22,497 research outputs found
The lost sunspot cycle: New support from Be10 measurements
It has been suggested that the deficit in the number of spots on the surface
of the Sun between 1790 and 1830, known as the Dalton minimum, contained an
extra cycle that was not identified in the original sunspot record by Wolf.
Though this cycle would be shorter and weaker than the average solar cycle, it
would shift the magnetic parity of the solar magnetic field of the earlier
cycles. This extra cycle is sometimes referred to as the 'lost solar cycle' or
'cycle 4b'. Here we reanalyse Be10 measurements with annual resolution from the
NGRIP ice core in Greenland in order to investigate if the hypothesis regarding
a lost sunspot cycle is supported by these measurements. Specifically, we make
use of the fact that the Galactic cosmic rays, responsible for forming Be10 in
the Earth's atmosphere, are affected differently by the open solar magnetic
field during even and odd solar cycles. This fact enables us to evaluate if the
numbering of cycles earlier than cycle 5 is correct. For the evaluation, we use
Bayesian analysis, which reveals that the lost sunspot cycle hypothesis is
likely to be correct. We also discuss if this cycle 4b is a real cycle, or a
phase catastrophe, and what implications this has for our understanding of
stellar activity cycles in general.Comment: accepted for publication in A&
Building a Sample of Distant Clusters of Galaxies
Candidate clusters of galaxies drawn from the sample identified from the
moderately deep I-band data of the ESO Imaging Survey (EIS), have been used for
follow-up optical/infrared imaging and spectroscopic observations. The
observations were conducted to assess the nature of these candidates over a
large range of redshifts. Currently, 163 EIS candidates have (V-I) colors, 15
have (I-K) and 65 cluster fields have been observed spectroscopically. From a
preliminary analysis of these data, we find that > 65% of the candidates
studied show strong evidence of being real physical associations, over the
redshift range 0.2<z<1.1. The evidence in some cases comes directly from
spectroscopic measurements, in others indirectly from the detection of
overdensities of objects with either the same color or the same photometric
redshift, or from a combination of color and spectroscopic information.
Preliminary results also suggest that the redshift derived from the
matched-filter algorithm is a reasonable measure of the cluster's redshift,
possibly overestimating it by Delta z ~0.1, at least for systems at z<0.7.
Overdensities of red objects have been detected in over 100 candidates, 38 of
which with estimated redshifts >0.6, and six candidates in the interval
0.45<z<0.81 have either been identified directly from measured redshifts or
have been confirmed by the measurement of at least one redshift for galaxies
located along a red-sequence typical of cluster early-type galaxies. Lastly,
five candidates among those already observed in the infrared have (I-Ks) colors
consistent with them being in the redshift interval 0.8<z<1.1. The sample of
"confirmed" clusters, already the largest of its kind in the southern
hemisphere, will be further enlarged by ongoing observations.Comment: To appear in "Large Scale Structure in the X-ray Universe", ed. M.
Plionis and I. Georgantopoulos (Paris: Editions Frontieres), in pres
Silicon MINP solar cells
The MINP solar cell concept refers to a cell structure designed to be a base region dominated device. Thus, it is desirable that recombination losses are reduced to the point that they occur only in the base region. The most unique feature of the MINP cell design is that a tunneling contact is utilized for the metallic contact on the front surface. The areas under the collector grid and bus bar are passivated by a thin oxide of tunneling thickness. Efforts must also be taken to minimize recombination at the surface between grid lines, at the junction periphery and within the emitter. Results of both theoretical and experimental studies of silicon MINP cells are given. Performance calculations are described which give expected efficiencies as a function of base resistivity and junction depth. Fabrication and characterization of cells are discussed which are based on 0.2 ohm-cm substrates, diffused emitters on the order of 0.15 to 0.20 microns deep, and with Mg MIS collector grids. A total area AM 1 efficiency of 16.8% was achieved. Detailed analyses of photocurrent and current loss mechanisms are presented and utilized to discuss future directions of research. Finally, results reported by other workers are discussed
Tripartite entanglement and threshold properties of coupled intracavity downconversion and sum-frequency generation
The process of cascaded downconversion and sum-frequency generation inside an
optical cavity has been predicted to be a potential source of three-mode
continuous-variable entanglement. When the cavity is pumped by two fields, the
threshold properties have been analysed, showing that these are more
complicated than in well-known processes such as optical parametric
oscillation. When there is only a single pumping field, the entanglement
properties have been calculated using a linearised fluctuation analysis, but
without any consideration of the threshold properties or critical operating
points of the system. In this work we extend this analysis to demonstrate that
the singly pumped system demonstrates a rich range of threshold behaviour when
quantisation of the pump field is taken into account and that asymmetric
polychromatic entanglement is available over a wide range of operational
parameters.Comment: 24 pages, 15 figure
Investigation of high efficiency GaAs solar cells
Investigations of basic mechanisms which limit the performance of high efficiency GaAs solar cells are discussed. P/N heteroface structures have been fabricated from MOCVD epiwafers. Typical AM1 efficiencies are in the 21 to 22 percent range, with a SERI measurement for one cell being 21.5 percent. The cells are nominally 1.5 x 1.5 cm in size. Studies have involved photoresponse, T-I-V analyses, and interpretation of data in terms of appropriate models to determine key cell parameters. Results of these studies are utilized to determine future approaches for increasing GaAs solar cell efficiencies
Electro-optical characterization of GaAs solar cells
The electro-optical characterization of gallium arsenide p/n solar cells is discussed. The objective is to identify and understand basic mechanisms which limit the performance of high efficiency gallium arsenide solar cells. The approach involves conducting photoresponse and temperature dependent current-voltage measurements, and interpretation of the data in terms of theory to determine key device parameters. Depth concentration profiles are also utilized in formulating a model to explain device performance
Multi-object spectroscopy of low-redshift EIS clusters IV. Reliability of matched-filter results at z~0.3-0.4
abridged) In this paper we present new redshifts for 747 galaxies in 23 ESO
Imaging Survey (EIS) cluster fields. We use the "gap"-technique to search for
significant overdensities in redshift space for identifying groups/clusters of
galaxies. In this way we spectroscopically confirm systems in 10 of the 23
cluster candidate fields with a matched-filter estimated redshift z_MF=0.3-0.4
and with spectroscopic redshifts in the range from z=0.158 to z=0.534. We find
that the systems identified in the present paper span a broad range of
one-dimensional velocity dispersion (175-497 km/s) and richness
(12L*<=L<=65L*). Both undersampling and contamination by substructures
contribute to the uncertainty of these measurements. From the analysis of the
colours of the galaxy populations we find that ~60% of the spectroscopically
confirmed systems have a "significant" red sequence with a colour matching
passive stellar evolution models. With this paper we complete our spectroscopic
survey of the fields of 58 EIS cluster candidates with estimated redshifts
z<=0.4. We have measured a total of 1954 galaxy redshifts in the range z=0.0065
to z=0.6706. Of the 58 systems we confirm 42 (~75%) with redshifts between
z=0.095 and z=0.534.Comment: Accepted for publication in A&A, 19 pages, 11 figure
Quadripartite continuous-variable entanglement via quadruply concurrent downconversion
We investigate an intra-cavity coupled down-conversion scheme to generate
quadripartite entanglement using concurrently resonant nonlinearities. We
verify that quadripartite entanglement is present in this system by calculating
the output fluctuation spectra and then considering violations of optimized
inequalities of the van Loock-Furusawa type. The entanglement characteristics
both above and below the oscillation threshold are considered. We also present
analytic solutions for the quadrature operators and the van Loock-Furusawa
correlations in the undepleted pump approximation.Comment: 9 pages, 5 figure
Microbial Activity Response to Solar Radiation across Contrasting Environmental Conditions in Salar de Huasco, Northern Chilean Altiplano
Indexación: Web of Science; Scopus.In high altitude environments, extreme levels of solar radiation and important differences of ionic concentrations over narrow spatial scales may modulate microbial activity. In Salar de Huasco, a high-altitude wetland in the Andean mountains, the high diversity of microbial communities has been characterized and associated with strong environmental variability. Communities that differed in light history and environmental conditions, such as nutrient concentrations and salinity from different spatial locations, were assessed for bacterial secondary production (BSP, H-3-leucine incorporation) response from short-term exposures to solar radiation. We sampled during austral spring seven stations categorized as: (a) source stations, with recently emerged groundwater (no-previous solar exposure); (b) stream running water stations; (c) stations connected to source waters but far downstream from source points; and (d) isolated ponds disconnected from ground sources or streams with a longer isolation and solar exposure history. Very high values of 0.25 mu E m(-2) s(-1), 72 W m(-2) and 12 W m(-2) were measured for PAR, UVA, and UVB incident solar radiation, respectively. The environmental factors measured formed two groups of stations reflected by principal component analyses (near to groundwater sources and isolated systems) where isolated ponds had the highest BSP and microbial abundance (35 microalgae taxa, picoeukaryotes, nanoflagellates, and bacteria) plus higher salinities and PO43- concentrations. BSP short-term response (4 h) to solar radiation was measured by H-3-leucine incorporation under four different solar conditions: full sun, no UVB, PAR, and dark. Microbial communities established in waters with the longest surface exposure (e.g., isolated ponds) had the lowest BSP response to solar radiation treatments, and thus were likely best adapted to solar radiation exposure contrary to ground source waters. These results support our light history (solar exposure) hypothesis where the more isolated the community is from ground water sources, the better adapted it is to solar radiation. We suggest that factors other than solar radiation (e.g., salinity, PO43-, NO3-) are also important in determining microbial productivity in heterogeneous environments such as the Salar de Huasco.http://journal.frontiersin.org/article/10.3389/fmicb.2016.01857/ful
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