6,628 research outputs found
Opposition diagrams for automorphisms of small spherical buildings
An automorphism of a spherical building is called
\textit{capped} if it satisfies the following property: if there exist both
type and simplices of mapped onto opposite simplices by
then there exists a type simplex of mapped onto
an opposite simplex by . In previous work we showed that if is
a thick irreducible spherical building of rank at least with no Fano plane
residues then every automorphism of is capped. In the present work we
consider the spherical buildings with Fano plane residues (the \textit{small
buildings}). We show that uncapped automorphisms exist in these buildings and
develop an enhanced notion of "opposition diagrams" to capture the structure of
these automorphisms. Moreover we provide applications to the theory of
"domesticity" in spherical buildings, including the complete classification of
domestic automorphisms of small buildings of types and
Opposition diagrams for automorphisms of large spherical buildings
Let be an automorphism of a thick irreducible spherical building
of rank at least with no Fano plane residues. We prove that if
there exist both type and simplices of mapped onto
opposite simplices by , then there exists a type simplex
of mapped onto an opposite simplex by . This property is
called "cappedness". We give applications of cappedness to opposition diagrams,
domesticity, and the calculation of displacement in spherical buildings. In a
companion piece to this paper we study the thick irreducible spherical
buildings containing Fano plane residues. In these buildings automorphisms are
not necessarily capped
Computer assisted performance tests of the Lyman Alpha Coronagraph
Preflight calibration and performance tests of the Lyman Alpha Coronagraph rocket instrument in the laboratory, with the experiment in its flight configuration and illumination levels near those expected during flight were successfully carried out using a pulse code modulation telemetry system simulator interfaced in real time to a PDP 11/10 computer system. Post acquisition data reduction programs developed and implemented on the same computer system aided in the interpretation of test and calibration data
Lyman alpha coronagraph research sounding rocket program
The ultraviolet light coronagraph was developed and successfully flown on three rocket flights on 13 April 1979, 16 February 1980 and 20 July 1982. During each of these flights, the Ultraviolet Light Coronagraph was flown jointly with the White Light Coronagraph provided by the High Altitude Observatory. Ultraviolet diagnostic techniques and instrumentation for determining the basic plasma parameters of solar wind acceleration regions in the extended corona were developed and verified and the understanding of the physics of the corona through the performance, analysis and interpretation of solar observations advanced. Valuable UV diagnostics can be performed in the absence of a natural solar eclipse
Satellite-derived ice data sets no. 2: Arctic monthly average microwave brightness temperatures and sea ice concentrations, 1973-1976
A summary data set for four years (mid 70's) of Arctic sea ice conditions is available on magnetic tape. The data include monthly and yearly averaged Nimbus 5 electrically scanning microwave radiometer (ESMR) brightness temperatures, an ice concentration parameter derived from the brightness temperatures, monthly climatological surface air temperatures, and monthly climatological sea level pressures. All data matrices are applied to 293 by 293 grids that cover a polar stereographic map enclosing the 50 deg N latitude circle. The grid size varies from about 32 X 32 km at the poles to about 28 X 28 km at 50 deg N. The ice concentration parameter is calculated assuming that the field of view contains only open water and first-year ice with an ice emissivity of 0.92. To account for the presence of multiyear ice, a nomogram is provided relating the ice concentration parameter, the total ice concentration, and the fraction of the ice cover which is multiyear ice
The X-ray spectrum of Fe XVII revisited with a multi-ion model
The theoretical intensities of the soft X-ray Fe XVII lines arising from
2l-3l' transitions are reexamined using a three-ion collisional-radiative model
that includes the contributions to line formation of radiative recombination
(RR), dielectronic recombination (DR), resonant excitation (RE), and
inner-shell collisional ionization (CI), in addition to the usual contribution
of collisional excitation (CE). These additional processes enhance mostly the
2p-3s lines and not the 2p-3d lines. Under coronal equilibrium conditions, in
the electron temperature range of 400 to 600 eV where the Fe XVII line
emissivities peak, the combined effect of the additional processes is to
enhance the 2p-3s lines at 16.78, 17.05, and 17.10 A, by ~ 25%, 30%, and 55%,
respectively, compared with their traditional, single-ion CE values. The weak
2p-3d line at 15.45 A is also enhanced by up to 20%, while the other 2p-3d
lines are almost unaffected. The effects of DR and RE are found to be dominant
in this temperature range (400 - 600 eV), while that of CI is 3% at the most,
and the contribution of RR is less than 1%. At lower temperatures, where the Fe
XVII / Fe XVIII abundance ratio is high, the RE effect dominates. However, as
the temperature rises and the Fe XVIII abundance increases, the DR effect takes
over.
The newly calculated line powers can reproduce most of the often observed
high values of the (I17.05 + I17.10) / I15.01 intensity ratio. The importance
of ionization and recombination processes to the line strengths also helps to
explain why laboratory measurements in which CE is essentially the sole
mechanism agree well with single-ion calculations, but do not reproduce the
astrophysically observed ratios.Comment: Submitted to Ap
Rocket spectrometer for investigation of the far ultraviolet solar spectrum
A rocket-borne Ebert spectrometer and telescope were used for analysis of the solar spectrum. The instrument was arranged in the high resolution line scanning mode. Selected emission lines between 1170 and 1640 A were scanned, and a complete wavelength scan was made from 1170 A to 1850 A. Accurate measurements were made of the line profiles of the He II lines at 1640 A, C IV lines at 1550 A, Si IV lines at 1400 A, C II lines at 1335 A, the N V lines at 1240 A, and the C III lines at 1175 A. Accurate intensity measurements of the quiet sun spectrum for wavelengths between 1174 A and 3220 A were obtained. Spectral resolution was better than 0.03 A over most of the range and spatial resolution was relatively low so that the observations are averaged over the chromospheric network. Plots of absolute intensity versus wave length were prepared for the full wavelength range of the observations
Visible and infrared photocurrent enhancement in a graphene-silicon Schottky photodetector through surface-states and electric field engineering
The design of efficient graphene-silicon (GSi) Schottky junction
photodetectors requires detailed understanding of the spatial origin of the
photoresponse. Scanning-photocurrent-microscopy (SPM) studies have been carried
out in the visible wavelengths regions only, in which the response due to
silicon is dominant. Here we present comparative SPM studies in the visible
( = 633nm) and infrared ( = 1550nm) wavelength regions for a
number of GSi Schottky junction photodetector architectures, revealing the
photoresponse mechanisms for silicon and graphene dominated responses,
respectively, and demonstrating the influence of electrostatics on the device
performance. Local electric field enhancement at the graphene edges leads to a
more than ten-fold increased photoresponse compared to the bulk of the
graphene-silicon junction. Intentional design and patterning of such graphene
edges is demonstrated as an efficient strategy to increase the overall
photoresponse of the devices. Complementary simulations and modeling illuminate
observed effects and highlight the importance of considering graphene's shape
and pattern and device geometry in the device design
Model selection in cosmology
Model selection aims to determine which theoretical models are most plausible given some data, without necessarily considering preferred values of model parameters. A common model selection question is to ask when new data require introduction of an additional parameter, describing a newly discovered physical effect. We review model selection statistics, then focus on the Bayesian evidence, which implements Bayesian analysis at the level of models rather than parameters. We describe our CosmoNest code, the first computationally efficient implementation of Bayesian model selection in a cosmological context. We apply it to recent WMAP satellite data, examining the need for a perturbation spectral index differing from the scaleinvariant (Harrison–Zel'dovich) case
New directions in EEG measurement: an investigation into the fidelity of electrical potential sensor signals
Low frequency noise performance is the key indicator in determining the signal to noise ratio of a capacitively coupled sensor when used to acquire electroencephalogram signals. For this reason, a prototype Electric Potential Sensor device based on an auto-zero operational amplifier has been developed and evaluated. The absence of 1/f noise in these devices makes them ideal for use with signal frequencies ~10 Hz or less. The active electrodes are designed to be physically and electrically robust and chemically and biochemically inert. They are electrically insulated (anodized) and have diameters of 12 mm or 18 mm. In both cases, the sensors are housed in inert stainless steel machined housings with the electronics fabricated in surface mount components on a printed circuit board compatible with epoxy potting compounds. Potted sensors are designed to be immersed in alcohol for sterilization purposes. A comparative study was conducted with a commercial wet gel electrode system. These studies comprised measurements of both free running electroencephalogram and Event Related Potentials. Quality of the recorded electroencephalogram was assessed using three methods of inspection of raw signal, comparing signal to noise ratios, and Event Related Potentials noise analysis. A strictly comparable signal to noise ratio was observed and the overall conclusion from these comparative studies is that the noise performance of the new sensor is appropriate
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