14,457 research outputs found
Quotients of E^n by A_{n+1} and Calabi-Yau manifolds
We give a simple construction, starting with any elliptic curve E, of an
n-dimensional Calabi-Yau variety of Kummer type (for any n>1), by considering
the quotient Y of the n-fold self-product of E by a natural action of the
alternating group A_{n+1} (in n+1 variables). The vanishing of H^m(Y, O_Y) for
0<m<n follows from the non-existence of (non-zero) fixed points in certain
representations of A_{n+1}. For n<4 we provide an explicit crepant resolution X
in characteristics different from 2,3. The key point is that Y can be realized
as a double cover of P^n branched along a hypersurface of degree 2(n+1).Comment: 9 page
Optical-radar imaging of scale models for studies in asteroid astronomy
During the past five years, delay-Doppler radar has become the primary technique for studying the structure of Earth-crossing asteroids. None of these objects has yet been visited by spacecraft, so ground-truth test cases are lacking. A laboratory system is described that provides optical-radar images at 0.1-mm resolution. These data are analogous to the highest-resolution asteroid radar images currently available and provided realistic test cases for developing signal-processing techniques. The system can be thought of as a 1/188,000 scale model of the Arecibo radar, or a 1/52,800 scale model of the Goldstone radar
Swept-wavelength mid-infrared fiber laser for real-time ammonia gas sensing
The mid-infrared (mid-IR) spectral region holds great promise for new
laser-based sensing technologies, based on measuring strong mid-IR molecular
absorption features. Practical applications have been limited to date, however,
by current low-brightness broadband mid-IR light sources and slow
acquisition-time detection systems. Here, we report a new approach by
developing a swept-wavelength mid-infrared fiber laser, exploiting the broad
emission of dysprosium and using an acousto-optic tunable filter to achieve
electronically controlled swept-wavelength operation from 2.89 to 3.25 {\mu}m
(3070-3460 cm^-1). Ammonia (NH3) absorption spectroscopy is demonstrated using
this swept source with a simple room-temperature single-pixel detector, with
0.3 nm resolution and 40 ms acquisition time. This creates new opportunities
for real-time high-sensitivity remote sensing using simple, compact mid-IR
fiber-based technologies.Comment: Invited article for APL Photonic
Density matrix renormalization group study of conjugated polymers with transverse pi-conjugation
We report accurate numerical studies of excited state orderings in long
hypothetical pi-conjugated oligomers in which the hydrogen atoms of
trans-polyacetylene are replaced with conjugated sidegroups, within modified
Hubbard models. There exists a range of the bare Coulomb repulsion for which
the excited state ordering is conducive to photoluminescence in the substituted
systems, even as this ordering is opposite in the unsubstituted polyenes of the
same lengths. Our work provides motivation to study real pi-conjugated polymers
with transverse conjugation and small optical gaps.Comment: 5 pages, 4 fig
Ozone measurements from the NOAA-9 and the Nimbus-7 satellites: Implications of short and long term variabilities
An overview is given of the measurements of total ozne and ozone profiles by the SBUV/2 instrument on the NOAA-9 spacecraft relative to similar measurements from the SBUV and TOMS instruments on Nimbus-7. It is shown that during the three year period from March 14, 1985, to February 28, 1988, when these data sets overlap, there have been significant changes in the calibrations of the three instruments which may be attributed to the drift of the NOSS-9 orbit to later equator crossing times (for SBUV/2). These changes in instrument characteristics have affected the absolute values of the trends derived from the three instruments, but their geophysical characteristics and response to short term variations are accurate and correlate well among the three instruments. For example, the total column ozone measured by the three instruments shows excellent agreement with respect to its day to day, seasonal, and latitudinal variabilities. At high latitudes, the day to day fluctuations in total ozone show a strong positive correlation with temperature in the lower stratosphere, as one might expect from the dynamical coupling of the two parameters at these latitudes
Preliminary results of the University of California X-ray experiment on the OSO-3
Cosmic and solar X ray data obtained by Orbiting Solar Observatory /OSO-3
Hard X-ray imaging facility for space shuttle: A scientific and conceptual engineering study
A shuttle-accommodated instrument for imaging hard X-rays in the study of nonthermal particles and high temperature particles in various solar and cosmic phenomena was defined and its feasibility demonstrated. The imaging system configuration is described as well as the electronics, aspect systems, mechanical and thermal properties and the ground support equipment
The Unpredictability of the Most Energetic Solar Events
Observations over the past two solar cycles show a highly irregular pattern
of occurrence for major solar flares, gamma-ray events, and solar energetic
particle (SEP) fluences. Such phenomena do not appear to follow the direct
indices of solar magnetic activity, such as the sunspot number. I show that
this results from the non-Poisson occurrence for the most energetic events.
This Letter also points out a particularly striking example of this
irregularity in a comparison between the declining phases of the recent two
solar cycles (1993-1995 and 2004-2006, respectively) and traces it through the
radiated energies of the flares, the associated SEP fluences, and the sunspot
areas. These factors suggest that processes in the solar interior involved with
the supply of magnetic flux up to the surface of the Sun have strong
correlations in space and time, leading to a complex occurrence pattern that is
presently unpredictable on timescales longer than active region lifetimes
(weeks) and not correlated well with the solar cycle itself.Comment: 4 page
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