10,617 research outputs found
The Milkman Rings Twice: Has Paul v. United States Given Federal Procurement Regulations the Force of Statutory Law?
Early detection of a solar flare - A study of X-ray, extreme ultraviolet, H-alpha, and solar radio emission from solar flares
X ray, extreme ultraviolet, H alpha, and radio emission from solar flares evaluated for use in satellite flare alarm syste
Dynamic Power Spectral Analysis of Solar Measurements from Photospheric, Chromospheric, and Coronal Sources
An important aspect in the power spectral analysis of solar variability is the quasistationary and quasiperiodic nature of solar periodicities. In other words, the frequency, phase, and amplitude of solar periodicities vary on time scales ranging from active region lifetimes to solar cycle time scales. Here, researchers employ a dynamic, or running, power spectral density analysis to determine many periodicities and their time-varying nature in the projected area of active sunspot groups (S sub act). The Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor (SMM/ACRIM) total solar irradiance (S), the Nimbus-7 MgII center-to-wing ratio (R (MgII sub c/w)), the Ottawa 10.7 cm flux (F sub 10.7), and the GOES background x ray flux (X sub b) for the maximum, descending, and minimum portions of solar cycle 21 (i.e., 1980 to 1986) are used. The technique dramatically illustrates several previously unrecognized periodicities. For example, a relatively stable period at about 51 days has been found in those indices which are related to emerging magnetic fields. The majority of solar periodicities, particularly around 27, 150 and 300 days, are quasiperiodic because they vary in amplitude and frequency throughout the solar cycle. Finally, it is shown that there are clear differences between the power spectral densities of solar measurements from photospheric, chromospheric, and coronal sources
Parity violation in quasielastic electron-nucleus scattering within the relativistic impulse approximation
We study parity violation in quasielastic (QE) electron-nucleus scattering
using the relativistic impulse approximation. Different fully relativistic
approaches have been considered to estimate the effects associated with the
final-state interactions. We have computed the parity-violating quasielastic
(PVQE) asymmetry and have analyzed its sensitivity to the different ingredients
that enter in the description of the reaction mechanism: final-state
interactions, nucleon off-shellness effects, current gauge ambiguities.
Particular attention has been paid to the description of the weak neutral
current form factors. The PVQE asymmetry is proven to be an excellent
observable when the goal is to get precise information on the axial-vector
sector of the weak neutral current. Specifically, from measurements of the
asymmetry at backward scattering angles good knowledge of the radiative
corrections entering in the isovector axial-vector sector can be gained.
Finally, scaling properties shown by the interference nuclear
responses are also analyzed.Comment: 15 pages, 11 figure
Vortex nucleation by collapsing bubbles in Bose-Einstein condensates
The nucleation of vortex rings accompanies the collapse of ultrasound bubbles
in superfluids. Using the Gross-Pitaevskii equation for a uniform condensate we
elucidate the various stages of the collapse of a stationary spherically
symmetric bubble and establish conditions necessary for vortex nucleation. The
minimum radius of the stationary bubble, whose collapse leads to vortex
nucleation, was found to be about 28 healing lengths. The time after which the
nucleation becomes possible is determined as a function of bubble's radius. We
show that vortex nucleation takes place in moving bubbles of even smaller
radius if the motion made them sufficiently oblate.Comment: 4 pages, 5 figure
Computational convergence of the path integral for real dendritic morphologies
Neurons are characterised by a morphological structure unique amongst biological cells, the core of which is the dendritic tree. The vast number of dendritic geometries, combined with heterogeneous properties of the cell membrane, continue to challenge scientists in predicting neuronal input-output relationships, even in the case of sub-threshold dendritic currents. The Green’s function obtained for a given dendritic geometry provides this functional relationship for passive or quasi-active dendrites and can be constructed by a sum-over-trips approach based on a path integral formalism. In this paper, we introduce a number of efficient algorithms for realisation of the sum-over-trips framework and investigate the convergence of these algorithms on different dendritic geometries. We demonstrate that the convergence of the trip sampling methods strongly depends on dendritic morphology as well as the biophysical properties of the cell membrane. For real morphologies, the number of trips to guarantee a small convergence error might become very large and strongly affect computational efficiency. As an alternative, we introduce a highly-efficient matrix method which can be applied to arbitrary branching structures
The Interface between Quantum Mechanics and General Relativity
The generation, as well as the detection, of gravitational radiation by means
of charged superfluids is considered. One example of such a "charged
superfluid" consists of a pair of Planck-mass-scale, ultracold "Millikan oil
drops," each with a single electron on its surface, in which the oil of the
drop is replaced by superfluid helium. When levitated in a magnetic trap, and
subjected to microwave-frequency electromagnetic radiation, a pair of such
"Millikan oil drops" separated by a microwave wavelength can become an
efficient quantum transducer between quadrupolar electromagnetic and
gravitational radiation. This leads to the possibility of a Hertz-like
experiment, in which the source of microwave-frequency gravitational radiation
consists of one pair of "Millikan oil drops" driven by microwaves, and the
receiver of such radiation consists of another pair of "Millikan oil drops" in
the far field driven by the gravitational radiation generated by the first
pair. The second pair then back-converts the gravitional radiation into
detectable microwaves. The enormous enhancement of the conversion efficiency
for these quantum transducers over that for electrons arises from the fact that
there exists macroscopic quantum phase coherence in these charged superfluid
systems.Comment: 22 pages, 7 figures; Lamb medal lecture on January 5, 2006 at the
Physics of Quantum Electronics Winter Colloquium at Snowbird, Utah; accepted
for publication in J. Mod. Optic
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