2,976 research outputs found
Solar wind observations with the ion composition instrument aboard the ISEE-3 ICE spacecraft
The principal observations obtained by the Ion Composition Instrument (ICI) flown on the ISEE-3/ICE spacecraft, which was in the solar wind from September 1978 to the end of 1982, before being directed to the far magnetotail of the Earth are discussed. Almost continuous observations were made of the abundances of 3He++, 4He++, O6+, O7+, Ne, Si and Fe in various charge states, and of their bulk speeds and temperatures. The results show that there is a strong tendency in the collisionless solar wind for the ionic temperatures to be proportional to the masses. For heavier ions these temperatures exceed typical coronal electron temperatures. 4He++, especially in high speed streams, moves faster than H+, and travels at the same speed as heavier ions. The mechanism leading to this heating and rapid streaming is still not entirely clear
Molecular line opacity of LiCl in the mid-infrared spectra of brown dwarfs
We present a complete line list for the X 1Sigma+ electronic ground state of
LiCl computed using fully quantum-mechanical techniques. This list includes
transition energies and oscillator strengths in the spectral region
0.3-39,640.7 cm-1 for all allowed rovibrational transitions in absorption
within the electronic ground state. The calculations were performed using an
accurate hybrid potential constructed from a spectral inversion fit of
experimental data and from recent multi-reference single- and double-excitation
configuration interaction calculations. The line list was incorporated into the
stellar atmosphere code PHOENIX to compute spectra for a range of young to old
T dwarf models. The possibility of observing a signature of LiCl in absorption
near 15.8 microns is addressed and the proposal to use this feature to estimate
the total lithium elemental abundance for these cool objects is discussed.Comment: 8 pages, 2 figures, 1 table. Accepted for publication in ApJ 613,
Sept. 20 200
Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows
The nature of dynamo action in shear flows prone to magnetohydrodynamic
instabilities is investigated using the magnetorotational dynamo in Keplerian
shear flow as a prototype problem. Using direct numerical simulations and
Newton's method, we compute an exact time-periodic magnetorotational dynamo
solution to the three-dimensional dissipative incompressible
magnetohydrodynamic equations with rotation and shear. We discuss the physical
mechanism behind the cycle and show that it results from a combination of
linear and nonlinear interactions between a large-scale axisymmetric toroidal
magnetic field and non-axisymmetric perturbations amplified by the
magnetorotational instability. We demonstrate that this large scale dynamo
mechanism is overall intrinsically nonlinear and not reducible to the standard
mean-field dynamo formalism. Our results therefore provide clear evidence for a
generic nonlinear generation mechanism of time-dependent coherent large-scale
magnetic fields in shear flows and call for new theoretical dynamo models.
These findings may offer important clues to understand the transitional and
statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review
Fundamental length in quantum theories with PT-symmetric Hamiltonians II: The case of quantum graphs
Manifestly non-Hermitian quantum graphs with real spectra are introduced and
shown tractable as a new class of phenomenological models with several
appealing descriptive properties. For illustrative purposes, just equilateral
star-graphs are considered here in detail, with non-Hermiticities introduced by
interactions attached to the vertices. The facilitated feasibility of the
analysis of their spectra is achieved via their systematic approximative
Runge-Kutta-inspired reduction to star-shaped discrete lattices. The resulting
bound-state spectra are found real in a discretization-independent interval of
couplings. This conclusion is reinterpreted as the existence of a hidden
Hermiticity of our models, i.e., as the standard and manifest Hermiticity of
the underlying Hamiltonian in one of less usual, {\em ad hoc} representations
of the Hilbert space of states in which the inner product is local
(at ) or increasingly nonlocal (at ). Explicit examples of
these (of course, Hamiltonian-dependent) hermitizing inner products are offered
in closed form. In this way each initial quantum graph is assigned a menu of
optional, non-equivalent standard probabilistic interpretations exhibiting a
controlled, tunable nonlocality.Comment: 33 pp., 6 figure
Online Forum Thread Retrieval using Pseudo Cluster Selection and Voting Techniques
Online forums facilitate knowledge seeking and sharing on the Web. However,
the shared knowledge is not fully utilized due to information overload. Thread
retrieval is one method to overcome information overload. In this paper, we
propose a model that combines two existing approaches: the Pseudo Cluster
Selection and the Voting Techniques. In both, a retrieval system first scores a
list of messages and then ranks threads by aggregating their scored messages.
They differ on what and how to aggregate. The pseudo cluster selection focuses
on input, while voting techniques focus on the aggregation method. Our combined
models focus on the input and the aggregation methods. The result shows that
some combined models are statistically superior to baseline methods.Comment: The original publication is available at
http://www.springerlink.com/. arXiv admin note: substantial text overlap with
arXiv:1212.533
Alpha Activity During Lucid Dreaming
We have been interested in the electrophysiological correlates of lucid dreaming (LD) since early work in this laboratory suggested a relationship between lucidity and alpha activity (Ogilvie, Hunt, Sawicki &McGowan, 1978; Ogilvie, Hunt, Tyson, Lucescu & Jeakins, 1982; Tyson, Ogilvie, & Hunt, 1984). Until now, this alpha-lucidity hypothesis had not been tested in our lab on high frequency lucid dreamers who signal while in REM sleep, and LaBerge (1980; 1981) has not observed any changes in alpha in signalled episodes of lucidity. The present report describes computer analyses of EEG activity obtained during eight laboratory nights from a frequent lucid dreamer. In addition to standard polysomnographic measures, CCTV monitoring with two cameras and a screen splitter permitted simultaneous display and videotaping of the subject's (RJS's) face and concurrent polygraphic activity. This was particularly useful during REM, since RJS had trained himself to sleep on his back and to try to signal from that position. In the month prior to the lab nights, RJS spend over an hour per day meditating. He also used LaBerge's lucidity induction or cognitive self-instruction techniques and reported LD rates in excess of one per night. He practiced signalling (using three blinks/ rapid vertical eye movements) and thought he had successfully signalled several LDs while at home
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Field-Weighted XML Retrieval Based on BM25.
This is the first year for the Centre for Interactive Systems Research participation of INEX. Based on a newly developed XML indexing and retrieval system on Okapi, we extend Robertson’s field-weighted BM25F for document retrieval to element level retrieval function BM25E. In this paper, we introduce this new function and our experimental method in detail, and then show how we tuned weights for our selected fields by using INEX 2004 topics and assessments. Based on the tuned models we submitted our runs for CO.Thorough, CO.FetchBrowse, the methods we propose show real promise. Existing problems and future work are also discussed
Turbulence and angular momentum transport in a global accretion disk simulation
The global development of magnetohydrodynamic turbulence in an accretion disk
is studied within a simplified disk model that omits vertical stratification.
Starting with a weak vertical seed field, a saturated state is obtained after a
few tens of orbits in which the energy in the predominantly toroidal magnetic
field is still subthermal. The efficiency of angular momentum transport,
parameterized by the Shakura-Sunyaev alpha parameter, is of the order of 0.1.
The dominant contribution to alpha comes from magnetic stresses, which are
enhanced by the presence of weak net vertical fields. The power spectra of the
magnetic fields are flat or decline only slowly towards the largest scales
accessible in the calculation, suggesting that the viscosity arising from MHD
turbulence may not be a locally determined quantity. I discuss how these
results compare with observationally inferred values of alpha, and possible
implications for models of jet formation.Comment: ApJ Letters, in press. The paper and additional visualizations are
available at http://www.cita.utoronto.ca/~armitage/global_abs.htm
Negatively Charged Strangelet Search using the E864 Spectrometer at the AGS
We provide a status report on the progress of searching for negatively
charged strangelets using the E864 spectrometer at the AGS. About 200 million
recorded events representing approximately 14 billion 10% central interactions
of Au + Pt at 11.5 GeV/c taken during the 1996-1997 run of the experiment are
used in the analysis. No strangelet candidates are seen for charges Z=-1 and
Z=-2, corresponding to a 90% confidence level for upper limits of strangelet
production of ~1 x 10^{-8} and ~4 x 10^{-9} per central collision respectively.
The limits are nearly uniform over a wide range of masses and are valid only
for strangelets which are stable or have lifetimes greater than ~50 ns.Comment: 6 pages, 4 figures; Talk at SQM'98, Padova, Italy (July 20-24, 1998
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