1,444 research outputs found
Analyzing seasonal temperature trends in forced climate simulations of the past millennium
Temperature observations from the Northern Hemisphere reveal a warming since 1861 which is larger in winter than in summer. Possible explanations for a decline in seasonal spread are discussed using the Earth system model CLIMBER-2. Simulations forced by natural and anthropogenic factors (Milankovitch forcing, solar variability, volcanism, atmospheric CO2 concentration, deforestation) generate specific seasonal responses. While the Milankovitch forcing increased the millennial seasonal spread, and solar variability and volcanism proved ancillary in reducing the spread on the centennial timescale, the anthropogenic factors appear the primary agents to attenuate the seasonal spread. The climatic effect of the anthropogenic factors is amplified by seasonally varying feedbacks related to the albedo of changing sea-ice and snow cover
Creel Survey on Newton Lake
Final Report issued May 1, 2001Report issued on: May 1, 2001INHS Technical Report prepared for Ameren Corporatio
Stability borders of feedback control of delayed measured systems
When stabilization of unstable periodic orbits or fixed points by the method
given by Ott, Grebogi and Yorke (OGY) has to be based on a measurement delayed
by orbit lengths, the performance of unmodified OGY method is expected
to decline. For experimental considerations, it is desired to know the range of
stability with minimal knowledge of the system. We find that unmodified OGY
control fails beyond a maximal Ljapunov number of
. In this paper the area of stability is
investigated both for OGY control of known fixed points and for difference
control of unknown or inaccurately known fixed points. An estimated value of
the control gain is given. Finally we outline what extensions have to be
considered if one wants to stabilize fixed points with Ljapunov numbers above
.Comment: 5 pages LaTeX using revtex and epsfig (4 figs included). Revised
versio
Self-organized pore formation and open-loop-control in semiconductor etching
Electrochemical etching of semiconductors, apart from many technical
applications, provides an interesting experimental setup for self-organized
structure formation capable e.g. of regular, diameter-modulated, and branching
pores. The underlying dynamical processes governing current transfer and
structure formation are described by the Current-Burst-Model: all dissolution
processes are assumed to occur inhomogeneously in time and space as a Current
Burst (CB); the properties and interactions between CB's are described by a
number of material- and chemistry- dependent ingredients, like passivation and
aging of surfaces in different crystallographic orientations, giving a
qualitative understanding of resulting pore morphologies. These morphologies
cannot be influenced only by the current, by chemical, material and other
etching conditions, but also by an open-loop control, triggering the time scale
given by the oxide dissolution time. With this method, under conditions where
only branching pores occur, the additional signal hinders side pore formation
resulting in regular pores with modulated diameter
Investigation of topographical stability of the concave and convex Self-Organizing Map variant
We investigate, by a systematic numerical study, the parameter dependence of
the stability of the Kohonen Self-Organizing Map and the Zheng and Greenleaf
concave and convex learning with respect to different input distributions,
input and output dimensions
Assessing climate forcings of the Earth system for the past millennium
[1] The effects of natural and anthropogenic forcings (solar activity, volcanism, atmospheric CO2 concentration, deforestation) on climate changes are estimated with the Earth system model of intermediate complexity, CLIMBER-2, for the past millennium. Simulated surface air temperatures for the Northern Hemisphere from the combined forcing correlate reasonably well with paleoclimatic data (r = 0.70). The largest negative anomalies occur when insolation minima coincide with volcanic eruptions. Anthropogenic forcings impose additional climate changes after 1850. The increasing warming from increasing CO2 concentrations is attenuated by the cooling effect from deforestation. Results from differently combined forcings suggest that the relatively cool climate in the second half of 19th century is largely attributable to cooling from deforestation
Polarization of Astronomical Maser Radiation. IV. Circular Polarization Profiles
Profile comparison of the Stokes parameters and is a powerful tool
for maser data analysis, providing the first direct methods for unambiguous
determination of (1) the maser saturation stage, (2) the amplification optical
depth and intrinsic Doppler width of unsaturated masers, and (3) the
comparative magnitudes of Zeeman splitting and Doppler linewidth. Circular
polarization recently detected in OH 1720 MHz emission from the Galactic center
appears to provide the first direct evidence for maser saturation.Comment: 14 pages, 1 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journa
Ground-State SiO Maser Emission Toward Evolved Stars
We have made the first unambiguous detection of vibrational ground-state
maser emission from SiO toward six evolved stars. Using the Very Large Array,
we simultaneously observed the v=0, J=1-0, 43.4-GHz, ground-state and the v=1,
J=1-0, 43.1-GHz, first excited-state transitions of SiO toward the oxygen-rich
evolved stars IRC+10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and the
S-type star chi Cyg. We detected at least one v=0 SiO maser feature from six of
the seven stars observed, with peak maser brightness temperatures ranging from
10,000 K to 108,800 K. In fact, four of the seven v=0 spectra show multiple
maser peaks, a phenomenon which has not been previously observed. Ground-state
thermal emission was detected for one of the stars, RX Boo, with a peak
brightness temperature of 200 K. Comparing the v=0 and the v=1 transitions, we
find that the ground-state masers are much weaker with spectral characteristics
different from those of the first excited-state masers. For four of the seven
stars the velocity dispersion is smaller for the v=0 emission than for the v=1
emission, for one star the dispersions are roughly equivalent, and for two
stars (one of which is RX Boo) the velocity spread of the v=0 emission is
larger. In most cases, the peak flux density in the v=0 emission spectrum does
not coincide with the v=1 maser peak. Although the angular resolution of these
VLA observations were insufficient to completely resolve the spatial structure
of the SiO emission, the SiO spot maps produced from the interferometric image
cubes suggest that the v=0 masers are more extended than their v=1
counterparts
A revised distance to IRAS 162932422 from VLBA astrometry of associated water masers
IRAS 16293-2422 is a very well studied young stellar system seen in
projection towards the L1689N cloud in the Ophiuchus complex. However, its
distance is still uncertain with a range of values from 120 pc to 180 pc. Our
goal is to measure the trigonometric parallax of this young star by means of
HO maser emission. We use archival data from 15 epochs of VLBA observations
of the 22.2 GHz water maser line. By modeling the displacement on the sky of
the HO maser spots, we derived a trigonometric parallax of mas,
corresponding to a distance of pc. This new distance is in
good agreement with recent values obtained for other magnetically active young
stars in the L1689 cloud. We relate the kinematics of these masers with the
outflows and the recent ejections powered by source A in the system.Comment: 14 pages, 6 tables, 8 figures. Accepted to be published in Astronomy
\& Astrophysic
Database management and analysis of fisheries in Illinois: Final report, 1 March 1999-28 February 2002
Issued May 2002; F-69-RReport issued on: May 200
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