution to them, and derive
empirical corrections for the UV-flux contribution to optical-IR based
bolometric light curves
Ultrafast control of inelastic tunneling in a double semiconductor quantum
In a semiconductor-based double quantum well (QW) coupled to a degree of
freedom with an internal dynamics, we demonstrate that the electronic motion is
controllable within femtoseconds by applying appropriately shaped
electromagnetic pulses. In particular, we consider a pulse-driven AlxGa1-xAs
based symmetric double QW coupled to uniformly distributed or localized
vibrational modes and present analytical results for the lowest two levels.
These predictions are assessed and generalized by full-fledged numerical
simulations showing that localization and time-stabilization of the driven
electron dynamics is indeed possible under the conditions identified here, even
with a simultaneous excitations of vibrational modes.Comment: to be published in Appl.Phys.Let
Entropic forces generated by grafted semiflexible polymers
The entropic force exerted by the Brownian fluctuations of a grafted
semiflexible polymer upon a rigid smooth wall are calculated both analytically
and by Monte Carlo simulations. Such forces are thought to play an important
role for several cellular phenomena, in particular, the physics of
actin-polymerization-driven cell motility and movement of bacteria like
Listeria. In the stiff limit, where the persistence length of the polymer is
larger than its contour length, we find that the entropic force shows scaling
behavior. We identify the characteristic length scales and the explicit form of
the scaling functions. In certain asymptotic regimes we give simple analytical
expressions which describe the full results to a very high numerical accuracy.
Depending on the constraints imposed on the transverse fluctuations of the
filament there are characteristic differences in the functional form of the
entropic forces; in a two-dimensional geometry the entropic force exhibits a
marked peak.Comment: 21 pages, 18 figures, minor misprints correcte
Stringy Effects During Inflation and Reheating
We consider inflationary cosmology in the context of string compactifications
with multiple throats. In scenarios where the warping differs significantly
between throats, string and Kaluza-Klein physics can generate potentially
observable corrections to the cosmology of inflation and reheating. First we
demonstrate that a very low string scale in the ground state compactification
is incompatible with a high Hubble scale during inflation, and we propose that
the compactification geometry is altered during inflation. In this
configuration, the lowest scale is just above the Hubble scale, which is
compatible with effective field theory but still leads to potentially
observable CMB corrections. Also in the appropriate region of parameter space,
we find that reheating leads to a phase of long open strings in the Standard
Model sector (before the usual radiation-dominated phase). We sketch the
cosmology of the long string phase and we discuss possible observational
consequences.Comment: 33pp, RevTeX4, v2. minor changes, added ref
Evaluation of electrode shape and nondestructive evaluation method for welded solar cell interconnects
Resistance welds of solar cell interconnect tabs were evaluated. Both copper-silver and silver-silver welds were made with various heat inputs and weld durations. Parallel gap and annular gap weld electrode designs were used. The welds were analyzed by light microscope, electron microprobe and scanning laser acoustic microscope. These analyses showed the size and shape of the weld, the relationship between the acoustic micrographs, the visible electrode footprint, and the effect of electrode misalignment. The effect of weld heat input on weld microstructure was also shown
Optimal flexibility for conformational transitions in macromolecules
Conformational transitions in macromolecular complexes often involve the
reorientation of lever-like structures. Using a simple theoretical model, we
show that the rate of such transitions is drastically enhanced if the lever is
bendable, e.g. at a localized "hinge''. Surprisingly, the transition is fastest
with an intermediate flexibility of the hinge. In this intermediate regime, the
transition rate is also least sensitive to the amount of "cargo'' attached to
the lever arm, which could be exploited by molecular motors. To explain this
effect, we generalize the Kramers-Langer theory for multi-dimensional barrier
crossing to configuration dependent mobility matrices.Comment: 4 pages, 4 figure
The Carrington event not observed in most ice core nitrate records
The Carrington Event of 1859 is considered to be among the largest space weather events of the last 150 years. We show that only one out of 14 well-resolved ice core records from Greenland and Antarctica has a nitrate spike dated to 1859. No sharp spikes are observed in the Antarctic cores studied here. In Greenland numerous spikes are observed in the 40 years surrounding 1859, but where other chemistry was measured, all large spikes have the unequivocal signal, including co-located spikes in ammonium, formate, black carbon and vanillic acid, of biomass burning plumes. It seems certain that most spikes in an earlier core, including that claimed for 1859, are also due to biomass burning plumes, and not to solar energetic particle (SEP) events. We conclude that an event as large as the Carrington Event did not leave an observable, widespread imprint in nitrate in polar ice. Nitrate spikes cannot be used to derive the statistics of SEPs
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