976 research outputs found
Early last glacial maximum in the southern Central Andes reveals northward shift of the westerlies at ~39 ka
The latitudinal position of the southern westerlies has been suggested to be a key parameter for the climate on Earth. According to the general notion, the southern westerlies were shifted equatorward during the global Last Glacial Maximum (LGM: ~24–18 ka), resulting in reduced deep ocean ventilation, accumulation of old dissolved carbon, and low atmospheric CO<sub>2</sub> concentrations. In order to test this notion, we applied surface exposure dating on moraines in the southern Central Andes, where glacial mass balances are particularly sensitive to changes in precipitation, i.e. to the latitudinal position of the westerlies. Our results provide robust evidence that the maximum glaciation occurred already at ~39 ka, significantly predating the global LGM. This questions the role of the westerlies for atmospheric CO<sub>2</sub>, and it highlights our limited understanding of the forcings of atmospheric circulation
A framework for habitat monitoring and climate change modelling: construction and validation of the Environmental Stratification of Estonia
Environmental stratifications provide the framework for efficient surveillance and monitoring of biodiversity and ecological resources, as well as modelling exercises. An obstacle for agricultural landscape monitoring in Estonia has been the lack of a framework for the objective selection of monitoring sites. This paper describes the construction and testing of the Environmental Stratification of Estonia (ESE). Principal components analysis was used to select the variables that capture the most amount of variation. Seven climate variables and topography were selected and subsequently subjected to the ISODATA clustering routine in order to produce relatively homogeneous environmental strata. The ESE contains eight strata, which have been described in terms of soil, land cover and climatic parameters. In order to assess the reliability of the stratification procedure for the selection of monitoring sites, the ESE was compared with the previous map of Landscape Regions of Estonia and correlated with five environmental data sets. All correlations were significant. The stratification has therefore already been used to extend the current series of samples in agricultural landscapes into a more statistically robust series of monitoring sites. The potential for applying climate change scenarios to assess the shifts in the strata and associated ecological impacts is also examined.</p
Twenty Years of Timing SS433
We present observations of the optical ``moving lines'' in spectra of the
Galactic relativistic jet source SS433 spread over a twenty year baseline from
1979 to 1999. The red/blue-shifts of the lines reveal the apparent precession
of the jet axis in SS433, and we present a new determination of the precession
parameters based on these data. We investigate the amplitude and nature of
time- and phase-dependent deviations from the kinematic model for the jet
precession, including an upper limit on any precessional period derivative of
. We also dicuss the implications of these results
for the origins of the relativistic jets in SS433.Comment: 21 pages, including 9 figures. To appear in the Astrophysical Journa
OAO-3 end of mission tests report
Twelve engineering type tests were performed on several subsystems and experiment(s) of the OAO 3 spacecraft near its end of mission. The systems tested include: Princeton experiment package (PEP), fine error system guidance, inertial reference unit, star trackers, heat pipes, thermal control coatings, command and data handling, solar array; batteries, and onboard processor/power boost regulator. Generally, the systems performed well for the 8 1/2 years life of OAO 3, although some degradation was noted in the sensitivity of PEP and in the absorptivity of the skin coatings. Battery life was prolonged during the life of the mission in large part by carefully monitoring the charge-discharge cycle with careful attention not to overcharge
Изучение каталитической активности ультрадисперсных порошков железа в процессе получения жидких углеводородов из синтез-газа
The propagation of linearly polarized large-amplitude electromagnetic waves in critical density plasmas is studied in the framework of the Akiezer-Polovin model. A new mechanism of mode conversion is presented. The well-known periodic solutions are generalized to quasiperiodic solutions taking into account additional electrostatic oscillations. Nearly periodic circle-like solutions are found to be stabilized by intrinsic mode coupling whereas for nearly periodic eight-like solutions an effective mode conversion mechanism is discovered. Finally, the modulation timescales are considered
Chance long-distance or human-mediated dispersal? How Acacia s.l. farnesiana attained its pan-tropical distribution
Acacia s.l. farnesiana, which originates from Mesoamerica, is the most widely distributed Acacia s.l. species across the tropics. It is assumed that the plant was transferred across the Atlantic to southern Europe by Spanish explorers, and then spread across the Old World tropics through a combination of chance long-distance and human-mediated dispersal. Our study uses genetic analysis and information from historical sources to test the relative roles of chance and human-mediated dispersal in its distribution. The results confirm the Mesoamerican origins of the plant and show three patterns of human- mediated dispersal. Samples from Spain showed greater genetic diversity than those from other Old World tropics, suggesting more instances of transatlantic introductions from the Americas to that country than to other parts of Africa and Asia. Individuals from the Philippines matched a population from South Central Mexico and were likely to have been direct, trans-Pacific introductions. Australian samples were genetically unique, indicating that the arrival of the species in the continent was independent of these European colonial activities. This suggests the possibility of pre-European human- mediated dispersal across the Pacific Ocean. These significant findings raise new questions for biogeographic studies that assume chance or transoceanic dispersal 2 for disjunct plant distributions
Large-Scale Sunyaev-Zel'dovich Effect: Measuring Statistical Properties with Multifrequency Maps
We study the prospects for extracting detailed statistical properties of the
Sunyaev-Zel'dovich (SZ) effect associated with large scale structure using
upcoming multifrequency CMB experiments. The greatest obstacle to detecting the
large-angle signal is the confusion noise provided by the primary anisotropies
themselves, and to a lesser degree galactic and extragalactic foregrounds. We
employ multifrequency subtraction techniques and the latest foregrounds models
to determine the detection threshold for the Boomerang, MAP (several microK)
and Planck CMB (sub microK) experiments. Calibrating a simplified biased-tracer
model of the gas pressure off recent hydrodynamic simulations, we estimate the
SZ power spectrum, skewness and bispectrum through analytic scalings and N-body
simulations of the dark matter. We show that the Planck satellite should be
able to measure the SZ effect with sufficient precision to determine its power
spectrum and higher order correlations, e.g. the skewness and bispectrum.
Planck should also be able to detect the cross correlation between the SZ and
gravitational lensing effect in the CMB. Detection of these effects will help
determine the properties of the as yet undetected gas, including the manner in
which the gas pressure traces the dark matter.Comment: 13 ApJ pages, 11 figures; typos and figure 5 revised; submitted to
Ap
Phase transitions in self-gravitating systems. Self-gravitating fermions and hard spheres models
We discuss the nature of phase transitions in self-gravitating systems both
in the microcanonical and in the canonical ensemble. We avoid the divergence of
the gravitational potential at short distances by considering the case of
self-gravitating fermions and hard spheres models. Three kinds of phase
transitions (of zeroth, first and second order) are evidenced. They separate a
``gaseous'' phase with a smoothly varying distribution of matter from a
``condensed'' phase with a core-halo structure. We propose a simple analytical
model to describe these phase transitions. We determine the value of energy (in
the microcanonical ensemble) and temperature (in the canonical ensemble) at the
transition point and we study their dependance with the degeneracy parameter
(for fermions) or with the size of the particles (for a hard spheres gas).
Scaling laws are obtained analytically in the asymptotic limit of a small short
distance cut-off. Our analytical model captures the essential physics of the
problem and compares remarkably well with the full numerical solutions.Comment: Submitted to Phys. Rev. E. New material adde
Optical and X-ray clusters as tracers of the supercluster-void network. I Superclusters of Abell and X-ray clusters
We study the distribution of X-ray selected clusters of galaxies with respect
to superclusters determined by Abell clusters of galaxies and show that the
distribution of X-ray clusters follows the supercluster-void network determined
by Abell clusters. We find that in this network X-ray clusters are more
strongly clustered than other clusters. Poor, non-Abell X-ray clusters follow
the supercluster-void network as well: these clusters are embedded in
superclusters determined by rich clusters and populate filaments between them.
We present a new catalog of superclusters of Abell clusters out to a redshift
of z_{lim}=0.13, a catalog of X-ray clusters located in superclusters
determined by Abell clusters, and a list of additional superclusters of X-ray
clusters.Comment: LaTex (sty files added), 16 pages, 3 ps figures, submitted to
Astronomical Journal. Animations of the 3D distribution of superclusters of
Abell and X-ray clusters at http://www.aai.ee/~maret/SCLVnet.ht
General-Relativistic Thomas-Fermi model
A system of self-gravitating massive fermions is studied in the framework of
the general-relativistic Thomas-Fermi model. We study the properties of the
free energy functional and its relation to Einstein's field equations. A
self-gravitating fermion gas we then describe by a set of Thomas-Fermi type
self-consistency equations.Comment: 7 pages, LaTex, to appear in Gen. Rel. Gra
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