3,199 research outputs found
Large-scale interaction of the solar wind with comets Halley and Giacobini-Zinner
In-situ measurements of comets Halley and Giacobini-Zinner have confirmed the accepted basic physics of comet/solar wind interaction. The solar wind magnetic field is captured by the comet through the mechanism of field-line loading by cometary ions and the field lines drape around the cometary ionosphere. With this basic model in hand, the large-scale structure of the plasma tail as revealed by submissions to the Large Scale Phenomena Network of the International Halley Watch is reviewed. The turn-on and turn-off of plasma activity seem consistent with theory. Some 16 obvious disconnection events (DEs) have been recorded. Preliminary results showed agreement with the sector-boundary model; a detailed analysis of all DEs will be required in order to make a definitive statement. A study of plasma activity around the time of the VEGA encounters provides strong support for the sector-boundary model and illustrates once again the power of simultaneous remote and in-situ measurements
Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs
In this paper we analyze the properties of a dc SQUID when the London
penetration depth \lambda is larger than the superconducting film thickness d.
We present equations that govern the static behavior for arbitrary values of
\Lambda = \lambda^2/d relative to the linear dimensions of the SQUID. The
SQUID's critical current I_c depends upon the effective flux \Phi, the magnetic
flux through a contour surrounding the central hole plus a term proportional to
the line integral of the current density around this contour. While it is well
known that the SQUID inductance depends upon \Lambda, we show here that the
focusing of magnetic flux from applied fields and vortex-generated fields into
the central hole of the SQUID also depends upon \Lambda. We apply this
formalism to the simplest case of a linear SQUID of width 2w, consisting of a
coplanar pair of long superconducting strips of separation 2a, connected by two
small Josephson junctions to a superconducting current-input lead at one end
and by a superconducting lead at the other end. The central region of this
SQUID shares many properties with a superconducting coplanar stripline. We
calculate magnetic-field and current-density profiles, the inductance
(including both geometric and kinetic inductances), magnetic moments, and the
effective area as a function of \Lambda/w and a/w.Comment: 18 pages, 20 figures, revised for Phys. Rev. B, the main revisions
being to denote the effective flux by \Phi rather than
AFC-Enabled Vertical Tail System Integration Study
This document serves as the final report for the SMAAART AFC-Enabled Vertical Tail System Integration Study. Included are the ground rule assumptions which have gone into the study, layouts of the baseline and AFC-enabled configurations, critical sizing information, system requirements and architectures, and assumed system properties that result in an NPV assessment of the two candidate AFC technologies
Probing the Complex and Variable X-ray Absorption of Markarian 6 with XMM-Newton
We report on an X-ray observation of the Seyfert 1.5 galaxy Mrk 6 obtained
with the EPIC instruments onboard XMM-Newton. Archival BeppoSAX PDS data from
18-120 keV were also used to constrain the underlying hard power-law continuum.
The results from our spectral analyses generally favor a double
partial-covering model, although other spectral models such as absorption by a
mixture of partially ionized and neutral gas cannot be firmly ruled out. Our
best-fitting model consists of a power law with a photon index of 1.81+/-0.20
and partial covering with large column densities up to 10^{23} cm**-2. We also
detect a narrow emission line consistent with Fe Kalpha fluorescence at
6.45+/-0.04 keV with an equivalent width of ~93+/-25 eV. Joint analyses of
XMM-Newton, ASCA, and BeppoSAX data further provide evidence for both spectral
variability (a factor of ~2 change in absorbing column) and
absorption-corrected flux variations (by ~60%) during the ~4 year period probed
by the observations.Comment: 7 pages, 2 figures. accepted for publication in the Astronomical
Journa
Electromagnetic Self-Duality in a Lattice Model
We formulate a Euclidean lattice theory of interacting elementary spin-half
electric and magnetic charges, which we refer to as electrons and magnetic
monopoles respectively. The model uses the polymer representation of the
fermion determinant, and exhibits a self-dual symmetry provided electric charge
and magnetic charge obey the minimal Dirac quantisation condition
. In a hopping parameter expansion at lowest order, we show that
virtual electron and monopole loops contribute radiative corrections of
opposite sign to the photon propagator. We argue that in the limit ,
fermion mass , the model describes QED together with strongly
interacting monopoles whose chiral symmetry is spontaneously broken. Prospects
for the existence of an interacting continuum limit at the self-dual point
are discussed.Comment: 29 pages plain TeX, 2 PostScript figures included using psfi
The collision of boosted black holes
We study the radiation from a collision of black holes with equal and
opposite linear momenta. Results are presented from a full numerical relativity
treatment and are compared with the results from a ``close-slow''
approximation. The agreement is remarkable, and suggests several insights about
the generation of gravitational radiation in black hole collisions.Comment: 8 pages, RevTeX, 3 figures included with eps
Human population growth offsets climate-driven increase in woody vegetation in sub-Saharan Africa
The rapidly growing human population in sub-Saharan Africa generates increasing demand for agricultural land and forest products, which presumably leads to deforestation. Conversely, a greening of African drylands has been reported, but this has been difficult to associate with changes in woody vegetation. There is thus an incomplete understanding of how woody vegetation responds to socio-economic and environmental change. Here we used a passive microwave Earth observation data set to document two different trends in land area with woody cover for 1992-2011: 36% of the land area (6,870,000 km2) had an increase in woody cover largely in drylands, and 11% had a decrease (2,150,000 km2), mostly in humid zones. Increases in woody cover were associated with low population growth, and were driven by increases in CO2 in the humid zones and by increases in precipitation in drylands, whereas decreases in woody cover were associated with high population growth. The spatially distinct pattern of these opposing trends reflects, first, the natural response of vegetation to precipitation and atmospheric CO2, and second, deforestation in humid areas, minor in size but important for ecosystem services, such as biodiversity and carbon stocks. This nuanced picture of changes in woody cover challenges widely held views of a general and ongoing reduction of the woody vegetation in Africa
Tidal interaction in binary black hole inspiral
In rotating viscous fluid stars, tidal torque leads to an exchange of spin
and orbital angular momentum. The horizon of a black hole has an effective
viscosity that is large compared to that of stellar fluids, and an effective
tidal torque may lead to important effects in the strong field interaction at
the endpoint of the inspiral of two rapidly rotating holes. In the most
interesting case both holes are maximally rotating and all angular momenta
(orbital and spins) are aligned. We point out here that in such a case (i) the
transfer of angular momentum may have an important effect in modifying the
gravitational wave ``chirp'' at the endpoint of inspiral. (ii) The tidal
transfer of spin energy to orbital energy may increase the amount of energy
being radiated. (iii) Tidal transfer in such systems may provide a mechanism
for shedding excess angular momentum. We argue that numerical relativity, the
only tool for determining the importance of tidal torque, should be more
specifically focused on binary configurations with aligned, large, angular
momenta.Comment: 5 pages, 2 figure
Three dimensional numerical relativity: the evolution of black holes
We report on a new 3D numerical code designed to solve the Einstein equations
for general vacuum spacetimes. This code is based on the standard 3+1 approach
using cartesian coordinates. We discuss the numerical techniques used in
developing this code, and its performance on massively parallel and vector
supercomputers. As a test case, we present evolutions for the first 3D black
hole spacetimes. We identify a number of difficulties in evolving 3D black
holes and suggest approaches to overcome them. We show how special treatment of
the conformal factor can lead to more accurate evolution, and discuss
techniques we developed to handle black hole spacetimes in the absence of
symmetries. Many different slicing conditions are tested, including geodesic,
maximal, and various algebraic conditions on the lapse. With current
resolutions, limited by computer memory sizes, we show that with certain lapse
conditions we can evolve the black hole to about , where is the
black hole mass. Comparisons are made with results obtained by evolving
spherical initial black hole data sets with a 1D spherically symmetric code. We
also demonstrate that an ``apparent horizon locking shift'' can be used to
prevent the development of large gradients in the metric functions that result
from singularity avoiding time slicings. We compute the mass of the apparent
horizon in these spacetimes, and find that in many cases it can be conserved to
within about 5\% throughout the evolution with our techniques and current
resolution.Comment: 35 pages, LaTeX with RevTeX 3.0 macros. 27 postscript figures taking
7 MB of space, uuencoded and gz-compressed into a 2MB uufile. Also available
at http://jean-luc.ncsa.uiuc.edu/Papers/ and mpeg simulations at
http://jean-luc.ncsa.uiuc.edu/Movies/ Submitted to Physical Review
Properties of Regge Trajectories
Early Chew-Frautschi plots show that meson and baryon Regge trajectoies are
approximately linear and non-intersecting. In this paper, we reconstruct all
Regge trajectories from the most recent data. Our plots show that meson
trajectories are non-linear and intersecting. We also show that all current
meson Regge trajectories models are ruled out by data.Comment: 30 pages, latex, 18 figures, to be published in Physical Review
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